Data from: Frost maintains forests and grasslands as alternate states in a montane tropical forest-grassland mosaic; but alien tree invasion and warming can disrupt this balance

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 regard-less 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 fol-lowing 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 in recruitment 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 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.

1. 林草镶嵌体（Forest-grassland mosaics）在全球范围内广泛分布，两类植被间的边界清晰明确。火与植食作用被普遍认为是维持这类镶嵌体的核心驱动因子，二者通过抑制草原生境中的树木定植发挥作用；而土壤因子与霜冻通常被视为强化该效应的次要辅助因素。不过，上述驱动因子的相对重要性可能因生态系统而异。具体而言，尽管已知霜冻在诸多山地热带林草镶嵌体中均有出现，但支持其作为驱动因子的实验证据仍较为匮乏。2. 本研究依托印度南部西高止山脉（Western Ghats）的一处热带山地林草镶嵌体草原生境，采用重复原位移植与增温实验方案，探究微气候（霜冻与低温冻融）及土壤因子对本土林木与外来入侵金合欢属（Acacia）植物的种子萌发及幼苗存活的影响。3. 无论土壤类型如何，本土与外来树木的种子在草原生境中的萌发率均更高，这表明种子萌发并非树木定植的限制阶段。然而，相较于毗邻未出现低温冻融的森林生境中的本土幼苗，草原生境中的本土幼苗在冬季霜冻与低温冻融事件后死亡率显著升高。因此，热带冬季的幼苗存活是本土林木在草原生境中定植的核心限制因子。与之形成鲜明对比的是，草原生境中的外来金合欢属幼苗的冬季死亡率则低得多。草原生境中的夜间实验增温显著提升了所有林木幼苗的越冬存活率，但外来金合欢属幼苗的种群补充量增幅远高于本土林木幼苗。4. 综合分析：本研究结果证实，霜冻与低温冻融在该热带林草镶嵌体的草原生境中，是限制树木定植的核心驱动因子。未来气温升高或可解除树木面临的这一限制，促进林木向草原生境扩张，且外来非本土金合欢属植物的扩张速率将远高于本土林木。我们建议，应在多种热带生态系统中开展霜冻对植物定植限制的相关研究，以重新评估霜冻作为热带地区植被转换驱动因子的普遍重要性。