Tropical ecosystems strongly influence Earth’s climate and weather patterns. Most tropical ecosystems remain warm year-round; nonetheless, their plants undergo seasonal cycles of carbon and water exchange. Previous research has shown the importance of water and light as drivers of the seasonality of photosynthetic activity in the tropics. Although data are scarce, field-based studies have found that seasonal cycles at a handful of tropical forest sites do not match those in land surface model simulations. A comprehensive understanding and model comparison of how seasonal variations in tropical photosynthetic activity relate to climate is lacking. In this study, we identify the seasonal relationships of precipitation and light availability with satellite-based photosynthetic activity. Three dominant and spatially distinct seasonal relationships emerge between photosynthetic activity and these two environmental drivers: photosynthetic activity that is positively correlated with both drivers (36% of tropical pixels), activity that increases following rain but decreases with light (28%), and activity that increases following bright seasons but decreases with rain (14%). We compare distributions of these observed relationships with those simulated by land surface models. In general, model simulations of gross primary productivity (GPP) overestimate the extent of positive correlations of photosynthetic activity with water and underestimate positive correlations with light. The largest discrepancies between simulations and observations are in the representation of the regions where photosynthetic activity increases with light and decreases with rain. Our clear scheme for representing the relationship between climate and photosynthetic activity can be used to benchmark tropical seasonality of GPP in land models.

Climate warming is projected to increase the frequency of climate extremes. The physiological response of tree species to these changes has not been well characterized. Particularly, how hydraulic parameters may adjust with temperature and what are the impacts during a severe drought in mesic forest tree species have limited study. Further, the pattern of responses to drought and recovery has been linked with isohydry-anisohydry strategies, but we do not know if these strategies are still relevant in the hotter and drier future. To fill this knowledge gap, we built a factorial experiment with ambient (AT) and high temperature (HT; ambient +4.5 0C) for two growing seasons, with six weeks of the initial acclimation period, four weeks of drought, and six weeks of recovery during the first year on seven tree species. Throughout, we followed water relations, leaf gas exchange, stem water potential, and hydraulic conductivity. Seedlings were acclimated to prolonged HT by adjusting key hydraulic traits and increasing anisohydry strategies, but these traits were not coordinated, as some isohydric species had surprisingly high stem water potentials at 50% loss of xylem conductivity (P50) and some anisohydric species had negative safety margins. In addition to this, anisohydric species reduced stem water potential during drought and delayed recovery, while isohydric species experienced earlier reductions in photosynthesis and increased dieback at HT. Our findings highlight that hydraulic traits may acclimate to HT and reduce some of the additive effects during drought in anisohydric species, but hotter drought intensified soil and atmospheric drought and increased mortality in isohydric species. Thus, acclimation of hydraulic traits to HT may help anisohydric species to avoid some of the consequences but can’t compensate for the negative effects.