Sun Shade

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Figure 2.28 Schematic representation of the relative differences in leaf form between sun and shade conditions based on data from Bongers & Popma (1988).

Figure 2.28 Schematic representation of the relative differences in leaf form between sun and shade conditions based on data from Bongers & Popma (1988).

cooling. Large leaves have a high boundary-layer resistance that reduces the rate of transfer of sensible heat between the leaf and the general atmosphere. Large leaves in full sun can reach lethally high temperatures. Therefore, small leaves are favoured in exposed conditions. Large-leaved species must be able to maintain rapid transpiration to prevent overheating. Leaf thickness may be greater in sun leaves because the high incident radiation means that sufficient light penetrates several layers of chloroplast-bearing mesophyll cells to allow them all to photosynthesise at high rates. Thicker leaves also tend to have better water-use efficiency because the carbon fixation rate rises more rapidly with thickness than does the rate of water loss.

Foliar defences may also vary with degree of shading. Mole et al. (1988) found that sun leaves generally had higher concentrations of phenolics, including condensed tannins, than shade leaves for four species in Sierra Leone. This may be more of a response to environmentally related changes in metabolism than alterations in plant investment in defence. Fibre concentrations were not influenced by shading.

A simplistic argument, often resorted to by students, when asked why shade leaves are bigger than sun ones, is to state that this allows the leaf in the shade to capture more light. Under the low irradiances of the forest interior light interception is probably directly proportional to leaf area, but a plant with many small leaves could still intercept as much light as one with few large ones. The operative feature under natural selection in the shade is probably the amount of light intercepted by the leaves per unit of investment (including that in terms of support and supply) per unit area of leaf. Low LMA and low N per unit area reflect low investment costs per unit area and more efficient shade leaves, although a lower limit on LMA may be set by susceptibility to damage. In the sun, where there is ample light, other factors such as efficient use of water, or the need to avoid overheating come to the fore in leaf design. The lower potassium concentrations of sun leaves have not yet received a physiological explanation.

Poorter et al. (1995) studied the variation in leaf optical properties in four canopy species in Costa Rica with leaf height in the canopy. They found small, but statistically significant, differences in leaf absorptance with height in the canopy. The more 'shade-tolerant' species (Lecythis ampla and Min-quartia guianensis) showed higher absorptance in the shade than the more 'light-demanding' species (Dipteryx panamensis and Simarouba amara), but the position was reversed in the bright light at 20 m up. The absorptance efficiency per unit dry mass decreased with height in the canopy. The total chlorophyll concentration on a mass basis decreased with height in the canopy for the two species studied. The results reflect changing limitations on photosynthesis with irradiance regime. Under the shaded conditions of the forest interior there is a shortage of light for photosynthesis, and efficient light capture is required with a greater amount of chlorophyll. Under bright light conditions there is often more than enough light to saturate the photosyn-thetic system of the leaf, and so carbon dioxide uptake may limit photosynthesis. Efficiency of photon capture may be less important than use of water or other factors in sun leaves.

The generally expected differences in photosynthetic performance between sun- and shade-grown leaves are for the sun leaf to have a higher dark respiration rate per unit area, higher light compensation point, higher saturating irradiance and higher maximal assimilation rate per unit area. There have not been many studies of leaves on mature tropical trees to confirm these expected responses to shade. Individuals of three pygmy-tree species of Psychotria growing in the shaded understorey and in gaps on Barro Colorado Island, Panama, were compared for photosynthetic performance (Mulkey et al. 1993). The gap plants showed higher respiration and assimilation rates than the shaded ones. However, available data indicate that there may be relatively little difference in physiological potential among leaves within the crowns of large rain forest trees in the upper canopy. For example, Ar-gyrodendron peralatum and Castanospermum australe in Queensland, Australia (Doley et al. 1987; Myers et al. 1987), Dryobalanops lanceolata in Brunei (Barker & Booth 1996) and Pentaclethra macroloba in Costa Rica (Oberbauer & Strain 1986) were found to show little or no consistent intra-crown variation. There were differences detected in leaf morphology and performance within the crown of a 20 m tree of Macaranga conifera in Borneo (Ishida et al. 1999b). These were largely between leaves in the upper canopy and the lowest layer more than 45 cm beneath the uppermost leaves. It is possible that the light gradients at the top of the forest canopy are not marked enough to lead to differentiation in leaf performance in most species, or the differences are too slight to distinguish from the large variation present among leaves on a branch.

The instantaneous irradiance impinging on a leaf in the rain forest varies rapidly and with a large magnitude. This short-term variation is particularly marked in the forest understorey where the dim foliage-filtered light is occasionally interspersed with brief periods of much higher irradiance when direct radiation finds its way through holes in the canopy (Chazdon 1988). Such sunflecks can make up 10-85% of the total irradiance received in the forest understorey (Chazdon et al. 1996). Technical improvements over the last decade have made it possible to study the efficiency of plant use of rapidly fluctuating irradiance. Among six Rubiaceae species of pygmy tree on Barro Colorado Island, three shade-tolerant species of Psychotria growing in the forest understorey exhibited more rapid induction of photosynthesis when exposed to a sunfleck and greater persistence of the induced state than three more light-demanding species growing in gaps and at the edge of clearings (Valladares et al. 1997). The understorey species made more efficient use of short sunflecks, but once sunfleck duration reached about 10 s there was little difference in performance between the species groups.

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