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

Questions: 1) How did seedling numbers and species composition change in the first year after a wildfire during drought, relative to pre-fire variation? 2) Has the community returned to pre-fire composition after five years? 3) Has the degree of dominance by exotic annual grasses changed? 4) Is there any evidence that drought conditions affected community cover, before or after fire?

Location: Exotic-dominated annual grassland in southern California, U.S.A.

Methods:  We monitored community cover and native annual forb densities for four years before and four (cover) to five (densities) years after an unintentional fire (fall 2013) coinciding with the spring 2012-2019 California drought. We also measured seedling emergence both before and during the first year post-fire. We assessed post-fire changes in cover and density relative to pre-fire variation, and tested correlations between community cover and annual rainfall measures.

Results: Seedling emergence declined strongly after fire for exotic grasses, but remained stable for exotic forbs. Seedling densities of the most common native forbs declined, but several previously-rare natives increased. Community cover initially shifted towards the exotic forbs Erodium spp., then returned to higher exotic grass densities. Yet the previously-dominant Bromus diandrus declined steeply, even as other exotic grasses and some native forbs increased. Up to five years after fire, relative cover and abundance of the most common exotic and native species still differed from pre-fire composition. Common species were uncorrelated with annual precipitation, but several may have responded to shorter growing seasons.

Conclusions: Immediate post-fire conditions favored exotic and native forbs over grasses, as predicted. Yet in contrast to many previous studies, the community did not return quickly to pre-fire composition but showed persistent changes that favored neither natives nor exotics. Our results suggest post-fire recovery in this habitat may be contingent on abiotic conditions, with drought one potential explanation for changes.

Methods
Please see published paper and metadata file.

Questions: 1) 干旱期间野火发生后的第一年，幼苗数量与物种组成相较于火前基线发生了怎样的变化？2) 野火发生五年后，群落是否恢复至火前的物种组成？3) 外来一年生草本（exotic annual grasses）的优势度是否发生改变？4) 是否有证据表明，干旱条件在火前或火后阶段对群落盖度（community cover）产生了影响？
Location: 美国加利福尼亚州南部以外来物种占优的一年生草本草原。
Methods: 我们在2012-2019年春季加利福尼亚干旱期间，于2013年秋季一场意外野火发生前后，分别开展了为期4年的群落盖度与本地一年生非禾本草本植物（native annual forb）密度监测；野火后对群落盖度的监测持续4年，对密度的监测持续5年。此外，我们还在火前与火后第一年期间测定了幼苗出苗（seedling emergence）情况。本研究以火前群落特征为参照基准，评估了火后群落盖度与密度的变化，并检验了群落盖度与年降雨量指标间的相关性。
Results: 外来一年生草本的幼苗出苗量在火后大幅下降，而外来非禾本草本植物的出苗量则保持稳定。最常见的本地非禾本草本植物的幼苗密度有所降低，但此前较为稀有的若干本地物种的密度有所上升。群落盖度最初转向以外来非禾本草本植物牻牛儿苗属（Erodium spp.）为优势类群，随后恢复至以外来草本占更高密度的状态。但此前占据优势的直雀麦（Bromus diandrus）出现了大幅减少，尽管其他外来草本与若干本地非禾本草本植物的多度有所增加。直至野火发生后五年，最常见的外来与本地物种的相对盖度与多度仍与火前群落组成存在显著差异。常见物种的多度与年降水量无相关性，但部分物种可能对更短的生长季产生了响应。
Conclusions: 正如预期，火后即刻的环境条件相较于草本植物，更有利于外来与本地非禾本草本植物的定植与生长。但与此前多项同类研究结果不同的是，该群落并未快速恢复至火前的物种组成，而是出现了既未偏向本地物种也未偏向外来物种的持续性群落变化。本研究结果表明，该生境的火后恢复可能依赖于非生物环境条件，而干旱或许是解释这些群落变化的潜在因素之一。
方法
请参阅已发表论文与元数据文件。