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 roots are generally woody and provide a mechanical and conductive service to the tree. Water and mineral nutrients are taken up by the fine roots that ramify through the mineral soil and sometimes the litter layer as well. There are a complex set of interactions between tree fine roots and soil micro-organisms, particularly fungi, that can result in symbioses, including mycorrhizas. These play an extremely important role in mineral nutrition and possibly in other areas of tree physiology.
The morphology of tree root systems is complex. Jenik (1978 and
references therein) has perhaps been the leading student of tropical tree root system form, although most of his papers have been concerned with aerial roots rather than with subterranean ones. Aerial roots are to be found in most tropical forests, and are particularly abundant in forests with periodic or continuous inundation by fresh or salt water, but they are far from common in most well-drained forests, unless buttresses are considered as aerial roots. A preliminary analysis of some tropical species showed there were possibilities that a Halle-and-Oldeman-type approach might be feasible for classifying root system architecture, and that processes such as reiteration do appear below ground (Atger & Edelin 1994). Halle (1995) has even argued that tree trunks and branches may be better considered as roots.
Another major means of classifying root systems is by their relative depth. Some trees have much of the root system close to the soil surface, sometimes with major roots running over the top of the soil. Other species have greater development of roots at depth. Tree roots may descend a long way into the soil. Nepstad et al. (1994) report roots at depths of up to 18 m in the southern, more seasonal, part of the Amazon Basin in Brazil. These deep roots reach to the permanent water table, allowing rain forest to exist in an area where the dry season is both prolonged and severe, averaging 5 months with less than 250 mm of rain. A considerable portion of the Amazon rain forests probably requires very deep roots to survive.
Becker & Castillo (1990) compared the root systems of three species of shrub or treelet with saplings of three tree species in the forest on Barro Colorado Island, Panama (BCI). They found that the shrub or treelets had shallower root systems (Fig. 2.16), with a greater proportion of the root system in the top 20 cm of the soil and more root area absolutely and per unit leaf area. The authors argued that this reflected a greater requirement for nutrients by the small-statured species because of reproduction. However, the relative scarcity of deep roots in the shrubs may make them more susceptible to drought than the saplings. Using stable hydrogen isotope analysis (SD) to fingerprint water supply, Jackson et al. (1995) found considerable variation in SD for shrubs on BCI, indicating that contrary to Becker & Castillo's observations, some species were probably deep-rooted. A repeat of the shrub versus sapling comparison for species at Andulau in Brunei (Becker et al. 1999a) failed to find a significant difference in dry mass allocation between the two life-form groups because some of the shrub or treelet species were deep-rooted.
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