Abstract Restoration of abandoned, high-elevation pastures is needed across many ecosystems. Diverse abiotic and biotic stressors often limit establishment of native trees species, however, justifying the need for novel approaches to alleviate such stressors. Freezing damage often negatively impacts survival of planted trees across temperate landscapes and on some high-elevation tropical restoration sites, such as for Acacia koa (koa) in Hawaii, USA. Koa performs poorly under forest canopies, a potential limitation to the use of nurse trees for establishment on frost-prone sites. Using a heterogeneous canopy of a non-native conifer, Cryptomeria japonica, we underplanted koa seedlings along a simulated range of canopy shelter levels in combination with field fertilization. We tested the effect of a canopy cover gradient and nutrient availability on frost avoidance and tolerance responses, as well as the potential to harness koa's developmental plasticity to optimize growth and survival. C. japonica canopy cover provided protection from frost damage, with increased sheltering under greater canopy closure. When combined with fertilization, increasing canopy closure reduced frost damage and increased koa growth. Although we observed limited frost damage in our study, leaf-level soluble sugars increased during the winter and in more open microsites, reflecting a potential mechanism for frost tolerance in this tropical species. We conclude that frost-tolerant conifers used as nurse trees represent a potential tool to help establish native tree species on high-elevation, frost-prone sites.

中文翻译：

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冠层开度影响高海拔热带牧场恢复中未种植树木的小气候和性能

摘要 许多生态系统都需要恢复废弃的高海拔牧场。然而，多样化的非生物和生物压力源通常会限制本地树种的建立，这证明需要采用新方法来缓解此类压力源。冻害通常会对温带景观和一些高海拔热带恢复地点的种植树木的生存产生负面影响，例如美国夏威夷的金合欢树 (koa)。Koa 在森林树冠下表现不佳，这是在易受霜冻的地点使用护生树建立的潜在限制。我们使用非本地针叶树 Cryptomeria japonica 的异质树冠，结合田间施肥，在模拟的树冠遮蔽层范围内种植 koa 幼苗。我们测试了树冠盖度梯度和养分可用性对避免霜冻和耐受反应的影响，以及利用 koa 的发育可塑性来优化生长和存活的潜力。C. japonica 树冠盖提供了免受霜冻损害的保护，在更大的树冠关闭下增加了遮蔽。当与施肥结合使用时，增加树冠关闭可减少霜冻损害并增加 koa 生长。虽然我们在研究中观察到有限的霜冻损害，但在冬季和更开放的微型场所，叶片水平的可溶性糖增加，这反映了这种热带物种抗霜冻的潜在机制。我们得出的结论是，用作护士树的耐霜针叶树代表了一种潜在的工具，可以帮助在高海拔、易受霜冻的地点建立本地树种。