Original articleEcosystem and decomposer effects on litter dynamics along an old field to old-growth forest successional gradient
Introduction
Identifying the mechanisms controlling decomposition is key to understanding ecosystem function. Controls on litter decomposition include abiotic and biotic factors such as temperature, moisture, and pH (Coûteaux et al., 1995, Murphy et al., 1998), litter quality (Cornelissen, 1996, Aerts, 1997), soil nutrient availability (Hobbie and Vitousek, 2000), and the abundance and diversity of the decomposer community (Seastedt, 1984, Blair et al., 1990). For example, litter quality (e.g. C:N and lignin content) varies among ecosystems depending upon soil and nutrient availability as well as plant community composition which, in turn, is dictated by successional stage (Fisk et al., 2002). Detritivores respond to litter quality by selectively consuming litter with nutrient concentrations (e.g. C:N) that satisfy their stoichiometric requirements (Hessen et al., 2004). Likewise, plant community composition may strongly but indirectly affect decomposition by influencing the macro-detritivore community structure and/or feeding patterns (Mayer et al., 2005). Conversely, litter biomass may affect plant production and community composition by serving as a nutrient pool or by inhibiting seedling growth (Foster and Gross, 1998, Mack and D'Antonio, 2003). In addition, variation in ecosystem structure due to successional processes is likely to alter the importance and interaction of these mechanisms (Köchy and Wilson, 1997, Shaw and Harte, 2001, Herman et al., 2003). Ecosystem type is known to influence decomposition via microclimate effects (Coûteaux et al., 1995, Köchy and Wilson, 1997, Shaw and Harte, 2001), litter production (Cebrian, 1999, Cebrian and Lartigue, 2004), and decomposer diversity (Seastedt, 1984, Mikola and Setälä, 1998, Hättenschwiler et al., 2005).
To distinguish among these factors and feedbacks, I studied litter dynamics along a successional gradient, measuring the interactive effects of ecosystem type (old field, transition forest, old-growth forest), litter type (herbaceous, leaf, wood) and decomposers (open vs restricted access litter bags) on litter mass loss and N flux. My objective was to quantify the effects of and the interactions among ecosystem type, litter type, and decomposers.
Based on a previous study at this site showing that macro-detritivores affect herbaceous litter decomposition in old fields (Mayer et al., 2005), I hypothesized that macro-detritivores would accelerate mass loss and N flux of all litter types in all ecosystem types (Hättenschwiler et al., 1999, González and Seastedt, 2001), by preferentially consuming litter with low C:N (Melillo et al., 1982) and litter collected in situ (Hunt et al., 1988). I expected that litter dynamics would reflect the gradient of ecosystem succession (Fisk et al., 2002, Xuluc-Tolosa et al., 2003, Vasconcelos and Laurance, 2005), and hypothesized that mass loss and N flux would increase with successional stage (i.e. rates in old field < transition forest < old-growth forest). Finally, I expected litter dynamics to be influenced by strong interactions among ecosystem type, litter type, and decomposers, but that the strength of these interactions would vary by successional stage (Wardle et al., 2004a, Wardle et al., 2004b) due to microclimate effects (i.e. temperature and moisture). Thus, I hypothesized that mass loss and N flux rates would be greatest in the warmest, wettest successional stage, especially where macro-detritivores were present (González and Seastedt, 2001).
Section snippets
Study site and methods
This study was conducted near Ada, Oklahoma, USA, at the Center for Subsurface Ecological and Assessment Research (CSEAR) site, in the cross timbers, a mosaic of mixed grasslands and oak-dominated forest (Hoagland et al., 1999). CSEAR encompasses ca. 45 ha covering a successional gradient of old fields on which cultivation was abandoned in about 1950, second-growth forest in various stages of succession, and old-growth fragments of oak (Quercus), hickory (Carya), and elm (Ulmus). Cattle grazed
Results
All main treatments (ecosystem type, litter type, and mesh size) significantly influenced litter dynamics prior to accounting for covariates (Table 1, Table 2). No interactions among main factors were evident.
Ecosystem effects
As hypothesized, litter mass loss and N accumulation were fastest in old-growth forests at CSEAR, which is consistent with other studies of the influence of successional stage and ecosystem type on decomposition (Hunt et al., 1988, Fisk et al., 2002, Xuluc-Tolosa et al., 2003). The effect of ecosystem at CSEAR was strongly related to microclimate. Soil temperatures in old fields, which exceeded 35 °C in summer at CSEAR, may have been inhospitable to many decomposers while cooler, wetter
Conclusions
To date, few studies have simultaneously examined the effects of litter type, successional stage, and decomposer type on decomposition dynamics. At CSEAR, ecosystem type plays an important role in dictating decomposition dynamics through the interactive effects of the decomposer community, litter production, and microclimate, emphasizing that old-growth forests function differently than old fields or immature, transitional forests.
Decomposition dynamics were similar in old fields and immature
Acknowledgements
I thank S. Holub, G. Silva, J. Spears, and two anonymous reviewers for constructive comments on early manuscript drafts. K. Burgess, J. Jones, P. Jones, W. Kirby, M. McCutchen, R. Neill, P. Nunn, J. Sheffield, and D. Walters provided field and lab assistance. M. Kennedy digitized aerial photos and calculated forest cover. Research funding was provided by US Environmental Protection Agency (EPA), Office of Research and Development and through the EPA, National Risk Management Research Lab,
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