Seedling shade tolerance

The concept of shade tolerance is, like many in ecology, one that appears simple on first acquaintance, but slowly reveals a hidden complexity. Various approaches to defining shade tolerance in tropical tree seedlings have been proposed (Whitmore 1996 Kitajima 1994 Popma & Bongers 1988). These include the following. (a) Minimum irradiance for seedling survival More shade-tolerant species can survive at lower irradiances than less shade-tolerant ones. It is easy to demonstrate that certain...

BOX 61 The nomenclature of the pioneerclimax dichotomy

Those fast-growing light-demanding species characteristic of large gaps in the forest, as well as forest edges and clearings, are the ecological group most frequently recognised by those attempting ecological classification of tropical trees. Various names have been coined for the group which reflect different aspects of their ecology. 'Pioneer' is perhaps the most widely used, and with 'early-succession' and 'secondary', refers to the abundance of such species in the early stages of secondary...

The ecological classification of tropical rainforest trees

An ecological classificatory system for tropical trees is needed in order to organise our information about the species concerned. The systematisation of knowledge is one of the most important activities of science. A logical organisation efficiently condenses information into a series of generalisations, and allows predictions to be made about members of groups recognised by the system. Without such a system, ecology is reduced to natural history employing the scientific method. This is not...

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...

Species groups based on leaf characteristics

The 61 species studied at Los Tuxtlas (Popma et al. 1992), were divided into three ecological groups based on their apparent light requirement for regeneration. The groups were 'obligate gap' species that were only ever found in gaps, 'gap-dependent' species that can survive in canopy shade as juveniles but need gaps to grow to large size, and 'gap-independent' species that can complete their life cycle in the shade (Popma et al. 1992). The obligate gap species had larger leaves than the other...

The pioneerclimax dichotomy

The pioneer-climax axis or dichotomy (Box 6.1) among tropical tree species has been associated with many features of their biology as summarised in Table 6.2. There has been a strong tendency to rely on anecdotal accounts, impressions from observation in the field and the application of circular arguments (Cecropia and Macaranga are pioneers, therefore features of these genera are characteristics of pioneers) to support these points. Other studies generally rely upon an act of faith by the...

Trees form mechanics and hydraulics Tree stature

Arborescent Prop

Individual trees of a large range of size are to be found in the tropical rain forest (see the profile diagram in Fig. 2.1). Each tree species also has a characteristic size at maturity and species are often referred to various stature classes, such as understorey trees, canopy trees and emergents, but, as Figure 2.2 shows, there is no discrete clustering of species in size classes. Maximum diameter for species on 50 ha in Pasoh Forest, Peninsular Malaysia (Fig. 2.2), was approximately a...

Water

The tropical rain forest is, as its name suggests, a generally wet place, and not one where studies of plant water relations readily spring to mind. However, many tropical rain forests do have dry seasons, and even the wettest of climates are liable to dry spells at intervals. During a dry spell small seedlings, with their limited root systems, are probably the woody plants most susceptible to drought. Reviewing the literature on factors influencing the species composition of lowland tropical...

Tree growth in the forest

Most studies of the growth of tropical forest trees have found that the vast majority of trees grow very slowly. For instance, 64 of trees in plots at Bukit Lagong and Sungei Menyala in Peninsular Malaysia had diameter growth rates averaging around 1 mm yr (Fig. 3.3) over the 20-30-year Figure 3.2 Fraction of plants surviving over the intercensus interval 198285 as a function of diameter class for sample tree species in the Barro Colorado Island plot. Many shade-tolerant species showed high and...

Leaf development coloured young leaves

A common feature of tropical rain forest trees is the production of new leaves of distinctive colours other than green. Young leaves are often a shade of red, varying from pale pink to bright scarlet, but white or even purple or blue can be observed. In a survey of 250 species across the tropics (Coley & Kursar 1996), 33 of species were found to have non-green young leaves. These included representatives of 61 of the families in the sample. Among tropical trees, coloured young leaves are...

The importance of phylogeny

There can be no doubt that a species is strongly influenced in many of its characters by its antecedents. We must therefore question whether the patterns ecologists see when they compare species, such as trees in the tropical rain forest, are not mostly reflections of phylogenetic relationships rather than recent ecological adaptations. Can we estimate the degree of 'phylogenetic constraint' on the ecology of species Or can we control for the influence of phylogeny when we design ecological...

Seed germination

The seeds of tropical rain-forest trees show a large interspecific range of time taken to germinate. For a sample of 330 species from the forests of Malaysia, 65 of species showed germination within 20 weeks of sowing fresh seed in a lightly shaded nursery (Ng 1980). The 35 of species that took longer than 20 weeks to germinate often had hard, thick seed coats or endocarps around the seed. Studies of the physiology of germination have shown that a number of factors can cause delayed germination...

Sudden changes in light availability

Any point in the forest is subject to considerable variation in the quantity and quality of photosynthetically active radiation it receives. The forest understorey probably shows the greatest short-term magnitude in variation, because of the occasional bright sunflecks set against the generally low irradiance of the forest interior. Despite typically being of short duration, their brightness means that sunflecks contribute a large proportion of the total daily irradiance. Physiological...

Seeds Seed size

The size of seeds interests comparative ecologists because it is so variable among species. The dry mass of seeds ranges over at least six orders of magnitude across species of tropical rain-forest tree. The Melastomataceae and Rubiaceae include tropical tree species with seeds of dry mass as little as 20 g (Metcalfe & Grubb 1995 Grubb & Metcalfe 1996). At the other extreme, the seeds of a number of trees, notably legumes, approach 100 g dry mass. Within any tropical forest site, most...

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). Figure 5.11 Mean relative growth rates ( 1 SE) in (a) height, (b)...

Mineral nutrients

Seedlings are generally the subjects of studies on the mineral nutrition of tropical trees. There appears to be a relationship between light-demandingness and growth response to increases in soil nutrient availability. Typically fast-growing species have been shown to respond readily to increased soil fertility by faster growth, but more shade-tolerant species often do not respond (Turner et al. 1993a Burslem et al. 1995 Raaimakers & Lambers 1996 Veenendaal et al. 1996), although positive...

Height at maturity

Forest ecologists have generally understated the rather obvious distinction between species in their height at reproductive maturity. Species are often divided into height classes in ecological analyses, but it seems more with the purpose of comparing like with like within the stature groups, than of making comparisons among the groups. However, some general trends do emerge from the literature, and these are summarised in Table 6.1 as a series of characteristics of small-statured species in...

Seedling mortality

The mortality of tree seedlings in the tropical rain forest is generally high, particularly for the new germinants. Many surveys of seedling population dynamics may well miss much of the earliest, and most intense, mortality because monitoring tiny seedlings and identifying them correctly is very difficult (Kennedy & Swaine 1992). A cohort of germinants often declines in numbers in a log-linear fashion, at least initially. That is, a straight line of negative gradient is produced by plotting...

Tropical rainforest diversity

Tropical rain forests are the most diverse of terrestrial ecosystems. Many lowland forests contain more than 100 species among the trees of 10 cm dbh or over on 1 ha (Fig. 1.2), and in some more than 200 species may Figure 1.2 Box-and-whisker plot of species richness for trees greater than 10 cm dbh of rain-forest sites for the major tropical regions (Asia-Pacific, America and Africa). The line inside the box represents the mean value of the average number of species per hectare. The box...

Root hemiparasites

A relatively few tropical rain-forest tree species, restricted to the order Santalales, produce haustoria from their roots (Fineran 1991). These Figure 2.16 Above- and below-ground architecture of saplings of canopy and subcanopy species (upper panel) and a treelet and shrubs (lower panel). After Becker & Castillo (1990). Copyrighted 1990 by the Association for Tropical Biology, P.O. Box 1897, Lawrence KS 66044-8897. Reprinted by permission. Figure 2.16 Above- and below-ground architecture...

Size shape and other structural characteristics

Leaf size has classically been dealt with in descriptive vegetation ecology by reference to leaf size classes. The Danish plant ecologist Raun-kiaer developed an arbitrary classification of leaf sizes based upon a geometric series 9 with a base leaf size of 25 mm2 (Raunkiaer 1916, 1934). He found that a series based on 9 produced a system that lent itself more meaningfully to the description of vegetation types across the globe than one based on 10. Each of the six size classes originally...

Mortality in trees

Death can take many forms in tropical trees. Natural disasters including hurricanes, landslides, earthquakes, floods or droughts, lightning strikes, high winds, and diseases, as well as old age, can all be causes of mortality. Tropical rain forests occur at sites differing considerably in incidence of intense natural disturbances. For instance, Carey et al. (1994) reported that 64 of mortality in Venezuela involved trees dying standing, whereas in Costa Rica (Lieberman et al. 1985b) and Central...

Mycorrhizas

A mycorrhiza is a sustainable, non-pathogenic, biotrophic interaction between a fungus and a root (Fitter & Moyersoen 1996). Most tropical tree species habitually possess vesicular-arbuscular mycorrhizas (VAM) (Alexander 1989), as can be seen from a survey of reports on mycorrhizal infection in different tropical rain-forest sites (Table 2.2). VAM involve primitive fungi from the Zygomycotina orders Endogonales and Glomales (Smith & Read 1997) (Table 2.3). There are probably about 200...

Roots

Our detailed knowledge of the tropical rain forest decreases both upwards and downwards from the forest floor. The relative difficulty of accessing the forest canopy means that we are only just beginning to learn of its biology. The subterranean portion of the forest is hidden from view and poorly understood. The excavation of the root systems of large trees is difficult and labour-intensive work. Tree root systems can be divided into two portions, fine and coarse, based on diameter. Coarse...

Reproductive biology

Trees Peat Swamp Forest

The high species diversity of the tropical rain forest is inevitably associated with low population densities for a majority of species. Most tree species will exist at densities below ten mature individuals per hectare of forest, and many will live in much sparser populations. Tropical ecologists have long been intrigued by the possible ability of tropical tree populations of widely separated individuals to outbreed successfully. In recent years, new techniques for genetically fingerprinting...

Sun Shade

Figure 2.28 Schematic representation of the relative differences in leaf form between sun and shade conditions based on data from Bongers & Popma (1988). Figure 2.28 Schematic representation of the relative differences in leaf form between sun and shade conditions based on data from Bongers & Popma (1988). cooling. Large leaves have a high boundary-layer resistance that reduces the rate of transfer of sensible heat between the leaf and the general atmosphere. Large leaves in full sun can...

Tests of the escape hypothesis

There have been many studies of the survival of seeds and seedlings of tropical trees with respect to conspecific density or distance to adult trees (Hammond & Brown 1998) (Table 4.10). Some of the research published had poor degrees of replication, studying just the offspring of one tree at one fruiting season. Despite this, the evidence seems to be in favour of the escape hypothesis. Seedlings of Dipteryx panamensis at La Selva, Costa Rica, showed a 100 mortality over the 7-20 month old...

The defences of tropical forest trees

Plants are attacked by a wide range of herbivores and pathogens. These vary in size, specificity and many other characteristics. It is therefore not surprising that plants normally possess many different sorts of defence. Some of these are outlined with examples from tropical forest trees below. More examples can be found in Chapter 5. Harborne (1993) and Bennett & Wallsgrove (1994) provide good reviews of plant defences. In order to ingest and digest plant material, herbivores need to...