Data from: The snow and the willows: earlier spring snowmelt reduces performance in the low-lying alpine shrub Salix herbacea

Current changes in shrub abundance in alpine and arctic tundra ecosystems are primarily driven by climate change. However, while taller shrub communities are expanding, dwarf shrub communities show reductions under climate warming, and the mechanisms driving the latter (such as warming temperatures or accelerated spring snowmelt) may be complex. To determine and disentangle the response of a widespread arctic-alpine prostrate dwarf shrub to both climate warming and changes in snowmelt time, we investigated phenology, clonal and sexual reproduction, leaf size, wood tissue carbon balance and leaf damage in 480 patches of Salix herbacea, along its elevational and snowmelt microhabitat range over three years in a space-for-time substitution. Earlier snowmelt was associated with longer phenological development periods, an increased likelihood of herbivory and fungal damage, lower stem density, smaller leaves and lower end-of-season wood reserve carbohydrates. Furthermore, while early snowmelt was associated with an increased proportion of flowering stems, the proportion of fruiting stems was not, as fruit set decreased significantly with earlier snowmelt. Warmer temperatures at lower elevations were associated with decreases in stem number and smaller leaves. Synthesis: Our study indicates that phenology, fitness proxies and fungal/insect damage of the dwarf shrub Salix herbacea (S. herbacea) are strongly influenced by snowmelt timing, and that earlier spring snowmelt reduced performance in S. herbacea. The likely mechanisms for many of the observed patterns are related to adverse temperature conditions in the early growing season. Reductions in clonal (stem number) and sexual reproduction (reduced fruit set) under earlier snowmelt, in addition to increasing damage probability, will likely lead to lower fitness and poorer performance, particularly in shrubs growing in early-exposure microhabitats. Further, we saw no concurrent benefits of higher temperatures for S. herbacea, particularly as warming was associated with lower clonal growth. As growing seasons become warmer and longer in arctic and alpine tundra ecosystems, early snowmelt is a critical mechanism reducing fitness and performance in a widespread dwarf shrub, and may ultimately reduce dwarf shrub communities in tundra biomes.

当前高山与北极冻原生态系统中灌木丰度的变化主要由气候变化驱动。尽管高大灌木群落呈现扩张态势，但矮灌木群落在气候变暖背景下却出现衰减，而驱动后者的机制（如气温升高或春季融雪加速）可能十分复杂。
为明确并解析一种广泛分布的北极-高山匍匐矮灌木——圆叶柳（Salix herbacea）对气候变暖和融雪时间变化的响应，我们采用空间代时间替代法，在三年时间内沿海拔梯度与融雪微生境范围，对480个圆叶柳斑块开展了物候特征、无性与有性繁殖、叶面积、木质组织碳平衡以及叶片受损情况的调查。
融雪提前与更长的物候发育期、更高的植食为害和真菌侵染概率、更低的茎密度、更小的叶片尺寸以及更低的季末木质贮藏碳水化合物水平相关。此外，尽管融雪提前与开花茎占比升高存在关联，但结果茎占比并未随之提升，因为坐果率随融雪提前显著下降。
低海拔区域的更高气温与茎数减少和叶片尺寸变小相关。
综合分析结果表明，圆叶柳（后续简称S. herbacea）的物候特征、适合度替代指标以及真菌/昆虫为害情况均受融雪时间的强烈影响，春季融雪提前会降低S. herbacea的生长表现。多数观测到的格局背后的潜在机制与生长季早期的不利温度条件有关。在融雪提前的情况下，无性繁殖（茎数减少）和有性繁殖（坐果率降低）的衰减，叠加为害风险上升，可能会降低植株适合度与生长表现，尤其是在融雪较早的微生境中生长的灌木。此外，我们未发现更高气温对S. herbacea存在协同益处，尤其因为升温与更低的无性生长速率相关。
随着北极与高山冻原生态系统的生长季愈发温暖且更长，融雪提前是降低一种广泛分布矮灌木适合度与生长表现的关键机制，并可能最终导致冻原生物群系中的矮灌木群落出现衰减。