The effects of anthropogenic nitrogen deposition on the gas exchange, specific leaf area, and leaf nitrogen concentration of Southern California coastal sage scrub and chaparral shrubs
The semi-arid shrublands of Southern California are subject to some of the highest rates of atmospheric nitrogen deposition in North America. Little is known about the effects of nitrogen (N) pollution on drought-deciduous coastal sage scrub species (CSS) such as California sage (Artemisia californica) and black sage (Salvia mellifera), and sclerophyllous chamise chaparral species such as chamise (Adenostoma fasciculatum) that inhabit this region. This study monitored the photosynthetic and respiration rates, leaf conductance, leaf nitrogen content and specific leaf area of these species quarterly over the course of two years in a mature CSS stand and a pre-and post-fire chaparral stand. An in-situ experiment was conducted; half of the 10 X 10 meter study plots at each site were subjected to an additional 50 kg N ha-1 in the form of NH4N03 fertilizer in October of 2003 and 2004. The remaining plots served as controls subject to ambient N deposition. Rainfall dependent seasonal changes of these physiological measures that are typical of Mediterranean-type ecosystems were observed, however a multiple analysis of variance (MANOVA) indicates that there were no significant differences between control and experimental plots due to N addition. This may be due to the temporal asynchrony of the artificial nitrogen addition and seasonal rainfall, as the nitrogen was added in the fall when the shrubs are physiologically dormant (CSS) or minimal (CC). The first rains may have flushed the N into downslope streams, away from the rooting zones of the shrubs. Alternatively, the lack of significant differences may be because the shrubs had been engaging in luxury consumption, or were already existing in a state of N saturation. Key words: Adenostoma fasciculatum, Artemisia californica, Salvia mellifera, coastal sage scrub, chaparral, nitrogen deposition.