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FIGURE 7.4 Cumulative DOC release from hemlock (*), cedar (O) and alder (A) litter. Data shown are means, and error bars represent one standard deviation. (Reproduced from McArthur and Richardson 2002.)

species of leaves in the terrestrial litter of a New England forest (Melillo et al. 1982) and of wood chips from five species of trees in streams in eastern Canada (Melillo et al. 1983). Leaf chemistry explained differences in decomposition of Croton gossypifolius (Euphorbiaceae) and a species of Clidemia (Melastomataceae) in an Andean stream (Mathuriau and Chauvet 2002). Higher decomposition rates in Croton than in Clidemia appeared to be related to lower tannin and higher N content in Croton, which resulted in earlier peaks in ergosterol (a compound found primarily in fungi), sporulation activity, and macroinvertebrate colonization. Lastly, chemical inhibitors impede leaf decay in several ways. Tough outer coatings such as the cuticle of conifer needles slow fungal invasion (Barlocher et al. 1978), and complexing of protein to tannins is a principal cause of slow breakdown in many broadleafed woody plants. Toxic effects of chemical constituents also may influence breakdown rates (Webster and Benfield 1986), just as secondary plant compounds defend against terrestrial herbivores, although evidence is scant. Somewhat surprisingly, chemical measures of tannins (total phenolics and condensed tannins) of 48 deciduous trees were unrelated to published breakdown rates (Ostrofsky 1993). However, Canhoto et al. (2002) found that oils of Eucalyptus globulus inhibited the growth of hyphomycetes and the activity of their enzymes, which could explain the delayed decomposition of eucalyptus in rivers.

As leaf processing advances, microbial populations colonize and proliferate on the leaf substrate. Microbial colonization plays an important role in altering the palatability of leaves for det-ritivores (Arsuffi and Suberkropp 1984) and in the fragmentation of leaf material. This colonization is primarily by fungi and bacteria, although protists also can be important (Ribblett et al. 2005). In addition to the conversion of leaf tissue into more edible material for shredders and the production of FPOM, microbial activity softens plant tissue, favoring the release of compounds that can be incorporated into microbial biomass (Gessner et al. 1999, Graca 2001). Fungi can degrade the polysaccharides present in the cell walls of the leaves by the production of extracellular enzymes (Jenkins and Suberk-ropp 1995), and their hyphae also penetrate leaf tissue, facilitating the softening process (Wright and Covich 2005).

Suberkropp and Klug (1976) followed in detail the succession of dominant microorganisms on oak and hickory in a Michigan stream from November until June. Fungi, primarily aquatic hyphomycetes, dominated during the first half (12-18 weeks) of the processing period. Bacteria, whose numbers gradually increased throughout, dominated the terminal processing stage and perhaps were benefited by fungal-induced changes in leaf surface area or by the release of labile compounds. In a stream in the Black Forest of Germany, fungal and bacterial biomass both increased during the first 4 weeks (Figure 7.5) and then remained constant for the next 4 weeks of the study (Hieber and Gessner 2002). Fungal colonization of leaves takes place primarily in the water, because freshly abscised leaves exhibit low fungal biomass (measured as ergosterol content) before entering the stream (Gessner and Chauvet 1997, Hieber and Gessner 2002). Propagules of soil fungi, although commonly carried into the stream on shed leaves, appeared to contribute little to decomposition (Suberkropp and Klug 1976). Bärlocher (1982) reported that typically 4-8 species of aquatic fungi dominate throughout the decomposition of leaves, while a similar or larger number of rare species appear erratically. Apparently no particular succession occurs on a single leaf; whichever fungal species arrives first as a waterborne spore establishes numerical dominance. Hieber and Gessner (2002) identified 30 species of hyphomycetes on decomposing leaves of alder and willow, but two species, Fla-gellospora curvula and Tetrachaetum elegans, were dominant. Bacteria from biofilms growing on decomposing leaves in a stream in Ohio were

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