Bluebunch wheatgrass (BBWG; Pseudoroegneria spicata [Pursh] À. Löve) and Snake River wheatgrass (SRWG; Elymus wawawaiensis J. Carlson & Barkw.) are perennial cool-season grasses commonly used in rangeland restoration in the Intermountain West. The annual downy brome (Bromus tectorum L.) is widespread in the region, compromising perennial-grass establishment. We conducted two winter and one summer greenhouse target-neighbor trials to evaluate single-seedling performance of perennials grown with or without this annual grass neighbor (AGN). Our hypotheses were 1) an AGN impacts perennial wheatgrass seedlings’ biomass and water-relations traits, 2) BBWG and SRWG differentially display traits when grown with and without an AGN, and 3) older wheatgrass cultivars differ from newer experimental populations for these traits. The AGN reduced shoot dry matter (DM) 28.2−33.1% and leaf area 32.5−35.9% across trials, but inconsistent differences for other traits suggested different drivers were operating among trials. In both winter trials, low humidity likely drove high vapor pressure deficits, leading to reduced water availability, with xylem pressure potential data suggesting greater water reduction in the third (winter) trial. In the second (summer) trial, heat rather than vapor pressure deficit was the likely driver. Both winter trials displayed higher shoot DM for BBWG under both AGN treatments, but in summer when the AGN was present, SRWG exceeded BBWG. In the summer trial, Goldar BBWG was sensitive to the combination of reduced water and heat. SRWG exhibited “faster” traits facilitating growth, while BBWG displayed “slower” traits conserving acquired resources. This may partially explain why SRWG's natural distribution is mostly restricted to rangelands with high soil fertility (e.g., the Palouse Prairie) while BBWG's distribution is more widespread. Newer experimental populations of both species often produced less shoot DM than older cultivars, suggesting the experimentals feature a more conservative growth strategy.

蓝bu麦草（BBWG；Pseudoroegneria spicata [Pursh]À。Löve）和斯内克河麦草（SRWG；Elymus wawawaiensis J. Carlson＆Barkw。）是多年生冷季草，通常用于西部山区间的草地恢复中。一年一度的霜霉病（Bromus tectorumL.）在该地区很普遍，损害了多年生草的生长。我们进行了两个冬季和一个夏季温室目标邻居试验，以评估有无该年生草邻居（AGN）的多年生植物的单苗性能。我们的假设是：1）AGN影响多年生小麦草幼苗的生物量和水分关系特征; 2）在有和没有AGN的情况下生长时，BBWG和SRWG分别显示不同的特征; 3）这些特征使较早的小麦草品种与较新的实验种群有所不同。在试验中，AGN减少了茎干物质（DM）28.2−33.1％和叶面积32.5−35.9％，但是其他性状的不一致表明在试验中使用不同的驱动程序。在两个冬季试验中，低湿度可能会导致较高的蒸气压不足，从而导致水供应量减少，木质部压力潜力数据表明在第三次（冬季）试验中水的减少量更大。在第二个（夏季）试验中，可能的原因是热量而不是蒸气压不足。两项冬季试验均显示在两种AGN处理下BBWG的茎干DM较高，但在存在AGN的夏季，SRWG超过了BBWG。在夏季试用中，Goldar BBWG对减少的水和热量的组合很敏感。SRWG表现出“较快”的特征以促进增长，而BBWG表现出“较慢的”特征以节省获得的资源。这可以部分解释为什么SRWG的自然分布主要限于土壤肥力较高的牧场（例如Palouse草原），而BBWG的分布更广泛。这两个物种的新实验种群通常比旧品种产生的茎干DM少，