Central Mesic Hardwood Forest (Eastern) - MHc36

Central Mesic Hardwood Forest (Eastern)

Forest description

Mesic hardwood forests dominated by basswood, northern red oak, and sugar maple. Present on loamy or sandy loam soils on hummocky stagnation moraines and rolling till plains.

Community Description

MHc36 is a relatively common hardwood forest community found in east-central and central Minnesota (see map; 146 relevés, 276 ECS worksheets). The majority of occurrences of this community are within the Western Superior Uplands (WSU) section and a fair portion are found in the Minnesota & Northeast Iowa Morainal (MIM) section. Few observations can be found in the southern portion of the Northern Minnesota Drift & Lake Plains section.

Distribution in Minnesota

Distribution map for MHc36

Vegetation Structure & Composition

Description is based on summary of vegetation data from 95 plots (relevés).

  • Ground-layer cover ranges from patchy to interrupted (25-75%). Common species include early meadow-rue (Thalictrum dioicum), lady fern (Athyrium filix-femina), large-flowered bellwort (Uvularia grandiflora), Clayton’s sweet cicely (Osmorhiza claytonii), Pennsylvania sedge (Carex pensylvanica), large-leaved aster (Eurybia macrophylla), wild sarsaparilla (Aralia nudicaulis), zigzag goldenrod (Solidago flexicaulis), hog peanut (Amphicarpaea bracteata), and rugulose and yellow violets (Viola canadensis/pubescens group).
  • Shrub-layer cover is variable; important species include chokecherry (Prunus virginiana), pagoda dogwood (Cornus alternifolia), prickly gooseberry (Ribes cynosbati), and beaked hazel (Corylus cornuta), along with sugar maple, ironwood, basswood, and northern red oak seedlings and small saplings.
  • Subcanopy cover is variable; sugar maple is the most important species, followed by ironwood and basswood.
  • Canopy is continuous (>75% cover), usually dominated by basswood, northern red oak, and sugar maple. Minor canopy species include green ash, paper birch, red maple, bur oak, and quaking aspen.

Landscape Setting & Soils

  • Stagnation moraines—Common. Landscape is hummocky with highly variable relief. Parent material is predominantly non-calcareous or weakly calcareous fine-textured till, often with a shallow cap of coarser, partially sorted drift. Soils have firm, sandy clay- loam subsoil horizons capable of perching snowmelt and rainfall well above the local water table. On upper slopes, soils do not perch water for long and are well drained with a fresh soil-moisture regime. On lower slopes, soils can perch water for prolonged periods and are moderately well drained with a very fresh or moderately moist soil-moisture regime. (WSU; Hardwood Hills in MIM)
  • Till plains—Occasional. Landscape is undulating to rolling. Parent material is non- calcareous gravelly, sandy-loam till. Soils have very firm and partially cemented lower horizons capable of perching snowmelt and rainfall. Gray soil colors just above the lower horizons indicate a thin (~10in [25cm]) zone of saturated soil that persists well into the growing season. Soils are well drained to moderately well drained. Soil-moisture regime is fresh or moderately moist. (WSU)

Tree Suitability

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 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 Index
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.


Tree suitability table for MHc36 trees

Legend for Tree suitability values
Suitability index valuesCrop Tree PotentialColor
4.0 - 5.0ExcellentGreen
3.0 – 3.99GoodBlue
2.0 – 2.99FairYellow

In general, trees with higher suitability indices are better choices as crop trees than trees with lower indices. MHc36 sites offer many options for crop trees, with 16 species having a fair, good, or excellent suitability. Sugar maple, northern red oak, basswood, green ash, bur oak, and quaking aspen are all ranked as excellent choices as crop trees by virtue of their frequent occurrence and moderately high cover-when-present on MHc36 sites. These species in any combination can be the management target. It is most common for sugar maple, basswood, or northern red oak to form the cover type; however, it is far more common for stands to be mixed. When that is the case, quaking aspen and northern red oak are prevalent in younger stands, and sugar maple and basswood commonly co-occur in older stands. Paper birch, white pine, red maple, black ash, big-toothed aspen, white oak, and bitternut hickory 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. Yellow birch, ironwood, and American elm 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. Low frequency and high mean cover-when-present is the hallmark of trees with greater potential for the site than is commonly observed. The loss of seed trees due to historic over-exploitation, lower frequency of a historic disturbance like fire, the arrival of new diseases/pests or changes in their virulence, and species’ range expansions due to climate change could all explain this pattern. For the MHc36 community white pine clearly fits this pattern, and white oak and yellow birch may fit. If the cause of low frequency is understood, then silvicultural correction is possible.

Tree Response to Climate Change

Due to global warming, land managers are bracing for local vegetation shifts to plants whose North American ranges are warmer and drier than their current habitat. Minnesota's climate has warmed approximately 2oF since temperature records have been collected and archived (ca. 1900). The rate of warming has noticeably accelerated since 1980, affecting winter-low temperatures more so than what is observed in other seasons. However, all seasons now have warmer temperatures than the long-term average. Unlike some parts of the continent, Minnesota has seen an increase of about 3" of rain annually. However the year-to-year fluctuation and rainfall in a particular storm have become incredibly variable and unpredictable. Minnesota is experiencing both drought and flooding extremes over several time scales. 

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. The table below presents our best guess as how trees currently suited to this NPC will react to a warmer and wetter climate. Keeping the full complement of trees on-site is a good hedge against future climate uncertainty.

Learn more about climate change

Climate Shift Calculations
Due to global warming, land managers are bracing for local vegetation shifts to plants whose North American ranges are warmer and drier than their habitat in Minnesota. An analysis of range climate 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 ... suggesting that basswood would greatly benefit if MHn35 sites get warmer. 

 


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 MHc36 trees to respond. The responses of “slight” or “significant” increases/decreases represents a full unit departure from the mean synecological score for the MHc36 community. 

Tree habitat response to climate change in MHc36

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 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 wit

Dave Anderson  |  Operations Services Division,  Drupal Developer
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h 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 IndexEquivalent PercentileDescriptor
0-10-20%none
1-220-40%Poor Suitability
2-340-60%Fair Suitability
3-460-80%Good Suitability
4-580-100%Excellent Suitability

Establishment and recruitment for MHc36 trees

Legend for Tree establishment values
Index valuesRatingColor
4.0 - 5.0ExcellentGreen
3.0 – 3.99GoodBlue
2.0 – 2.99FairYellow
1.0 – 1.99FairOrange
0.0 – 0.99FairWhite

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 MHc37 forests sugar maple, ironwood, basswood, and American elm are almost always present in usable abundance; however American elm is not long-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 MHc37 forests. Sugar maple, ironwood, basswood, American elm, and northern red oak readily establish by seed without any seedbed preparation. Quaking aspen suckers are almost always present when represented in the canopy. 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 MHc37 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 MHc37 forests. Common germination hurdles in MHc37 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. Sugar maple, basswood, ironwood, and red maple do this in MHc37 forests. Of these trees, ironwood and American elm have some difficulty recruiting to heights much over 10m. Ironwood is limited by its inherent short stature. American elm is first susceptible to Dutch elm disease at that approximate height.  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 may be the case for quaking aspen and balsam fir. This may be the case for bur oak, but its 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. This is clearly the case for northern red oak; and it may be the case for green ash. Trees like paper birch with poor understory indices may need to be regenerated in open habitat where recruitment under a canopy isn’t an issue.

Natural Disturbance

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 typesDisturbance typesRiverine typesFire maintained typesOpen types
Swamp 40Windthrow 72Bottomland 135Thicket 92Meadow 183
Forest 50Burned land 76Dry land 157Pine openings 113NOTA 192
Dry ridge 60  Oak openings 145Prairie 236
Grove 69  Scattering oak 166Marsh 278
Island 70  Barrens 177Wet 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. 

Figure 1

Figure 1. Frequency of 'forest' survey corners in 10-link distance classes by rounding mean distance (e.g., the 10-link class includes corners with mean distance of 5-15 links).

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.

Figure 2

Figure 2. Frequency of 'oak opening' survey corners in 10-link distance classes by rounding mean distance (e.g., the 10-link class includes corners with mean distance of 5-15 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< meanbetween mean and mean + SD>Mean + SD
Full complementUndisturbedUndisturbedMaintenance
Partial complementUndisturbedMaintenanceBurned
Assumed disturbed - Fire-maintained group< 50 linkslinks> 206 links
Full complementUndisturbedMaintenanceBurned
Partial complementMaintenanceMaintenanceBurned

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 ClassFire 15-year windowWind 15-year windowMaintenance 5-year windowUndisturbed
Barrens  1111
Dry ridge  129
Forest42 11110168
Grove   1
Bottoms   45
Scattering pine  716
Scattering timber2 2460
Swamp (misassigned)  5153
Thicket21 61143
Burned710   
Ravine   6
Windthrown 63  
No other tree around16   
Island  15
Totals7916322110637

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 MHc36 landscape in those conditions and the resulting calculated rotation. Maintenance disturbances were the most frequent disturbance type in this landscape; fire disturbances were relatively rare. 

Natural disturbance rotations for MHc36 landscapes

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 alter, accelerate, or allow 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

Stand Dynamics

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 MHc36 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 MHc36 community, the more probable species is the graph title and the less probable species is shown in parentheses (e.g. Quaking (big-toothed) aspen). 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 1% have dots representing their presence in that diameter class (e.g. Bitternut hickory). 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 MHc36 comprise 6,567 PLS corners and 18,198 witness trees. 

Natural dynamics model for MHc36

Natural dynamics model for MHc36

Compositional Succession
MHc36 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. However surface fires and windthrow chronically disturbed these forests and not many stands experienced long-term succession. If severe surface fire re-initiated the stand, quaking or big-toothed aspen, northern red oak, and paper birch comprised the initial cohort. Wind-origin stands had these species as well, but sugar or red maple were significantly present in the initial cohort. Succession was driven by significant declines in northern red oak and aspen, and replacement by sugar or red maple, basswood, bur or white oak, with some green or black ash. Most change occurred over the period when stand diameter increased from 5-11 inches, but the trend continued until stand diameter was 17 inches and the forest was about 100 years old. Sugar or red maple and basswood very gradually replaced the initial cohort over this period. By the time stand diameter reached 17 inches, MHc36 forests were compositionally stable. Northern red oak and bur or white oak were able to maintain significant presence in mature stands, and white pine continually became a larger component of the forest. This strongly suggests the presence of canopy gaps large enough for establishment and recruitment of mid-tolerant trees. MHc36 forests were affected by maintenance disturbance more so than any other central mesic hardwood community on a rotation of about 40 years, and it seems likely that the effects of surface fires and selective windthrow provided large-gap opportunities. White pine was the large-diameter dominant of this community, and as such its abundance in the large diameter classes is slightly exaggerated due to our method of setting the diameter class equal to that of the largest diameter tree. White pine establishment appeared during the transition as initial-cohort northern red oak, aspen, and birch died. White pine is sufficiently wind-firm and fire-resistant to gradually accumulate as MHc36 stands aged.

Structural Succession
Structural succession in MHc36 forests was not much like that of other mesic hardwood forests, but they were most similar to hardwood forests influenced by maintenance disturbance. In the course of stand maturation they changed considerably with regard to tree density and modestly regarding the evenness at which trees were distributed. Change was due mostly to mortality of initial-cohort northern red oak followed by rapid replacement of the oaks by sugar or red maple. By the time stand diameter reached 4-5 inches any self-thinning was essentially complete and trees of the young forest were approximately 44 feet apart, which is typical of central hardwood forests and sparser than northern hardwood forests. Tree spacing decreased over the period when stand diameter increased from 4-15 inches, and eventually reached a minimum at about 30 feet. This decrease coincided with the rapid decline of initial-cohort northern red oak and immediate replacement by sugar or red maple. This is not similar to most other mesic hardwood forests where ingress and eventual expansion of tolerant hardwoods was prolonged, but it is consistent with the idea that many MHc36 forests were wind-initiated resulting in released advance regeneration of trees able to fill small gaps such as sugar or red maple. As stands matured beyond 15 inches, inter-tree distance increased only slightly in a manner consistent with larger stand diameter and crown expansion. 

MHc36 forests showed modest variance in tree spacing as stands aged. Young forests were patchy and had ratios of standard deviations to mean inter-tree distances of about 1.4, as compared to uniform forests where this ratio is about 1.0 or less. Over the period when stand diameter increased from 4-7 inches this ratio increased and peaked at about 1.7. This was a very patchy distribution of trees and it coincided with the period of rapid decline of northern red oak and expansion of sugar or red maple. After stand diameter reached 8 inches MHc36 stands gradually returned to a more uniform tree distribution, and the ratio of standard deviations to their inter-tree means stabilized at about 1.1-1.2. This was still somewhat 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.

Sivicultural Strategies

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 short rotation of about 40 years. Stand replacing windthrow was an important disturbance with an estimated rotation of 380 years. Stand-replacing fire was not important with an estimated rotation of 1,150 years. 

The usual cause of tree mortality and replacement was species-specific disease or outbreaks of defoliating insects. Because MHc36 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. 
Stand-replacing windthrow and selective windthrow were comparatively common and played an important role as a stand-replacing event and formation of 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 severe 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. 

MHc36 forests rarely experienced stand-replacing fire or surface fires, but fires did occur. Trees common to MHc36 sites vary widely regarding their sensitivity to burning. Thus the effect of fires was to select for fire-tolerance and to considerably delay the ingress and expansion of fire-sensitive trees. Silvicultural emulation involves diminishing the local population of fire-sensitive trees and creating open conditions or large-gaps for intolerant or mid-tolerant trees respectively. Two strategies are envisioned:

  • Re-initiate a stand as would severe surface fire to create open to very large gap habitat lacking fire-sensitive trees
  • Maintain a stand as would light surface fire to create large-gap habitat for fire-resistant trees. 

Modern MHc36 forests are commonly affected by windthrow, either directly or as a consequence of trees weakened by disease, pests, or accumulated wounding and rot.  Climate change has increased the frequency and intensity of straight-line damaging winds, thus windthrow is expected to continue as an important disturbance and silvicultural consideration. Fire no longer affects the composition of the forest due to suppression efforts, and this has changed these forests more so than logging.  Commercial logging coupled with the lack of surface fire has resulted in MHc36 forests with far less northern red oak and far more quaking aspen, big-toothed aspen, sugar maple, red maple, and ironwood than was documented by PLS surveyors in the late 1800s.  Managing MHc36 stands primarily for quaking aspen or big-toothed aspen increases the risk of windthrow because these trees are not wind-firm. Managing MHc36 stands in the absence of surface fire increases the risk of losing white pine, white oak, bur oak, and northern red oak as potential crop trees.

 

Silvicultural Strategy for re-initiating MHc36 forests as would severe windthrow
Emulating windthrow to favor northern red oak, quaking aspen, paper birch, or big-toothed aspen

Re-initiation Concept
Occasionally catastrophic winds would flatten MHc36 forests, thus creating open and large-gap habitat. At the stand-scale it was a highly variable mess, with a variety of outcomes including some that 1) initiated some aspen suckering, 2) prompted some stump sprouting of all present hardwoods, 3) selected for sugar maple, northern red oak, green ash, bur oak, red maple, white oak, and bitternut hickory as 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 sugar maple, basswood, red maple, black ash, American elm, northern red oak, bitternut hickory, and green ash, 5) provided little in the way of mineral seedbeds except on tip-ups, and 6) left abundant coarse woody debris. Although aspen and birch were a significant portion of the initial cohort, oaks and tolerant trees had better survival and reproductive success throughout the self-thinning stage.

 

Silvicultural Strategy to accelerate natural succession in MHc36 forests as would selective windthrow
Emulating initial-cohort mortality to favor sugar maple, basswood, red maple, black ash, bitternut hickory, or green ash

Accelerating Concept
Following windthrow, natural succession occurred rapidly in 30-60 year-old MHc36 stands when initial-cohort northern red oak and aspen died. The transition was short because late-successional, tolerant trees were part of the initial cohort. Slower-growing sugar maple, basswood, green ash, red maple, black ash, and bitternut hickory saplings grew up beneath the emerging canopy, and were poised to immediately replace canopy aspen and paper birch. These tolerant trees also built seedling banks throughout the years when initial-cohort aspen northern red oak were self-thinning. Basswood, sugar maple, and red maple are especially adept at recruiting to the mid-story under either a full canopy or in sub-canopy gaps left when suppressed aspen and northern red oak died. By the time stands were ~60 years old, the sub-canopy and mid-story was filled with tolerant species awaiting release. 

 

Silvicultural Strategy for maintaining a MHc36 forest as would natural senescence, disease, or selective windthrow
Emulating fine-scale disturbance to favor sugar maple, basswood, red maple, black ash, northern red oak, bitternut hickory, or green ash

Small-gap Concept
Mature and old MHc36 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 MHc36 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 sugar maple, basswood, red maple, black ash, northern red oak, bitternut hickory, or green ash.

 

Silvicultural Strategy for re-initiating MHc36 forests as would severe surface fire
Emulating severe surface fire to favor northern red oak, quaking aspen, paper birch, or big-toothed aspen

Re-initiation Concept
Very rarely severe surface fires killed most of the canopy trees in MHc36 forest, thus creating open and large-gap habitat. Such fires 1) stimulated aspen suckering and rapid growth of oak seedling-sprouts, 2) initiated stump sprouting of all present hardwoods, 3) selected for northern red oak, bur oak, white pine, and white oak as fire-resistant residuals, 4) prepared mineral seedbeds that favored trees like bur oak, paper birch, white pine, and white oak, and 5) killed advance regeneration of fire-sensitive, tolerant hardwoods such as sugar maple, red maple, black ash, American elm, and bitternut hickory.

 

Silvicultural Strategy for maintaining MHc36 forests as would light surface fire
Emulating surface-fire to favor northern red oak, green ash, bur oak, white pine, and white oak

Large-gap Concept
Commonly surface fires killed scattered patches of fire-sensitive trees in MHc36 forests, thus creating large-gap habitat. Such fires 1) stimulated aspen suckering and rapid growth of oak seedling-sprouts, 2) initiated stump sprouting of all present hardwoods, 3) selected for northern red oak, bur oak, white pine, and white oak as fire-resistant residuals, 4) prepared mineral seedbeds that favored trees like bur oak, paper birch, white pine, and white oak, and 5) killed advance regeneration of fire-sensitive, tolerant hardwoods such as sugar maple, red maple, black ash, American elm, and bitternut hickory.