Rehm Evan M, Feeley Kenneth J
Department of Biological Sciences, International Center for Tropical Botany, Florida International University, Miami, FL, USA.
The Fairchild Tropical Botanic Garden, Coral Gables, FL, USA.
Oecologia. 2016 Aug;181(4):1233-42. doi: 10.1007/s00442-016-3619-0. Epub 2016 Apr 12.
The elevations at which tropical treelines occur are believed to represent the point where low mean temperatures limit the growth of upright woody trees. Consequently, tropical treelines are predicted to shift to higher elevations with global warming. However, treelines throughout the tropics have remained stationary despite increasing global mean temperatures. The goal of the study reported here was to build a more comprehensive understanding of the effects of mean temperature, low-temperature extremes, shading, and their interactions on seedling survival at tropical treelines. We conducted a seedling transplant study using three dominant canopy-forming treeline species in the southern tropical Andes. We found species-specific differences and contrasting responses in seedling survival to changes in mean temperature. The most abundant naturally occurring species at the seedling stage outside the treeline, Weinmannia fagaroides, showed a negative relationship between the survival of transplanted seedlings and mean temperature, the opposite of a priori expectations. Conversely, Clethra cuneata showed increased survival at higher mean temperatures, but survival also increased with higher absolute low temperatures and the presence of shade. Finally, the survival of Gynoxys nitida seedlings was insensitive to temperature but increased under shade. These findings show that multiple factors can determine the upper distributional limit of species forming the current tropical treeline. As such, predictions of future local and regional tropical treeline shifts may need to consider several factors beyond changes in mean temperature. If the treeline remains stationary and cloud forests are unable to expand into higher elevations, there may be severe species loss in this biodiversity hotspot.
热带树线出现的海拔高度被认为代表了低平均温度限制直立木本植物生长的临界点。因此，随着全球变暖，热带树线预计会向更高海拔移动。然而，尽管全球平均温度不断上升，热带地区的树线却一直保持稳定。本文报道的这项研究的目的是更全面地了解平均温度、极端低温、遮荫及其相互作用对热带树线幼苗存活的影响。我们在安第斯山脉南部热带地区，对三种形成树冠层的主要树线物种进行了幼苗移植研究。我们发现，幼苗存活对平均温度变化存在物种特异性差异和不同的反应。树线外幼苗期最常见的天然物种，即法加罗叶肖榄，其移植幼苗的存活率与平均温度呈负相关，这与先验预期相反。相反，楔叶桤叶树在平均温度较高时存活率增加，但在绝对低温较高且有遮荫的情况下存活率也会增加。最后，亮叶蚊母草幼苗的存活对温度不敏感，但在遮荫条件下存活率增加。这些发现表明，多种因素可以决定构成当前热带树线的物种的上限分布范围。因此，预测未来局部和区域热带树线的变化可能需要考虑平均温度变化之外的几个因素。如果树线保持稳定，云雾森林无法向更高海拔扩展，那么这个生物多样性热点地区可能会出现严重的物种丧失。
