Climate and plant resource controls on coastal sage scrub ecohydrology and succession
Abstract
The ability of ecosystems to recover from disturbance likely has limits, and shifting disturbance regimes or altered recovery conditions may cause ecosystem degradation and increase vulnerability to species invasion and type conversion. Human activities affect disturbance-prone ecosystems in many ways, including fire frequency, introduction of invasive species, and shifts in water, nutrients, and other plant resources. This dissertation investigates the effects of altered precipitation and nitrogen input on adjacent coastal sage scrub (CSS) and grassland communities in the Santa Ana Mountain foothills during post-fire recovery.
Chapter two examines the ecohydrology of CSS and grassland to determine their respective water use strategies and investigate how they coexist and differ under similar climates. Deeper roots and access to stable deep-water stores allow CSS to employ a conservative growth and water use strategy, whereas grassland follows a “live fast, die young” strategy with rapid, intense water withdrawal over a shallower depth range and a shorter growing season. While a mature CSS community tolerates the presence of invasive annuals, a disturbance pattern that kills CSS individuals completely and prevents post-fire resprouting may increase vulnerability to invasion.
Chapter three examines the impact of altered resources on CSS successional patterns. Altered water input is an important controller of successional rate for drought-adapted ecosystems, with reduced water decelerating and increased water accelerating succession. The combination of restricted water and added nitrogen may produce a fundamental shift in CSS succession away from a shrub-dominated community, leading to increased vulnerability of CSS to invasion by Eurasian grassland.
Chapter four investigates the transient maximum ecosystem productivity hypothesis. A transient productivity maximum occurs in the CSS independent of nitrogen or water inputs, indicating this phenomenon is likely driven by light availability and dominance by plant species that have been selected to rapidly capture space. Increased nitrogen and reduced water availability both shift the vegetation type responsible for the transient maximum from the native nitrogen-fixing subshrub Acmispon glaber to Eurasian grasses. Increases in drought and nitrogen deposition may reduce the importance of A. glaber during CSS fire recovery, with cascading effects on ecosystem function.