Amazonian savannas are isolated patches of open habitats found within an extensive matrix of tropical forest. Climate in Amazonia is transitioning to a state where savannas are more likely to occur than forests, but the hydraulic functioning of savannas species in Amazonian vegetation remain unknown and unexplored. We measured 22 leaf, wood and hydraulic traits, including embolism resistance (as P50), hydraulic safety margin and isotope-based water use efficiency, for the seven dominant woody species (75% of biomass) in an Amazonian savanna plants. We found a wide variation in resistance to embolism (-1.66 ±0.06 MPa and -5.07±0.46 MPa) and structural anatomy with species investing in three different functional strategies to tolerance the drought. (1) species with high leaf succulence have less efficient water use increase Gmax, minimizing the risk for xylem. (2) Species that have higher embolism resistance by reinforcement of the intervascular walls of the longest vessels. (3) Species that decrease the spread of embolism by thickening of the membrane of the intervascular pit of short vessel elements. Our results suggest no unique dominant functional strategy among Amazonian savanna species illustrating the multiple ways to adapt to increasing of seasonality in those environments.

Leaf water potential is a good indicator of tree species responses to rainfall seasonality, allowing to better understand water stress effects on vegetation. We investigated daily and seasonal patterns on leaf water potentials of 21 dominant species in three distinct vegetation types from the Cerrado-Amazonia transition to assess how they respond to seasonal changes in water availability. We found that the variation on leaf water potential (Ψl) across species was higher in the dry compared to the rainy season, for all vegetation types. Typical cerrado and cerradão species showed similar patterns, with higher Ψl regulation under higher VPD and lower variation in predawn Ψl (ΔΨpd), indicating root access to soil water even during intense drought. In contrast, seasonal forest species showed lower regulation under lower VPD effects in both seasons, and higher ΔΨpd, indicating lower access to deep water in the soil. These differences revealed two main groups of species: higher Ψl regulators with a strategy isohydric occurring in both typical cerrado (savannah) and cerradão (woodland), and lower Ψl regulators with anisohydric strategy, occurring in seasonal forest. Our results suggest that seasonal tropical forest species will become very vulnerable to predicted increases on drought severity in the region.

Lianas are a distinctive component of tropical forests and their increase in abundance may have profound ramifications for forest composition and ecosystem functioning. However, so far, the current view considers lianas as a single, functional plant type and, therefore, ignores the life history differences among species resulting from their climbing mechanisms. Here, we integrate data from the main functional traits used to characterize plant form and function with global abundance data to reveal that lianas consistently have differences associated with their main climbing mechanisms. Overall, lianas with active climbing were characterized by an acquisitive strategy, showing higher specific leaf area, foliar nitrogen and slightly higher (marginally significant) maximum photosynthetic rates compared to lianas with passive climbing. Using structural equation modeling, we show that across the tropics the abundance of lianas with active climbing is mainly determined by forest structure (stem size distribution). Our study shows that active and passive climbing lianas clearly diverge in their functional traits and factors affecting their distribution. We conclude that a shift in the current view, where lianas are considered a single, functional plant type, is urgently needed to increase our predictability of their effects on tropical forests in the future.

Carbon allocation to non-structural carbohydrates (NSC) is essential for plant metabolism playing an important role in tree responses to drought. It is still unclear if and how interspecific hydraulic trait variation modulates NSC concentration dynamics in different plant organs, particularly in tropical tree species. We investigated whether drought-resistance strategies (inferred from hydraulic traits) explain seasonal and interannual NSC dynamics in leaves, branches, trunks, and roots in seasonal eastern Amazon tree species in Brazil. We measured NSC concentration in eight abundant species during three years, including the end of the wet and dry seasons of the typical regular years (2013-2014) and the extreme drought induced by El Niño–Southern Oscillation in 2015 (ENSO). Organs have an important contribution to explain the starch (ST), soluble sugar (SS), and NSC variance among trees. We showed seasonal and year-to-year homeostasis in ST and SS concentrations in a majority of organs during 2013 and 2014, but SS increased in all organs during the extreme ENSO drought, while the ST concentration did not. The increase in SS concentration was more evident in woody organs from species with intermediate and tolerant drought strategies. The drought-tolerant species maintain higher root starch concentrations and mobilize more SS during extreme drought.

The future of tropical forests is dependent on the capacity of young trees to adjust to drought. We evaluated multiple hydraulic traits indicative of the drought tolerance of small trees across nine common genera at the world’s longest-running tropical throughfall exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6% in response to the drought treatment. However, understorey trees in both a control and the throughfall exclusion treatment had significantly lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to the large trees, as well as significantly greater hydraulic safety margin (HSM) and PLC and embolism resitance, occupying a distinctly different hydrualic niche. The greater HSM of small understorey trees relative to large canopy trees likely enables them to adjust other aspects of their hydraulic systems to take advantage of increases in light availability in the understorey, driven by drought-induced mortality of canopy trees. Our results suggest that small understorey trees can adjust their hydraulic systems in response to changes in water and light availability and this has major implications for the regeneration potential of tropical forests following droughts.