Relative influences of landscape-level drivers on insect-mediated forest processes
Forest canopies are critically important to numerous biological processes that shape the global environment. As the major consumers in forests, herbivorous insects substantially influence forest functioning. Insect herbivores are influenced by a complex of factors, including plant quality, predation, and climate. Recent advances in hyperspectral remote sensing are making it possible to quantify determinants of canopy traits that influence herbivorous insect populations at landscape scales. We sought to determine the relative contributions of canopy quality, predation pressure, and climate on rates of herbivory and herbivore-mediated materials flux along a latitudinal gradient of trembling aspen (Populus tremuloides) in northern Wisconsin and Michigan, USA.
In 2011, 18 plots were established from mid-central Wisconsin to the Upper Peninsula of Michigan, USA. We assessed canopy damage, herbivore fluxes (i.e., frass and greenfall), and insect parasitoid densities throughout the growing season (early June-mid September). AVIRIS (Airborne Visible InfraRed Imaging Spectrometer) data were used to estimate canopy quality. Climate data were acquired from PRISM (www.prism.oregonstate.edu). Partial least squares path modeling (PLS-PM) was used to determine relationships among either 1) canopy quality, parasitoid densities, and climate with canopy damage and herbivore fluxes or 2) only AVIRIS-derived canopy quality with canopy damage and herbivore fluxes.
We found that canopy quality had the largest effects on canopy damage. Foliar C:N had the strongest positive relationship with canopy damage, while foliar salicinoids (toxic defense compounds) had the strongest negative relationship. Lower average minimum temperatures had a negative relationship with damage rates, but the response was not statistically significant. Parasitoid pressure and stand structure had had relatively less influence on ecosystem processes mediated by insect herbivores, but older, compared with younger, stands were of lower nutritional value. Comparison of a full model including parasitoids, climate, stand structure, and AVIRIS measurements to a model relating only AVIRIS-derived information with canopy damage and herbivore fluxes resulted in a nominal loss of explanatory power.
Models built using only AVIRIS derived canopy quality performed similarly to models that included additional landscape-level variables and AVIRIS derived foliar trait estimates had the strongest relationships with canopy damage and herbivore fluxes. This study reveals the possibility for hyperspectral remote sensing to estimate parameters that drive variation in insect-mediated ecosystem processes across landscapes.