Abstract
Canopy species need to shift their adaptive strategy to acclimate to very different light environments as they grow from seedlings in the understory to adult trees in the canopy. However, research on how to quantitively detect ecological strategy shifts in plant ontogeny is scarce. In this study, we hypothesize that changes in light and tree height levels induce transitions in ecological strategies, and growth phases representing different adaptive strategies can be classified by leaf trait variation. We examined variations in leaf morphological and physiological traits across a vertical ambient light (represented by the transmittance of diffuse light, %TRANS) and tree height gradient in Parashorea chinensis, a large canopy tree species in tropical seasonal rainforest in Southwestern China. Multivariate regression trees (MRTs) were used to detect the split points in light and height gradients and classify ontogenetic phases. Linear piecewise regression and quadratic regression were used to detect the transition point in leaf trait responses to environmental variation and explain the shifts in growth phases and adaptive strategies. Five growth phases of P. chinensis were identified based on MRT results: (i) the vulnerable phase, with tree height at less than 8.3 m; (ii) the suppressed phase, with tree height between 8.3 and 14.9 m; (iii) the growth release phase, with tree height between 14.9 and 24.3 m; (iv) the canopy phase, with tree height between 24.3 and 60.9 m; and (v) the emergent phase, with tree height above 60.9 m. The suppressed phase and canopy phase represent “stress-tolerant” and “competitive” strategies, respectively. Light conditions drive the shift from the “stress-tolerant” to the “competitive” strategy. These findings help us to better understand the regeneration mechanisms of canopy species in forests.
Highlights
Ecological strategies represent adaptations to the intensity of environmental filters involving competition, stress and disturbance [1]
As a canopy tree species, P. chinensis adapts to light environments both in the understory and canopy throughout its life history
According to CSR theory [6] and ontogenetic shifts in leaf traits [1,8,10], the suppressed phase and canopy phase could be classified as “stress-tolerant” strategy and “competitive” strategy, respectively, these strategies were inferred only relative to ontogenetic adaptation in P. chinensis
Summary
Ecological strategies represent adaptations to the intensity of environmental filters involving competition, stress and disturbance [1]. When considered as an operating process for resource allocation, plants’ ecological strategies can be expected to shift from seedling to mature stages [1]. Forests 2020, 11, 1145 light requirements among tree species underlie the major changes in vegetation composition during succession [2,3]. Compared with pioneers and understory species, canopy tree species exhibit a greater change in tree stature, and very different light environments occur as they grow up from seedlings and saplings in the understory to adult trees in the canopy. Several forest tree species have different light requirements throughout their developmental stages [4], and ontogenetic variation in traits has received research attention in evaluating the adaptive strategies of specific species [5]. The adaptive strategies of canopy species throughout their ontogeny cannot be characterized by a single style
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