PS 6-54
The response of photosynthetic biomass production to experimental rainfall manipulations differs between native shrubs and exotic herbaceous species in a coastal sage scrub ecosystem
Many native shrublands in Southern California are increasingly invaded by exotic herbaceous species, with the potential to significantly alter ecosystem functioning. This area is a predicted hotspot for climate change which could promote or prevent further invasion; specifically, precipitation in this region is expected to decline overall, but with increasing interannual variability. Invasion by exotic annual grasses into woody shrublands has reduced carbon stocks in some areas of the Western United States, though these patterns may change depending on the precipitation regime. Here we asked whether invasion altered the ecosystem response to drought versus high rainfall, as quantified by the normalized difference vegetation index (NDVI) – a measure photosynthetic biomass production. To do this we performed a rainfall manipulation experiment with paired plots dominated by either native shrubs or exotic herbaceous species, subjected to treatments of 50%, 100%, or 150% of ambient rainfall. The study site was located in a coastal scrub sage ecosystem, with patches dominated by native shrubs and exotic grasses located near Rainbow, CA, USA. Leaf photosynthetic rates for the native shrubs were also collected using a Licor 6400-XT Portable Photosynthesis System.
Results/Conclusions
During two growing seasons, we found that NDVI varied significantly among rainfall treatments (F=9.14, p=0.006) and vegetation types (F=128.85, p<0.001). Post-hoc tests showed that in the exotic-dominated plots the 150% rainfall treatment had significantly higher NDVI than either the 100% or 50% treatments (p=0.005 and p=0.003, respectively), but that the 50% and 100% treatments are not significantly different. Accounting for pre-treatment measures, exotic dominated communities increased their photosynthetic biomass production over the course of a growing season in the 150% watering treatment more than the native shrubs. This trend was reversed for the 50% drought treatment. Higher drought tolerance in the native species likely allows for their continued growth during drought, while the shallowly rooted exotic species are more negatively affected. Rates of photosynthesis measured for the native shrubs also did not vary among watering treatments (p>0.05), further demonstrating their tolerance to drought. Our results suggest that ecosystem responses to drought vary depending on community composition, and that increased rainfall may promote greater carbon storage in exotic dominated communities when compared to native dominated communities, but with drought, this trend is reversed.