Data from: Competitor or facilitator? The ambiguous role of alpine grassland for the early establishment of tree seedlings at treeline

Alpine treelines are expected to move upslope with a warming climate. However, so far treelines have responded inconsistently and future shifts remain difficult to predict since many factors unrelated to temperature, such as biotic interactions, affect responses at the local scale. Especially during the earliest regeneration stages, trees can be strongly influenced by alpine vegetation via both competition and facilitation. We aimed to understand the relative importance of these two types of interaction in different vegetation structures for treeline regeneration dynamics. Effects of herbaceous alpine vegetation on seedling emergence and first-year performance were studied in a field experiment in the French Alps (2100 m a.s.l.) with five important European treeline tree species: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia. Total emergence and locally-germinated seedling survival were not affected, but for seedlings planted at two months of age, negative vegetation impacts dominated for all response parameters: first-year survival, growth and carbohydrate accumulation. However, in the winter half-year, evergreen tree seedlings increased carbohydrate reserves under the protection of senescent herbs. Also, responses of locally-germinated seedlings suggest facilitative vegetation effects in the first two months after emergence. Thus, the interaction switched between competition and facilitation according to ontogenetic stage and seasons. Still, the net outcome after one year was negative, but species differed in their susceptibilities. Because initial establishment is the first bottleneck determining whether treelines remain stable or move upslope, understanding establishment, including site-, life-stage and species-specific processes, is essential for understanding observed treeline spatial patterns and dynamics. When developing predictive models of treeline dynamics, all these ‘local’ aspects should be incorporated in addition to more global drivers like changes in temperature.

随着气候变暖，高山林线（alpine treelines）预计将向坡上迁移。然而迄今为止，林线的响应并不一致，且未来的迁移趋势仍难以预测——因为诸多非温度相关的因素，例如生物间相互作用（biotic interactions），会在局地尺度影响林线的响应过程。尤其在树木更新的最早阶段，高山植被可通过竞争与促进两种作用方式，对树木产生强烈影响。本研究旨在明确不同植被结构下，这两类相互作用对林线更新动态的相对重要性。研究团队在法国阿尔卑斯山区（海拔2100米a.s.l.）开展了野外实验，探究高山草本植被对5种欧洲重要林线乔木的幼苗出苗率及一年生生长表现的影响，供试物种分别为欧洲落叶松（Larix decidua）、欧洲云杉（Picea abies）、瑞士五针松（Pinus cembra）、欧洲黑松（Pinus uncinata）与花楸（Sorbus aucuparia）。实验结果显示，总出苗率与本地萌发幼苗的存活率未受显著影响，但对于移栽时已生长2个月的幼苗而言，植被的负向影响在所有响应指标中占主导地位，包括一年生存活率、生长量及碳水化合物积累量。不过在冬半年，常绿乔木幼苗可在衰老草本植被的庇护下提升碳水化合物储备。此外，本地萌发幼苗的响应表明，在出苗后的前两个月，植被可发挥促进作用。由此可见，植被与树木的相互作用会随个体发育阶段与季节发生切换。尽管一年后的净效应仍为负，但不同树种的敏感性存在差异。由于初始定植是决定林线能否维持稳定或向坡上迁移的首个瓶颈，因此理解定植过程——包括局地生境、生活史阶段及物种特异性的相关机制——对于解析已观测到的林线空间格局与动态至关重要。在构建林线动态的预测模型时，除温度变化等全球驱动因子外，上述所有局地过程均应被纳入考量。