1. Forest–grassland mosaics, with abrupt boundaries between the two vegetation types, occur across the globe. Fire and herbivory are widely considered primary drivers that maintain these mosaics by limiting tree establishment in grasslands, while edaphic factors and frosts are generally considered to be secondary factors that reinforce these effects. However, the relative importance of these drivers likely varies across systems. In particular, although frost is known to occur in many montane tropical mosaics, experimental evidence for its role as a driving factor is limited.
2. We used replicated in situ transplant and warming experiments to examine the role of microclimate (frost and freezing temperatures) and soil in influencing germination and seedling survival of both native forest trees and alien invasive Acacia trees in grasslands of a tropical montane forest–grassland mosaic in the Western Ghats of southern India.
3. Seed germination of both native and alien tree species was higher in grasslands regardless of soil type, indicating that germination was not the limiting stage to tree establishment. However, irrespective of soil type, native seedlings in grasslands incurred high mortality following winter frosts and freezing temperatures relative to native seedlings in adjoining forests where freezing temperatures did not occur. Seedling survival through the tropical winter was thus a primary limitation to native tree establishment in grasslands. In contrast, alien Acacia seedlings in grasslands incurred much lower levels of winter mortality. Experimental night‐time warming in grasslands significantly enhanced over‐winter survival of all tree seedlings, but increases were much greater for alien Acacia than for native tree seedlings.
4. Synthesis. Our results provide evidence for a primary role for frost and freezing temperatures in limiting tree establishment in grasslands of this tropical montane forest–grassland mosaic. Future increases in temperature are likely to release trees from this limitation and favour tree expansion into grasslands, with rates of expansion of non‐native Acacia likely to be much greater than that of native trees. We suggest that studies of frost limitation to plant establishment are needed across a range of tropical ecosystems to re‐evaluate the general importance of frost as a driver of vegetation transitions in the tropics.

中文翻译：

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弗罗斯特（Frost）在山地热带森林-草原马赛克中维持森林和草原的交替状态。但是外来树木的入侵和变暖会破坏这种平衡

1. 森林-草地马赛克在两种植被类型之间都有突然的界线，遍布全球。人们普遍认为火和草食是通过限制草原树木的形成来维持这些镶嵌的主要驱动力，而普遍认为食道因素和霜冻是加强这些影响的次要因素。但是，这些驱动程序的相对重要性可能会因系统而异。特别是，尽管已知霜冻会发生在许多山地热带马赛克中，但有关其作为驱动因素的实验证据有限。
2. 我们使用复制的原位移植和增温实验，研究了小气候（霜冻和冷冻温度）和土壤对热带山地林-草地马赛克的草原中天然林木和外来入侵相思树的发芽和幼苗存活的影响。印度南部的西高止山脉。
3. 无论土壤类型如何，草原上本地和外来树种的种子发芽率均较高，这表明发芽不是树种建立的限制阶段。但是，与土壤类型无关，相对于没有发生冻结温度的毗邻森林中的本地幼苗，冬季霜冻和冻结温度后草原上的本地幼苗会导致较高的死亡率。因此，整个热带冬季的幼苗生存是草原原生树种的主要限制。相反，草原上的外来相思树苗的冬季死亡率要低得多。在草原进行的夜间夜间实验性实验大大提高了所有树木幼苗的越冬存活率，但外来相思树的增加幅度更大 而不是本地树苗。
4. 综合。我们的结果提供了证据，表明霜冻和冰冻温度在限制这种热带山地森林-草原马赛克的草地中树木的建立方面起着主要作用。未来温度的升高很可能使树木摆脱这种限制，并有利于树木向草原的扩展，非本地相思树的扩展速度可能会比原生树大得多。我们建议需要对一系列热带生态系统进行霜冻限制以建立植物的研究，以重新评估霜冻作为热带地区植被转变驱动力的一般重要性。