Facilitation drives tree seedling survival at alpine treelines

A correction has been published: Journal of Plant Ecology, Volume 17, Issue 4, August 2024, rtae054, https://doi.org/10.1093/jpe/rtae054.

[EN]Alpine treelines are considered ecological monitors recording the impacts of climate change on trees and forests. To date, most treeline research has focused on how climate change drives treeline dynamics. However, little is known about how biotic interactions mediate treeline shifts, particularly in the case of tree recruitment, a bottleneck of treeline dynamics. We hypothesized that inter- and intraspecific facilitation determined the establishment and survival of tree seedlings at alpine treelines. To test this hypothesis, 630 Abies georgei var. smithii seedlings with different ages (4–6, 7–9 and 10–15 years old) were transplanted into three growth habitats (canopy-in, canopy-out and meadow) across the alpine treeline ecotone (4300–4500 m) in the Sygera Mountains, on the southeastern Qinghai-Tibetan Plateau. Microclimate, height growth, mortality rates and leaf functional traits of transplanted seedlings were measured over 3 years. We found that the variations in leaf functional traits were driven by microclimate. After the transplantation, the leaf concentrations of soluble sugars and starch and C:P ratio increased, whereas leaf size decreased. The resource use of seedlings gradually shifted to a more conservative strategy as indicated by changes in non-structural carbohydrates and nutrient concentrations. Radiation, temperature and moisture conditions, mediated by plant interactions, influenced seedling mortality and annual growth by affecting leaf morphological traits. Our findings illustrate how facilitation plays a crucial role in altering solar radiation and leaf trait functioning, determining seedling survival and growth at alpine treelines. We provide new insights into the underlying mechanisms for tree establishment and alpine treeline shifts in response to climate change.

[CN]互利作用促进了高山树线幼苗的存活 高山树线被视为记录气候变化对树木和森林影响的生态监测器。迄今为止，大多数树线研究都集中在气候变化如何驱动树线动态的变化。然而，关于生物相互作用如何调节树线变化，特别是在树木更新这一树线动态的瓶颈方面，人们知之甚少。本研究假设种间和种内促进作用决定了高山树线树苗的定居和存活。为了验证这一假设，我们在青藏高原东南部的色季拉山高山树线交错区(海拔4300–4500 m)的3个生长生境(林冠内、林冠外和草地)中共计移植了630株不同年龄(4–6年、7–9年和10–15年)的急尖长苞冷杉(Abies georgei var. smithii)幼苗，连续3年监测了移植后幼苗的微气候、高度生长、死亡率和叶片功能性状。结果发现，叶片功能性状的变化是由微气候驱动的。移植后，叶片可溶性糖和淀粉浓度以及C:P比增加，而叶面积减小。幼苗的资源利用逐渐转向更保守的策略，其背后机制主要受到了生理性状(非结构性碳水化合物和植物养分)的介导。同时，由结构方程模型发现太阳辐射、温度和水分条件在植物相互作用下，通过介导叶片形态性状的变化最终影响了幼苗的死亡率和年生长。上述研究结果表明，植物间的互利作用通过改变太阳辐射和叶片功能性状，最终在决定幼苗生长和存活上起到至关重要的作用，同时也为理解全球气候变化背景下高山树线动态变化的驱动机制提供了新的见解。

This research was supported by the National Natural Science Foundation of China (42030508), the Science and Technology Major Project of Tibetan Autonomous Region of China (XZ202201ZD0005G02) and the Long-Term Ecological Observation Study of Alpine Pine in Southeast Tibet (Science and Technology Innovation Base) (XZ202301JD0001G).

Peer reviewed