Complementary Strategies of Hydraulic Variability and Conservative Stomatal Regulation Enable Widespread Distributions in a Heterogeneous Karst Landscape

Karst ecosystems display pronounced elevational heterogeneity driven by steep water and soil gradients, fostering niche differentiation as a biodiversity maintenance mechanism. While hydraulic trait divergence underpins such distribution patterns, certain species thrive across diverse microhabitats. Intraspecific variability in drought-resistant hydraulic traits may enable such broad-niche species to exploit habitat heterogeneity, but whether it is a key mechanism in karst ecosystems is unclear. Specifically, locally widespread species lacking hydraulic variability may compensate through conservative stomatal regulation to maintain hydraulic safety, thereby colonizing extreme arid hilltops. For three widespread woody species in a tropical karst forest (<i>Delavaya toxocarpa</i>, <i>Tirpitzia ovoidea</i>, <i>Litsea glutinosa</i>), we quantified the intraspecific variability of key hydraulic traits (leaf area, LA; leaf mass per area, LMA; leaf turgor loss point, Ψ_TLP; stem vulnerability to embolism, Ψ₁₂ and Ψ₅₀; hydraulic conductivity, <i>K</i> _s and diameter of stem vessels, <i>D</i> _h) and the stringency of stomatal regulation (isohydricity index, <i>σ</i>; hydroscape area) along a water availability gradient. Results revealed that as water availability decreased, all three species shifted toward more conservative stomatal regulation (decreasing <i>σ</i> and hydroscapes area), but their hydraulic trait adjustment patterns differed. <i>D. toxocarpa</i> and <i>T. ovoidea</i> exhibited high intraspecific variability in leaf (reduced LA, increased LMA, more negative Ψ_TLP) and stem traits (declining <i>D</i> _h, <i>K</i> _s, Ψ₁₂, Ψ₅₀), enabling colonization of xeric hilltops via hydraulic safety prioritization. Their high variability indices (<i>PPI</i> _TLP, <i>PPI</i> _Ψ50) were consistent with superior community importance. Conversely, <i>L. glutinosa</i> maintained static hydraulic traits and relied primarily on a consistently conservative stomatal strategy (maintaining a stable hydraulic safety margin, HSM₅₀), which facilitates wide distribution but limits competitiveness in karst forest, as reflected in its low importance value (a composite metric of abundance, basal area, and distribution). These findings demonstrate that variability-mediated tradeoffs between hydraulic efficiency and safety, coupled with leaf-stem trait coordination, are key drivers of niche partitioning in karst forests. The study advances understanding of plant adaptive strategies in stochastic environments, emphasizing the role of trait variability in ecological dominance under resource heterogeneity.