Light and shade

3.2.1 Influence of shade on tree development

Owing to their height and complex structure, there can be less light near the ground in forests than in any other terrestrial vegetation type. Shade can thus be a very important factor in determining the forest dynamics. Very great variations occur in the light levels reaching the floors of, for example, Scandinavian forests in which periodic storms produce mosaic patterns of light and shade. Tree fall greatly increases the amount of light reaching many young saplings and dwarf trees of Norway spruce Picea abies, some of which develop on the rotting logs of trees that fell long ago, and are thus able to grow away rapidly after decades of relative inactivity. Careful inspection, however, often reveals skeletons of saplings for which the additional light came too late.

Light intensities below a forest (normally expressed as a percentage of full sunlight outside the forest) vary greatly. In Europe, an open birch forest may still have 20-50% of sunlight at ground level, dropping to 2-5% under beech Fagus sylvatica. As beech is deciduous, there is of course more light available during the leafless winter (see Section 1.4.2), but the trunks and branches still block some light such that light levels are still likely to be below 70-80% full sun. Evergreen forests tend to cast similar shade year-round (although the absolute amount of radiation reaching the ground is less in winter since the sun's apparent output is less and the lower sun angle leads to more interception by the atmosphere and a longer pathway through the forest). In Europe, summer light levels below natural Scots pine Pinus sylves-tris forests are usually around 11-13% and below Norway spruce Picea abies they can be as little as 2-3%. In tropical rain forests, light levels at the forest floor may be even lower, reaching just 0.2-2.0%.

The shade cast by mature evergreen spruce forest, or by deciduous beech Fagus sylvatica forest in summer, is very intense and really active growth beneath it is virtually impossible. Far more light reaches the understorey of the relatively open eucalypt forests of Australia. The mature leathery (sclerophyl-lous) leaves of mountain ash Eucalyptus regnans are long and narrow, and hang downwards so preventing heating in the hot Australian sun. This keeps water loss within reasonable bounds and allows the growth of other species beneath it. Beech would overheat and suffer impossibly large water losses in such an environment.

Most plants need 20% of full sunlight for maximum photosynthesis (the saturation point) and require 2-3% sunlight to reach the compensation point where respiration costs are just balanced by photosynthesis, i.e. the plant is just breaking even and starts to grow actively. These are gross generalizations (variations are discussed below) but it demonstrates that the average light intensity at the forest floor is often below the compensation point of most woodland plants and seedlings, and yet they survive. Part of the solution to this paradox rests in sunflecks, patches of sunlight passing through gaps in the canopy, which can give up to 50% of full sunlight. Evans (1956) found that in a tropical rain forest in Nigeria 20-25% of ground area was illuminated by sunflecks at solar noon (when the sun is directly overhead) and that these made up 70-80% of the total solar energy reaching the ground. These flecks are thus seen to be important in a forest, especially to shade plants (see Section 3.2.2) that are capable of responding quickly to these brief flurries of light. Another consideration is that plants vary in their compensation point; Bates and Roeser (1928) found that while the pinyon pine Pinus edulis of the open forests of the arid American south-west has a compensation point of 6.3% full sunlight, the coastal redwood Sequoia sempervirens that grows in deep shade requires just 0.62% sunlight. Plants may also persist in light conditions below their compensation point (i.e. they are sustaining a net loss of energy) while their food reserves last. For some plants of the woodland flora, this may be a seasonal problem, enduring deep shade during the summer and growing in the spring and possibly the autumn (see Section 3.2.3). For seedlings it may be a waiting game, forming a persistent seedling bank (see Section 4.2.2) able to take advantage of an opening in the canopy if it should come before they die from lack of light.

Many investigations of rain-forest tree seedlings have emphasized the contrasts between those which are strongly light-demanding and those which are strongly shade-tolerant. A comparison of seedlings of 15 species belonging to the latter group, grown at three different light levels (10%, 0.8% and 0.2% full sunlight) for up to a year, affords an insight into the nature of competition between its members. Seedlings of the 15 species differed in their height and mass (weight) at the beginning of the experiment. Figure 3.4 shows final harvest values for seedling dry mass and height. Those of Prunus turneriana (labelled pt in the figure) were tallest at the start and remained so in both the 10% and 0.8% treatments. As seed size varied so much, the relative growth rate of biomass (RGRm) was critical. At 0.8% that of the fastest-growing species, Gillbeea adenopetala (ga) was 30 times that of the slowest-growing species Cryptocarya murrayi (cm, a true laurel) even though the final mass of the latter is higher (Fig. 3.4). Intuitively, it might be expected that big seeds would result in fast-growing seedlings but the seed reserves of G. adenopetala were the smallest of all the species and 118 times smaller than those of C. murrayi. This suggests that in these forests rapid growth to outcompete other seedlings is more important than maintaining reserves of energy to last longer in deep shade, perhaps because gaps appear less frequently than in other forests. Differences in growth, especially height, between species at 10% full light were much less; seedling mortality in both these treatments over the full period of the experiment were negligible. At the lowest light level (0.2%) mortality increased greatly, and in a number of species reached 100% by the end of the experiment, reinforcing the idea of a desperate upward race for light.

Some species gained height much more rapidly than others, which they might well shade out under appropriate circumstances. However, the results also showed a change in height ranking (and thus in the competitive hierarchy) across these shade-tolerant species over time, especially under high-light regimes. Thus, the impact of a changing environment upon such competitors is likely to favour different species in different situations. At the lower end of the height-increase scale, species with greater survival rates under adverse conditions have an improved chance of outliving their neighbours and growing into adult trees when a forest gap forms. This study thus helps our understanding of how different combinations of morphological and physiological traits possessed by these 15 species of tree seedlings enables their continued co-existence in the same shade-tolerant niche.

Aerial photographs make it possible to investigate long-term canopy dynamics over considerable periods; indeed Fujita et al. (2003) have already done so over a period of 32 years in a 4-ha permanent plot in an old-growth evergreen broadleaved forest in the Tatera Forest Reserve, south-west Japan. This reserve is dominated by Japanese chinquapin Castanopsis cuspidata and the dawn isu tree Distylium racemosum at low altitudes and the Japanese evergreen oak Quercus acuta on the hills. In the low-altitude study plots of this truly primeval forest, canopy height varies between 20 and 30 m, while the diameter at breast height (dbh) of some trees exceeds a metre. Aerial photographs taken in 1966, 1983, 1993 and 1998 enabled the creation of digital elevation models on a 2.5 m grid of the canopy surface of a 10-ha area. This

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Figure 3.4 Final harvest values of (a) seedling dry mass and (b) seedling height for 15 shade-tolerant Australian tropical rain forest species grown from seed in either 10% (open bars) or 0.8% (dark bars) of full sunlight. Species are arranged according to length of their growing season, from 7 months on the left to 12 months on the right (From Bloor and Grubb, 2003. Journal of Ecology 91, Blackwell Publishing.)

Gillbeea adenopetala

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Darlingia darlingiana

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Cryptocarya murrayi

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Prunus turneriana

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C. aff triplinervis

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Flindersia bourjotiana

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C. mackinnoniana

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Castanospora alphandii

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Argyrodendron trifoliolatum

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Cupaniopsis flagelliformis

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Aglaia sapindina

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Guioa lasioneura

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Athertonia diversifolia

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Synima macrophylla

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Cardwellia sublimis

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included the 4-ha plot which was surveyed in order to create a topographic map of the ground surface in 1990. The 3.4% of the 6400 2.5 x 2.5m quadrats which remained as gaps throughout the 32-year period tended to be in the centres of large old gaps, whereas those experiencing re-disturbance (2.5%) were along the edges of old gaps; 74.5% of the area, in which gaps were constantly forming and closing, remained as closed canopy throughout. The establishment of deciduous and shade-intolerant pioneer species in predominantly evergreen broadleaved forests like this was seen to be dependent on the existence of long-term, large openings.

The important influence of shade upon the development of young trees has been thoroughly investigated, but complications resulting from herbivory have been less well recognized. Survival of shade-tolerant juvenile trees in the understorey depends on resource allocation strategies which divert resources to the recovery from damage inflicted by deer and other herbivores, but slow their upward growth. In contrast, resource allocation should enable rapid height growth in canopy gaps where competitor saplings grow rapidly. Using the shade-tolerant dipterocarp Shorea quadrinervis, a dominant canopy tree in Borneo, Blundell and Peart (2001) investigated the interaction between canopy gaps and simulated herbivory on juvenile plants (<1 cmdbh) in an area where caterpillars and grasshoppers cause the most damage to its foliage. Under natural conditions S. quadrinervis juveniles often spend many years under low-light conditions in the understorey, where on average they grow upwards at a rate of only 1 cm a year, and experience an average mortality rate of approximately 10% a year. Such young trees allocate a much higher proportion of their resources to the roots than do those growing in gaps and consequently have higher root/shoot ratios. While this slows their growth towards the light, this less-vulnerable below-ground tissue can be used as a source of energy for leaf replacement in the event of herbivory. These tendencies were clearly shown in the results of a shading experiment depicted in Fig. 3.5, which involved the use of 24 juveniles that were collected from the understorey around an adult tree in the forest used in the main experiment. Half were subsequently grown under shade conditions corresponding to the understorey and the other half placed in an artificial gap for 8 months.

In the same study, simulated herbivory experiments involved the removal of the apical meristem or 10, 50 or 90% of the foliage from plants in the forest itself. Survival of juvenile plants in the gaps was high over the 8 months of the experiment and not significantly affected even by removal of 90% of the foliage. Of the plants growing in the understorey beneath the forest canopy survival of those suffering up to 50% defoliation was again not significantly affected, but some 70% of plants suffering 90% foliage removal died within

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Canopy cover

Figure 3.5 Resource allocations and root/shoot ratios in juveniles (<1cm dbh) of the dipterocarp tree Shorea quadrinervis grown in gaps (n = 12) and the understorey (n = 12). Values shown are means ±1 SE. Experiments were run over 8 months at Gunung Palung National Park, Kalimantan Barat, Indonesian West Borneo. (From Blundell and Peart, 2001. Journal of Ecology 89, Blackwell Publishing.)

the 8 months. The authors suggest that most understorey individuals experiencing a single severe defoliation would probably die before they had been observed in an area survey.

In the understorey, growth was negligible even in controls (0% of leaf removal) and there was no effect of the removal treatments (Fig. 3.6). Height growth in the juveniles placed in the gaps was actually increased by herbivory, particularly removal of the apical meristem or 10% of leaf tissue and was reduced only after 90% removal. Conversely, under the shade of the canopy, height growth of all individuals, both control and experimental, was negligible (Fig. 3.6). Shorea quadrinervis can be seen to be adapted to rapid growth in open gaps. But the increased growth of gap plants after herbivory is unexpected considering that the reduced leaf area must lead to less photosynthesis. This indicates that the plant is adapted to invest as much energy as possible in new height growth in order to maintain its competitiveness for light against its neighbours. Higher damage levels increased subsequent net leaf loss. Although leaf production was much higher, leaf retention was much lower in the gaps than in the understorey.

Traditionally, trees and other plants have been classified as shade tolerant or intolerant (see Box 9.3) which gives useful information to foresters and others interested in regeneration in forests. Trees such as European beech Fagus sylvatica and the sugar maple Acer saccharum from North America are very tolerant of deep shade while birches and poplars grow best under high light intensities. However, it is now apparent that the ability to tolerate shade can

Acer Leaf Damage

Removal treatment

Figure 3.6 Effect of simulated herbivory on change in height over 8 months of the dipterocarp tree Shorea quadrinervis at Gunung Palung National Park. Plants in gaps are indicated by thick lines, those in the understorey by thin lines. Simulated herbivory involved removing just the apical meristem or removing various percentages of tissue from all leaves; 0% removal were the untouched controls. Change in height plotted as residuals (means ±1 SE) of change in height vs. initial height. Different letters indicate statistically significant differences. (From Blundell and Peart, 2001. Journal of Ecology 89, Blackwell Publishing.)

Removal treatment

Figure 3.6 Effect of simulated herbivory on change in height over 8 months of the dipterocarp tree Shorea quadrinervis at Gunung Palung National Park. Plants in gaps are indicated by thick lines, those in the understorey by thin lines. Simulated herbivory involved removing just the apical meristem or removing various percentages of tissue from all leaves; 0% removal were the untouched controls. Change in height plotted as residuals (means ±1 SE) of change in height vs. initial height. Different letters indicate statistically significant differences. (From Blundell and Peart, 2001. Journal of Ecology 89, Blackwell Publishing.)

change through the life span of a tree. The ways in which the light requirements of a developing individual tree change as it progresses from a seed to a seedling to the juvenile (sapling) stage and finally to an adult are discussed by Poorter et al. (2005a). They measured crown exposure (i.e. degree of exposure to direct sunlight) for 7460 trees belonging to 47 different species in a Liberian lowland rain forest on the Atlantic coast of West Africa, following individual trees for periods varying from 2.8-9.8 years. Figure 3.7 illustrates the way in which the average irradiance of light at population level can be low, intermediate or high, and how this can change as the forest matures. A species with a low light requirement throughout its life could have a height-light trajectory (HLT, a fitted curve relating canopy exposure to tree height) running from bottom left to top left, while one with a high light requirement throughout its life would have a vertical trajectory on the right side of the diagram. In practice, as the diagonal arrows on the diagram indicate, light requirements of particular species often vary between different stages.

Poorter et al. (2005a) were particularly concerned with the transition between the juvenile and adult stages, and found that all nine of the height-light trajectories indicated on the diagram were represented in the 47 tree species they examined. This is important because it shows that later requirements of

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Irradiance

Figure 3.7 Relative commonness of different height-light trajectories for 47 rain-forest tree species. The relative light levels of juveniles (below) and adults (above) are shown. Light levels are classified as relatively low (lower than the average forest level), intermediate or relatively high. The number of species that follow a particular light trajectory is indicated by the thickness of the arrow and the corresponding percentages. (From Poorter et al., 2005a. Journal of Ecology 93, Blackwell Publishing.)

species classified as pioneer or shade tolerant on the basis of seed and seedling behaviour may be very different. Two strategies were common. The majority (57%) of species started in intermediate light environments in the juvenile stage and passed to low light environments as adults; these are typically shorter trees that become progressively stuck under taller canopy trees. The other common strategy was to stay in intermediate light conditions from juvenile to adult. The whole-life shade-tolerant and the whole-life shade-intolerant niches were filled by just one species each (2% of the 47 species investigated). Large stature species tend to possess relatively slender stems and narrow crowns, grow rapidly and so achieve increased crown exposure earlier.

It was shown above that the adult trees differ in their degree of tolerance to shade and that the degree of shade tolerance may change in an individual tree over its life. It should also be noted that trees can differ in their shade tolerance within an individual canopy. Trees that live in shaded environments may have just one layer of leaves on the outside of the canopy like the fabric on an umbrella - mono-layered trees. Most shade-intolerant trees, however, have several layers (multi-layered trees) one inside the other. In fact it may be difficult to distinguish discrete layers but nevertheless multi-layered trees have a thick canopy where some leaves are grown below others. Thus, the

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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