CASE STUDY 12.1 Interaction of light and nutrients on rainforest seedlings

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Paul Kriedemann

Different species of rainforest seedlings can show strong contrasts in growth response to light x nutrient inputs which relate to their gap-colonising ability and successional status. For example, wide gaps in tropical rainforests of North Queensland that result from major disturbance during forest operations are often colonised by early-successional species such as Toona australis (red cedar of commerce). By contrast, narrow disturbance gaps within established forests that have become species rich become occupied by later-successional species such as Flindersia brayleyana or Argyrodendron sp. These differences in gap-colonising ability relate, inter alia, to inherent differences in vigour and thus growth response to light x nutrient inputs, and can be reconciled with leaf attributes. Toona is sun loving and fast growing compared with later-successional species of Argyrodendron (tulip oak of commerce) which colonise narrow gaps and are shade tolerant as seedling trees. Species in this genus are adapted to life as seedlings in weak light with occasional sunflecks on the forest floor (Figure 12.14). This distinction in seedling vigour between Toona and Argyrodendron fits their respective successional status in forest dynamics and, as shown here, is further expressed in growth and photosynthetic response to light x nutrient inputs.

A priori, strong vigour should have a selective advantage for those species that are recruited to major disturbances where strong sunlight combines with a flush of plant-available soil nutrients to promote fast growth. However, such vigour would be selectively neutral on a forest floor where daily photon irradiance is a major limitation (notwithstanding sunflecks). In effect, leaf longevity plus resistance to predation would matter more than light-saturated photosynthetic capacity in promoting a positive carbon balance and hence survival. Moreover, strong sunlight may well prove supra-optimal for seedlings of shade-adapted species of Argyrodendron, especially when strong sunlight is imposed upon a low supply of nutrients, conditions that would predispose to photodamage.

Thompson and co-workers explored these issues for a number of species including Flindersia brayleyana, a rainforest tree with broad tolerance to sun and shade (Thompson et al. 1988), Toona australis, which is sun loving, and two species of Argyrodendron that are shade adapted (Thompson et al. 1992a, b). Tree seedlings were held for six months in growth cabinets under a factorial combination of three light ¥ two nutrient treatments (30, 130 and 535 µmol quanta m–2 s–1 x 1/3 strength or 1/200 strength (modified) Hoagland nutrient solution). Leaf attributes were measured periodically, and dry mass taken at final harvest. The three light treatments provided 1.3, 5.6 and 23 mol quanta m–2 d–1 respectively, and were designed to correspond with daily photon irradiance on forest floor, mid-canopy and upper crown of a North Queensland rainforest.

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Figure 1 Figure 1 Growth response to light in (a) Toona australis (sun loving) and (b) Argyrodendron sp. (shade adapted). Photographs show, left to right, low versus high nutrient supply under strong light for both (a) and (b). Seedlings illustrated here came from a population held for six months under factorial combination of weak, medium or strong light x either low or high nutrient supply. Plants correspond to those described in Thompson et al. 1992a, b. Scale bar = 10 cm (see Colour Plate 49) (Photographs courtesy P.E. Kriedemann)

Consistent with colonising behaviour on disturbed sites in nature, Toona seedlings held under strong light showed a spectacular growth response to nutrient supply (Figure 1a). This growth response was accompanied by marked changes in leaf anatomy and light-saturated rates of photosynthesis (Pmax). Mesophyll depth increased substantially with photon irradiance (Figure 2a), resulting in higher lamina mass:area ratio (Table 1), while Pmax (area basis) in high-light plants was doubled by high nutrient supply (Figure 12.13).

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Figure 2 Transverse sections of leaves from Toona australis (a) and Argyrodendron sp. (b) grown on high nutrient supply under either weak light (left side) or strong light (right). Toona (sun loving) produces a much greater depth of mesophyll tissue under strong light compared to weak light, which accounts for enhanced photosynthetic capacity. Argyrodendron sp. (shape adapted) is much less responsive to photo irradiance during growth, producing consistently thicker leaves regardless of light level, but with lower photosynthetic capacity. Shade adaptation in Argyrodendron is clearly not predicated on leaf adjustments, but probably depends more on carbon retention and leaf longetivity in deep shade than on short-term carbon gain. Scale bar = 100 µm

(Micrographs courtesy I.E. Craig)

Fast growth in Toona contrasted with slow growth in Argyrodendron (Figure 1b). A light x nutrient interaction on seedling size was apparent in both cases (Figure 1 in Thompson et al. 1992a), but overall growth was much more modest in Argyrodendron (5 g plant–1 in Argyrodendron compared with 20g plant–1 in Toona; both on high light x high nutrients). Furthermore, high light proved supraoptimal for Argyrodendron on low nutrient supply, resulting in some photo-oxidative damage and even leaf necrosis (Figure 1b).

Nutrient effects on growth relate mainly to supply and/or utilisation of nitrogen, and nitrogen use efficiency for photo-synthesis by Toona, Flindersia and Argyrodendron was inferred from the slope of relationships between light-saturated photosynthesis and leaf nitrogen (both on an area basis; Figure 6 in Thompson et al. 1992b). This term was always highest in Toona, especially in seedlings grown under strong light, and would contribute to the colonising success of this species in wide disturbance gaps.

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Table 1 Lamina mass:area ratio (g m-2) in Argyrodendron sp. and Toona australis as a function of nutrient supply and photon irradiance during growth

Leaves of sun-loving Toona showed major adjustments to light x nutrient treatments (Figure 2a, Table 1) whereas healthy leaves on Argyrodendron sp. were much less affected in terms of either Pmax (Figure 12.13), cross-sectional anatomy (Figure 2b) or lamina mass:area ratio (Table 1). Notably, Argyrodendron had consistently thicker leaves (evident in Figure 2, and implied by a greater mass:area ratio in Table 1).

Environmental plasticity in leaf properties of Toona underpin this species’ responsiveness to well-exposed and nutrient-rich sites where competitive success hinges on carbon gain. Muted responses in Argyrodendron seedlings are consistent with requirements for shade tolerance in a closed forest where leaf longevity and carbon retention favours survival.

References

Thompson, W.A., Kriedemann, P.E. and Craig, I.E. (1992a). ‘Photosynthetic response to light and nutrients in sun-tolerant and shade-tolerant rainforest trees. I. Growth, leaf anatomy and nutrient content’, Australian Journal of Plant Physiology, 19, 1–18.

Thompson, W.A., Huang, L.-K. and Kriedemann, P.E. (1992b). ‘Photosynthetic response to light and nutrients in sun-tolerant and shade-tolerant rainforest trees. II. Leaf gas exchange and component processes of photosynthesis’, Australian Journal of Plant Physiology, 19, 19–42.

Thompson, W.A., Stocker, G.C. and Kriedemann, P.E. (1988). ‘Growth and photosynthetic response to light and nutrients of Flindersia brayleyana F. Muell., a rainforest tree with broad tolerance to sun and shade’, Australian Journal of Plant Physiology, 15, 299–315.

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