Data from: Herbivory and pollen limitation at the upper elevational range limit of two forest understory plants of eastern North America

Studies of species’ range limits focus most often on abiotic factors, although the strength of biotic interactions might also vary along environmental gradients and have strong demographic effects. For example, pollinator abundance might decrease at range limits due to harsh environmental conditions, and reduced plant density can reduce attractiveness to pollinators and increase or decrease herbivory. We tested for variation in the strength of pollen limitation and herbivory by ungulates along a gradient leading to the upper elevational range limits of Trillium erectum (Melanthiaceae) and Erythronium americanum (Liliaceae) in Mont-Mégantic National Park, Québec, Canada. In T. erectum, pollen limitation was higher at the range limit, but seed set decreased only slightly with elevation and only in one of two years. In contrast, herbivory of T. erectum increased from <10% at low elevations to >60% at the upper elevational range limit. In E. americanum, we found no evidence of pollen limitation despite a significant decrease in seed set with elevation, and herbivory was low across the entire gradient. Overall, our results demonstrate the potential for relatively strong negative interactions (herbivory) and weak positive interactions (pollination) at plant range edges, although this was clearly species-specific. To the extent that these interactions have important demographic consequences – highly likely for herbivory on Trillium, based on previous studies – such interactions might play a role in determining plant species’ range limits along putatively climatic gradients.有关物种分布边界的研究大多聚焦于非生物因子（abiotic factors），尽管生物交互作用（biotic interactions）的强度也可能沿环境梯度发生变化，并对种群动态产生显著影响。例如，受严苛环境条件制约，传粉者（pollinator）的丰度可能在分布边界处下降；而植物密度降低会削弱其对传粉者的吸引力，并可能增加或减少植食作用（herbivory）的强度。我们沿加拿大魁北克省蒙梅甘蒂克国家公园内的海拔梯度，针对延龄草（*Trillium erectum*，黑药花科（Melanthiaceae））与美洲猪牙花（*Erythronium americanum*，百合科（Liliaceae））的海拔上限分布边界开展研究，检测了花粉限制（pollen limitation）与有蹄类植食作用的强度变化。就延龄草而言，其花粉限制在分布边界处更为显著，但种子结籽率仅在两个观测年份中的一年里随海拔升高出现小幅下降。与之形成对照的是，延龄草的植食损伤率从低海拔处的不足10%升至海拔上限分布边界处的60%以上。而就美洲猪牙花而言，尽管其种子结籽率随海拔升高出现显著下降，但未观测到花粉限制现象，且整个海拔梯度内的植食损伤率均处于较低水平。整体而言，我们的研究结果表明，在植物分布边界处可能存在相对较强的负向交互作用（植食作用）与较弱的正向交互作用（传粉作用），不过这一现象显然具有物种特异性。若这类交互作用对种群动态具有重要影响——结合以往研究，延龄草所受的植食作用极大概率符合这一情形——那么此类交互作用可能在以气候梯度为假定前提的植物物种分布边界调控过程中发挥关键作用。