Data from: Seasonality of precipitation interacts with exotic species to alter composition and phenology of a semi-arid grassland

While modeling efforts suggest that invasive species will track climate changes, empirical studies are few. A relevant and largely unaddressed research question is: how will the presence of exotic species interact with precipitation change to alter ecosystem structure and function? We studied the effects of changes in seasonal timing of precipitation on species composition and resource availability in a grassland community in Colorado, USA. We examined how seasonal precipitation patterns affect the abundance of historically present (native) and recently-arrived (exotic) plant species, as well as soil moisture, nitrogen, and above-ground biomass. Over four years, we applied four precipitation treatments based on climate model predictions for the study area: winter-wet/summer-ambient, winter-wet/summer-dry, winter-wet/summer-wet and winter-dry/summer-wet. Cover of exotic winter-active grasses was greater in winter-wet treatments than in control or winter-dry treatments. Cover of native warm-season grasses and forbs was greatest in the winter-dry/summer-wet treatment, and lowest in the winter-wet/summer-dry treatment. These results support the expectation that increased winter precipitation benefits new arrivals, whereas increased summer precipitation benefits later-growing native plants. Structural equation models showed that interactive effects of increased winter precipitation and increased cover of winter-active grasses reduced growing season soil water content and species diversity. In addition, the dominant winter-active species, Bromus tectorum, flowered and senesced earlier in plots receiving increased winter precipitation and reduced summer precipitation, suggesting that earlier growth of winter-active grasses decreases available soil resources and impacts later-growing native plants. Peak above-ground biomass was lowest in the treatment receiving reduced summer precipitation, but only in years with dry springs. Plant-available nitrogen in spring was lower in plots receiving supplemental winter precipitation, and highest in plots with reduced winter precipitation. Synthesis. Our results indicate that altering the seasonality of precipitation can have large direct effects on plant community composition and phenology, as well as significant indirect effects, mediated through exotic species, on plant-available resources and plant interactions.

尽管模型研究表明入侵物种（invasive species）会随气候变化发生分布迁移，但相关实证研究仍较为匮乏。一个具有重要研究价值且尚未得到充分解决的科学问题是：外来物种（exotic species）的存在将如何与降水变化相互作用，进而改变生态系统的结构与功能？我们针对美国科罗拉多州的一处草原群落，研究了降水季节格局变化对物种组成与资源可获得性的影响。我们考察了季节降水模式如何影响历史留存的本土（native）植物与新近入侵的外来植物的丰度，同时测定了土壤湿度、土壤氮含量与地上生物量。在四年的研究周期内，我们依据该区域的气候模型预测设置了四种降水处理方案：冬季湿润/夏季常态、冬季湿润/夏季干旱、冬季湿润/夏季湿润以及冬季干旱/夏季湿润。冬季活跃型外来草本的盖度在冬季湿润处理组中显著高于对照组或冬季干旱处理组。本土暖季草本与非禾本科草本的盖度在冬季干旱/夏季湿润处理组中最高，而在冬季湿润/夏季干旱处理组中最低。上述结果印证了这一预期：冬季降水增加会使外来入侵物种获益，而夏季降水增加则会惠及后期生长的本土植物。结构方程模型（structural equation models）分析显示，冬季降水增加与冬季活跃草本盖度提升的交互作用，会降低生长季土壤含水量与物种多样性。此外，优势冬季活跃物种——旱雀麦（Bromus tectorum）在冬季降水增加且夏季降水减少的样地中，开花与衰老时间均提前，这表明冬季活跃草本的早期生长会消耗可利用土壤资源，并对后期生长的本土植物产生负面影响。地上生物量峰值在夏季降水减少的处理组中最低，但该现象仅出现在春季干旱的年份。春季植物可利用氮含量在冬季补充降水的样地中更低，而在冬季降水减少的样地中最高。综合来看，我们的研究结果表明，降水季节格局的改变不仅会对植物群落组成与物候产生显著的直接影响，还会通过外来物种介导，对植物可利用资源与植物间相互作用产生重要的间接效应。