Lianas in tropical dry seasonal forests have a high hydraulic efficiency but not always a higher embolism resistance than lianas in rainforests.

BACKGROUND AND AIMS: Lianas have higher relative abundance and biomass in drier seasonal forests than in rainforests, but whether this difference is associated with their hydraulic strategies is unclear. Here, we investigate whether lianas of seasonally dry forests are safer and more efficient in water transport than rainforest lianas, explaining patterns of liana abundance. METHODS: We measured hydraulic traits on five pairs of congeneric lianas of the tribe Bignonieae in two contrasting forest sites: the wet 'Dense Ombrophilous Forest' in Central Amazonia (~2 dry months) and the drier 'Semideciduous Seasonal Forest' in the inland Atlantic Forest (~6 dry months). We also gathered a broader database, including 197 trees and 58 liana species from different tropical forests, to compare hydraulic safety between habits and forest types. KEY RESULTS: Bignonieae lianas from both forests had high and similar hydraulic efficiency but exhibited variability in resistance to embolism across forest types when phylogenetic relationships were taken into account. Three genera had higher hydraulic safety in the seasonal forest than in the rainforest, but species across both forests had similar positive hydraulic safety margins despite lower predawn water potential values of seasonal forest lianas. We did not find the safety-efficiency trade-off. Merging our results with previously published data revealed a high variability of resistance to embolism in both trees and lianas, independent of forest types. CONCLUSIONS: The high hydraulic efficiency of lianas detected here probably favours their rapid growth across tropical forests, but differences in hydraulic safety highlight that some species are highly vulnerable and may rely on other mechanisms to cope with drought. Future research on the lethal dehydration threshold and the connection between hydraulic resistance strategies and liana abundance could offer further insights into tropical forest dynamics under climatic threats.

Thresholds and alternative states in a Neotropical dry forest in response to fire severity.

Neotropical xerophytic forest ecosystems evolved with fires that shaped their resilience to disturbance events. However, it is unknown whether forest resilience to fires persists under a new fire regime influenced by anthropogenic disturbance and climate change. We asked whether there was evidence for a fire severity threshold causing an abrupt transition from a forest to an alternative shrub thicket state in the presence of typical postfire management. We studied a heterogeneous wildfire event to assess medium-term effects (11 years) of varying fire severity in a xerophytic Caldén forest in central Argentina. We conducted vegetation surveys in patches that were exposed to low (LFS), medium (MFS), and high (HFS) fire severities but had similar prefire woody canopy cover. Satellite images were used to quantify fire severity using a delta Normalized Burning Ratio (dNBR) and to map prefire canopy cover. Postfire total woody canopy cover was higher in low and medium than high severity patches, but the understory woody component was highest in HFS patches. The density of woody plants was over three times higher under HFS than MFS and LFS due to the contribution of small woody plants to the total density. Unlike LFS and MFS patches, the small plants in HFS patches were persistent, multistem shrubs that resulted from the resprouting of top-killed Prosopis caldenia trees and, more importantly, from young shrubs that probably established after the wildfire. Our results suggest that the Caldén forest is resilient to fires of low to moderate severities but not to high-severity fires. Fire severities with dNBR values > ~600 triggered an abrupt transition to a shrub thicket state. Postfire grazing and controlled-fire treatments likely contributed to shrub dominance after high-severity wildfire. Forest to shrub thicket transitions enable recurring high-severity fire events. We propose that repeated fires combined with grazing can trap the system in a shrub thicket state. Further studies are needed to determine whether the relationships between fire and vegetation structure examined in this case study represent general mechanisms of irreversible state changes across the Caldenal forest region and whether analogous threshold relationships exist in other fire-prone woodland ecosystems.

Embolism resistance drives the distribution of Amazonian rainforest tree species along hydro-topographic gradients.

Species distribution is strongly driven by local and global gradients in water availability but the underlying mechanisms are not clear. Vulnerability to xylem embolism (P50 ) is a key trait that indicates how species cope with drought and might explain plant distribution patterns across environmental gradients. Here we address its role on species sorting along a hydro-topographical gradient in a central Amazonian rainforest and examine its variance at the community scale. We measured P50 for 28 tree species, soil properties and estimated the hydrological niche of each species using an indicator of distance to the water table (HAND). We found a large hydraulic diversity, covering as much as 44% of the global angiosperm variation in P50 . We show that P50 : contributes to species segregation across a hydro-topographic gradient in the Amazon, and thus to species coexistence; is the result of repeated evolutionary adaptation within closely related taxa; is associated with species tolerance to P-poor soils, suggesting the evolution of a stress-tolerance syndrome to nutrients and drought; and is higher for trees in the valleys than uplands. The large observed hydraulic diversity and its association with topography has important implications for modelling and predicting forest and species resilience to climate change.

Drought-induced mortality patterns and rapid biomass recovery in a terra firme forest in the Colombian Amazon.

Extreme climatic events affecting the Amazon region are expected to become more frequent under ongoing climate change. In this study, we assessed the responses to the 2010 drought of over 14,000 trees ≥10 cm dbh in a 25 ha lowland forest plot in the Colombian Amazon and how these responses varied among topographically defined habitats, with tree size, and with species wood density. Tree mortality was significantly higher during the 2010-2013 period immediately after the drought than in 2007-2010. The post-drought increase in mortality was stronger for trees located in valleys (+243%) than for those located on slopes (+67%) and ridges (+57%). Tree-based generalized linear mixed models showed a significant negative effect of species wood density on mortality and no effect of tree size. Despite the elevated post-drought mortality, aboveground biomass increased from 2007 to 2013 by 1.62 Mg ha-1  yr-1 (95% CI 0.80-2.43 Mg ha-1  yr-1 ). Biomass change varied among habitats, with no significant increase on the slopes (1.05, 95% CI -0.76 to 2.85 Mg ha-1  yr-1 ), a significant increase in the valleys (1.33, 95% CI 0.37-2.34 Mg ha-1  yr-1 ), and a strong increase on the ridges (2.79, 95% CI 1.20-4.21 Mg ha-1  yr-1 ). These results indicate a high carbon resilience of this forest to the 2010 drought due to habitat-associated and interspecific heterogeneity in responses including directional changes in functional composition driven by enhanced performance of drought-tolerant species that inhabit the drier ridges.