Seedling and sapling form

The form of newly germinated seedlings has been classified, along the lines of earlier systems, by Garwood (1996). Her system is based on cotyledon (hidden or not, foliaceous or purely storage) and germination (epigeal or hypogeal) characters and recognises five main seedling types as follows.

Phanerocotylar-epigeal-foliaceous (PEF) Phanerocotylar-epigeal-reserve (PER) Phanerocotylar-hypogeal-reserve (PHR) Cryptocotylar-hypogeal-reserve (CHR) Cryptocotylar-epigeal-reserve (CER)

Surveys of seedling types from various tropical areas show a generally similar pattern of relative abundance of the different seedling types. PEF is usually the commonest in terms of number of species; PHR and CER are relatively rare. A search for the ecological relevance of the seedling types has not been very rewarding till now (Garwood 1996). The clearest correlation is with seed size. Nearly all small seeds are phanerocotylar and epigeal; for example, 100% of seeds less than 3 mm long in a sample from Malaysia (Ng 1978). At the other end of the size spectrum the PE (F or R) seedling group becomes much rarer with only 5% of species with seeds > 40 mm long in Malaysia (Ng 1978) and 19% of species with seeds > 20 mm long in Gabon (Miquel 1987; Hladik & Miquel 1990). It is possible that cryptocotyly is favoured in big seeds as a means of protecting the large cotyledons or endosperm from animals.

Seedling and sapling morphology and architecture are quite varied, ranging from erect unbranched juveniles to branching plants with arching shoots. In an analysis of sapling form of common species on Barro Colorado Island, King et al. (1997) found that more than half of the species had

Table 5.1. Frequency of occurrence of different general architectural forms among species with 50 or more individuals (> 1 cm dbh) in 50 ha on Barro Colorado Island

Architectural form No. species Percentage of total tiers of plagiotropic branches on a vertical stem 21 11 plagiotropic branches (not in tiers) on a vertical stem 33 18

plagiotropic branches on an arching stem 40 21

orthotropic forms 80 43

intermediate forms 12 6

unexamined 2 1

strongly orthotropic main stems (Table 5.1). Such saplings tend to have distinctly three-dimensional foliage whereas plagiotropic seedlings usually hold their leaves in flat arrays with relatively little depth of foliage on each branch (King 1998a). Leaf form is related to sapling architectural form (King & Maindonald 1999). Species with compound leaves are generally ortho-tropic. In species with simple leaves, orthotropic saplings tend to have larger leaves with longer petioles than plagiotropic saplings. Many species show a switch from plagiotropy as saplings to orthotropy as canopy trees. This may reflect a more efficient harvesting of the widely scattered light in the tropical rain-forest understorey by the plagiotropic sapling design.

Reviewing a large body of published data on the growth of tropical tree seedlings, Veneklaas & Poorter (1998) concluded that stem mass ratio increases with plant mass during the course of development. There is also a concomitant increase in average leaf mass per unit area (LMA), which progressively reduces the seedling leaf area ratio (LAR) and results in a trend of decreasing relative growth rate (RGR) with plant size. They argued that species follow individual allometric trajectories during the course of sapling growth. However, Kohyama & Hotta (1990) found that for nine species from the Sumatran rain forest with saplings 60-300 cm tall there were no significant interspecific differences in slope for the major allometric relationships of architectural form.

Seedling and sapling leaves can be very different from those found on the adult tree. In some species this is marked enough to be regarded as hetero-phylly; for example, Scaphium macropodum has deeply lobed leaves in juveniles, but simple leaves in mature trees (Fig. 5.7). It is clearly unlikely that small seedlings will be able to support leaves of adult size, so there is generally an increase in leaf size with age initially. However, size may peak at the sapling stage and then decline, as seen in Cecropia obtusifolia (Alvarez-Buylla &

Figure 5.7 The leaf shapes of Scaphium macropodum. The heights of the trees from which the leaves were collected are indicated in the figure. After Yamada & Suzuki (1996).

46 cm 107 cm 124 cm 850 cm 600 cm 3670 cm

Figure 5.7 The leaf shapes of Scaphium macropodum. The heights of the trees from which the leaves were collected are indicated in the figure. After Yamada & Suzuki (1996).

Martinez-Ramos 1992) (Fig. 5.8). This probably reflects a maximum leaf size in saplings before they start to branch. The unbranched main stem bears very big leaves, but after branching has taken place such big leaves would tend to self-shade and could probably not be supported by the narrower axis of a secondary branch. There was a highly significant positive correlation between height at first branching and leaf length for saplings of 70 species spread across four different tropical rain-forest sites (King 1998b). Thomas & Ickes (1995) studied ontogenetic changes in leaf size in 51 species at Pasoh, Peninsular Malaysia. They found that half (26/51) exhibited larger leaves on saplings than adults. Thirteen species had larger adult leaves. These were mostly understorey treelets. Microscopic leaf-venation patterns differ between juvenile and mature trees (Roth 1996) with leaves of young trees having less dense venation with fewer vein endings and less ramification per mesh. This is probably mostly an effect of denser shade on juvenile leaves.

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