UC Irvine

Global environmental change is impacting ecosystems. Drought, nitrogen deposition, and invasion by annual grasses (GL) are causing extensive loss of Southern California's coastal sage scrub (CSS) community. Warmer temperatures, changes in precipitation, and altered fire regimes are impacting interior Alaska’s boreal forest function and productivity. In this dissertation, I explore these two contrasting ecosystems with the goal of developing a mechanistic understanding of how ecosystems respond to multiple global environmental change drivers.

Chapter 2 examines an apparently stable vegetation boundary between CSS and GL. This chapter utilizes historical aerial photography and measures of above and belowground properties to examine how vegetation type impacts soil development. CSS had more deeply distributed root biomass, denser surface soil, and greater total stocks of soil organic carbon and nitrogen than GL. Vegetation type creates unique soil properties that may reinforce the stable boundary by preventing invasion of vegetation with different growth, rooting, and/or water-use strategies. Chapter 3 explores how CSS and GL respond to drought and added nitrogen. CSS was more sensitive to global change than GL and drought was the main driver of ecosystem change.

In CSS, drought and drought × +N reduced native shrub biomass and altered litter composition, consistent with a vegetation shift from CSS to GL. These changes were mirrored belowground, with reductions in soil bulk density, soil organic carbon content, and soil C/N ratio in CSS drought plots. The surface soil properties in CSS drought plots were more similar to GL than to CSS, indicating that drought induced vegetation shifts from CSS to GL impact ecosystem properties and processes, both above and belowground.

Chapter 4 combines measures of remotely-sensed NDVI with forest surveys to explore the drivers of recent NDVI trends. Recent fire led to sharp NDVI decreases, while early recovery of deciduous species led to NDVI increases. The mid-succession transition from deciduous to evergreen forests led to weak NDVI decreases, and the mid-to-late successional thinning of evergreen canopies to weak NDVI increases. Thus, both increasing and decreasing NDVI stands occur naturally across the landscape, and do not necessarily reflect a large-scale shift in boreal forest productivity.