The tropical Andean forest are reported to be going through compositional changes. Data from the Colombian Andes demonstrates compositional changes in juvenile and adult trees which are driven by mortality of highland-affiliated species in the warm extreme of their thermal ranges and increased abundance of lowland-affiliated species, with evidence suggesting that it is warming and not low precipitation that is the main cause. However specific mechanisms behind such changes remain unknow. Here we use a 2-year field transplant experiment on an elevation gradient in the Colombian Andes to study the impact of warming on growth of dominant Andean tree species. Specifically, we exposed the study species to a range of temperatures within their thermal niches (14°C, 22°C, 26°C), with the trees growing in a common irrigated soil. We planted 24 x 2 year old saplings of 15 of the most dominant tree species of intermediate succession from the Colombian Andes. The plants included one lowland affiliated (Inga), and seven highland affiliated (Clusia, Clethra, Guatteria, Ilex, Miconia, Tibouchina and Weinmannia) genera. We hypothesised that there would be differential responses of lowland affiliated and highland affiliated species. Tree growth and leaf foliar traits were monitored on a quarterly basis for three years. We used (i) logistic regression to assess survival probability as a function of temperature and species identity, (ii) two-way ANOVA to evaluate sapling growth and leaf trait differences between temperature treatments and species. We found sapling survival and growth rate decreased with warming in all highland affiliated species but increased in all lowland affiliated species. Ten of the studied species grew best at 14°C but did survive and grow at + 22°C, albeit at a lower rate. In addition, some species that showed reduced growth and survival with warming invested in leaf traits, showing an increased number of leaves or leaf mass per area at warmer locations. Our findings suggest the high survival of lowland affiliated species across the studied temperature range. However, highland affiliated species were unable to grow and survive at the extreme of their current thermal ranges.
Andean forest, highland-affiliated, thermal ranges, transplant experiment, survival and growth