Climate‐driven diversity change in annual grasslands: Drought plus deluge does not equal normal

Abstract

Climate forecasts agree that increased variability and extremes will tend to reduce the availability of water in many terrestrial ecosystems. Increasingly severe droughts may be exacerbated both by warmer temperatures and by the relative unavailability of water that arrives in more sporadic and intense rainfall events. Using long-term data and an experimental water manipulation, we examined the resilience of a heterogeneous annual grassland community to a prolonged series of dry winters that led to a decline in plant species richness (2000-2014), followed by a near-record wet winter (2016-2017), a climatic sequence that broadly resembles the predicted future in its high variability. In our 80, 5-m observational plots, species richness did not recover in response to the wet winter, and the positive relationship of richness to annual winter rainfall thus showed a significant weakening trend over the 18-year time period. In experiments on 100, 1-m plots, wintertime water supplementation increased and drought shelters decreased the seedling survival and final individual biomass of native annual forbs, the main functional group contributing to the observed long-term decline in richness. Water supplementation also increased the total cover of native annual forbs, but only increased richness within nested subplots to which seeds were also added. We conclude that prolonged dry winters, by increasing seedling mortality and reducing growth of native forbs, may have diminished the seedbank and thus the recovery potential of diversity in this community. However, the wet winter and the watering treatment did cause recovery of the community mean values of a key functional trait (specific leaf area, an indicator of drought intolerance), suggesting that some aggregate community properties may be stabilized by functional redundancy among species.

Seed banks of native forbs, but not exotic grasses, increase during extreme drought

“…Long‐term work at this site also showed that dry years cause high‐SLA forbs to decline in aboveground cover and diversity relative to low‐SLA forbs (Harrison et al. , ). Thus, based simply on changes in aboveground abundance and seed input, we predicted that (1) abundance of exotic annual grass seeds in the seed bank would decline more during the severe drought than the abundance of native annual forb seeds, and (2) high‐SLA native annual forbs would decline more in the seed bank than low‐SLA forbs, leading to lower community weighted mean SLA of native annual forbs in the seed bank in 2014 than 2012.…”

“…A previous study at the same site found that the extreme 2012-2014 drought caused aboveground cover of exotic annual grasses, but not native forbs, to decline even more than expected based on community responses to normal interannual variability (Copeland et al 2016). Long-term work at this site also showed that dry years cause high-SLA forbs to decline in aboveground cover and diversity relative to low-SLA forbs (Harrison et al 2015(Harrison et al , 2017. Thus, based simply on changes in aboveground abundance and seed input, we predicted that (1) abundance of exotic annual grass seeds in the seed bank would decline more during the severe drought than the abundance of native annual forb seeds, and (2) high-SLA native annual forbs would decline more in the seed bank than low-SLA forbs, leading to lower community weighted mean SLA of native annual forbs in the seed bank in 2014 than 2012.…”

“…(), and Harrison et al. () for further details and previous analyses of aboveground data from these plots. All vegetation surveys used in the present study were done using a 1‐m 2 sampling frame and were carried out by the same trained and experienced person with a minimum estimate threshold of 0.1%.…”

“…Each plot consists of five permanently marked 1-m 2 subplots along a 40-m transect where visual estimates of species cover ("aboveground data") were recorded twice annually in April and June to capture peak cover for both early-and lateflowering species. See Harrison (1999), Elmendorf andHarrison (2009), Fernandez-Going et al (2012), and Harrison et al (2017) for further details and previous analyses of aboveground data from these plots. All vegetation surveys used in the present study were done using a 1-m 2 sampling frame and were carried out by the same trained and experienced person with a minimum estimate threshold of 0.1%.…”

Invasive species interact with climatic variability to reduce success of natives

“…These findings underscore that the long-term decline in native annuals with high specific leaf 311 area, i.e., acquisitives, that has been observed in this study system (Harrison et al 2015(Harrison et al , 2017 is 312 likely neither a normal fluctuation nor strictly a consequence of a drier climate. Instead, intense 313 competition from invasive grasses during dry conditions is likely exacerbating the decline of 314 these drought-intolerant species.…”