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Forest succession, composition and dynamics are often explained based on the light requirements of tree species for establishment, survival and growth (Finegan 1984; Pacala et al. 1996). Traditionally, foresters in both temperate and tropical zones have classified tree species into groups according to their shade tolerance in the regeneration phase, leading to the classical dichotomy of pioneer vs. shade-tolerant species (Swaine & Whitmore 1988). Strong early environmental requirements in combination with high mortality rates may determine when, where and under what conditions seedlings of tree species are found. However, species may differ in their tolerance of shade at successive stages in their life cycle (Oldeman & van Dijk 1991; Grubb 1996; Bazzaz 1998). The ‘regeneration niche’ of a species, as defined by Grubb (1977), embraces all stages in the regeneration process, and up till now there have been very few quantitative studies of the requirements of tree species beyond the juvenile stage (e.g. Clark & Clark 1992).
While every tree may have its own unique light trajectory from seed to adult tree, a general classification can be used to indicate the possible alternative pathways during a tree's life (individual ontogenetic trajectories). In Fig. 1 we simplify these possibilities by considering four critical life-history stages and three light levels. After dispersal, a seed may be found under low, intermediate or high light conditions. The seedling growing out of this seed may be found in lower, similar or higher light conditions than the seed, as may be the juvenile and adult stage of that individual, depending on the changes in the individual tree's light environment. Trajectories can be described in the same way at the population level (population or species height–light trajectory, HLT), where the light levels experienced by seeds, seedlings, juveniles and adults may differ, depending on the light requirements for germination and establishment, and on the light dependency of survival, growth and reproduction. Species thus may show different shifts in their light requirements in different stages of their life cycle. The most extreme trajectories are formed by the outer pathways in Fig. 1: on the left, vertical arrows represent species that grow up and mature in the low light conditions of the forest understorey (traditionally referred to as strict shade-tolerant species), whereas species that establish and grow up in the bright light environment of gaps (light demanding or pioneer species) are on the far right. In between there is a whole gamut of potential light trajectories: 81 hypothetical pathways result from only four discrete life-history stages and three light levels (Fig. 1) and, in reality, the two axes are continuous, leading to an infinite number of possible pathways.
Figure 1. Potential height–light trajectories for species in different stages of their life cycle. Average irradiance at the population level can be low, intermediate or high and light levels may either remain constant from seed to adult stage (vertical arrows) or shift from one stage to the next (diagonal arrows). The nine possible pathways between juvenile and adult (i.e. above the broken line) are the focus of this paper.
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In tropical rain forest there is a strong and predictable vertical gradient in light availability: a progressive exponential extinction of light occurs when incoming radiation from above is intercepted by successive leaf layers in the forest canopy. Trees therefore should encounter brighter light conditions as they increase in height. The null hypothesis for a height–light trajectory thus would be to change from shade when small to bright light when tall. What processes lead to possible deviations of species HLTs from this null model? Species may either deviate from the average light environment right from the beginning of their life cycle, or such differences may only become apparent in later stages. Experimental evidence shows that both pioneer and non-pioneer species may germinate under a wide range of environmental conditions (e.g. Kyereh et al. 1999; Peña-Claros 2001; Pearson et al. 2002), contradicting the classical ideas (cf. Swaine & Whitmore 1988). Selection occurs therefore mainly in later stages of the life cycle, when the species are growing towards the canopy. From a physiological point of view, trees need brighter light conditions when growing in size: the ratio between photosynthesizing and respiring tissue declines with plant size, leading to an enhanced whole-plant light compensation point (Givnish 1988). The height-dependent changes in the whole-plant light compensation point may differ amongst species, and the species may follow therefore different height–light trajectories.
Despite its central role as a working model in forest ecology, little quantitative information is available on the shade tolerance of tree species. Species are often subjectively classified in two or three groups, based on the ecological knowledge and best-educated guesses of the researchers. The last decade has seen a growing body of publications, in which the light environments of tropical tree species have been quantified using PAR sensors, hemispherical photographs, or qualitative estimates of the light environment (e.g. Welden et al. 1991; Clark & Clark 1992; Clark et al. 1993; Lieberman et al. 1995; Davies et al. 1998; Rose 2000; Poorter & Arets 2003). Most studies, however, focus only on one life-history stage, or consider only few species (but see Hawthorne 1995). A community-wide approach is needed to provide the necessary resolution for revealing ecologically and statistically sound patterns, and to gain insight into the extent to which tree species partition the light gradient. Lieberman et al. (1995) took such a community-wide perspective, and considered trees over a large size range. They found that 14% of the species indeed occurred in darker or brighter conditions than expected, whereas 86% had a random distribution with respect to light. However, by the nature of their analysis, they excluded a priori the possibility that species may switch light requirements with tree height (cf. Fig. 1).
If such height-related shifts exist, they may have profound repercussions for our current views on plasticity and adaptation (Reich 2000), light partitioning and species coexistence (Hubbell et al. 1999), and silviculture and management (Lamprecht 1990). Species that experience large changes in light requirements with height might be expected to have greater trait plasticity than species that consistently occur under high or low light conditions (Popma et al. 1992; Grubb 1998; Reich 2000). Similarly, the influence of light partitioning might seem weak when only the seedling stage is considered, but might be stronger when crossovers in light requirements occur as species increase in size (Sack & Grubb 2001). Finally, silvicultural interventions such as liberation thinning might be more refined if we know for what species it is needed, and at what ontogenetic stage it should be applied.
Here we draw on a large data set from a Liberian rain forest, in which height and crown exposure were measured for 7460 trees. Temporal changes in crown exposure were followed for a 6-year period. We describe the height–light trajectories of 47 species using multinomial logistic regression analysis. Monitored trees had a diameter at breast height of 10 cm or over, and our analysis is therefore confined to the juvenile and adult stage (Fig. 1).
We address two questions. First, do species differ in their height–light trajectories from the average vertical light profile in the forest, and if so, how common is each of the hypothetical pathways identified? Secondly, are differences in these trajectories related to the maximal adult stature and allometry of the species?