The rate of organic matter decomposition is called the "k" value. It is calculated as follows: if X0 is the original amount of litter on the forest floor, X is the amount of litter remaining at a later time, e is the base of natural logarithms, and t is the time elapsed between X0 and X, then

In an ecosystem where rates of litter fall and decomposition rates are approximately equal, and the stock of litter is thus in a steady state, k is related to standing stock of litter (X) and rate of litter fall (L) by the equation:

The time required for 95% of the standing crop to decompose is estimated by 3/k, and for 99%, 5/k (Olson 1963). Comparison of k values in various biomes indicates that decomposition is significantly higher in tropical forests (Table 2.2).

Anderson et al. (1983) challenged the conclusion that the decomposition rate of litter is higher in the tropics. They presented a series of k values for leaf litter from a variety of habitats and concluded that variation within single regions is too great to permit statements about differences between tropical and temperate latitudes. There certainly is a wide variation in decomposition rates in the tropics. Values of k in lowland tropical forests range from

Table 2.2. Decomposition constants k and 3/k in six ecosystem types. See text for definitions. (Adapted from Swift et al. 1979)

Tundra Boreal Temperate Temperate Savannah Tropical forest deciduous grassland moist fo forest moist forest k year-1 0.03 0.21 0.77 1.5 3.2 6.0

0.76 in a heath forest to greater than 3.0 on soils of intermediate quality (Jordan 1985). However, decomposition follows the same pattern as primary production, and while primary production varies widely in the tropics it is still higher than at high latitudes when the forests that are compared receive similar amounts of rain and are on soils of similar quality. Thus if litter fall is higher in lowland moist tropical forests on rich soils than litter fall in lowland moist temperate forests on rich soils, then decomposition also must necessarily be higher, since, on average, decomposition must equal production. If decomposition in tropical forests were lower than litter fall, there would have to be an ever-increasing stock of organic matter on the forest floor. This is clearly not the case.

It is important to recognize that stocks of organic matter on the forest floor do not indicate the rate of decomposition. Carbon accumulation in soils of the lowland wet tropics ranges from 12-18 kg/m3 of soil and may be as high as 24 or more in montane forests (Zinke et al. 1984) (organic litter is approximately 50% carbon). These high stocks do not mean that litter is accumulating or that decomposition is slow. Decomposition rate is a function of the amount of undecomposed material present (Olson 1963). The greater the amount of organic matter present, the greater the rate of decomposition. A larger biomass of undecomposed material will support a greater mass of decomposers. Thus, as the amount of organic matter increases, the number of decomposers increases, until the rate of decomposition equals the rate of input.

The high rate of organic matter decomposition in tropical forests has important implications for management and conservation (Box 2.4).

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