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丛丰度的变化主要由气候变化驱动。尽管高大灌丛群落持续扩张，但矮灌丛群落在气候变暖背景下却呈现出衰退趋势，其背后的驱动机制（如温度升高或春季融雪加速）可能较为复杂。 为明确并解析一种广泛分布的北极-高山匍匐矮灌丛对气候变暖与融雪时间变化的响应，本研究采用空间替代时间法，在三年时间尺度上沿海拔梯度与融雪微生境范围，对480处圆叶柳（Salix herbacea）斑块的物候、无性与有性繁殖、叶面积、木质组织碳平衡及叶片损伤情况展开调查。 提前融雪与更长的物候发育周期、更高的植食性与真菌侵染损伤概率、更低的茎密度、更小的叶面积以及更低的季末木质部储备碳水化合物水平相关。此外，尽管提前融雪会增加开花茎的占比，但结果茎的占比并未同步提升，因为坐果率会随融雪提前显著下降。 海拔更低区域的更高温度与茎数量减少、叶面积更小相关。 研究结果表明，圆叶柳（S. herbacea）的物候、适合度替代指标以及真菌/昆虫损伤程度均受融雪时间的强烈调控，春季融雪提前会降低圆叶柳的生长表现。多数观测到的模式背后的潜在机制，与生长季早期的不利温度条件相关。在融雪提前的条件下，无性繁殖（茎数量减少）与有性繁殖（坐果率降低）的衰退，加上损伤概率上升，很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丛。此外，本研究未发现高温对圆叶柳存在协同益处，尤其是变暖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延长，提前融雪是降低一种广泛分布矮灌丛适合度与生长表现的关键驱动因素，并可能最终导致冻原生物群系中的矮灌丛群落衰退。