Differential nutrient limitation and tree height control leaf physiology, supporting niche partitioning in tropical dipterocarp forests

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 and kerangas) occurring along an <5 km 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 and 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. Read the free Plain Language Summary for this article on the Journal blog.

Automated summary

Understanding the mechanisms of environmental niche partitioning in lowland tropical forests is crucial for studying species distributions and vulnerability to environmental change. This study examined how leaf physiological function and morphology vary with nutrient availability across different forest types and along a fine-scale edaphic gradient in Borneo. The results showed that leaf traits responded consistently to soil nutrient availability, and unique combinations of leaf nutrients were related to variations in photosynthesis and respiration rates. Additionally, generalist species were able to adapt to nutrient-poor environments through changes in leaf mass per area. These findings highlight the importance of leaf physiological and morphological adaptations for dipterocarp species in response to environmental gradients.