Southern Wet-Mesic Hardwood Forest - MHs49
Rich, wet-mesic lowland hardwood forests on level silty alluvium in stream valleys and on level glacial till bordering lakes. Sites are protected from fire, and soils remain moist throughout the growing season.
MHs49 forests are a somewhat uncommon hardwood community located across southern Minnesota (see map; 50 relevés, 39 ECS worksheets). Most occurrences of this community are found in the Paleozoic Plateau (PPL) section. Other minor amounts of observances are found in the Minnesota & Northeast Iowa Morainal (MIM) and North Central Glaciated Plains (CGP) sections.
Vegetation Structure & Composition
Description is based on summary of vegetation data from 39 plots (relevés).
- Ground-layer cover is mostly continuous (75-100%). Important species include false rue anemone (Enemion biternatum), blue phlox (Phlox divaricate), stemless blue violets (Viola sororia and V. missouriensis), hispid buttercup (Ranunculus hispidus), appendaged waterleaf (Hydrophyllum appendiculatum), Virginia spring beauty (Claytonia virginica), tall coneflower (Rudbeckia laciniata), white trout lily (Erythronium albidium), yellow trout lily (E. americanum), white bear sedge (Carex albursina), and hairy-leaved sedge (C. hirtifolia). Other common and often abundant species include Virginia waterleaf (Hydrophyllum virginianum), cleavers (Galium aparine), and wood nettle (Laportea canadensis).
- Shrub-layer cover is variable, ranging from sparse to continuous (5-100%); typical species are chokecherry (Prunus virginiana), Missouri gooseberry (Ribes missouriense), basswood, sugar maple, black ash, hackberry, bitternut hickory, American elm, red elm, and rock elm.
- Subcanopy is generally patchy to continuous (25-100% cover), with sugar maple, basswood, hackberry, ironwood, black ash, and elms the most common species.
- Canopy cover is mostly interrupted to continuous (50-100%). Species composition is variable, but basswood, black ash, sugar maple, American elm, red elm, rock elm, green ash, hackberry, box elder, and bur oak are common. Butternut and black walnut are present in some stands.
Landscape Setting & Soils
- Stream valleys and stagnation moraines — Common. Present on modern alluvium in rugged landscapes, including dissected sedimentary bedrock terrain, stagnation moraines, and deep tributary stream valleys of major rivers. Local relief is sufficiently high for alluvium to be deposited as terraces, fans, and benches at toes of slopes. Parent material is silty on the surface with complex stratification of silt, coarse sand, gravel, and buried organic matter below. Soil is typically leached of free carbonates. Soil colors 50-80in (125-200cm) deep indicate prolonged periods of saturation. Soils are moderately well drained. Soil moisture regime is moist to very moist. (Anoka Sand Plain, Big Woods, and Oak Savanna in MIM; PPL; Minnesota River Prairie in CGP)
- Moraines and till plains — Occasional. Present on lake peninsulas, benches along lakeshores, and as wet inclusions on low sites within larger areas of Mesic Hardwood Forest Communities. Parent material is clayey to loamy, calcareous till with free carbonates present below 30-40in (75-100cm). Clayey subsoil horizons and generally fine-textured parent material help to perch snowmelt and rainfall near the soil surface. Most soils have rather black surface horizons, suggesting a past history of prairie on these sites. Gray soil colors or mottles are commonly present but at depths greater than 30in (75cm). Soils are moderately well drained. Soil moisture regime is moist to very moist. (Big Woods and Oak Savanna in MIM; Minnesota River Prairie in CGP)
The suitability index is our estimate of a tree's ability to compete with all plants in a particular NPC Class without silvicultural assistance. The data come from forests approaching rotation age or older. The raw index is based upon the product of percent presence and mean cover-when-present (below) within the set of relevés classified as that NPC. Plants are ranked by their raw index and the full rank order is partitioned into 5 equal groups of plants and re-scaled to yield the suitability index ranging from 1 to 5. This is done so that the indices can be compared within the NPC (below) or with other NPC classes (statewide suitability table[AJ(1]).
It is important to note that the table presented below is a landscape summary of how trees perform on average in a NPC Class. At the stand-scale current stocking and any knowledge of the stand's disturbance/management history should inform how the suitability table can be used. Discussion of stand-scale application of the table follows below the table.
The table is also useful at the landscape-scale when there are restoration or conservation needs for the NPC Class itself, or when forest plan directives call for a management emphasis of a particular species. Species with a high suitability index that are not currently present on the site can be introduced to the site with less risk than species with a lower index.
- Learn more about tree suitability
A tree species is 'suited' to a site when its physical and genetic makeup allow for it to survive and reproduce given the constraints of a site's physical environment AND co-occurring vegetation. Ecologists call this the 'realized niche' of a tree. Our suitability index is based upon the assumption that a tree is highly suited to a site when we see it often and in great abundance in its Native Plant Community Class.
These tables are intended to help foresters decide which tree species to silviculturally favor or introduce on sites that have been classified using the Field Guides to the Native Plant Communities of Minnesota1. Trees with excellent suitability should grow well with very little silvicultural treatment other than providing the correct light and seedbed environments for establishment and recruitment. Trees with poorer suitability for a site can be grown to meet specific objectives, but the forester should expect progressive increases in cost and risk for trees with good to fair to poor suitability rankings. The underlying assumption for using these tables is that when trees are naturally suited to their site, they are vigorous. Vigor should translate to superior quality, resistance to disease, capacity for natural regeneration, and the ability to withstand fluctuations in climate.
Suitability is a mathematical calculation. The data for this calculation come from 6,303 vegetation plots that have been classified as belonging to one of 54 forested NPCs. Two metrics -- commonness and local abundance -- are the elements of suitability.
A plant is 'suited' to a NPC when we often find it there. Percent presence was our metric of commonness. Similarly, a plant is 'suited' to a NPC when it tends to occur in abundance when present. Mean percent cover-when-present was our metric of local abundance. The suitability index is the product of percent presence and mean percent cover-when-present.
Example: Of the 6,306 sample plots, 757 were classified as Northern Mesic Hardwood Forest (MHn35). Basswood trees occur in 483 of the 757 plots. Thus, its percent presence as a tree is (483/757)*100= 63.8%. The mean cover of basswood trees on those 757 plots is 16.5%. Thus, its raw suitability index is 63.8*16.5=1,053. There are 158 species with >3% presence in MHn35 forests and basswood's rank order on a scale of 1-5 is 4.8, its standardized suitability index. The index is standardized so that basswood's suitability can be compared among different NPC classes.
- Legend for tree suitability values
Suitability index values Crop Tree Potential Color 4.0 - 5.0 Excellent Green 3.0 – 3.99 Good Blue 2.0 – 2.99 Fair Yellow
In general, trees with higher suitability indices are better choices as crop trees than trees with lower indices. MHs49 sites offer many options for crop trees, with 16 species having a fair, good, or excellent suitability. Basswood, sugar maple, black ash, bur oak, red elm, black walnut, American elm, hackberry, boxelder, green ash, and rock elm are all ranked as excellent choices as crop trees by virtue of their frequent occurrence and moderately high cover-when-present on MHs49 sites. These species in any combination can be the management target; however, all elms are not long-lived after reaching the height of 10 meters. With so many species, it is incredibly rare for one to form a cover type with over half of the tree cover. The common condition is for young stands to be a mixture of bur oak, black ash, and green ash, and for older stands to be dominated by basswood and any of the elms (historically). Bitternut hickory, butternut, and northern red oak are ranked as good crop trees, and stands can be managed to perpetuate these trees as co-dominants whether for their inherent value or as a future seed source. Ironwood and black cherry are ranked as just fair choices of crop trees, but stands can be managed to maintain their presence as minor trees for purposes other than timber production. If stands are to be silviculturally manipulated to favor one species over another, mean cover-when-present is the more important element of the index, with the higher covers more likely to result in higher stocking following treatment.
Tree Response to Climate Change
Land managers are adapting management strategies to position the forested landscape into one that will be resilient in Minnesota's changing climate. Trends in climate data have shown that Minnesota is getting warmer and wetter, please view the DNR's climate trends webpage for more information. Differences among the 52 forested NPCs are related very much to water availability for trees and understory plants. Forested communities have different capacities for interception, infiltration, storage, and runoff, thus we expect them to react differently to changes in the hydrologic regime -- whatever that may be.
Climate change considerations for forest management can be thought of as a 'funnel' that guides a landowner to make decisions at a successively narrower spatial extent from landscape, to region, then site. The land manager must evaluate information for an entire ecosystem (e.g., hydrology, soils, and forest health) and how manipulations to that ecosystem may be reflected in the tree species present. Generally, the goal is to select tree species that will be resilient to the changing climate and survive through a full stand rotation.
The widest part of the funnel is the landscape-level, here we keep in mind the full suite of species that currently exist in the forested landscape, those that may arrive, and those who's importance across the landscape may decrease. To understand climate change at this broad level read the Minnesota Forest Ecosystem Vulnerability Assessment and Synthesis, where you will learn about how a warmer and wetter climate will impact ecosystem functions and operability concerns (e.g., frozen ground conditions, length of growing season, and prevalence of drought).
The middle part of the funnel is the region-level. At this stage of climate change analysis researchers have defined changes land managers can assess for System Groups (i.e. forest types), instead of impacts to forests as a whole. The Climate Change Field Guide for Northern Minnesota Forests highlights how changes in precipitation and temperature may result in ecosystem changes such as hydrology modifications, increase/decrease in frost days, and drought stress, and apply those concepts to the species adapted to our System Groups. Regionally, land managers can also start brainstorming how the System characteristics may make it well suited for adaptation or vulnerable to climate stressors. Please view the Site-level Considerations for the six main forested systems in the Climate Change Field Guide for Northern Minnesota Forests publication to gain knowledge about site characteristics that would increase or decrease climate risk.
The smallest spatial extent a land manager must think about for climate change is the site-level. Here users must apply the regional Site-level Considerations with the conditions currently found on site. The land manager should assess the climate change risk and how a prescription may be modified in order to favor or disfavor certain tree species. The information provided below is our analysis of the competitive advantage a single species has within a community if the integrity of the community is expected to be maintained into the future. Overall, some of the species we present having a higher heat/moisture tolerance within the community may well be expected to diminish in habitat suitability across the region due to climate change. The species listed also do not represent any tree species changes as forests adapt through time.
It is important to remember that individual site conditions will vary and opportunities to create resilient forests will be a site-by-site analysis. Overall, keeping the full complement of suitable trees on-site is a good hedge against future climate uncertainty.
- Learn more about tree response to climate change
Incorporating climate change information into a site prescription can be a complicated web of understanding information at multiple scales. Most climate change prediction data answers questions about how a tree species will perform across a broad heterogeneous landscape, but decisions about species risk need to be made for individual sites. The following guides in the Learn More expansion provide useful information about vulnerabilities across forested-Minnesota. Please pay close attention to the Climate Change Field Guide as it highlights System Group-specific information and site-level considerations for each NPC System within the Laurentian Mixed Forest Province. Lastly, tree species concerns are neatly explained in the Northwoods Tree Handouts. All of these resources together can help a forester develop the desired future conditions for a forest.
Synecological score calculations
An analysis of habitat range climate data was used to assign and adjust synecological scores for our plants with regard to moisture (M) and temperature (H). The scores range from 1 (dry/cool) to 5 (wet/warm). The difference between a plant's individual synecological score and the mean synecological score of its community provides some insight as to whether that plant would benefit or suffer should its local environment become warmer or wetter.
Example: For each of the 256 MHn35 vegetation plots, the M score of all component plants was summed and averaged to yield a score for each plot. Then the plot scores were summed and averaged to yield an M score for the community, which in this case was 2.3. The adjusted M score for basswood is 2.01, which is drier than 2.3. Thus, we assume that basswood would benefit from a slightly drier conditions. Similarly, the H score for basswood is 4.03, which is substantially warmer than the 2.9 mean for the MHn35 community, which suggests basswood would greatly benefit if MHn35 sites get warmer.
For more information about synecological scores, please view the following references:
Bakuzis, E.V. and Kurmis, V. 1978. Provisional list of synecological coordinates and selected ecographs of forest and other plant species in Minnesota. Staff Series Paper 5. Department of Forest Resources, University of Minnesota. St. Paul, MN.
Brand, G.J., and Almendinger, J.C. 1992. Synecological coordinates as indicators of variation in red pine productivity among TWINSPAN classes: A case Study. Research Paper NC-310. North Central Forest Experiment Station, U.S. Department of Agriculture, St. Paul, MN.
For information about regional forest vulnerability, please view Minnesota Forest Ecosystem Vulnerability Assessment and Synthesis:
Handler, Stephen; Duveneck, Matthew J.; Iverson, Louis; Peters, Emily; Scheller, Robert M.; Wythers, Kirk R.; Brandt, Leslie; Butler, Patricia; Janowiak, Maria; Shannon, P. Danielle; Swanston, Chris; Barrett, Kelly; Kolka, Randy; McQuiston, Casey; Palik, Brian; Reich, Peter B.; Turner, Clarence; White, Mark; Adams, Cheryl; D'Amato, Anthony; Hagell, Suzanne; Johnson, Patricia; Johnson, Rosemary; Larson, Mike; Matthews, Stephen; Montgomery, Rebecca; Olson, Steve; Peters, Matthew; Prasad, Anantha; Rajala, Jack; Daley, Jad; Davenport, Mae; Emery, Marla R.; Fehringer, David; Hoving, Christopher L.; Johnson, Gary; Johnson, Lucinda; Neitzel, David; Rissman, Adena; Rittenhouse, Chadwick; Ziel, Robert. 2014. Minnesota forest ecosystem vulnerability assessment and synthesis: a report from the Northwoods Climate Change Response Framework project. Gen. Tech. Rep. NRS-133. Newtown Square, PA; U.S. Department of Agriculture, Forest Service, Northern Research Station. 228 p.
Available online at https://www.fs.usda.gov/nrs/pubs/gtr/gtr_nrs133.pdf
For information about site-level considerations and vulnerabilities for NPC System Groups, please view the Climate Change Field Guide for Northern Minnesota Forests:
Handler, S., K. Marcinkowski, M. Janowiak, and C. Swanston. 2017. Climate change field guide for northern Minnesota forests: Site-level considerations and adaptation. USDA Northern Forests Climate Hub Technical Report #2. University of Minnesota College of Food, Agricultural, and Natural Resource Sciences, St. Paul, MN. 88p. Available at www.forestadaptation.org/MN_field_guide
Individual tree species adaptation characteristics and climate change projections across subsection-level areas can be found here:
Climate in Minnesota has been getting warmer and wetter. If this trend continues the descriptions in the table forecast the direction and magnitude how we expect MHs49 trees to respond. The responses of “slight” or “significant” increases/decreases represents a full unit departure from the mean synecological score for the MHs49 community.
Tree Establishment and Recruitment
The vertical structure of releves was used to interpret the ability of trees to establish themselves and recruit to taller strata under the canopy of a mature forest and on seedbeds associated with older forests. The goal was to develop an appreciation of which trees are capable of developing enough advance regeneration to fully stock a future stand by natural regeneration. For trees with modest advance regeneration, we wanted to figure out if the problem seems to be related to poor establishment or poor recruitment -- issues that can be silviculturally resolved. For trees with little or no advance regeneration but good or excellent suitability as a tree, we assume that even-aged systems would be required to perpetuate them in that community.
Establishment and recruitment indices are calculations designed to estimate how a tree performs in different size classes with no silvicultural assistance:
1. small regenerant <10cm tall, R-index
2. seedling 10cm -- 2m, SE-index
3. sapling 2m -- 10m, SA-index
4. tree >10m, T-index
The index is the product of percent presence, mean percent cover-when-present, and mean number of reported strata. The index is re-scaled to run from 0 to 5 so that suitability can be compared among different NPCs.
- Learn more about tree establishment and recruitment indices
The tree height data from releves was transformed into 4 standard height strata: regenerants <10cm tall, seedlings 10cm -- 2 m tall, saplings 2 -- 10m tall, and trees >10m. These height breaks were used because they are the most frequently used on releves to describe the natural structural breaks in forests. Still, some releves report strata that span our standard height seams and we had to apportion the presence of the tree and its percent cover into our standard classes. This was done by splitting the reported strata into the 8 individual height classes and evenly splitting the cover among the classes. For example, sugar maple reported in a D3-6 layer (0.5-20m) comprises four individual height classes that need to contribute cover to our standard seedling, sapling, and tree strata. The cover of sugar maple in that stratum was class 3 (25-50% cover). Using the mid-point rule as for suitability (see above), cover class 3 is converted to 37.5%, and the apportionment is 37.5% / 4 = 9.37% cover awarded for sugar maple in each height class. After cover was awarded to all individual height classes in a releve, they were then lumped into the standard strata and the individual covers summed.
For each standard stratum we calculated an index of 'regeneration success' for the tree species. We settled on three measures of success:
First, trees were considered successful if they were common in a particular stratum. Presence is our measure of stratum commonness, and below is how seedling presence was calculated. The parallel calculation was done also for regenerants, saplings, and trees.
SE Presence = (# of releves with the tree present as a seedling / total # of releves for the community) * 100
Second, trees were considered successful if we found them to be abundant in a particular stratum. Mean cover-when-present (MCWP) was our measure of stratum abundance, and below is how seedling MCWP was calculated. The parallel calculation was done also for regenerants, saplings, and trees.
SE MCWP = sum of all seedling cover of tree / number of releves with the tree present as a seedling
Third, trees were considered successful recruiters if we often found it in multiple strata. As a measure of recruitment complexity we calculated the mean number of strata when present (MSWP) reported in the original releves (not our standard strata) for a species. We used this number as a weighting factor to help segregate species that develop a presence in many layers from those that don't develop a lot of strata because they probably need some kind of disturbance to develop an understory cohort.
MSWP = sum of all reported strata for a species / number of releves in which the species occurs
From these three measures of stratum success we calculated the raw recruitment index by multiplying the numbers together. Below is how the raw seedling index was calculated.
Raw SE Index = SE presence * SE MCWP * SE MSWP
For each stratum -- regenerants, seedlings, saplings, and trees -- the ranges of raw index scores are different and not comparable between strata and between communities. To allow comparison, the raw scores were ranked and then re-scaled so that the lowest raw score was zero and the maximum was five.
The indices of regeneration were placed into classes as for suitability so that in tables, foresters can quickly identify the species that tend to have poor, fair, good, or excellent regeneration in mature forests that have not been silviculturally manipulated in the recent past.
Regeneration Index Equivalent Percentile Descriptor 0-1 0-20% none 1-2 20-40% Poor Suitability 2-3 40-60% Fair Suitability 3-4 60-80% Good Suitability 4-5 80-100% Excellent Suitability
- Legend for tree establishment values
Index values Rating Color 4.0 - 5.0 Excellent Green 3.0 – 3.99 Good Blue 2.0 – 2.99 Fair Yellow 1.0 – 1.99 Fair Orange 0.0 – 0.99 Fair White
In general, trees with high understory presence and excellent R-, SE-, and SA-indices can be depended upon to produce enough advance regeneration to stock a stand after removal of canopy trees. In MHs49 forests basswood, black ash, bitternut hickory, and red elm are almost always present in usable abundance; however, red elm is short-lived after reaching the height of 10m. At maturity these species dominate both the understory and overstory, and the forest is self-sustaining.
Trees with excellent R-index values have no problem establishing on an undisturbed forest floor in mature MHs49 forests. Basswood, sugar maple, black ash, bitternut hickory, and red elm readily establish by seed without any seedbed preparation. The remainder of trees in the table need to establish by seed, but have lower R-index values. This indicates a germination need that is not fully met in a mature MHs49 forest. The need is often species-specific, thus silvicultural improvement requires knowledge of the tree’s silvics and its competitive context following silvicultural treatment in MHs49 forests. Common germination hurdles in MHs49 forests are: freedom from root competition, insufficient light/heat due to proximal shade from shrubs and subcanopy trees, lack of well-decomposed nurse logs, thick duff in stands unaffected by earthworms, and lack of seed source.
Trees with excellent SE- and SA-index need no silvicultural assistance recruiting to tree size in an unmanaged forest. Basswood, black ash, bitternut hickory, and red elm do this in MHs49 forests. Of these trees, bitternut hickory has some difficulty recruiting to heights much over 10m. Trees with either a good or fair SE- or SA-index would likely benefit from intermediate silvicultural treatment as long as establishment isn’t a problem. If the SE-index is the lower of the two then early survival and growth is the issue, and treatments like cleaning and spacing may help to diminish proximal competition. This is clearly the case for sugar maple and green ash, and it may be the case for hackberry and American elm. Boxelder, ironwood, and rock elm may fit this pattern, but their R-index is also low, suggesting that establishment and early growth are both a problem. If the SA-index is the lower of the two then the issue is most likely sufficient light as the trees try to emerge from the shrub layer, and overhead release may help to promote the advance regeneration. No trees fit this pattern on MHs49 sites. Trees like black cherry, bur oak, black walnut, butternut, and northern red oak with poor understory indices generally need to be regenerated under more open conditions where recruitment under a canopy isn’t an issue.
Understanding natural disturbance regimes is prerequisite for designing silvicultural systems and treatments that emulate natural processes. Theoretically, 'natural' treatments favor trees adapted to the site, conserve local gene pools by relying on natural regeneration, maintain native plants in the understory, are less risky than agricultural approaches, and cheaper to implement. Because clear-cutting and other stand-regenerating systems are so often employed, it is important to determine which NPCs were maintained by stand-replacing disturbances and, further, to estimate the natural rotation. For many NPCs the natural rotation far exceeds commercial rotation, and this requires us to look to other silvicultural strategies for harvest and regeneration. Thus, we must also estimate the frequency and intensity of disturbances that maintain these kinds of forest communities.
Natural rotation of catastrophic and maintenance disturbances were calculated from Public Land Survey (PLS) records. The records provide a point-in-time estimate (ca. 1846-1908 AD) of just how much of the historic landscape was recently disturbed by fire, windthrow, or gap-forming events such as surface fires, disease pockets, etc. To some extent these regional trends can be applied to management at the stand-scale. The kind of natural disturbance can inform site preparation; the comparative frequency of stand-replacement and maintenance events informs canopy retention and entry schedules/rotation.
- Learn more about calculating natural disturbance rotations
Natural Disturbance Regimes
The goal of the PLS analysis was to estimate the rotation of stand-replacement and maintenance disturbances unique to each NPC class. The surveyors explicitly described burned and windthrown land when working within the forested regions of Minnesota. When in the prairie region and especially along the prairie/forest border, the surveyors used a variety of terms to describe wooded vegetation understood to be maintained by frequent disturbance. Most often this was fire, but in some regions wind was important as well. Thus, geographic context is an important consideration when trying to determine if a surveyor's comments are indicating that the corner was 1) undisturbed, 2) catastrophically disturbed, or 3) recently affected by a less intense, maintenance disturbance. Placing corners in these three categories is the critical step that allows the calculation of disturbance regimes. To get at this, we must understand the surveyor's physiognomic descriptions of the vegetation at the corners: prairie, grove, bottoms, barrens, burned lands, windthrown timber, etc. Our rules for assessing disturbance at survey corners were individually set for each physiognomic vegetation type across the state.
Stocking (i.e., tree density) is the most important element of their physiognomic descriptions. Our initial step in the analysis was to understand how the distances to bearing trees affected the surveyor's vocabulary. For all of the types, we calculated the mean distance to bearing trees which allowed us to rank and group the types in some sensible fashion.
Wooded types Disturbance types Riverine types Fire maintained types Open types Swamp 40 Windthrow 72 Bottomland 135 Thicket 92 Meadow 183 Forest 50 Burned land 76 Dry land 157 Pine openings 113 NOTA 192 Dry ridge 60 Oak openings 145 Prairie 236 Grove 69 Scattering oak 166 Marsh 278 Island 70 Barrens 177 Wet prairie 411
Table 1. Vegetation types mentioned by surveyors and their mean distances in links to their bearing trees. Columns roughly ranked by range of distances. (NOTA means 'no other tree around.')
Wooded and riverine types (Table 1) were assumed to be undisturbed forest. The short distances to trees in the wooded types are indicative of naturally stocked forest where tree density is largely set by competition for space among trees. The riverine types have longer distances than the wooded types because bottomland and dry land corners are intermingled with river channels and marsh at a fine scale. It is common for these linear, treeless features to occupy a full quadrant at a corner in bottomland forest requiring a bearing tree be found across the channel or meadow if possible. For each wooded type, we examined the frequency distribution of corners in 10-link distance classes to get a general sense of distances that would indicate natural stocking. Figure 1 is an example for the forest-type distribution associated with 77,506 corners.
In Figure 1 about 80% of all corners fall in the first 6 classes (up to 55 links). The mean distance for all forest corners is 50 links. Our interpretation is that somewhere around the mean there is a change in the nature of the distribution. Classes under 50 links are common and likely represent the natural range of variation in stocking (perhaps due to age). Classes over 50 links are infrequent and most likely represent a corner where at least one quadrant lacked nearby trees or had damaged trees due to disturbance. Thus, for our 'undisturbed' wooded and riverine classes, it just turns out that the mean distance usually falls in the last or one of the last abundant distance classes, and classes with longer distances were assumed to be disturbed to some extent. To make a simple rule, we arbitrarily set the minimum distance indication disturbance to the mean for vegetation that the surveyors described as swamp, forest, dry ridge, grove, island, bottomland, or dry land.
Setting the upper limit, above which we assume stand-replacement, was also a guess. It is clear that mean distances over about 180 links are typical of open, treeless environments (Table 1.). Even at distances of about 110-180 links it is clear that trees were scarce enough that the surveyors noted that the vegetation wasn't forest. The distribution in Figure 1 is incredibly smooth over the longer mean-distance classes and there is no gap in classes to suggest a natural break for our higher distance threshold. To make a simple rule, we arbitrarily set the maximum distance indicating stand-replacing disturbance as the mean plus one standard deviation for swamp, forest, dry ridge, grove, island, bottomland, or dry land. For most classes, this number is close to the mean distances for open types that we know had very few trees.
The frequency distributions of fire-maintained types are different from the wooded and riverine types. At distances greater than the peak class, the fall in frequency is nearly linear, an example of which is for oak openings (Figure 2.). There is no obvious point of inflection to set the lower, naturally-stocked, undisturbed limit, nor are there breaks in the distribution that can help us set the upper limit for catastrophic disturbance. It is important to remember that we are interpreting the use of terms like 'openings' and 'scatterings' to corners that we believe from modern vegetation to be capable of forest stocking. Almost certainly, these terms were used to describe recent disturbance that caused trees to be sparser than normal 'forest.' To help us interpret the use of these terms to describe forest, we returned to the coarser analysis. Corners with distances under 50 links were almost certainly in places one would describe as undisturbed forest. Corners with distances over 200 links were in places where tree density was low and comparable to open habitats like prairie and meadow. To make a simple rule for corners described as thicket, pine openings, oak openings, scattering timber, and barrens, we arbitrarily set the minimum distance indicating disturbance to 50 links, and we set the maximum distance indicating stand-replacing disturbance at 200 links.
In addition to distance, we found it important to consider also missing bearing trees as evidence of disturbance. A common survey note is 'NOTA' meaning 'no other tree around,' which was the surveyor's explanation for not marking all of the required bearing trees (i.e., 4 at section corners and 2 at quarter-section corners). Most often this note appeared at corners described as one of the fire-maintained or open community groups (Table 1.). NOTA was also used at corners described as burned or windthrown. Within the context of interpreting corners modeled as forest or woodland, NOTA almost certainly was relating to some kind of disturbance that left dead trees or trees too small to scribe. Table 2 describes our model for assigning a disturbance class based on both distance and complement of bearing trees. Within their type, survey corners were assigned their final disturbance class -- undisturbed, partially disturbed, catastrophically disturbed -- by a combination of the corner's mean distance to its bearing trees and whether it had its full complement of 2 or 4 bearing trees.
Assumed undisturbed - Wooded and Riverine groups < mean between mean and mean + SD >Mean + SD Full complement Undisturbed Undisturbed Maintenance Partial complement Undisturbed Maintenance Burned Assumed disturbed - Fire-maintained group < 50 links links > 206 links Full complement Undisturbed Maintenance Burned Partial complement Maintenance Maintenance Burned
Table 2. Rules for assigning a disturbance class to survey corners not explicitly described as burned or windthrown.
Adjusting the Model -- Window of Recognition
It is obvious that several pragmatic decisions and rules were made in order to assign corners to disturbance categories. Even if these rules are reasonable, one must still set a 'window of recognition' in order to make quantitative estimates of stand-replacing and maintenance rotations. The window of recognition is the span of years for which a surveyor would have bothered to describe a disturbance. Would a surveyor recognize and care to report that a stand had been burned 5, 10, 15, or 20 years after the fact? We believe that mention of fire and windthrow was more an excuse for not marking bearing trees than any conscientious effort to alert potential buyers to fire- or wind-damaged timber. Consider the fact that quaking aspen is the early successional species for nearly all terrestrial forests in Minnesota. The surveyors actually marked and scribed some 390, 2-inch aspen bearing trees and some 3,039 three-inch trees. Clearly, surveyors would bother to scribe 2-3' trees if that was their only choice. Our age/diameter models for 2-3' aspen trees suggest that these trees were between 11 and 18 years old respectively. If commenting about fire and wind was an excuse, then the window of recognition should be somewhere in the 11-18 year range because that is when trees reach a minimum diameter for marking.
Alternatively, a window of recognition is empirically set to 'force' the rotation model to match the estimates from studies using more reliable methods. In the Great Lakes States, there are reconstructions of disturbance regimes from fire-scar studies (Frissell 1973), stand-origin mapping (Heinselman 1996), and charcoal analysis of varved sediments (Clark 1988). When we model disturbance regimes from bearing trees in these same regions, a window of 15 years tends to yield results similar to the other methods for stand-replacing disturbance. We used a 15-year window of recognition because it yields rotations comparable to rotations calculated from fire-scars, stand-origin maps, and varved lake sediments.
Many detailed investigations of forest disturbance do not calculate rotations of maintenance disturbance, but recognize its confounding effect on estimating stand-replacing events. Trees with multiple fire-scars attest that some forest types are affected more by maintenance surface fire than catastrophic crown fires. Dendrochronological reconstructions of stand history also attest that maintenance events (fire and non-fire) are common and important, releasing cohorts of advance regeneration and providing some growing space in the canopy (e.g. Bergeron et al. 2002). Minor peaks in varve charcoal are also more common than major ones, possibly recording maintenance fires. Calculating maintenance disturbance is more complicated than stand-replacement because the signal is weaker, reliable studies are fewer, and the cause less obvious. However, some estimate is absolutely required to provide guidance in applying intermediate silvicultural treatments to the right NPCs.
As was the case for estimating stand-replacing rotations, adjusting the window of recognition is the easiest way to adjust the model. Logic would suggest that the window should be shorter for maintenance events because the disturbance is less intense and evidence of it might be gone in 15 years. If the surveyors really used terms like burned or windthrown to explain the lack of bearing trees, it is likely that they did so less often on lands lightly disturbed because there were trees around -- they just had to go a little farther to find bearing trees and might not always find a suitable tree in all quadrants. We found that a 5-year window produced rotations that matched what one might guess from multiple-scarred trees. Also, the ratio of maintenance events to catastrophic ones seemed within the range of what one might expect from the ratio of strong charcoal peaks to minor ones in varve studies. A 5-year recognition window was used to calculate maintenance rotations because it seems to fit fire-scar and varve studies.
Calculating Rotation by Example -- Northern Mesic Mixed Forest (FDn43)
Having settled on windows of recognition and having assigned disturbance classes to the corners associated with an NPC, it is possible to calculate rotation. This is easiest to understand by example.
Northern Mesic Mixed Forest (FDn43) is a fire-dependant NPC that is the matrix vegetation for much of northeastern Minnesota. Our model assigned 11,712 PLS survey corners to this community because 1) they fall on landforms (LTAs) where we have modern samples of FDn43 forests, 2) the attending bearing trees were typical of the community (>70% frequency), and 3) they lacked trees atypical of the community (<30% frequency).
Each corner was assigned one of 4 disturbance classes based upon the distance and complement rules set up for each physiognomic vegetation class (Table 2.). The tallies for each class are shown in Table 3.
Vegetation Class Fire 15-year window Wind 15-year window Maintenance 5-year window Undisturbed Barrens 11 11 Dry ridge 1 29 Forest 42 111 10168 Grove 1 Bottoms 45 Scattering pine 7 16 Scattering timber 2 24 60 Swamp (misassigned) 5 153 Thicket 21 61 143 Burned 710 Ravine 6 Windthrown 63 No other tree around 16 Island 1 5 Totals 791 63 221 10637
Table 3. Counts of survey corner assignment to disturbance classes by physiognomic vegetation class for the FDn43 community.
The FDn43 landscape of 11,712 survey corners provides the base area for calculating rotation of a NPC. In Table 3, 791 of those corners were interpreted as having been catastrophically burned, representing 6.75% of the area.
(791 burned corners/11,712 total corners)*100=6.75% of the landscape
Presumably, surveyors recognized burned lands for 15 years after the event, meaning that the annual percent of the landscape that catastrophically burned is 1/15th of 6.75%.
6.75% of landscape burned/15-year recognition window=0.45% burned annually
The rotation is the time required to catastrophically burn the entire area represented by 11,712 corners. Because we have calculated this as a percent, the time it takes to achieve that is:
100%/0.45% burning annually=222 year rotation of catastrophic fire
There is no need to calculate acres or percent of landscape, but it makes the calculation easier to understand given the area concept of rotation in forestry. The easier formulas to use are:
(Total # corners / # corners in disturbance category)*recognition window=rotation
(11,712/791burned)*15 years=222 year rotation of catastrophic fire
(11,712/63 windthrown)*15 years=2,788 year rotation of catastrophic windthrow
(11,712/221 maintenance)*5 years=265 year rotation of maintenance disturbance
It is also useful to calculate the rotation of all fire (or wind), regardless if it was catastrophic or maintenance. To make this calculation it is easiest to sum the annual percents.
0.45% burned catastrophically each year
0.37% burned in maintenance event
0.45%+0.37%=0.82% annual=122 year rotation for all types of fires
It is the rotation of all fires that tends to reasonably match the published estimates of return intervals. For example, in the BWCAW Heinselman (1996) reports return intervals for the common forest types: Aspen-Birch-Conifer (70-110 years), Red Pine (<100), and White Pine (>100). These cover types, especially the white pine, are predominantly the FDn43 community for which we calculate a 122 year rotation for all fires.
The table below shows the frequency of PLS survey corners assigned to four different disturbance categories. Shown also is the percent of the MHs49 landscape in those conditions and the resulting calculated rotation. Maintenance disturbances were the most frequent disturbance type. There is no evidence of stand re-initiating fire disturbances.
Stand Dynamics & Growth Stages
Understanding natural stand dynamics is the essence of prescription writing. Without some understanding of how dynamics affect tree establishment, thinning, recruitment, competitive ability, form, longevity, and succession during 'unsupervised' stand maturation — foresters cannot write worthwhile prescriptions. For the most part, prescriptions are written to re-initiate, transition, or maintain the natural course of events in a forest in order to meet a management objective.
Public land survey records were used to develop a natural model of compositional succession and structural change. The figure below orders PLS section and quarter-sections by diameter class, assigned as the diameter of the largest attending bearing tree at the corner. Presumably this is chronological ranking along the y-axis. The rough age of each diameter class was estimated by FIA diameter/age models for a tree common in all diameter classes.
For every tree, its relative abundance in a diameter class is graphed along the x-axis. This shows the compositional change as forests mature from small diameter classes to larger ones. The inter-tree distances provide some insight concerning initial tree density and how that changes as forests age. Low standard deviation of the inter-tree distances indicate uniform tree spacing; high standard deviations indicate a patchy distribution of trees. A cluster analysis (CONISS) groups contiguous diameter classes into periods of stability (growth-stages) and change (transitions).
- Learn more about natural dynamics and growth stages
PLS data are not inherently temporal. To create a model of stand dynamics we must somehow rank survey corners assigned to a particular NPC Class in a way that is reflective of time. The best that can be done is to assign each corner to a diameter-class equal to the diameter of the largest bearing tree at that corner. This allows us to reasonably rank corners from smaller-diameter/presumably younger forests to larger/presumably older forests the first 100 years or so. We refer to the diameter of the largest tree at a corner as 'stand-diameter.' We believe that this ranking is good enough to make some broad interpretations about dynamics throughout the early years of stand maturation. The modeled age of the largest tree at a corner is a minimum estimate of how long the stand has avoided a catastrophic disturbance. This is NOT true stand age because no corner can be assigned a diameter/age beyond the biological size or longevity of its old-growth species. For forest classes with long rotations of stand-replacing disturbance, the largest tree at any point can easily belong to the 2nd, 3rd, or subsequent cohort. Survey corners were assigned to 'stand-diameter' classes equal to the diameter of the largest witness tree at that corner.
For the set of corners in a diameter class we calculated the relative abundance of each bearing tree taxon. This allows us to see how composition changes as stand-diameter increases over time. For each tree taxon, we plotted its relative abundance by stand diameter to see if its relative abundance tends to decrease, increase, or peak over the range of stand-diameters (below). Also, for the set of corners in a diameter class we calculated the mean distance of trees to the corner and the standard deviation of that mean. This allows us to understand how tree density and variance in density changes as stand diameter increases over time. Because these dynamic models are useful for forest planning the estimated age of a tree that diameter is provided. This is done for a tree species that is abundant and present in all stand-diameter classes. The model is based upon diameter and age measurements of FIA site trees. Stand diameter age was estimated by using a quadratic equation that was fit to FIA site-index trees.
Age=C+A*dbh+B*dbh2 (where C is a constant, A&B are coefficients from the FIA model)
A constrained clustering routine (CONISS, constrained incremental sum of squares) was applied to the stand-diameter classes as characterized by the relative abundance of trees in that class. This method is constrained in that stand-diameter classes must be grouped to its adjoining classes or clusters that include the adjoining classes. The result is a hierarchical grouping of contiguous diameter classes based upon the similarity of their tree composition (below). Tight groups represent a span of stand-diameters where we infer little compositional change. We call these 'growth-stages.' Separating growth-stages are clusters of contiguous diameter classes that are not necessarily very similar, but are the last to be clustered. Such spans of diameter classes represent periods of species turnover called 'transitions.' This analysis provides some insight into the timing and rate of dynamic change.
Graphed for each of the common MHs49 trees is their relative abundance (percent composition; x-axis) as PLS bearing trees by diameter class. In cases where the PLS surveyors did not distinguish species common to the MHs49 community, the more probable species is the graph title and the less probable species is shown in parentheses (e.g. Black (green) ash). Also shown is the mean distance of trees to the survey corner (inter-tree distance, white inset bars) and the standard deviation about that mean (black bars) for each diameter class. All data were smoothed using a 5-sample running average. Curves for taxa not exceeding 5% have a shaded 5X exaggeration to better illustrate trends (e.g. Butternut). Curves for taxa not exceeding 1% have dots representing their presence in that diameter class (e.g. Boxelder). A constrained cluster analysis (CONISS) groups diameter classes with similar species composition; the groups are interpreted as either stable growth-stages (e.g. young, mature) or periods of change (transition). Data for MHs49 comprise 10,925 PLS corners and 30,326 witness trees.
MHs49 forests were among several upland communities that exhibited compositional change during the course of stand maturation. The long-term trend was consistent with tolerance-based models of succession, whereby young stands were dominated by intolerant species, and dominance shifted to more tolerant trees in mature forests because of canopy closure and the formation of sub-canopy strata. Disturbances that resulted in stand re-initiation left young MHs49 forests that were nearly pure oak. Bur oak and northern red oak were strongly favored, and these trees together with minor amounts of black or green ash comprised the initial cohort. Succession was driven by a significant decline in bur oak, and replacement by red or American or rock elm, sugar maple, and basswood, with some bitternut hickory and butternut. Most compositional change occurred over the period when stand diameter increased from 5-10 inches. By the time stand diameter reached 12 inches and the forest was about 65 years old, most canopy trees belonged to the second cohort or were old initial-cohort bur oaks. At that time composition stabilized at a roughly equal mixture of bur oak, elm, basswood and sugar maple. Although the early dynamics were the result of bur oak decline it was able to maintain importance in mature MHs49 forests. Apparently on wet-mesic sites, bur oak was quite able to compete with more tolerant species and contribute to second or subsequent cohorts.
Structural succession in MHs49 forests was typical of mesic hardwood forests that were influenced by maintenance disturbances such as surface fires, partial windthrow, or swamping. In the course of stand maturation they changed modestly with regard to tree density and the evenness at which trees were distributed. Change was very slow due mostly to the gradual establishment and ultimate dominance of fire-sensitive trees. By the time stand diameter reached 4-5 inches any self-thinning was essentially complete and trees of the young forest were approximately 67 feet apart, which is typical of southern hardwood forests and far sparser than northern or central hardwood forests. The initial spacing of trees changed very little until stand diameter reached 10 inches. Following that, spacing very gradually increased to about 76 feet over the period when stand diameter increased from 10-19 inches. This decrease in density coincided with some decline of bur oak and a substantial increase in red, American or rock elm along with some basswood and sugar maple. In other southern mesic hardwood forests, the replacement of oak by tolerant hardwoods resulted in an increase in density at maturity because tolerant trees were able to fill small canopy gaps. The slight increase in spacing in MHs49 forests is more consistent with crown expansion of long-lived, large-diameter bur oak and red, rock, or American elm. Also, mature MHs49 forests are much sparser than other southern mesic hardwood forests and are more similar to chronically disturbed fire-dependent forests. However, there was no historic evidence of fire and little evidence of windthrow as important disturbance agents. MHs49 forests occur on alluvial bottomlands with overflow channels, water-filled depressions, and often seepage zones. It is possible that MHs49 forests were sparse because the land surface itself was not entirely available as suitable tree habitat due to prolonged ponding. Thus, we are uncertain about the cause of low density in MHs49 forests, but our observations are consistent with those of Curtis (1959) in Wisconsin, where southern wet-mesic forests are described as having high basal area but low density with just 85 trees per acre in mature stands.
MHs49 forests showed modest variance in tree spacing as stands aged. Young forests were somewhat patchy having ratios of standard deviations to mean inter-tree distances of about 1.2, as compared to more uniform forests where this ratio is 1.0 or less. Over the period when stand diameter increased from 8-15 inches this ratio increased and peaked at about 1.4. This tendency of a patchier distribution of trees coincided with a slight decline of bur oak and establishment of more tolerant trees. After stand diameter reached 15 inches MHs49 stands returned to a slightly more uniform tree distribution, and the ratio of standard deviations to their inter-tree means stabilized at about 1.2. This was still patchy and suggests that the formation of both large and small canopy gaps provided recruitment opportunities in mature forests for both mid-tolerant oaks and tolerant hardwoods.
Silvicultural strategies are sequences of treatment outcomes designed to emulate natural stand dynamics and promote natural regeneration. They are not silvicultural systems in the traditional sense because they do not cover a full rotation or have attached the implied goal of maintaining a particular species or cover-type indefinitely. Most involve 1-2 stand entries over a short period of time that will move a stand towards a forest plan objective -- with enough inertia that little silvicultural intervention will be needed to meet long-term goals. We describe management outcomes rather than silvicultural treatments because there are usually several treatments that might achieve the desired outcome. All strategies are based upon our understanding of NPC-specific natural stand dynamics and disturbance regimes. The sequence of outcomes follows the natural pattern; the timing is foreshortened because we intend to harvest sound trees rather than allowing natural senescence.
The most common, natural pattern of tree mortality and replacement was the partial loss of trees on a rotation of about 160 years. Stand-replacing windthrow was rare with an estimated rotation of 2,890 years. There was no evidence of fire affecting this community.
The most likely cause of tree mortality and replacement was hydrologic stress followed by dieback, species-specific disease, or outbreaks of defoliating insects. Because MHs49 forests were mostly mature, of mixed composition, and multiple-aged the loss of canopy trees was partial, limited to a few (or single) species at the age at which they are susceptible. Insect and disease mortality is ultimately apparent as selective windthrow because it affects the vitality and structural soundness of individual trees. Different species of trees, even when sound, vary regarding wood strength and canopy architecture. Thus damaging wind and to some extent heavy snow can impart selective mortality in mixed stands, creating large canopy gaps. Silvicultural emulation involves species selection and varying gap size or residual density. This can be done to establish early-successional trees or to promote existing advance regeneration. Three strategies are envisioned:
- Re-initiate a stand as would prolonged ponding and ultimate windthrow to create open to very large gap habitat.
- Accelerate natural succession as would selective windthrow by removal of initial-cohort trees.
- Maintain a stand as would natural senescence, disease, or selective windthrow to create small-gap habitat.
Modern MHs49 forests are commonly affected by windthrow, either directly due to ponding or as a consequence of trees weakened by disease, pests, or accumulated wounding and rot. According to climate experts, climate change is expected to increase the frequency of storms in Minnesota, but current research shows this will likely translate to greater precipitation and not increases in frequency or intensity of wind events. Fire never did affect the composition of these forests. Commercial logging coupled with compromised drainage and Dutch elm disease has resulted in MHs49 forests with far less American elm and more sugar maple, ironwood, and bur oak than was documented by PLS surveyors in the late 1800s. Managing MHs49 stands primarily for butternut increases the risk of windthrow because these butternut is not wind-firm. Managing MHs49 stands in the absence of surface fire increases the risk of losing northern red oak and bur oak as potential crop trees.
Silvicultural strategy for re-initiating MHs49 forests as would prolonged ponding and ultimate windthrow
Emulating windthrow of flood-stressed trees to favor black ash, bur oak, or green ash
Rarely flood-stressed trees would lose most of their crowns or were killed by secondary diseases and pests over broad areas, thus creating open and large-gap habitat. Ultimately, the standing dead or trees with dieback were toppled by wind because their root system was as adversely impacted as their crowns. At the stand-scale it was a highly variable mess, with a variety of outcomes including some that 1) prompted some stump sprouting of all present hardwoods, 3) selected for sugar maple, bur oak, butternut, green ash, bitternut hickory, and northern red oak wind-firm residuals, 4) selected for trees of smaller stature with the general effect of releasing advance regeneration and mid-canopy trees dominated by tolerant hardwoods such as black ash, sugar maple, basswood, red elm, bitternut hickory, hackberry, American elm, and green ash, 5) provided little in the way of mineral seedbeds except on tip-ups, and 6) left abundant coarse woody debris.
Silvicultural Strategy to accelerate natural succession in MHs49 forests as would selective windthrow
Emulating initial-cohort mortality to favor sugar maple, basswood, American elm, red elm, hackberry, or bitternut hickory
Following windthrow, natural succession occurred rapidly in 30-50 year-old MHs49 stands when initial-cohort oaks died. The transition was fairly rapid because late-successional, tolerant trees were part of the initial cohort. Slower-growing black ash, sugar maple, basswood, red elm, bitternut hickory, American elm, and hackberry saplings grew up the emerging canopy, and were poised to immediately replace canopy oaks and ash. These tolerant trees also built seedling banks throughout the years when initial-cohort northern red oak, bur oak, black ash, and green ash were self-thinning. Basswood, black ash, and red elm were especially adept at recruiting to the mid-story under either a full canopy or in sub-canopy gaps left when suppressed oaks died. By the time stands were ~50 years old, the sub-canopy and mid-story was filled with tolerant species awaiting release.
Silvicultural Strategy for maintaining MHs49 forests as would natural senescence, disease, or selective windthrow
Emulating fine-scale disturbance to favor black ash, sugar maple, basswood, red elm, bitternut hickory, hackberry, American elm, or green ash
Mature and old MHs49 forests were maintained by fine-scale disturbances. Stands were a mixture of many species and thus, disturbances – disease, defoliation, ice storms, snow load, etc. – differentially killed individual or small clusters of canopy trees by species. Mature MHs49 forests were also mixed regarding tree age and thus, there was little synchrony of canopy tree mortality relating to senescence or accumulated damage. Fine-scale mortality events continuously created small-gaps that released advance regeneration of black ash, sugar maple, basswood, red elm, bitternut hickory, hackberry, American elm, or green ash.