High tolerance of tropical sapling growth and gas exchange to moderate warming

The effects of global warming on tropical forest growth and carbon storage are uncertain. While observations on canopy trees indicate negative correlations between temperature and growth, some seedling studies suggest the opposite. These contrasting results may reflect ontogenetic differences in temperature responses, or differences between the performance of potted plants under controlled conditions and plants under more variable conditions in the field. To try to bridge the gap between highly controlled experiments on small seedlings and field observations on canopy trees we conducted two sets of outdoor experiments on saplings up to 2.5 m tall; one set to study the effects of night warming, and another set on the effects of day warming. To test the hypothesis that night warming would reduce growth in tropical saplings through stimulation of respiration, we grew the early‐successional species Ochroma pyramidale in large 380‐L soil containers under ambient night‐time temperature and ambient +4.5°C. To test the hypothesis that day warming would reduce growth by reducing photosynthesis we compared plants in multi‐species and single‐species mesocosms rooted in the ground under ambient and passively warmed daytime conditions. In all experiments we monitored growth and measured foliar physiology and plant biomass allocation. Neither night warming nor day warming significantly affected biomass accumulation and allocation. Height growth increased with night warming in O. pyramidale , but decreased with day warming in late‐successional species. Night warming resulted in acclimation of dark respiration. Day warming resulted in acclimation of photosynthesis in early‐successional species, but warming decreased photosynthesis in late‐successional tree species. The seedling‐to‐sapling transition is a critical stage in the life of trees. We found no evidence that in this juvenile growth phase moderate increases in mean temperature reduce the performance of tropical trees, although increases in peak daytime temperature may negatively impact photosynthesis, especially in late‐successional species. A plain language summary is available for this article.