My research aims to understand the mechanisms that promote coexistence and the effects of climate change on plant communities through changes in species interactions.
Small changes in rainfall drive substantial changes in coexistence in an annual grassland
For a species to persist in its current range, it must not only survive the direct effects of climate changes but also the indirect effects of altered densities and competition strengths of neighbors. We don’t know what the indirect effects of biotic interactions are for many communities making it difficult to predict how plants will be affected by future climate. To address this question, I set up a large rainfall manipulation experiment where I grew combinations of six annual plant species with varying densities of competitors under two precipitation treatments. I tracked the germination, seed production and competitive neighborhood of over 1500 plants. With this data I tested the direct effects of changes in rainfall on species individually, as well as parameterized an established population growth model to derive the competitive effect of each species on one another in each treatment. I then quantified the stabilizing niche differences and fitness differences which determine long-term coexistence for each species pair under each treatment. I found that the rainfall changes had little direct impact on plants when grown in isolation, but nonetheless could still change the competitive interactions between pairs. Pairs that were predicted to coexist long-term under one treatment could not coexist under another and this was impossible to know from their individual responses. These results show the importance of considering changes in species interactions when predicting consequences of global change. (Experimental set up with a celeb appearance from my mom – an excellent field tech ↓)
Phenological sensitivity to precipitation – implications for coexistence and global change
One of the clearest ecological consequences of climate change thus far, is the observed shift in the phenology (timing) of life history events such as germination, leaf out, and flowering. These changes have great potential to affect interactions between species if their processes shift at different rates. I am exploring whether changes in water availability are shifting flowering times and whether that may be a driver of the shifts in competitive interactions I found previously. Southern California grasslands are dominated by annual plants whose lifespans are tightly tied to rainfall. Most annuals germinate around December when the rainy season starts and senesce sometime between February and June as precipitation wanes. I hypothesized that interspecific variation in phenology may maintain coexistence through reducing competition by temporally partitioning resource uptake. To explore these ideas, I sowed experimental plots with seventeen annual species, to examine whether species in this community are temporally separating their flowering times across the growing season, and if their flowering phenology responds to changes in rainfall. After germination, I 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 duration for all species. Initial analyses show some species are shifting their peak flowering time to earlier while others are not. Idiosyncratic phenological responses to changes in rainfall, may cause new overlap of resource use which could affect competition or open up niche space for invasives to take advantage of, resulting in possible significant changes to the composition of future plant communities.
Community demographic responses to long term variation in climate
Southern California experiences high interannual variability in rainfall due to the irregular El Niño/La Niña cycles. Modern coexistence theory suggests that coexistence mechanisms, such as the temporal storage effect, may be important in communities experiencing fluctuating abiotic conditions like the annual grassland at Sedgwick Reserve. The Kraft lab and collaborators have collected data on several annual species’ germination rates and seed production for the past fifteen years. Precise rainfall data is also available for the site and shows considerable interannual variation. Preliminary analyses of eight annual plant species over 12 years show evidence that species demographic responses are not always correlated across years, indicating that species differ in which years they perform best. Variation in response concentrates intraspecific interactions relative to interspecific interactions and favors coexistence. We are working on quantifying the effect of the interannual variation on coexistence with the intention to understand how future increases in variability and prolonged drought may impact the community composition.
Plant functional traits
In all my projects, I have tried to use plant functional traits to broaden the impact of my research. Functional traits provide a general way to organize the substantial variation in structure and function found across plant species and are a useful tool for comparing processes across communities. The Kraft lab has collected substantial functional trait data of the annual plants in the grassland community including traits such as specific leaf area, maximum height, seed size, rooting depth, specific root length, water use efficiency, and more. I am working on comparing species responses to the competition experiment, the phenology experiment and across years to test whether species with similar functional traits respond to changes in the environment or competitors in similar ways.