Effects of environmental filters on early establishment of cloud forest trees along elevation gradients: Implications for assisted migration

Cloud forest tree species are reported to be shifting and retracting their distributional ranges in response to increasing temperatures. However, there is limited information regarding the impact of increasing temperatures on the recruitment of cloud forest trees, a critical phase in population dynamics. Evaluating the establishment of introduced seedlings along an elevation gradient offers an opportunity to assess the influence of environmental change on the early establishment and potential response of cloud forest tree species to assisted migration into higher elevations as a mitigation strategy. We evaluated the early seedling establishment of 3240 seedlings of 12 cloud forest tree species introduced into nine degraded forests subjected to traditional selective logging along an elevation gradient (1250-2517 m a.s.l.) in southern Mexico. We examined (1) how the probability of successful seedling establishment varies along the elevation gradient and (2) how temperature, canopy cover, herbaceous cover and humidity affect seedling survival and growth. Seedling survival, relative growth rates and environmental factors were recorded over a period of two years. Most species displayed high survival (∼90%), and survivorship was most strongly related to canopy cover and temperature. Survival probability increased with canopy cover in five species. Seedling survival and growth decreased with increasing temperature in five species, while the opposite trend was found in two species. Growth rates increased with herbaceous cover in seven species. Humidity had no effect on seedling performance. Our experiment shows that most of the study species have a high probability of seedling survival under canopy cover, even outside the limits of their reported elevational ranges. The results indicate that five of the studied cloud forest species may already benefit from assisted migration to cooler climatic conditions at higher elevations. This study supports the design of management guidelines for assisting the migration of cloud forest tree species with narrow distributions, as a climate change mitigation strategy.