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

Abstract

Extreme droughts such as the one that affected California in 2012-2015 have been linked to severe ecological consequences in perennial-dominated communities such as forests. In annual communities, drought impacts are difficult to assess because many species persist through facultative multiyear seed dormancy, which leads to the development of seed banks. Impacts of extreme drought on the abundance and composition of the seed banks of whole communities are little known. In 80 heterogeneous grassland plots where cover is dominated by ~15 species of exotic annual grasses and diversity is dominated by ~70 species of native annual forbs, we grew out seeds from soil cores collected early in the California drought (2012) and later in the multiyear drought (2014), and analyzed drought-associated changes in the seed bank. Over the course of the study we identified more than 22,000 seedlings to species. We found that seeds of exotic annual grasses declined sharply in abundance during the drought while seeds of native annual forbs increased, a pattern that resembled but was even stronger than the changes in aboveground cover of these groups. Consistent with the expectation that low specific leaf area (SLA) is an indicator of drought tolerance, we found that the community-weighted mean SLA of annual forbs declined both in the seed bank and in the aboveground community, as low-SLA forbs increased disproportionately. In this system, seed dormancy reinforces the indirect benefits of extreme drought to the native forb community.

Long‐term monitoring of a highly invaded annual grassland community through drought, before and after an unintentional fire

“…Not all findings are consistent across studies though. In contrast to our results, LaForgia et al () found greater drought tolerance for Bromus diandrus than Bromus madritensis and Bromus hordeaceus . Our data may only represent local patterns, and so reinforce the value of research from multiple sites in helping resolve how habitat and species characteristics shape drought and fire responses.…”

“…We also cannot rule out the possibility that responses were contingent on conditions specific to the fire year, although total precipitation immediately after fire (2013–2014) was closer to average than for any other growing season in the study (Figure ). Still, our findings are consistent with declines in both above‐ground cover and seed banks of exotic grasses in other California grasslands during the recent drought, paralleled by increases in some native forbs (Harrison, Gornish, & Copeland, ; LaForgia, Spasojevic, Cse, Latimer, & Harrison, ). Similarly, in a predominately native California grassland the recent drought corresponded with disproportionate declines of the most common species and compensatory increases in some rarer competitors (Prugh et al, ).…”

“…Similar to our data, Prugh et al () identified Calandrinia menziesii as a drought ‘winner’, while Erodium spp., Bromus madritensis and Bromus hordeaceus remained unchanged. Likewise, Lupinus bicolor increased strongly both at our site and those monitored by LaForgia et al () and Prugh et al (). Not all findings are consistent across studies though.…”

“…Other recent research similarly suggests that species‐specific responses to abiotic stress and disturbance are more important than broad differences between invasive and native species (Funk & Zachary, ). Encouragingly, species trait‐based approaches show promise for improving predictions (Harrison et al, ; LaForgia et al, ; Prugh et al, ). Harrison et al () found a decline in mean community specific leaf area with drought.…”

Recent advances in understanding grasslands

“…They found that invasive species responded more strongly to carbon dioxide enrichment and elevated temperature (and did not respond significantly to nitrogen deposition and increased precipitation), indicating that the problems caused by invasive plant species are likely to get worse under a changing climate. This study suggested that drought may be detrimental for invasive species, as did a seedbank study in Californian annual grasslands, which found that seeds of exotic annual grasses declined whilst native annual forbs increased 15 . Invasive species have very large impacts on grassland communities in some parts of the world, meaning that understanding and predicting these impacts is a priority.…”