Data from: Winter warming effects on tundra shrub performance are species-specific and dependent on spring conditions

1. Climate change driven increases in winter temperatures positively affect conditions for shrub growth in arctic tundra by decreasing plant frost damage and stimulation of nutrient availability. However, the extent to which shrubs may benefit from these conditions may be strongly dependent on the following spring climate. Species-specific differences in phenology and spring frost sensitivity likely affect shrub growth responses to warming. Additionally, effects of changes in winter and spring climate may differ over small spatial scales, as shrub growth may be dependent on natural variation in snow cover, shrub density and cryoturbation. 2. We investigated the effects of winter warming and altered spring climate on growing-season performance of three common and widespread shrub species in cryoturbated non-sorted circle arctic tundra. By insulating sparsely vegetated non-sorted circles and parts of the surrounding heath with additional snow or gardening fleeces we created two climate change scenarios: Snow addition increased soil temperatures in autumn and winter and delayed snowmelt timing without increasing spring temperatures, whereas fleeces increased soil temperature similarly in autumn and winter, but created warmer spring conditions without altering snowmelt timing. 3. Winter warming affected shrub performance, but the direction and magnitude were species-specific and dependent on spring conditions. Spring warming advanced, and later snowmelt delayed canopy green-up. The fleece treatment did not affect shoot growth and biomass in any shrub species despite decreasing leaf frost-damage in E. nigrum. Snow addition decreased frost damage and stimulated growth of V. vitis-idaea by approximately 50%, while decreasing in B. nana growth (P < 0.1). All of these effects were consistent in the mostly barren circles and surrounding heath. 4. Synthesis. In cryoturbated arctic tundra, growth of V. vitis-idaea may substantially increase when a thicker snow cover delays snowmelt, whereas in longer-term, warmer winters and springs may favor E. nigrum instead. This may affect shrub community composition and cover, with potentially far-reaching effects on arctic ecosystem functioning via its effects on cryoturbation, carbon cycling and trophic cascading. Our results highlight the importance of disentangling effects of winter and spring climate change timing and nature, as spring conditions are a crucial factor in determining the impact of winter warming on plant performance.

1. 气候变化驱动的冬季气温升高，通过降低植物霜冻损害、提升养分有效性，对北极苔原（arctic tundra）的灌木生长条件产生积极影响。然而，灌木从这些条件中获益的程度，很大程度上取决于后续的春季气候。物候学（phenology）与春季霜冻敏感性的物种特异性差异，很可能会影响灌木对变暖的生长响应。此外，冬季与春季气候变化的影响在小空间尺度上可能存在差异，因为灌木生长依赖于积雪覆盖、灌木密度以及冻融扰动（cryoturbation）的自然变异。
2. 本研究针对冻融扰动（cryoturbation）发育的非分选环北极苔原中的3种常见广布灌木物种，探究了冬季变暖与春季气候改变对其生长季表现的影响。我们通过额外积雪或园艺防寒布（gardening fleeces）对植被稀疏的非分选环及周边部分石楠灌丛进行保温处理，构建了两种气候变化情景：积雪添加组可提升秋冬季节土壤温度并延迟融雪时间，但不会升高春季气温；而防寒布组同样能提升秋冬土壤温度，却可在不改变融雪时间的前提下营造更温暖的春季环境。
3. 冬季变暖会影响灌木的生长表现，但其影响方向与强度存在物种特异性，且依赖于春季气候条件。春季变暖会提前冠层返青，而延迟融雪则会推迟冠层返青。尽管防寒布处理可降低E. nigrum的叶片霜冻损害，但并未对任何灌木物种的枝条生长与生物量产生影响。积雪添加可降低霜冻损害，并使V. vitis-idaea的生长提升约50%，却会抑制B. nana的生长（P < 0.1）。上述所有效应在植被稀疏的非分选环及周边石楠灌丛中均保持一致。
4. 综合分析：在冻融扰动北极苔原中，当更厚的积雪延迟融雪时，V. vitis-idaea的生长可能会显著提升；而从长期来看，暖冬与暖春则更有利于E. nigrum。这可能会改变灌木群落的组成与覆盖度，并通过影响冻融扰动、碳循环以及营养级联效应，对北极生态系统功能产生深远影响。本研究结果强调了厘清冬季与春季气候变化的时间节点与本质特征的重要性，因为春季气候条件是决定冬季变暖对植物生长表现影响的关键因素。