Asymmetrical application of ecological theory along a forest landscape ecosystem gradient

Background/Question/Methods

Understanding responses of biotic communities to perturbations and environmental gradients is fundamental to managing ecosystems and conserving biodiversity. Ecologists have made progress in developing theories for forecasting change to communities, yet have been frustrated that theories often are only accurate for some communities, vary with scale, and apply only under some sets of conditions (e.g., when multiple perturbations do not interact). We used Pinus ponderosa forests of northern Arizona as a model system for evaluating theory regarding responses of plant communities to reducing abundance of a dominant species (Pinus), grazing, and adding a surrogate (smoke) for natural disturbance. In a split-split plot experimental design, we thinned Pinus overstories, allowed or excluded grazing, and applied liquid smoke at 36, 0.05-ha plots at 9 field sites across a landscape ecosystem gradient. We collected plant community data in 2003 before treatment and in 2006 and 2008, three and five years after treatment. 

Results/Conclusions

Influences of factors were hierarchical, with ecosystem type, at the broadest level, constraining responses to other treatments. Ecosystem influences were followed by those of thinning, grazing (which only exhibited effects on thinned plots), and lastly by smoke, which exhibited no or inconsistent effects. Thinning increased species richness the most in lower productivity ecosystems, and, on thinned plots, grazing further increased richness the most in the least productive ecosystem. In contrast, thinning increased plant cover the most in more productive ecosystems. Results suggest that no symmetrical theory regarding influences of reducing a dominant or grazing was applicable to all ecosystems. When allowing for asymmetry, however, ecosystem classification reflected how different areas of the landscape responded differently to perturbations. This may help ecologists refine theory and accommodate the often-observed asymmetrical conformity of system responses to ecological theory.