Global precipitation patterns are expected to change in the coming decades showing increased intensity and frequency of events, and southern California is predicted to be a regional global change hotspot. Rainfall scenarios impact invasion dynamics in this system, with native shrublands facing increasing invasion by exotic annual grasses in wet years. Here we focus on green-up date and growing season length as phenological responses to altered precipitation with implications on ecosystem level functioning. To do this, we measured normalized difference vegetation index (NDVI) on a plot-level throughout three growing seasons within a rainfall manipulation experiment. Paired plots were dominated either by native shrubs or exotic herbaceous species and subjected to treatments of 50% (low), 100% (ambient), or 150% (high) of ambient rainfall. The study site was located in a coastal sage scrub ecosystem, with patches dominated by native shrubs and exotic grasses located in San Diego County.
Results/Conclusions
Green-up date was significantly affected by rainfall treatment, though the magnitude of this response depended on plot community composition and varied between growing seasons (3-way interaction, p=0.037). Green-up date was delayed in the 50% rainfall treatment when compared to the 150% treatment in both 2014 and 2015 (p<0.01 for both, by 55 and 14 days, respectively). In 2014, the green-up date of the 50% treatment was also significantly later than the 100% treatment (p=0.048, by 49 days). In both years, green-up date was negatively correlated with growing season length measured from onset of green-up to date to when peak NDVI was recorded, for both native and exotic dominated plots (p<0.01, r<-0.70 for all) suggesting that delayed green-up date does not shift the growing season but rather truncates the period of plant activity. Thus, later green-up may result in reduced ecosystem net primary production; this is supported by the finding that the low rainfall treatment significantly reduced maximum green-up for exotic dominated plots (p=0.001). Interestingly, native dominated communities did not vary in maximum green-up, indicating that native dominated communities may be more resilient to altered precipitation regimes with the capacity to increase the rate of green-up despite impacted growing season length.