Variation in crown light utilization characteristics among tropical canopy trees

Abstract

Background and aims: Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent).

Methods: Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K).

Results: Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves.

Conclusion: Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.

Fig. 1.

Line drawings of typical mature trees (A–E) and terminal shoots (a–e) of the study species in the reversed order of successional status: Anacardium excelsum (late successional species with the greatest leaf longevity among the study species (A and a); Luehea seemannii (B and b); Antirrhoea trichantha (C and c); Castilla elastica (D and d); Cecropia longipes (shortest-lived pioneer; E and e). The whole trees are drawn to the scale of the tower crane (42 m tall) to show their relative sizes and crown shapes. Scales for branch drawings are shown as bars at the bottom (0·2 m, shared for a–d, separate for e).

Fig. 2.

Mean cumulative leaf area index (horizontal axis) as a function of depth from the crown surface (vertical axis) for five canopy trees. Leaf contacts were counted for every 0·25–1 m down to the lowest leaf along each vertical transect through each crown. The cumulative LAI was estimated by dividing cumulative leaf contact with cosine of the median leaf angle of each species.

Fig. 3.

Light extinction as a function of cumulative leaf area index (LAI) through crowns of five neotropical tree species (see Table 1 for full names). Mean % PFD transmission (vertical axis) at a given depth from the canopy surface is plotted against mean LAI (horizontal axis) above that depth.

Fig. 4.

Foliage and light distribution at the surface (open circles) and 0·5 m inside (closed circles) of the crown crust of an emergent Anacardium excelsum tree. (A, B) Canopy surface topography along two transects laid from the apex of the crown to south-east (125°) and north-west (300°); height from the ground and horizontal distance from the apex was measured at every 1-m interval along the transects. (C, D) LAI estimated optically with a canopy analyser (LAI-2000, Li-Cor) as a function of the height from the ground. (E, F) Percentage PFD transmission as a function of optically estimated LAI within the canopy crust.

Fig. 5.

The relationship of % total daily PFD at individual leaf surface (100 × total daily PFD per unit leaf area divided by above-canopy PFD on the horizontal plane, vertical axis) to leaf position within the terminal shoots (indicated by the number of distal leaves relative to the focal leaf, horizontal axis) for three canopy tree species with contrasting branch architecture. (A) Anacardium with steeply angled leaves around orthotropic shoots, (B) Luehea with variously angled leaves along plagiotropic shoots, and (C) Cecropia with minimally overlapping leaves at the tips of orthotropic shoots. Open circles, upper canopy branches; closed circles, shade branches in the lower part of the crown. No shade branch existed for Cecropia. Solid and broken lines indicate significant (P < 0·05) and non-significant regressions, respectively, for sun and shade branches.