Climate warming effect on Acer rubrum seedling susceptibility to foliar herbivory

Background/Question/Methods

Climate warming could affect tree seedling susceptibility to foliar herbivory via changes in leaf chemistry and quality for consumption.  Insect herbivory often responds to changes in foliar nitrogen content and the ratio of carbon:nitrogen, an indicator of the quality of foliage to invertebrate herbivores.  Lower C:N ratios are thought to be of greater nutritional value to herbivores.  The C:N ratio is also used as an indicator for changes in leaf chemistry in response to warming-induced increases in soil N availability and plant uptake of nitrogen.  Soil warming was linked to foliar decreases in C:N in canopy Acer rubrum trees at the Barre Woods experimental warming site (Harvard Forest, MA) over a six-year period.  The question is whether elevated soil temperatures lead to changes in foliar herbivory on Acer rubrum seedlings.  Hypotheses: 1) C:N will be lower in leaves growing at elevated temperatures (due to greater soil N availability) 2) With lower C:N ratios, foliar herbivory could respond by a) increasing since more nitrogen can be gained per amount consumed or b) decreasing if sufficient nitrogen is gained with less biomass consumed. Herbivory was measured on individual A. rubrum seedlings growing in soil and air warming chambers (ambient, +3°C, +5°C) under two light conditions (closed-canopy, open-canopy) at Harvard Forest, MA. Leaf area and leaf biomass consumed were measured using visual estimates and photographic methods.  Leaf samples were taken for SLA and chemical (C,N, C:N, isotopes) analyses.  

Results/Conclusions

Preliminary results of visual estimates in the closed-canopy treatment show decreased herbivory with higher temperatures (p=0.027).  While the C:N at +3°C was lower (p=0.078) than the C:N at ambient temperatures as expected by hypothesis 1, the C:N at +5°C was higher ( p=0.069) than the C:N at ambient temperatures.  A potential explanation for greater C:N and decreased herbivory in the +5°C treatment may due to a leaf age difference caused by earlier bud breaks at warmer temperatures.  Budbreak was 6-9 days earlier on seedlings growing at elevated temperatures in a similar warming experiment at Duke Forest, and nitrogen content decreases as leaves age.  For this ubiquitous eastern tree species, summer insect herbivory diminishes with increasing temperature, but not necessarily in-line with altered foliar C:N ratios.