Gymnosperms vs. angiosperms in an intact mixed forest ecosystem: Resources or beavers

Background/Question/Methods

The relative role of abiotic and biotic factors in determining the spatial distributions of tree species remains an outstanding question generally, but has been little studied in unmanaged temperate forest ecosystems, particularly those that have an intact flora and fauna including large mammals.  We examined the spatial distributions of common tree species in a mapped 13.5-ha old-growth forest plot (~46,000 mapped trees ≥ 1 cm diameter) at Haliburton Forest in Central Ontario, Canada.  A dominant feature in this system is a zone near lake margins in which coniferous trees, including a mix of hemlock (Tsuga canadensis), balsam fir (Abies balsamea) and white pine (Pinus strobus) dominate; in contrast a mixture of deciduous species occurs in areas away from the lake margin, with dominant species including sugar maple (Acer saccharum) and beech (Fagus grandifolia).  We examined the relative role of edaphic factors (topographic position, drainage, soil texture, pH, profile depth, and major mineral nutrients) vs. beaver (Castor canadensis) impacts in determining tree spatial distributions in this system.  A spatial foraging model for beavers was parameterized on the basis of observed species preferences and probability of stem encounter.  Spatial models of species occurrence as a function of edaphic factors and beaver impacts were then compared in a likelihood framework.

Results/Conclusions

Strong effects of both edaphic factors and beaver foraging were detected in model analyses.   Among edaphic factors, topography and drainage effects had the strongest detectable effects on tree distribution patterns.  Edaphic factors alone did not explain the prevalence of conifers near the lake margin.   Beaver foraging impacts extended nearly 100 m from the lake margin, and in agreement with prior studies beavers showed pronounced tree species preferences, and largely avoided conifers.  The spatial pattern of beaver foraging corresponded closely to both the zone in which conifers dominate in the system, and the area in which edaphic models alone poorly predicted tree species distribution patterns generally.  We conclude that beaver foraging is a main factor structuring the forest community in this system, profoundly altering the relative balance of conifers and angiosperms.  As wolves are the main predators of beavers in this region, there is a high potential for top-down regulation of forest community composition as well.