The interactive effects of herbivory and rising temperatures on Quercus alba seedling demography: An experimental and observational test

Background/Question/Methods

There is little doubt that climate change alters species interactions within communities. However, few experimental studies have provided insights on the effect of rising temperatures on White Oak (Quercus alba) seedling demography, a dominant tree species found in temperate deciduous forests in the eastern US. In this experimental manipulation, we investigate the potential for combined and interactive effects of insect herbivory and increasing temperatures on Q. alba seedling survival. During the summer of 2010, we surveyed naturally occurring Q. alba seedlings in an array of open-top chambers (OTCs) actively warmed year round, ranging from ambient conditions to 5.5ºC above ambient in half degree increments. Temperature in the OTCs is increased in a regression design in order to delineate potential nonlinearities in the effects of increasing temperature. For the past two summers, we have recorded the intensity of insect herbivory, height, and number of leaves for each individual seedling (mean no. of seedling plot^-1 = 78). In April 2011 we resurveyed the seedlings to determine survival. We used a multivariate logistic regression model that included temperature treatment, overall herbivory, height (cm), and the number of leaves as predictors of seedling survival.

Results/Conclusions

Both temperature (p = 0.001) and herbivory (p = 0.04) affected Q. alba seedling survival such that the risk of seedling death was higher with rising temperatures and higher levels of herbivory. However, there did not appear to be an interaction between herbivory and temperature on seedling survival, indicating that the effect of herbivory on seedling survival is not exacerbated by rising temperatures. Overall, the proportion of Q. alba seedlings surviving after their first growing season (summer 2010) was approximately 33 percent higher at ambient temperatures than at the warmest temperature treatment and average herbivory was approximately 41 percent higher at ambient temperatures than at the warmest temperature.  Taken together, these results could suggest a potential decline in this dominant tree species and an overall shift in the temperate deciduous tree community if temperatures continue to rise.