|Abstract or Summary
- Future scenarios of global climate change rely on large-scale climate envelope models that do not account for local climatic conditions to which organisms most closely respond. Shifts in species distributions and phenology driven by climate change are well-documented, yet we lack a strong understanding of how climate change will influence the demographic rates of animal populations, which directly determine the likelihood of species persistence. Land cover change, an agent of global change that is increasing in extent and intensity to meet the needs of a growing human population, can combine with climate change to stress animal populations by altering microclimatic conditions, as well as changing the availability of resources such as food and cover from predators. Thus, identifying the individual and combined effects of climate and land management practices is essential to accurately predict the responses of animal populations to global change.
Intensive forest management results in land cover change by suppressing the growth of competing plant species in favor of commercial species and altering the abundance and composition of forest vegetation. In turn, changes in the abundance and composition of forest vegetation can modify local air temperatures. I hypothesized that herbicide application would alter the thermal environment for early-seral forest organisms (Chapter 2). If this were the case, then air temperature could be expected to increase in magnitude and variability along a gradient of herbicide treatment intensity. To test this hypothesis, I used iButton dataloggers to monitor air temperature at 160 nest boxes on 20 intensively managed early-seral forest stands (10.4 - 18.9) in the northern Oregon Coast Range, USA representing a gradient in intensive forest management (i.e., no-spray control, light, moderate and intensive herbicide application). I also measured the amount and composition of vegetation cover to test for herbicide effects on vegetation among application intensities. Using linear mixed models, I compared three measures of air temperature (mean daily minimum, mean, and maximum) and their associated coefficients of variation (CVs). Additionally, I used linear mixed models to confirm differences in total vegetation and broadleaved vegetation cover. Although mean total vegetation cover generally decreased, it did not significantly differ among herbicide treatments; in contrast, mean broadleaved vegetation cover was significantly reduced in the moderate and intensive treatments. Herbicide treatment was a significant predictor of maximum and mean temperatures, but minimum temperatures did not differ with herbicide treatment. Although there was an effect of herbicide treatment on air temperature, corrected pairwise comparisons indicated no significant differences among treatments. I note that though my power to detect statistical differences among treatments was limited, these differences were quite small (< 0.5°C) and confidence intervals were
generally relatively narrow (< 1.5°C), suggesting that temperature did not differ among herbicide treatments in biologically meaningful ways. Furthermore, I did not detect an effect of herbicide treatment on temperature variability. Estimated differences in temperature variability among treatments were small (< 0.5%) and confidence intervals covered a relatively broad range of values, indicating that I did not have enough statistical power to detect effects. I found no uniform pattern in the direction (positive or negative) of the effect of herbicide treatment on temperature or CVs among treatment intensities.
Daily air temperatures can strongly influence the reproductive output of early-seral songbirds if temperatures exceed physiological tolerances of offspring, decreasing physiological performance (e.g., excessively high and low temperatures can alter metabolic rates) and survival. Moreover, the abundance and composition of early-seral forest vegetation can influence songbird reproductive output through changes in nest predator communities, food resources, or both. Thus, I further hypothesized that intensive forest management practices could combine with intraseasonal air temperatures to impact reproductive output in an insectivorous cavity-nesting songbird, the House Wren (Trogolodytes aedon) (Chapter 3). If this were the case, then House Wren nest survival, the number of offspring produced, and the quality of those offspring would decline with greater management intensity and increasing air temperatures. To test these predictions, I monitored 283 nests on 24 intensively managed early-seral forest stands (8.6 - 18.9 ha) in the northern Oregon Coast Range, USA representing a gradient in intensive forest management (i.e., no-spray control, light, moderate and intensive herbicide application).
I used data from Chapter 2 to test for combined effects of air temperature and herbicide-driven vegetation changes on House Wren reproductive output. Using linear mixed models within a model selection framework, I did not find support for combined effects of temperature and herbicide treatment on nest survival. After accounting for the effects of herbicide-driven vegetation changes on reproductive output, air temperature effects were negligible. My results suggest that post-harvest vegetation management likely does not influence the number of young produced nor their quality (as indicated by body condition) in intensively managed early-seral forests, but may influence nest survival. However, nest survival did not decline along a gradient of herbicide intensity as I expected. Instead, mean nest survival was greatest in the control and most intensively managed stands (failure was greatest in the light treatment); however, these effects were so variable as to not be statistically significant.
My results suggest that post-harvest vegetation management in intensively managed forests may be linked to minor changes in microclimatic air temperatures, but that there is high variation in these effects and effects are likely small. Therefore, there appears to be limited potential for vegetation control strategies through herbicides to buffer expected climate change effects on organisms in early-seral forest. My results also suggest that the potential for combined effects of herbicide application and air temperature on early-seral cavity-nesting songbirds is limited. Under current local climate patterns, air temperature appears to exert negligible effects on House Wren reproductive output after accounting for changes in vegetation cover. The effects of herbicide-driven vegetation changes on early-seral songbirds may be large, though highly
variable, and not an increasing function of herbicide intensity. These effects may be primarily predator-mediated, as indicated by the large effects of herbicide treatment on House Wren nest survival but not the number of offspring produced nor their quality. My finding of limited temperature effects on House Wren reproductive output compared to the effects of forest management intensity supports predictions that, despite increasing concerns over the impacts of advancing climate change on animal populations, land cover change driven by anthropogenic land use will continue to be the primary global change driver impacting animal populations in the near future.