| Thunderstorm winds (estimated at 25-35 m/sec) often damage scattered trees in two northwestern Minnesota forest stands (Itasca State Park, Clearwater County). Windstorms have different consequences for populations of shade-intolerant trees in the two stands. In the pine-maple stand, sandy loam soils support a well-developed understory of Acer saccharum, Ostrya virginiana, and tall shrubs, which usually survive wind damage and continue to shade the forest floor. Damage and mortality rates are highest for Populus tremuloides, Pinus resinosa and Pinus strobus, populations of which are infected with wood-rotting fungi. Windstorm-related microsites (mounds, pits, stumps, logs) have only 6% areal coverage and are dominated by seedlings of Acer saccharum and Acer rubrum, the same species that dominate control plots. Hence colonization opportunities for shade-intolerant trees rarely form when trees blow down.;The two stands differ in relative frequency of tree breakage and uprooting. However, mounds and stumps do not differ in colonizing flora, hence damage mode has little influence on forest composition.;This work illustrates two problems with the concept of ecological disturbance. First, many definitions of "disturbance" are based upon biotic responses and would include windstorms in the pine-fir stand but would exclude windstorms in the pine-maple stand where species diversity is not enriched. Second, fires and windstorms, both generally considered disturbances, have very different consequences and are not sufficiently analogous to be grouped together.;In contrast, the pine-fir stand is on sandy soils and Acer saccharum and Ostrya virginiana are absent. The most shade-tolerant taxon, Abies balsamea, has weak wood and is prone to wind damage. Light gaps thus form more commonly than in the pine-maple stand. Microsites also have greater (18%) areal coverage. Rotting logs and stumps are major establishment substrates for Betula papyrifera, Abies balsamea, Populus tremuloides, Picea spp., and Pinus strobus in this stand. The resulting patchy distribution of understory trees makes for higher light levels and provides colonization opportunities for shade-intolerant species, which are well-represented in smaller size classes in this stand. |
