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, kerangas) occurring along an < 5km edaphic gradient in
North Borneo. The traits measured included saturating photosynthesis
(Asat), maximum photosynthetic capacity (Vcmax), leaf dark respiration
(Rleaf), leaf mass per area (LMA), leaf thickness, minimum stomatal
conductance (gdark) 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 Rleaf_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 Vcmax_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 Asat, [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.