Photosynthesis and growth of invasive plants exposed to extreme drought in California coastal grassland

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Michael E. Loik, Environmental Studies, University of California, Santa Cruz, CA, Justin Luong, Katherine M. Ross and Daniel O. Hastings, Environmental Studies, University of California, Santa Cruz, Santa Cruz, CA

Background/Question/Methods
Longer droughts will be a particularly problematic outcome of global climate change for California. The ecological consequences of longer droughts are numerous, and extend from individuals to communities and ecosystems. Severe drought reduces plant water access and decreases photosynthetic ability, which can reduce fitness and productivity. The goal of this study was to determine the sensitivity of photosynthesis and growth of coastal prairie vegetation to extreme drought. Measurements were conducted between 2015, before drought shelters were constructed, and 2018, after three years of treatment. Comparisons were also made at 15, 150 and 300 m to test for an elevation effect on the drought response of photosynthesis and growth. Plant water potential, stomatal conductance to water vapor, photosynthetic CO2 assimilation of leaves and canopies, and aboveground Net Primary Productivity (ANPP) were measured for drought shelter and open (“control”) plots. Drought shelters were built according to International Drought Experiment guidelines. Based on 100 years of precipitation records, shelters removed 60% of ambient precipitation. Measurements were made in April and May each year, when soil moisture, photosynthesis and growth tend to be maximal.
Results/Conclusions
Biomass was generally lower on shelter compared to open, ambient rainfall plots across years. The differences between sites significantly affected ANPP, growth of grasses and forbs, and the amount of residual biomass (thatch) from the previous year. Year was a significant factor for residual biomass, and growth of grasses and forbs. The drought treatment significantly affected grass ANPP and left-over forb biomass from the prior year. Drought plots were invaded by the non-native Raphanus sativus (Brassicaceae), which accumulated substantial biomass in 2017. Leaf-level photosynthesis and stomatal conductance to water vapor were slightly lower for R. sativus on drought compared to ambient rainfall plots. Leaf and air temperatures and leaf-to-air vapor pressure deficit were not different. For the dominant invasive grass Avena barbata (Poaceae), photosynthesis was 30% lower and stomatal conductance was 65% lower on drought plots. At the canopy level, Net Ecosystem Exchange was about 30-fold lower, and ET was about eight-fold lower, on drought compared to open plots. Results are consistent with reduced stomatal conductance and photosynthetic CO2 assimilation under the drought shelters compared to open, ambient rainfall plots. This portends diminished carbon sink capacity for invaded California coastal grasslands during extreme droughts.