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Webb, S.L. 1998. Windstorms in Itasca state park, Minnesota: consequences for the pines, for forest dynamics, and for biodiversity. Final report to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 32+ pp.


This project investigated the consequences of windstorms on the forests of Itasca State Park, Minnesota. Itasca Park has unique value as a preserve where old growth stands of pines, now 200 - 350+ years old, escaped the logging that cleared nearly all other forests in the state. Following the infamous storm of July 1995, I undertook surveys of wind damage to trees and of windstorm related regeneration, building on my past research on windstorms at Itasca, ongoing since 1983. The objectives were (1) to understand the consequences of windstorms for tree populations, succession, and species diversity in this unique natural area; (2) to continue to monitor changes in Itasca's vegetation, as it responds to windstorms and to global warming; (3) to evaluate the consequences of windstorm events for old growth pine populations. The focus is on moderate windstorms that damage scattered trees every year.

On the basis of three damage surveys (1983, 1987, 1995), it is clear that Itasca Park's older red and white pines sustain heavy mortality in windstorms. In a typical old pine/hardwoods forest, approximately 10% of each pine population was damaged, half the white pines surviving their damage but most red pines killed outright. This figure includes pines damaged in two major storm years (1983 and 1995) plus pines damaged over another three years (1984 1987) as surveyed in 1987; thus it represents five years of windstorm effects. In a younger pine/spruce/fir stand, red pines sustained less damage but white pines were heavily damaged as in the older forest. The loss of ancient pines is not compensated for by pine regeneration. Windstorm events are not triggering pulses of pine reproduction in the way that fires do, judging from regeneration surveys repeated over time. Widely scattered white pine seedlings grow only on rotting logs, while red pine seedlings are exceedingly rare. Where storms admit sufficient light for pine reproduction, we see instead very rapid growth of aspen and other hardwood sprouts. Elsewhere, smaller treefall areas are apparently too shady for these pines. Efforts by park managers to obtain pine regeneration show a devastating role for deer.

More generally, the windstorm disturbance regime has very different consequences in different forest types, depending primarily on the wood strength of the understory trees. Although disturbances are theoretically expected to enrich diversity by setting back succession in open patches, this dynamic does not occur in the pine/hardwood forest type where a windfirm understory of maples, ironwood, and shrubs survive windstorms to grow upward toward the canopy, shading the ground and precluding colonization of light demanding species. Here windstorms accelerate succession by removing early successional aspen, birch, and pine from the canopy. The overall effect is a depletion of diversity. In contrast, the pine/spruce/fir forest type has a weak wooded understory and thus more frequent formation of discrete canopy light gaps. The highest damage rates within the park are seen for aspen (in overmature, fungus-infested stands) and for white spruce. Patterns of mortality differ between sites and between storms.

Moderate windstorms change the forest by imposing uneven mortality amongst the canopy trees, but have surprisingly little effect on tree regeneration, beyond the rotting log substrates they leave behind. Twelve years of observing regeneration plots (1984 1996, in the pine/hardwoods forest type) showed dramatic changes over time, but those changes did not differ between treefall and control areas. Instead, alterations in the lower layers of the forest might result from a severe drought event in 1988 or might represent a general successional shift in composition as the aspen and pine canopies deteriorate. Major changes include thinning of small seedlings of sugar maple, red maple, and lowbush blueberry; decline in diversity of shrubs; a pulse of red oak seedling germination (long after the key windstorm, between 1992 and 1996), and increased numbers of tall (height >2 m, diameter <2.5 cm) seedlings of shade tolerant species: sugar maple, red maple, and ironwood. The loss of shrub diversity across the drought period could be a harbinger of global warming consequences.

These findings have management implications. First, to preserve and replicate the old growth pine forests of Minnesota, there is a compelling need for more and larger preserves. The rarity and fragmentation of ancient forest remnants puts the biodiversity of these forests at grave risk of destruction by catastrophic windstorms. Second, salvage logging should be avoided since the rotting logs represent major germination sites for the pines and other trees of Itasca, particularly the conifers and paper birch; these logs also support remarkable diversity of fungi and bryophytes. Third, continuing study is needed to elucidate the interactions between fire, windstorms, climate change, and deer as they together structure the forests of Itasca. Many species reach the limits of their natural ranges at this ecotone between conifer forest, deciduous forest, and prairie. Thus Itasca State Park is an invaluable laboratory for monitoring ecological sensitivity and forest dynamics in the future.

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