Shallow water table and high fertility selects for trees with low embolism resistance across the Amazon basin

Aim
Understanding tree mortality under climate change-induced droughts requires knowledge of hydraulic trait distribution across environmental gradients. The spatial distribution of embolism resistance (i.g., P50), a key trait linked to hydraulic failure, is currently unknown across the Amazon. Here we tested how precipitation, water table depth (WTD), and soil fertility interact as filters modulating the geographic hydraulic trait distribution.

Location
Four sites in three key regions across the Amazon basin: Central, Western peripheral area, and Brazilian Shield.

Time period
Present

Major taxa studied
Tree species

Method
We measured hydraulic vulnerability curves of 64 tree species (165 individuals) using a hierarchical design contrasting seasonal precipitation gradients, soil fertility and WTD. To produce the geographical distribution of hydraulic traits for the basin scale, we use the coefficients of the best based on generalized linear models, projected on a 1 km grid in each environmental layer.

Results
We show that WTD and soil fertility are the main drivers of embolism resistance, while precipitation has a secondary effect. Trees in shallow WTD and fertile soils had riskier hydraulic strategies. Our spatial projection identified the Amazonian southern band and valleys across the basin as the most vulnerable areas. High soil fertility in these regions increases the variability of hydraulic traits and thus uncertainty in predictions of drought responses.

Main conclusion
We show, at the large scale of the Amazon basin, a convergence of dominant tree species towards hydraulic resistance traits when resource levels (water, nutrients) are low, but a high variability when those increase, even if their means are on the risky side of the spectrum. From that, we highlight the importance of considering WTD and soil fertility when assessing hydraulic traits and their effects on tree mortality under climate change-induced droughts.