A seed germinating in a rainforest understorey starts life in a low light environment. This will not present major problems to an obligate shade species which cannot tolerate strong sunlight; such species have adapted to life in an understorey. However, many rainforest species are better described as either shade tolerant (i.e. able to germinate and persist in low light, but requiring higher light to reach maturity) or shade intolerant (unable to germinate or grow in low light). In successional terms, shade tolerance is a feature associated with climax species and shade intolerance with pioneer species.
Shade-tolerant species can persist as seedlings in the understorey, often for years, while still being able to respond to an increase in light availability when it occurs.
By comparison, shade-intolerant (early-successional) species can only germinate and grow where there is ample sunlight, and consequently they tend to occur in wide gaps and on forest edges. Wide gaps are relatively rare in old-growth rainforests which have been undisturbed by logging or slash and burn agriculture. Shade-intolerant species are unable to maintain a positive carbon balance when growing in low light. The change from a high light to a low light environment requires a change in allocation of plant resources as described in the quantitative growth chapter (Section 6.2). Shade-intolerant plants are unable to make this change and are burdened with the higher costs of construction and maintenance of leaves better suited to strong sunlight
Germination
Shade-intolerant species tend to produce numerous small seeds throughout the year which are widely dispersed. Their seeds are also able to remain viable for long periods (years) by going through a period of dormancy. This is often broken by high temperature or strong direct sunlight with a high ratio of red to far-red irradiance (R:FR ratio decreases with sunlight attenuation through canopies). Such environmental cues for germination are all experienced in wide gaps. Following germination, seedlings show rapid growth to maturity, allowing them to become well established in a gap before other slower growing species. These characteristics increase the probability of success for shade-intolerant species in the heterogeneous light environment of a rainforest.
Shade-tolerant species, on the other hand, have evolved a different suite of characteristics. They tend to produce a few large seeds which are generally not well dispersed, with little or no dormancy. However, the seeds have the ability to germinate in low light and persist in the understorey as seedlings for years. A rarity of gaps and a lack of dormancy found in most shade-tolerant species increases the probability of establishing in a low-light understorey environment. In situations like this, the larger seed provides seedlings with a reserve which they can draw upon during early establishment. In rainforests, tree seedlings survival in understorey habitats is positively correlated with seed size, especially in the first few months following germination.
Following establishment in the understorey, seedlings of shade-tolerant species may have to wait a long time before a gap appears overhead. Many species succumb to attack from herbivores or pathogens or may be crushed by large animals (including humans!). Those that do survive must be able to acclimate to the new conditions arising on gap formation; their ability to do this will depend on the nature of the new microclimate and the acclimation potential of each species.
Growth response to high light
Emergent trees of tropical rainforests have to endure strong sunlight, and leaves comprising the crowns of such trees will have acclimated to full sun. In young-growth forests, canopy emergents are early-successional fast-growing species that are adapted for fast growth in full sun on large disturbances. Such species represent an initial phase in forest dynamics that might last 10–20 years. By contrast, in old-growth forests, early-successional species have long since completed their life cycles, and will have been replaced by later-successional species whose seedlings initially tolerated deep shade on the forest floor, but now endure full sun as canopy emergents. Such remarkable plasticity is an adaptive feature of late-successional species and involves sun/shade acclimation by individual leaves.
The differences in growth rate of early-successional fast-growing species versus later-successional and shade-adapted species is illustrated in Figure 12.13 by two rainforest species that are important in the timber industry: the sun-loving red cedar (Toona australis) and the shade-adapted tulip oak (Argyrodendron sp.).
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Figure 12.13 Growth of (left) Toona australis (sun loving) and right Argyrodendron sp. (shade adapted). Plants are the same age (6 months) and grown in same size pots (15 cm) in high light and high nutrient supply. (Thompson et al., 1992a; photographs courtesy P.E. Kriedemann)
A detailed study of three rainforest species from North Queensland was conducted by Thompson et al. (1992a, b). The red cedar Toona australis (an early-successional species) and two species of tulip oak Argyrodendron (late-successional species) showed different acclimation potentials when grown under a range of light conditions (Figure 12.14).
When grown under high light, 535 µmol quanta m-2 s-1,which is typical of average canopy radiation in a tropical rainforest, T. australis achieved a higher Pmax and light-saturation point than either of the Argyrodendron species. However, T. australis was more sensitive to nutrient levels, being unable to acclimate to the same degree in low-nutrient compared to high-nutrient regimes. Moreover, fast growth in T. australis was greatly promoted by a positive light x nutrient interaction on leaf expansion and photosynthetic capacity; adaptive features with a clear selective advantage on open sites where soil disturbance liberates nutrients.
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Figure 12.14 Photosynthesis versus light response curves for seedlings of a shade-adapted rainforest tree species (Argyrodendron) and a sun-loving tree (Toona australis). Seedlings were grown under factorial combinations of weak, medium or strong light (shown left to right) × either high or low nutrient supply (solid lines with filled symbols, and dashed lines with open symbols respectively). (W.A. Thompson et al., 1992b)
Toona (sun loving) produced a much greater depth of mesophyll tissue under strong light compared to weak light (Figure 12.15b), which accounted for enhanced photosynthetic capacity. Argyrodendron sp. (shade adapted) was much less responsive to photo irradiance during growth, producing consistently thicker leaves regardless of light level, but with lower photosynthetic capacity.
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Figure 12.15 Transverse sections of leaves from Toona australis (a) and Argyrodendron sp. (b) grown on high nutrient supply under either weak light (left) or strong light (right). Scale bar = 100 µm (W.A. Thompson et al., 1992a; micrographs courtesy I.E. Craig)
These adaptations result in the more vigorous growth of Toona than Argyrodendron at high light.