Spatial and temporal variation in herbivory can differentially influence forest dynamics with long-lasting legacies. To examine the possibility of such an influence, we examined the differential response of balsam fir (Abies balsamea) and white spruce (Picea glauca) to historic patterns of moose (Alces alces) abundance on Isle Royale National Park (ISRO) with dendroecology. These species contrast in palatability and use by moose, with fir comprising 60% of the winter diet of moose on ISRO whereas spruce occurs in only trace amounts. We collected 198 cross-sections and 338 increment cores. Trees were sampled using a spatially-stratified, random selection procedure. We developed allometric-height-to-diameter models and tested for differences between species and among sites. We examined ring width patterns by using tree ring chronologies to test for historic growth anomalies that might be indicative of times and places with high local moose abundance. These events were compared to independent estimates of annual moose abundance dating back to 1959.
Results/Conclusions
Balsam fir and white spruce growing on shallow soils on the east side of ISRO displayed similar allometric relationships between height and diameter (Adj. R2=0.865, F3,251=537.12, P<0.001). However, on deeper soils on the west side where greater rates of herbivory have been observed over time, fir was significantly shorter at a given diameter than fir growing on the east side (P=0.003). White spruce growing on the west side were taller at a given diameter than fir growing close by (P=0.003), but still shorter than those on the east side. For example, a 30cm diameter spruce on the west side was predicted to be 0.53m shorter in height than a tree of equal diameter growing on the east side. By contrast, the difference for fir was 2.9m. Our results suggest that fir and spruce growing on the west are generally shorter than those found on the east. Deeper fertile soils on the west side favor hardwoods and a robust field layer whereas shallower soils on the east may be more favorable to conifers. Nevertheless, the legacy of browsing on fir reduced its stature relative to spruce and our results suggest this disparity persists throughout the tree’s life, which may influence subsequent competitive dynamics and canopy recruitment. Dendroecological approaches for detecting periods of intense herbivory from the tree-ring record will be discussed.