Functional traits determine trade-offs and niches in a tropical forest community

Abstract

How numerous tree species can coexist in diverse forest communities is a key question in community ecology. Whereas neutral theory assumes that species are adapted to common field conditions and coexist by chance, niche theory predicts that species are functionally different and coexist because they are specialized for different niches. We integrated biophysical principles into a mathematical plant model to determine whether and how functional plant traits and trade-offs may cause functional divergence and niche separation of tree species. We used this model to compare the carbon budget of saplings across 13 co-occurring dry-forest tree species along gradients of light and water availability. We found that species ranged in strategy, from acquisitive species with high carbon budgets at highest resource levels to more conservative species with high tolerances for both shade and drought. The crown leaf area index and nitrogen mass per leaf area drove the functional divergence along the simulated light gradient, which was consistent with observed species distributions along light gradients in the forest. Stomatal coordination to avoid low water potentials or hydraulic failure caused functional divergence along the simulated water gradient, but was not correlated to observed species distributions along the water gradient in the forest. The trait-based biophysical model thus explains how functional traits cause functional divergence across species and whether such divergence contributes to niche separation along resource gradients.

Fig. 1.

Carbon gain landscapes for 13 co-occurring tree species of a Bolivian forest. Carbon gain was calculated as the difference between diurnal gross photosynthesis rate minus diurnal maintenance respiration rate. (A) Carbon gain landscapes along gradients of light availability as the average vertical light intensity in (μmol⋅m⁻²⋅s⁻¹) units, and water availability as the soil water potential in (Pa) units. Different plane colors represent different species (Fig. S2). (B) Zero carbon gain isoclines along the light and water availability gradient, indicating the border of the fundamental niche. (C) Carbon gain at saturated soil water availability (soil water potential = 0 Pa) along a light availability gradient. The cross-points with the dashed line (carbon gain = 0) represent the estimated light compensation points of the species. (D) Carbon gain at saturating light availability (light intensity = 1,500 μmol⋅m⁻²⋅s⁻¹) along a water availability gradient. Cross-points with the dashed line (carbon gain = 0) represent the estimated water compensation points of the species. In B–D, gray lines represent species known as pioneers and black lines show species known as shade tolerant. These two species groups are shown for illustration purposes only, because no formal tests among species groups were performed.

Fig. 2.

Possible trade-offs across performance traits. (A) Minimum vs. maximum carbon gain; (B) maximum carbon gain vs. light compensation points; (C) maximum carbon gain vs. water compensation points. Actual trade-offs are suggested by a negative relationship for A and positive relationships for B and C. Increasing trends were fitted with a linear model (y = A + B × x), a quadratic model (y = A + B × x + C × x²), and a sigmoidal model (y = e^(A+B/x)), and only the most significant model fit is shown (if P < 0.05).

Fig. 3.

Actual species distributions in relation to maximum carbon gain. Distributions were defined by indexes for light and water availability. For the light index we took the average population-level crown exposure of saplings 2 m tall (43), which is a strong predictor of incident radiation (48) (Methods and SI Text S3). For the water index, we quantified the relative position of saplings of each species along slopes, which is a strong predictor of the soil water availability in this forest (29) (Methods and SI Text S3). As such, species with a high light index are mainly found in high light habitats and species with a high water index are mainly found in wet valleys. See Fig. 2 for the procedure of fitting lines.