R

Forest succession i i i i i

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Fig. 2.7. Trends of gross production (PG), respiration (R), net production (PN=PG-R), and biomass accumulation (B) in a forest as a function of time during secondary succession. (Redrawn from Kira and Shidei 1967, with permission of the Ecological Society of Japan)

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Fig. 2.7. Trends of gross production (PG), respiration (R), net production (PN=PG-R), and biomass accumulation (B) in a forest as a function of time during secondary succession. (Redrawn from Kira and Shidei 1967, with permission of the Ecological Society of Japan)

and high concentrations of phenolic compounds in the leaves (Bruijnzeel and Veneklaas 1998). However, on some tropical mountains, primary production can be highest at mid-elevations, if production at lower elevations is limited by high temperatures, nutrient deficiencies in the soil, or a high ratio of evapotranspiration to precipitation (Webb et al. 1983). Nutrient availability also influences primary production. Forests on richer soils produce wood and leaves at almost twice the rate of those on nutrient-poor soils (Jordan 1985).

Productivity also tends to change with time in successional forests. At first, productivity is low, due to small biomass. As the structure of the forest increases, productivity increases, but eventually it declines as costs of maintenance increase (Fig. 2.7). The costs of maintenance include a wide range of defenses against competition, disease, and herbivory, as well as repair of tissues damaged by these factors, wind, lightning, and other physical factors.

Productivity and the accumulation of biomass that occurs depend upon the intensity, size, and duration of the disturbance that initiates the succession (Jordan 1985):

• If the disturbance is of high intensity, large size, and short duration, as would occur on a landslide of a tropical mountain, or after a strong volcanic eruption, then primary succession will occur. It may take decades or even centuries for soil to develop in which even the pioneer tree species can become established. Algae and possibly some grasses will colonize the area. Tree ferns may occupy a stage in humid, montane areas.

• If the disturbance is a hurricane, then the disturbance could be of low to high intensity, possibly large in size, and of short duration. Mature trees that are damaged by hurricanes often have a capacity to resprout. Secondary stages of succession may be entirely bypassed, except in forest gaps where mature trees have been uprooted.

• If the disturbance is of moderate intensity, small size, and short duration, as would occur from a few years of shifting cultivation, then early and late secondary species as well as mature (climax) species may immediately become established. Due to the small size of the forest opening, seeds from the surrounding forest fall into the gap and become established. Large-scale agricultural activities such as banana plantations are much slower to recover once abandoned, because of long distances to seed trees and lack of seed dispersers.

• If the disturbance is one of light intensity, of variable size, and short duration, as would occur with shelterwood forestry (see Chap. 5), then the secondary successional stages would be mostly bypassed. Selective logging as in the shelterwood or similar systems (Fox 1976; de Graaf and Poels 1990) mimics the impact of natural tree fall, with the exception of soil disturbance along the skid trail (path to remove the log).

• If the disturbance is of moderate intensity, large size, and duration long enough to eliminate all sprouting trees from the original forest, then we would expect colonization by pioneer species. Such a disturbance might result from extended periods of agriculture, followed by abandonment with no further disturbance.

• If the disturbance is of moderate intensity, large size, and long duration, as would occur with conversion of forest to pasture followed by periodic burning, then there will be a fire "disclimax". For example, in the Gran Pajonal of Peru, degraded and burned land is first invaded by the fern Pteridium aquilinum. If fire continues, the aggressive grass Imperata bra-siliensis invades. As soil erosion increases, grasses of the genus Andropo-gon become dominant (Scott 1978). Regeneration of forest can be slow, due to lack of seed rain, low seed germination, unfavorable microclimate, and compact and/or nutrient-poor soil (Holl 1999). Recovery to the original forest type may never occur. For example, it is possible that the huge baba9u palm (Orbignya phalerata) forests in the Amazonian state of Mar-anhao (Dubois 1990) represent a "fire disclimax", that is, a community of lesser biomass and species diversity than the original forest in that area. In areas of Southeast Asia, an aggressive grass, Imperata spp., often establishes, and is favored by fire (Lal 1987). Once a dense colony of this grass establishes at a site, recovery to forest may be slow or impossible.

Uhl et al. (1990) illustrated the rate of biomass recovery as a function of disturbance type (Fig. 2.8).

Fig. 2.8. Accumulation of biomass during succession as a function of disturbances of varying intensity. (Adapted from Uhl et al. 1990, with permission of Columbia University Press)
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