Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

Abstract

The relationship between rooting depth and above‐ground hydraulic traits can potentially define drought resistance strategies that are important in determining species distribution and coexistence in seasonal tropical forests, and understanding this is important for predicting the effects of future climate change in these ecosystems. We assessed the rooting depth of 12 dominant tree species (representing c. 42% of the forest basal area) in a seasonal Amazon forest using the stable isotope ratios (δ18O and δ2H) of water collected from tree xylem and soils from a range of depths. We took advantage of a major ENSO‐related drought in 2015/2016 that caused substantial evaporative isotope enrichment in the soil and revealed water use strategies of each species under extreme conditions. We measured the minimum dry season leaf water potential both in a normal year (2014; Ψnon‐ENSO) and in an extreme drought year (2015; ΨENSO). Furthermore, we measured xylem hydraulic traits that indicate water potential thresholds trees tolerate without risking hydraulic failure (P50 and P88). We demonstrate that coexisting trees are largely segregated along a single hydrological niche axis defined by root depth differences, access to light and tolerance of low water potential. These differences in rooting depth were strongly related to tree size; diameter at breast height (DBH) explained 72% of the variation in the δ18Oxylem. Additionally, δ18Oxylem explained 49% of the variation in P50 and 70% of P88, with shallow‐rooted species more tolerant of low water potentials, while δ18O of xylem water explained 47% and 77% of the variation of minimum Ψnon‐ENSO and ΨENSO. We propose a new formulation to estimate an effective functional rooting depth, i.e. the likely soil depth from which roots can sustain water uptake for physiological functions, using DBH as predictor of root depth at this site. Based on these estimates, we conclude that rooting depth varies systematically across the most abundant families, genera and species at the Tapajós forest, and that understorey species in particular are limited to shallow rooting depths. Our results support the theory of hydrological niche segregation and its underlying trade‐off related to drought resistance, which also affect the dominance structure of trees in this seasonal eastern Amazon forest. Synthesis. Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.

Seasonal and drought‐related changes in leaf area profiles depend on height and light environment in an Amazon forest

“…A recent study conducted in the TNF by Brum et al . () concluded that rooting depth, hydraulic stress resilience, and tree size all fell along a single niche axis, likely influencing the functional diversity of this seasonally dry forest. The results of our study suggest that phenological strategy may also be arrayed along this niche axis; in this case, the combination of high light, high VPD, and low soil moisture in short‐statured, shallow‐rooted, canopy surface trees could lead to faster or intensified leaf abscission or reduced leaf expansion and maturation in drought, or a higher prevalence of deciduousness.…”

“…The work of Brum et al . () suggests that large trees are less tolerant of water stress, operating with small safety margins, such that when deep soil water is depleted the consequences may be severe; this helps reconcile short‐term resilience (deep soil water still present) with long‐term large tree drought die‐off. Intriguingly, Leitold et al .…”

“…Since they surveyed with LiDAR 3 months after the measurements made by Brum et al . (), their study may capture delayed drought‐induced tree or branch die‐back. Liu et al .…”

Estimation of Evapotranspiration of Amazon Rainforest Using the Maximum Entropy Production Method

“…Specifically, when stomata close to prevent water loss, this results in a difference between q s and q sat within the stomatal cavity, expected to be particularly pronounced during droughts. In this study, above-canopy air specific humidity (q a ) was used as a surrogate of q s in the site-level analysis since surface soil moisture is unlikely to be a suitable metric to characterize water stress of the Amazon where plants may take up water from very deep soil (Brum et al, 2018;Ivanov et al, 2012;Nepstad et al, 1994). In this study, above-canopy air specific humidity (q a ) was used as a surrogate of q s in the site-level analysis since surface soil moisture is unlikely to be a suitable metric to characterize water stress of the Amazon where plants may take up water from very deep soil (Brum et al, 2018;Ivanov et al, 2012;Nepstad et al, 1994).…”

Phenological and structural linkages to seasonality inform productivity relationships in the Amazon Rainforest

“…With basin‐wide drying and more frequent and severe droughts projected across Amazonia (Malhi et al ., ), it is critical to understand root‐to‐canopy structural and functional responses to water limitation. This is non‐trivial as understanding how forest structure, demography, and water and light availability interact to influence productivity requires a multi‐pronged approach (Leitold et al ., ; Brum et al ., ). As Smith et al .…”