Revealing the mechanisms of environmental niche partitioning within lowland tropical forests is important for understanding the drivers of current species distributions and potential vulnerability to environmental change. Tropical forest structure and species composition change across edaphic gradients in Borneo over short distances. However, our understanding of how edaphic conditions affect tree physiology and whether these relationships drive niche partitioning within Bornean forests remains incomplete.

This study evaluated how leaf physiological function changes with nutrient availability across a fine-scale edaphic gradient and whether these relationships vary according to tree height. Furthermore, we tested whether intraspecific leaf trait variation allows generalist species to populate a wider range of environments.

We measured leaf traits of 218 trees ranging in height from 4 to 66 m from 13 dipterocarp species within four tropical forest types (alluvial, mudstone, sandstone, kerangas) occurring along an < 5km edaphic gradient in North Borneo. The traits measured included saturating photosynthesis (A_sat), maximum photosynthetic capacity (V_cmax), leaf dark respiration (R_leaf), leaf mass per area (LMA), leaf thickness, minimum stomatal conductance (g_dark) and leaf nutrient concentrations (N, P, Ca, K, Mg).

Across all species, leaf traits varied consistently in response to soil nutrient availability across forest types except R_{leaf_mass}, [Mg]_leaf and [Ca]_leaf. Changes in photosynthesis and respiration rates were related to different leaf nutrients across forest types, with greater nutrient-use efficiency in more nutrient-poor environments. Generalist species partially or fully compensated reductions in mass-based photosynthesis through increasing LMA in more nutrient-poor environments.

Leaf traits also varied with tree height, except V_{cmax_mass}, but only in response to height-related modifications of leaf morphology (LMA and leaf thickness). These height-trait relationships did not vary across the edaphic gradient, except for A_sat, [N]leaf, [P]_leaf and [K]_leaf.

Our results highlight that modification of leaf physiological function and morphology act as important adaptations for Bornean dipterocarps in response to edaphic and vertical environmental gradients. Meanwhile, multiple nutrients appear to contribute to niche partitioning and could drive species distributions and high biodiversity within Bornean forest landscapes.