Introduction
Historical aspects
Significant reductions in the depth of organic horizons only occur after approximately a decade of debris removal (Dzwonko & Gawronski, 2002a). Total microbial biomass in the top 25 cm of mineral soil decreased by an average of 67% after seven years of litter removal treatments in a young secondary montane tropical rainforest (Li et al., 2004). Over time, a positive feedback occurs as nutrient concentrations in the leaves decrease and litter quality deteriorates, so that the effects persist even after cessation of litter removal.
Removal of debris decreases the concentration of nutrients in the soil, especially on nutrient-poor soils such as sand. The first study of the effects of long-term litter ripping on soil nitrogen concentration showed an overall decrease of 43% in total nitrogen and a reduction of nearly 90% in the top 0.5 m of soil (Ebermayer, 1876). There is great variation (5-78%) in the loss of nitrogen caused by long-term waste disposal.
However, a 20% decrease in litter nitrogen concentration in one of the short-term studies, despite no changes in leaf nitrogen concentration, demonstrated greater nitrogen retranslocation after three years of litter removal (McLeod et al., 1979). ). Such 'litter gap deisers' can survive in the soil seed bank for at least six months (Metcalfe & Turner, 1998), and can survive even longer, until disturbance provides litter-free sites for germination (Putz, 1983; Vazquez-Yanes). & Orozco Segovia, 1992). The species composition of the forest floor in the Vienna Woods, Austria, differed between plots after only one year of litter removal and addition treatments (Onno, 1969) and the species composition of a mixed deciduous forest in southern Poland changed significantly after only three years. years of treatment and by 66% over a period of 16 years (Dzwonko & Gawronski, 2002a,b).
Restoration of the above-ground ectomycorrhizal flora in stands of Pinus sylvestris (Scots pine) in the Netherlands by removing litter and humus.
The forest floor environment
The carbon cycle
Substrate-induced respiration (SIR) is commonly used as a measure of active microbial biomass (Anderson & Domsch, 1978); long-term litter harvesting in Eucalyptus forests in tropical China decreased SIR in the top 25 cm of the soil by an average of 40% and by 61% during the rainy season (Penget al., . 2003). One study showed no decrease in microbial biomass with litter removal, but there was a strong decrease in the active microbial biomass in the soil in treatments in which the O and A horizons were removed (Nadelhofer et al., 2004).
The nutrient cycle
Long-term use of litter as mulch in the US or for fuel in China has similar negative effects on total soil phosphorus and calcium availability. The nature and magnitude of the effects of litter removal on tree growth through changes in water balance differ between wetter and drier years, with reduced tree growth in dry periods (Mitscherlich, 1955) and even increased growth in wetter years, possibly due to greater root-soil contact with soil compaction (Gomezet al., 2002), as long as the nutrient supply is sufficient. This may be attributed to the facilitated establishment of some species in the absence of litter cover and increased competitive ability with litter cover in others.
The third group, endogeous earthworms, build lateral burrows in the upper layers of mineral soil and rarely come to the surface. Experiments with Laccaria bicolor showed that the removal of organic layers stimulated the formation of fruiting bodies from the mycelium present in the soil (Baar, Ozinga & Kuyper, 1994a). Partial restoration of fungal and plant species diversity by removing litter and humus layers in Scots pine stands in the Netherlands.
Differential responses of seedlings to post-Hurricane Hugo litter in the Luquillo Experimental Forest, Puerto Rico. Changes in the community structure of forest floor vegetation after repeated disturbance of litter by raking. Silva Fennica17, 289–300. Effects of litter burning and raking on certain animals in Duke Forest. American Midland Naturalist 29, 406–424.
Effect of litter layer on natural regeneration of companion tree species in Korean pine forest. Environmental and Experimental Botany27, 53-65.
Direct and indirect effects on vegetation
Soil and litter fauna
Litter removal generally leads to reductions in micro- and macroarthropod populations, while adding litter does not always have a clear effect. The reduction of soil fauna populations by litter removal was often the result of factors other than substrate reduction, as the relatively small responses to litter addition treatments (Poser, 1990; Davidet al., 1991) indicate that litter is not a limiting food source (Davidet et al. , 1991). In another study, litter removal caused a significant decline in the number of individuals and total biomass of saprophagous bipeds and isopods, followed by a decrease in the number of zoophagous bipeds (Davidet al., 1991).
Disturbances in the litter layer affect soil fauna by changing the temperature and moisture regime (David et al Reynolds, Crossley & Hunter, 2003); the decline of populations in litter removal plots in spring, but the increase compared to controls in autumn may be a consequence of such changes (Arpinet al., 1995). Seasonal variation in spider species abundance in litter removal and addition treatments has been attributed to changes in soil water content, soil temperature, or the effect of litter thickness on prey density (Uetz, 1979). Slight increases in abundance and diversity of soil fauna have been shown in litter addition treatments (Poser, 1990; Davidet al., 1991; Pongeet al., 1993; Arpinet al., 1995), but they were generally not as highlighted as expected. given the size of the response to the waste removal treatment; The addition of litter also caused population declines in some species (Uetz, 1979; Poser, 1990).
Fungi and mycorrhizae
A third treatment, in which the top 15 cm of the A horizon was removed and the litter replaced, did not affect earthworm activity (Nielsen & Hole, 1964), indicating that litter is a more important resource for this species than older organic matter in the soil. The abundance and diversity of fungal species characteristic of early succession increased the most, indicating that removal of the forest floor returns the soil to an earlier successional stage (De Vrieset al., 1995). Nitrogen depletion with removal of organic matter was likely one of the driving factors for increased ectomycorrhizal infection and fruiting body production (Baar & Ter Braak, 1996), although doubling the thickness of the forest floor had little effect on fruiting body production (Baar & Ter Braak, 1996). & Kuyper De Vrieset al., 1995; Baar & Ter Braak, 1996).
Decomposing fungi are likely to be at a disadvantage in litter removal treatments due to the lack of substrate; Five out of seven decomposer species had fewer fruiting bodies in the litter removal plots in one study (Tyler, 1991). The microclimatic conditions determined by the litter layer play a decisive role in fungal fruiting body production; fungi whose fruiting bodies originate from deeper in the soil, such as Russula species, may not be as strongly affected by adverse changes in soil water content and temperature caused by litter manipulation than other species (Tyler, 1991). The main cause of these changes appeared to be a reduction in the colonization of roots by one of the common ectomycorrhizal species, possibly due to the loss of its ability to compete with the higher soil water content associated with litter cover (Brearley et al., 2003).
Conclusions
In a beech forest, the number of ectomycorhizal fruiting bodies of Russula species was 79% higher in raked plots compared to controls after two years of litter removal, while the abundance of other mycorrhizal species was 90% lower in the raked plots (Tyler, 1991 ). Even when litter was solarized to kill existing spores, ectomycorrhizal infection in the surface soil layer increased by 10% and diversity decreased by 31% after litter addition, while perlite application had no effect (Cullingset al., 2003 ). The difference in soil nutrient declines may simply be a methodological artifact, as short-term studies have measured available phosphorus and exchangeable bases, but also total organic nitrogen, which may respond more slowly to treatments. iii).
However, in light of global climate change and the current discussion on carbon sequestration, there will be an increasing demand for information on the effects of changes in primary production in boreal and tropical forests. Tropical forests in particular need more attention as many lowland tropical forest soils lack a thick organic horizon and are generally. poor in nutrients; Litter may therefore play a relatively more important role in carbon and nutrient cycling in the tropics. Octagons indicate variables influenced by leaf litter as a protective layer; ovals indicate variables influenced by leaf litter that form part of the nutrient and carbon cycle; rectangles indicate variables influenced by leaf litter either through its protective function or through its role in carbon and nutrient cycling; SOM is soil organic matter.
Acknowledgements
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Effects of litter from a tropical rainforest on tree seed germination and establishment under controlled conditions. Species effects on earthworm density in tropical tree plantations in Hawaii. Biology and Fertility of Soils15, 35-38.
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