Assisted migration of cloud forest trees: Unearthing the effects of climatic transfer distance

Global climate change is impacting tree species recruitment. For many tropical montane tree species, rising temperatures are driving mortality at lower elevations as well as upward shifts towards higher elevations. Shade-tolerant trees are a key functional group in the structure and functioning of tropical forests, but their potential migration pathways might be inaccessible because of deforestation and limited dispersal. Assisted migration of key tree species could help to overcome the uncoupling between species population dynamics and climate change, but our knowledge of tropical tree species responses to climate is extremely limited. We evaluated the effect of Climatic Transfer Distance (CTD; the difference between the historical climate at the seed source and the current climate at translocation sites) on the survival and growth of 13 shade-tolerant tropical montane cloud forest tree species. A decline in the survival and growth of young tree life stages was predicted to occur with higher CTD. In each of eight forest sites along an elevation gradient (1250-2429 m a.s.l.) in Mexico, 30 seedlings per species were planted (3120 in total). Individual survival and growth were recorded for eight years. We tested the survival and growth response to CTD in terms of Mean Annual Temperature (CTD_MAT), Maximum and Minimum Temperatures (CTD_Tmax and CTD_Tmin), Mean Annual Precipitation (CTD_MAP), and Climate Moisture Deficit (CTD_CMD). Sapling survival (all species considered together) was 56.1 ± 2.8%, with an estimated mortality rate of 5.4 ± 0.8 % y^-1. Survival declined with increasing CTD_MAT, CTD_MAP, and CTD_CMD, while growth declined with increasing CTD_Tmax, CTD_MAT, CTD_MAP, and CTD_CMD, and in the warmer and drier sites. However, the results show a relatively wide tolerance to the broad range of CTD in the functional group studied, offering an optimistic forecast for their establishment at newly available sites and under changing climates. These findings provide a scientific basis for the design of management strategies and policies to mitigate climate change impacts.