Litter Quality

The litters collected in Fallopia and native vegetation stands were chemically very different (table 1). Senescent Fallopia leaves and stems had extremely low N concentrations (0.63 and 0.30 % respectively), and accordingly high C/N ratios (72 and 151 respectively) while the indigenous litter mixture had a low C/N ratio (33) due to a high N concentration (1.38 %). The Fallopia litter C/N ratio is exceptionally high compared to most deciduous broadleaves species and even conifer tree species. For instance, litter of Fagus sylvatica and Picea abies, which are known to have very slow decomposing litter, have a C/N ratio of 56 and 49 respectively (Hobbie et al., 2006), which is still lower than that of Fallopia organs. On the other hand, the C/N ratio of the indigenous litter mixture is lower than that of Tilia cordata (37), which is known to have a particularly rapid decomposing litter (Hobbie et al., 2006). The C/N ratio found in the native litter mixture is comparable to that of typical temperate grassland litter. Whitehead (1995) found that C/N ratio in dead grass herbage varies between 19 in N fertilized grassland to 44 in grassland with no or moderate N fertilization. The lignin concentration is also a good predictor of litter decay rate (Funk et al., 2005). In Fallopia organs, it was higher compared to indigenous leaves (14.5 % in Fallopia leaves and 18.8 % in stems vs. 12.1 % in indigenous mixture, table 1). Compared to indigenous herbaceous species, Fallopia has thus a low quality litter. For instance, Poa pratensis has only 5.7 % lignin in its senescent leaves and Bromus tectorum, which is known to decrease litter decomposition rate in the Great plains of North America has 7.4 % lignin in its senescent leaves (Ogle et al., 2003).

Table 1. Chemical composition of the three litter types

Fallopm leaves 45.7 ± 0.2 0.63 ± 0.01 72.5 ± 1.2 14.53 ± 0.04 23.1 ± 0.4 Fallopia stems 45.4 ± 0.3 0.30 ± 0.01 151.3 ± 5.1 18.82 ± 0.05 62.7 ± 2.1

Indigenous mixture 45.5 ± 0.2 1.38 ± 0.02 33.0 ± 0.5 12.07 ± 0.03 8.7 ± 0.1

Mean ± standard deviation (N=2 analytical repetitions for C, N, lignin and cellulose and 3 for Ca, Mg,

According to its lower quality, Fallopia litter decomposed more slowly than indigenous litter (figure 4). After one year, the indigenous litter lost circa 90 % of its initial mass while Fallopia leaves and stems lost only 39 and 55 % of their initial mass, respectively. Moreover, contrarily to Fallopia, indigenous litter was more rapidly attacked by decomposers, the structure of the litter being hardly recognizable after only a few weeks. Some invasive species (mostly grass species) have been shown to also have a recalcitrant litter with a slow decomposition rate (Ehrenfeld et al., 2001, Ogle et al., 2002; Drenovsky & Batten, 2007). For instance, Aegilops triuncalis litter have a higher lignin/N ratio and decomposed much more slowly than the litter of the native grassland species (Drenovsky & Batten, 2007). However, many other studies reported (Ehrenfeld et al., 2001; Allison & Vitousek, 2004; Rothstein et al 2004; Standish et al., 2004; Ashton et al., 2005; Funk, 2005; Hughes and Uowolo, 2006...) higher litter quality (higher N concentration, lower C/N ratio, lower lignin/N ratio) and faster decomposition rates for invasive compared to native species.

Time (days)

Figure 4. Decomposition of Fallopia leaves (triangles) and stems (squares) and indigenous litter (diamonds) during one year (between October 2006 and October 2007). All litter types were incubated in invaded (black) and uninvaded (white) environments; Decomposition is expressed as the percentage of initial mass loss Values are means ± standard deviation.

Time (days)

Figure 4. Decomposition of Fallopia leaves (triangles) and stems (squares) and indigenous litter (diamonds) during one year (between October 2006 and October 2007). All litter types were incubated in invaded (black) and uninvaded (white) environments; Decomposition is expressed as the percentage of initial mass loss Values are means ± standard deviation.

The slow decomposition rate of Fallopia compared to indigenous mixture may be accounted for by its much lower quality (higher C/N and lignin/N ratios). Indigenous litter could also have benefited for a "mixture effect". Indeed, in their review on litter mixture decomposition, Gartner and Cardon (2004) found that, most of the time (67 % cases) mixtures decompose faster than expected with additive decomposition rate of the components of the mixture. The synergistic effect of mixture can be explained by nutrients transfer between nutrient rich and nutrient poor components. For instance, in a mixture experiment, the litter of the N fixing Vicia lathyroides helped the decomposition of the less decomposable (N poorer) Calamagrostis epigejos (Hoorens et al., 2002). A similar mechanism could explain the high decomposition rate found in our native mixture. The relatively N rich Eupatorium accelerated the decomposition of Calamagrostis.

The C/N ratio of indigenous litter did not vary during decomposition and the N release followed the decomposition curve (figure 5) indicating that N is not limiting for the decomposers community development. On the other hand, N concentration in Fallopia litter increased from 0.6 % at the beginning of the experiment to 1.4 % after one year. This increased N concentration was not due to a corresponding C loss. Rather, it is most likely the result of net N fixation by the decomposers community (figure 5). Gallardo and Merino (1992) showed that litter N concentration increased during decomposition until litter C/N ratio was equal to that of the decomposers. Lignin is, in this case, a sink for N, forming the precursors of humus.

Time (days)

Figure 5. Evolution of the N stock (=remaining mass x N concentration) in Fallopia leaves (triangles) and stems (squares) and indigenous litter (diamonds during decomposition in invaded (black) and uninvaded (white) environment. Values are means ± standard deviation.

Time (days)

Figure 5. Evolution of the N stock (=remaining mass x N concentration) in Fallopia leaves (triangles) and stems (squares) and indigenous litter (diamonds during decomposition in invaded (black) and uninvaded (white) environment. Values are means ± standard deviation.

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