Plant diversity patterns along elevational gradients and across forest ecotones in the northeastern United States

Background/Question/Methods

While climate change is generally expected to shift species distributions toward higher latitudes and elevations, ecosystem adaptation and resilience to climate-induced stress depends on the diversity of primary producers—plants (biodiversity as insurance hypothesis). Climate-induced transition zones (ecotones) between plant communities can be either (i) sensitive to climate change because many species occur at their range limits, or (ii) resilient to climate change because species richness is potentially high due to overlapping distinct floras. We tested if species richness peaked at the ecotone, or declined with elevation with gradual compositional changes across the transition from low-elevation temperate deciduous forests to high-elevation conifer forests in four states in the northeastern United States. We surveyed vegetation on ten mountains at six elevations (from 500 to 1,000m above sea level). Understory vegetation was surveyed on 1,710 plots (each 1x1m) in total (30 plots per elevation; 15 in canopy gaps and 15 under canopy). Overstory trees were surveyed using the point-centered quarter method. Site species richness trends over elevation were tested for the main plant functional types (overstory trees, all understory plants, tree seedlings, herbs) using ANOVA and post-hoc Tukey HSD tests. Compositional changes were examined with non-metric multidimensional scaling (NMDS).

Results/Conclusions

While species composition changed gradually with elevation as ecotone communities occupied intermediate positions between the low- and high-elevation communities, our results did not support either of our species richness hypotheses. Intermediate community compositions at the ecotone did not lead to a peak in species richness there. Gradual community changes with elevation did not lead to a gradual decline in species richness. Instead, species richness of both overstory trees and understory plants was nearly constant in the deciduous forest and through the ecotone, although species richness of both groups declined significantly with elevation within conifer forests above the ecotone. Species richness was nearly constant across all elevations for both tree seedlings and herbs considered separately. Interestingly, seedling abundance of dominant tree species was lowest at the ecotone, while functional diversity of tree seedlings was highest there. Overall, understory species richness was greater in canopy gaps than under forest canopy, but species richness trends over elevation did not differ between these two canopy environments. If species richness connotes resilience to changing climate, our results suggest that temperate deciduous forest and the ecotone environment may have similar potential for climate adaptation, while conifer forests may be less adaptable, particularly at higher elevations.