Significance: Combining observations and model results, we show that soil salinization does affect the long-term soil–water balance of saline river basins, eventually acting as an aridity enhancer. The strength of this control depends on the capability of vegetation to cope with salt stress through specific functional traits—i.e., salt tolerance—and is key to the understanding of vegetation–water relations in arid and transitional watersheds. Our approach can also be applied to infer crucial data about the salinization level and vegetation salt tolerance of river basins. Such information is essential to develop reverse-engineering measures to mitigate catchment-scale aridity and salinization, based on salt-tolerance plant traits.

Aim: Mangrove canopy height is a key metric to assess tidal forests' resilience in the face of climate change. In terrestrial forests, tree height is primarily determined by water availability, plant hydraulic design, and disturbance regime. However, the role of water stress remains elusive in tidal environments, where saturated soils are prevalent, and salinity can substantially affect the soil water potential.Location: Global.Time Period: The canopy height dataset provides a global snapshot of the maximum mangrove height geographical distribution for the year 2000. Climate and environmental variables extend over the period 1970–2018.Major Taxa Studied: Mangroves.Methods: We use global observations of maximum canopy height, species richness, air temperature, and seawater salinity—a proxy of soil water salt concentration—to explore the causal link between salinity and mangrove stature.Results: Our findings suggest that salt stress limits mangrove height. High salinity favours more salt‐tolerant species, narrowing the spectrum of viable traits. Highly salt‐tolerant mangroves have evolved to cope with high salt concentrations in the soil, but this adaptation comes at a cost. They typically have lower rates of photosynthesis and growth, resulting in reduced productivity and smaller stature compared to more salt‐sensitive mangrove species. This suggests a causal link between salinity, biodiversity, and tree height, where high salinity selects for more salt‐tolerant species that tend to be less productive and shorter.Conclusions: We hypothesize that the salinity‐induced limit to mangrove canopy height is the direct result of a reduction of primary productivity, an increment in the risk of xylem cavitation, and an indirect consequence of the decrease in biodiversity. As sea‐level rise enhances coastal salinisation, failure to account for these effects can lead to incorrect estimates of future carbon stocks in Tropical coastal ecosystems and endanger preservation efforts.

Summary: Salinity is known to affect plant productivity by limiting leaf‐level carbon exchange, root water uptake, and carbohydrates transport in the phloem. However, the mechanisms through which plants respond to salt exposure by adjusting leaf gas exchange and xylem–phloem flow are still mostly unexplored. A physically based model coupling xylem, leaf, and phloem flows is here developed to explain different osmoregulation patterns across species. Hydraulic coupling is controlled by leaf water potential, ψ_l, and determined under four different maximization hypotheses: water uptake (1), carbon assimilation (2), sucrose transport (3), or (4) profit function – i.e. carbon gain minus hydraulic risk. All four hypotheses assume that finite transpiration occurs, providing a further constraint on ψ_l. With increasing salinity, the model captures different transpiration patterns observed in halophytes (nonmonotonic) and glycophytes (monotonically decreasing) by reproducing the species‐specific strength of xylem–leaf–phloem coupling. Salt tolerance thus emerges as plant's capability of differentiating between salt‐ and drought‐induced hydraulic risk, and to regulate internal flows and osmolytes accordingly. Results are shown to be consistent across optimization schemes (1–3) for both halophytes and glycophytes. In halophytes, however, profit‐maximization (4) predicts systematically higher ψ_l than (1–3), pointing to the need of an updated definition of hydraulic cost for halophytes under saline conditions.

In salt‐affected soils, salt stress causes a drastic reduction of plant transpiration. The extent of this reduction is a function of vegetation salt tolerance and can result in highly species‐specific controls on soil water budget and salinization levels. Despite their crucial role in regulating the hydrology of saline soils, the effects of salt tolerance on transpiration remain mostly unexplored. Here we propose a minimalist stochastic model of primary salinization which reproduces the hitherto overlooked feedback of salinity on the soil water balance and the active role of plant salt tolerance in modulating such an effect. We show that vegetation can exert significant control on leaching occurrence and soil salinization, thus imposing a species‐specific upper limit, C _max , to the concentration of soluble salts in the soil. Since C _max increases with plant salt tolerance, salt‐resilient species have the potential to sustain more saline conditions in the soil, eventually creating a favorable environment for their ecological success.