How changes in phenology affect coexistence in an annual grassland

Mary Van Dyke, Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA and Nathan J. B. Kraft, Department of Ecology & Evolutionary Biology, University of California, Los Angeles, CA

Background/Question/Methods:
As scientists attempt to understand the effects of climate change on plant communities, a crucial question is how species interactions will be affected by changes in the abiotic environment. It is already clear that some plants are adjusting the timing of key life history events such as flowering in response to temperature and precipitation changes. These adjustments may in turn have complex consequences for the outcome of species coexistence and thus the composition of future communities. Southern California is expected to become more volatile with increasing occurrences of extremely dry and extremely wet years. Examining how precipitation affects coexistence through changes in phenology is therefore key to understanding how plant communities in this region may react to future change. In an annual grassland, we’ve found that precipitation regimes can alter coexistence outcomes for competing plants. We hypothesize that interspecific variation in phenology may maintain coexistence through reducing competition by temporally partitioning resource uptake. If species phenologies change at different rates, there may be overlapping resource use that was not occurring before which could affect competition. Senescence of annual plants is strongly linked with a cessation of resource uptake from the soil and thus greatly reducing competitive effects on neighbors. We set up twenty 1m2 plots with seventeen annual species evenly sowed in October 2019 to examine whether species in this community are temporally separating their flowering times across the growing season, and if their flowering phenologies respond to changes in rainfall. After germination, we reduced 50% of incoming rain for half of the plots. Each plot had a camera set up to take a picture every morning to obtain data on first flowering, peak flowering and total flowering times for all species.
Results/Conclusions:
Preliminary results show that there is a significant difference between species peak flowering times in the control plots providing evidence that these annuals temporally separate their flowering times throughout the growing season. So far we’ve found one grass species, Hordeum murinum, shifted first flower date by an average of eleven days earlier in the reduced rain plots. A few other species are also trending earlier, but for many species there was no detectable shift. Idiosyncratic phenological responses to changes in rainfall, may cause new overlap of resource use and may even open up niche space for invasives to take advantage of, resulting in possible significant changes to the composition of future plant communities.