12.1.3 Sun/shade acclimation and rainforest gaps

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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, such canopy emergents represent 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contrast, in old-growth forests, early-successional species have long since completed their life cycles, and will have been replaced by later-successional species whose seedlings initially tolerated deep shade on the forest floor, but now endure full sun as canopy emergents.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 in photosynthesis–light relations is an adaptive feature of late-successional species and involves sun/shade acclimation by individual leaves as discussed below.

A seed germinating in a rainforest understorey (or beneath any closed canopy) starts life in a low light environment. This will not present major problems to an obligate shade species which cannot tolerate strong sunlight. Precluding any other lethal stresses, such species have adapted to life in an under-storey. 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. In conjunction with variation in the light environment within a rainforest, and in particular with the formation of gaps, these two types of physiology are in some part responsible for the patterns of distribution of rainforest species and changes in these patterns through time (Denslow 1987; Whitmore 1989).

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Figure 12.13 Photosynthesis-light response curves for seedlings of a shade-adapted rainforest tree species (Argyrodendron sp.) and a sunloving tree species (Toona australis). Seedlings were grown under factorial combination of weak, medium or strong light (shown left to right) x either high or low nutrient supply (solid lines with filled symbols, and dashed lines with open symbols respectively). Light-saturated photosynthesis (Pmax) was little affected by treatment in Argyrodendron sp., but Pmax in T. australis was greatly enhanced when cultured under high-nutrient supply x strong light. Highly conserved photosynthetic attributes in Argyrodendron sp. fit the late-successional status of this species, while photosynthetic responsiveness to strong light and abundant nutrients in T. australis in consistent with successful colonisation of disturbance sites by this early-successional species. (See Case study 12.1 for further details)

(Based on Thompson et al. 1992b)

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 be found in wide gaps and on forest edges. When grown in low light, shade-intolerant species are unable to maintain a positive carbon balance. The change from a high light to a low light environment requires a change in allocation of plant resources as described earlier (Section 6.2). Shade-intolerant plants seem to be unable to make this change and are burdened with the higher costs of construction and maintenance of leaves better suited to strong sunlight. (Taken more broadly, factors beyond photosynthesis contribute to gap dynamics, and shade tolerance in rainforest plants is also associated with features that reduce herbivory and increase resistance to pathogens; Kitajima 1994.)

Wide gaps are relatively rare in old-growth rainforests which have been undisturbed by human activities such as logging or slash and burn agriculture. However, rainforest plants have evolved a range of characteristics which improve their probability of encountering a gap and thus their survival to reproductive maturity. 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 which 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 likely to be 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 but with the ability to germinate in low light and persist in the understorey as seedlings for long periods (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 them with a reserve which they can draw upon during early establishment. Osunkoya et al. (1993)

In a study of factors affecting survival in North Queensland rainforest tree seedlings found that seedling survival in understorey habitats was 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 for a very long time before a gap appears overhead. Many 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.

As a case in point for rainforest species from North Queensland, Thompson et al. (1992b) found that Toona australis (an early-successional species) and two species of Argyrodendron (late-successional species) showed different acclimation potentials when grown under a range of light conditions (Figure 12.13). When grown under high light, 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 (see Case study 12.1).

 

 

 

 

 

 

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