Growth and survival with respect to light

The influence of light on the growth and survival of seedlings has been the aspect of the ecology of tropical trees most often investigated. There

Figure 5.11 Mean relative growth rates (±1 SE) in (a) height, (b) number of leaves, (c) branch length and (d) stem thickness of trenched (TR) and untrenched (CT) saplings of 13 species, measured in understorey (U) and gaps (G), in Venezuelan caatinga. After Coomes & Grubb (1998a).

is a considerable range in the degree of 'naturalness' of such studies. Observations can be made on wild seedlings in situ across a variety of natural microsites in the forest. Seedlings can be planted or placed in pots into the forest, or grown in shade-houses imitating the natural microclimates of the forest, or controlled-environment chambers can be used (often well outside the tropics). Natural forest seedlings are very variable in size, age and environment. The greater degree of control and replication of artificial environments may lack the reality of the natural forest. For instance, growth chambers usually provide uniform light to plants, but in the forest the light received by a seedling on the forest floor varies almost continuously in intensity, and to some extent in spectral composition.

Natural seedlings in situ

Seedling growth and survival is nearly always promoted by increased light (King 1991b, 1994; Coomes & Grubb 1998b; van der Meer et al. 1998), except at very high levels (Zagt & Werger 1998). For example, Turner (1990b) observed that seedlings less than 1 m tall grew little under canopy shade conditions in Malaysian dipterocarp forest. There was a positive correlation between height growth and 1 m2 seedling-plot light climate as estimated by using hemispherical photography. Clark et al. (1993) found similar results for saplings (0.5-5 m tall) of nine species at La Selva, Costa Rica, as did Dalling et al. (1998a) for eight out of ten pioneer species growing in gaps on Barro Colorado Island. However, shade-tolerant understorey species with seedlings that survive well in low light conditions may show little difference in survival between gap and understorey sites. For instance, Fraver et al. (1998) found no significant difference in survival over a four-year period for seedlings of Protium panamense and Desmopsis panamensis growing in gaps and under a continuous forest canopy in a Panamanian rain forest.

Seedlings planted into the forest

The range of growth possible in different light environments is exemplified by the results of an experiment by Thompson et al. (1988) in Queensland. Seedlings of four species were planted into tiny, medium and big gaps (0.6%, 9% and 40% full sun). After seven years, one species, Darlingia darlingiana, persisted in the shadiest site where its seedlings were about 15 cm tall. The largest Toona australis individuals in the big gap had reached 10 m in height by this time. Boot (1996) grew seedlings of Guyanan tree species in understorey and gap-edge and gap-centre sites (2.2%, 9.4% and 30.2% full-sun PAR), and performed serial harvests of seedlings in order to conduct classic growth analysis of the species. Some species could not survive in the understorey site. These tended to have the highest relative growth rate in the highest light. Most species did not show much improvement in growth rate between the gap-edge and gap-centre sites. Rooted cuttings of seven shrubby species did show faster growth in gap centres (9-23% full sun) compared with gap edges (3-11% full sun) (Denslow et al. 1990). Ashton et al. (1995) grew seedlings of four species of Shorea in forest gaps at three topographic positions in a forest in Sri Lanka. The valley gap was the biggest, the ridge gap the smallest and the slope gap intermediate in size. The faster growers in the big valley gap, Shorea megistophylla and S. trapezifolia, showed poorest survival in the shade, particularly on the ridge top. Another species, S. worthingtonii, was the best performer in the slope gap. Kobe (1999) planted small seedlings of four species of Moraceae (sensu lato) into various forest microsites ranging from less than 1% to 85% full sun and followed seedling survival and stem-base radial increment for one year. All the species showed a positive relationship between light availability and growth. Three (Castilla elastica, Cecropia obtusifolia and Trophis racemosa) also exhibited declining mortality with increasing light availability. The fourth, Pourouma aspera, had a peak survival at about 20% full sun. Combining growth with expectation of survival from a population of two seedlings at each light level, Kobe (1999) demonstrated that the four species neatly partitioned the light range into sections of approximately 20% full-sun increments in which each showed the best performance. The author reported that partitioning was maintained if larger seedling population sizes (n = 5 or 10) were employed, although Pourouma steadily increased the range of light availabilities at which it was expected to grow the fastest.

Shade-house experiments

A common approach to studying tropical tree seedling response to light has been to grow seedlings in nurseries under different degrees of shading. Many early experiments of this type used a shading material that reduced the photon flux density incident on the plants beneath, but did not alter the spectral quality of the shade cast. Special paints or filters can be used to alter the colour as well as the quantity of light in the shade house. Thus, forest conditions can be imitated more closely, although the accuracy of the simulation is still not necessarily high.

Many experiments have been conducted where seedlings of different species were grown under a range of light conditions, usually spanning the irradiance regimes of large gaps to the forest understorey. I have summarised the results of some of these experiments by plotting growth irradiance as a percentage of full direct sun against seedling relative growth rate (Fig. 5.12). The general response to increased light availability is one of improved growth up to relatively high intensities (about 20% full sun), after which there may be

Figure 5.12 Summary of seedling growth responses to light availability (% full sun on a logarithmic scale). Each line represents a different species in the experiment. Data from (a) Osunkoya et al. (1994), (b) Popma & Bongers (1988), (c) Boot (1996), (d) Agyeman et al. (1999), (e) Kitajima (1994), (f) Poorter (1999).

Figure 5.12 Summary of seedling growth responses to light availability (% full sun on a logarithmic scale). Each line represents a different species in the experiment. Data from (a) Osunkoya et al. (1994), (b) Popma & Bongers (1988), (c) Boot (1996), (d) Agyeman et al. (1999), (e) Kitajima (1994), (f) Poorter (1999).

levelling off, or even a decrease in performance. Most of the studies where growth declined at the highest light intensities included a treatment that involved a period of direct sunlight. Most rain-forest tree species decline in performance when exposed to such conditions. In many cases (see, for example, Kitajima 1994; Poorter 1999) there was little, if any, change in the relative performance of species between shade treatments within an experiment. Some species grow well in all light treatments, and others grow poorly in them all. Ecologists have often argued that tropical tree species may partition the forest environment by being different in their optimal conditions for growth. In terms of light, one might then expect changes in relative performance between species across different shade treatments. A very shade-tolerant species would do best in the deepest shade, a shade-intolerant in the brightest light, and intermediate species would take over at intermediate light intensities. The experimental studies reviewed here offer surprisingly little support for this concept. Of the studies summarised in Fig. 5.12, that of Agyeman et al. (1999) on 16 West African species showed the closest match to the predicted changes in relative performance of species across a light gradient. In the study by Popma & Bongers (1988), Cecropia obtusifolia switched from growing the fastest of 10 species in the high light treatment, to the slowest in the deepest shade. Denslow et al. (1990) found switches in relative performance between light treatments in rooted cuttings of three Miconia species. Another example is the study by Ashton (1995) of four species of Shorea from Sri Lanka. In this experiment three of the four species were the best performers in one or more shade treatment. Probable reasons for this greater variation in outcome in these compared to other studies include a relatively long time period of observations and a wide range of light treatments used including both deep shade (2% full sun or less) and some direct sun treatment.

It must be concluded that subtle differences in light availability (of say a few per cent above 3% full sun) are probably not sufficient to cause major shifts in relative performance between the seedlings of tropical tree species. The studies also show that tolerance of periods of direct full sun may also be an important factor differentiating between species.

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