Midday Depression vs. Midday Peak in Diurnal Light Interception: Contrasting Patterns at Crown and Leaf Scales in a Tropical Evergreen Tree

Abstract

Crown architecture usually is heterogeneous as a result of foraging in spatially and temporally heterogeneous light environments. Ecologists are only beginning to identify the importance of temporal heterogeneity for light acquisition in plants, especially at the diurnal scale. Crown architectural heterogeneity often leads to a diurnal variation in light interception. However, maximizing light interception during midday may not be an optimal strategy in environments with excess light. Instead, long-lived plants are expected to show crown architectures and leaf positions that meet the contrasting needs of light interception and avoidance of excess light on a diurnal basis. We expected a midday depression in the diurnal course of light interception both at the whole-crown and leaf scales, as a strategy to avoid the interception of excessive irradiance. We tested this hypothesis in a population of guava trees (Psidium guajava L.) growing in an open tropical grassland. We quantified three crown architectural traits: intra-individual heterogeneity in foliage clumping, crown openness, and leaf position angles. We estimated the diurnal course of light interception at the crown scale using hemispheric photographs, and at the leaf scale using the cosine of solar incidence. Crowns showed a midday depression in light interception, while leaves showed a midday peak. These contrasting patterns were related to architectural traits. At the crown scale, the midday depression of light interception was linked to a greater crown openness and foliage clumping in crown tops than in the lateral parts of the crown. At the leaf scale, an average inclination angle of 45° led to the midday peak in light interception, but with a huge among-leaf variation in position angles. The mismatch in diurnal course of light interception at crown and leaf scales can indicate that different processes are being optimized at each scale. These findings suggest that the diurnal course of light interception may be an important dimension of the resource acquisition strategies of long-lived woody plants. Using a temporal approach as the one applied here may improve our understanding of the diversity of crown architectures found across and within environments.

Keywords: Psidium guajava; crown architecture; crown openness; diurnal course of light interception; foliage clumping; leaf angle; light stress; temporal variation.

FIGURE 1

Field sampling design. Crown-scale variables were quantified in crown sector B. Leaf-scale variables were quantified in crown sectors A, N, E, S and W, where 10 leaves per sector were sampled (50 leaves per tree; squares). Hemispheric photographs were taken in the six sampling points. Modified from Hallé et al., 1978.

FIGURE 2

Diurnal courses of light interception at the crown and leaf scales. Crown scale light interception is the percentage of incident PAR intercepted by the crown. Leaf scale potential light interception is represented with STAR (%). Bars denote the SE of the mean for N = 9 trees.

FIGURE 3

Relationship between the clumping index and the gap fraction across zenith rings (A). Closed circles represent each of 8 zenith regions spanning 9° elevation angles, indicated with 𝜃. 𝜃₁ is the range between 0 and 9° elevation angles (zenith, top central part of the crowns in the picture) and 𝜃₈ is the range between 72 and 81° (horizon, lateral parts of the crown in the picture). Bars denote 1 standard deviation for N = 9 trees. (B) Correlation between the CV of the clumping index and annual light interception. CV, coefficient of variation calculated over 216 crown segments. N = 9 trees.

FIGURE 4

Rose diagrams for the distribution of leaf lamina course (A) and tilt angles (B) per crown sector. Course angles are shown in degrees clockwise from North (0°), and tilt angles range from a vertical leaf position with the leaf tip pointing toward zenith (0°), to horizontal (90°), and to vertical with the leaf tip pointing downward (180°). The bins represent the frequencies in each direction and blue dots show single leaf data. For leaf course, 95% confidence intervals of the mean direction of the population and their p-values correspond to an underlying Von Mises distribution with the mean course direction predicted to be at 180°. For leaf tilt, the underlying distribution was a Von Mises without a specific prediction for the mean direction. For each crown sector, we indicate the number of leaves sampled (n), and mean direction (μ), and concentration parameter (κ) with their standard errors. N, E, S, W are the four basal crown sectors in the main compass directions, and A is the central upper crown sector.