Department of Environmental Studies, University of California, Santa Cruz, CA 95064, USA.
Glob Chang Biol. 2013 Jan;19(1):161-72. doi: 10.1111/gcb.12032. Epub 2012 Oct 29.
Global change is likely to affect invasive species distribution, especially at range margins. In the eastern Sierra Nevada, California, USA, the invasive annual grass, Bromus tectorum, is patchily distributed and its impacts have been minimal compared with other areas of the Intermountain West. We used a series of in situ field manipulations to determine how B. tectorum might respond to changing climatic conditions and increased nitrogen deposition at the high-elevation edge of its invaded range. Over 3 years, we used snow fences to simulate changes in snowpack, irrigation to simulate increased frequency and magnitude of springtime precipitation, and added nitrogen (N) at three levels (0, 5, and 10 g m(-2) ) to natural patches of B. tectorum growing under the two dominant shrubs, Artemisia tridentata and Purshia tridentata, and in intershrub spaces (INTR). We found that B. tectorum seedling density in April was lower following deeper snowpack possibly due to delayed emergence, yet there was no change in spikelet production or biomass accumulation at the time of harvest. Additional spring rain events increased B. tectorum biomass and spikelet production in INTR plots only. Plants were primarily limited by water in 2009, but colimited by N and water in 2011, possibly due to differences in antecedent moisture conditions at the time of treatments. The threshold at which N had an effect varied with magnitude of water additions. Frequency of rain events was more influential than magnitude in driving B. tectorum growth and fecundity responses. Our results suggest that predicted shifts from snow to rain could facilitate expansion of B. tectorum at high elevation depending on timing of rain events and level of N deposition. We found evidence for P-limitation at this site and an increase in P-availability with N additions, suggesting that stoichiometric relationships may also influence B. tectorum spread.
全球变化可能会影响入侵物种的分布，尤其是在分布范围的边缘。在美国加利福尼亚州内华达山脉东部，入侵性一年生草本植物——多枝赖草呈斑块状分布，与山间西部的其他地区相比，其影响微乎其微。我们使用一系列现场操作来确定多枝赖草在其入侵范围的高海拔边缘，可能会如何应对气候变化和增加的氮沉降。在 3 年的时间里，我们使用雪栅来模拟积雪的变化，通过灌溉来模拟春季降水频率和幅度的增加，并在两种主要灌木——三齿蒿和糙隐子草下的多枝赖草自然斑块以及灌木间空间（INTR）中添加三种水平的氮（0、5 和 10 g m(-2)）。我们发现，4 月份多枝赖草幼苗密度较低，可能是由于积雪较深导致的延迟出苗，但在收获时，小穗产量或生物量积累没有变化。额外的春雨事件仅增加了 INTR 斑块中多枝赖草的生物量和小穗产量。2009 年，植物主要受到水分的限制，但 2011 年，植物受到氮和水分的共同限制，这可能是由于处理时的前期水分条件不同。氮产生影响的阈值随水分添加量的变化而变化。降雨频率比降雨量对多枝赖草生长和繁殖的影响更大。我们的结果表明，降雨从雪到雨的预测变化可能会根据降雨时间和氮沉降水平，促进高海拔地区多枝赖草的扩张。我们在这个地点发现了磷限制的证据，并且氮添加增加了磷的可利用性，这表明化学计量关系也可能影响多枝赖草的传播。
