Info

detritus

Hymenoptera (order)

Nests

Predators, detritus, other

Coleoptera (order)

Litter

Predation, varied

Scarabaedidae

Litter

Wood, litter, dung

Aranea (order)

Litter

Predation

Pseudoscorpionida

Litter, top soil

Predation on small invertebrates

Acari (order)

Litter, top soil

Nematodes, hyphae, detritus

immediate vicinity. Large carcasses, such as ungulate species, constitute an intense localized disturbance, which releases a concentrated pulse of nutrients into the soil, and changes the vegetation for years after (Towne, 2000). It is unclear to what extent soil nitrogen supply is derived from invertebrate and vertebrate animal cadavers and excreta. The effect of cadavers (or animal detritus) has been much less studied than plant-derived litter, presenting opportunities for further research.

The impact of macroinvertebrates on litter decomposition is usually demonstrated using mesh bags enclosing leaf litter. Leaf litter is placed on the field site, using mesh sizes to bias against size classes of the litter fauna (exclusion of fauna), and against larger mesh sizes that do not (control litter bags) (Fig. 4.2). Using mesh of 1-2 mm, most macrofauna are excluded, but this often increases moisture content in the litter bag

Fig. 4.2. Litter bags of plastic mesh holding decomposing leaf litter. The large mesh allows access to macroarthropods and the larger organisms, while fine mesh bags limit access of macroinvertebrates.

(Vossbrink et al., 1979). The rate of litter mass loss in control and exclusion litter bags is obtained from weight loss through time. This is approximated by a simple decay equation Wt = Wo e-kt, with the decay constant k representing the rate of mass loss through time t, and Wt and Wo representing the final and initial dry weight of litter. The decay constant k can be obtained from the slope of the natural logarithm of the graph, from lnWt - lnWo = -ktlne. However, the actual pattern of mass loss is often multiphasic, with periods of rapid loss and periods of stable or slow mass loss (Fig. 4.3). The effect is caused in part by seasonal variations and by species succession on the litter. The pattern mostly reflects plant-specific differences in litter nutrient composition, timing of leachate of toxins and rate of decomposition of the cell walls. The procedure can overestimate the rate of decomposition, because macroinvertebrates will chew on the litter, but defecate the unassimilated portion at a distance outside the litter bag. Furthermore, a portion of litter fragments falls out of wide mesh bags that permit access to macroinvertebrates. The rate of mass loss can be underestimated by a variable amount, due to an increase in mass of organisms in the litter tissues. This value is normally low, but can be more significant in some cases. For instance, pine needles can be embedded with enchytraeids, Collembola, oribatids and other organisms (Ponge, 1991; Edslung and Hagvar, 1999). The presence of primary and secondary saprotrophs inside the litter does, however, contribute significantly to the litter nutrient content (such as elemental nitrogen).

Months in litter

Fig. 4.3. Rate of mass loss over time in several leaf litter species. Representative data from single species of leaf litter placed in large mesh litter bags, in a temperate forest in North Carolina.

Months in litter

Fig. 4.3. Rate of mass loss over time in several leaf litter species. Representative data from single species of leaf litter placed in large mesh litter bags, in a temperate forest in North Carolina.

Litter mass loss rates

The rate of initial litter decomposition from mass loss studies in litter bags varies with climate (temperature and moisture), litter chemistry and faunal composition (Table 4.1) (Cadish and Giller, 1997). Comparison of leaf mass loss across climatic regions shows that the effect of climate is the principal predictor of the initial decay rate (Swift et al., 1979). This was shown further in a review of leaf litter decomposition from 44 varied geographic locations (Aerts, 1997). The climate was characterized from the actual évapotranspiration (AET) from monthly temperature and precipitation data (Thornthwaite and Mather, 1957). This value is superior to temperature or moisture values used alone. It is an index of climatic energy and the availability of capillary water from soil and litter (Meentemeyer, 1978). Several generalities were observed from the comparisons reported by Aerts (1997) (Tables 4.3 and 4.4).

1. After the actual evapotranspiration value (climate effect), leaf litter chemistry was the next most important parameter determining initial decomposition rate. Climate exerts the strongest influence on initial litter decay rate (k) between geographic regions, but litter chemistry (nutrient content, lignin and secondary metabolites) is a better descriptor of variations in decay rate locally.

2. A threefold increase in AET from temperate to tropical regions is accompanied by a sixfold increase in the decay constant.

Table 4.3. Summary of data of 44 leaf litter decomposition studies, from temperate, Mediterranean and tropical sites (modified from Aerts, 1997).

Region

Temperate

Mediterranean

Tropical

Lignin (%)

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