does litter from invasive plant species differ from native species in quality and decomposition rate?
Our study revealed only small differences between litter quality of native and invasive species. The nitrogen and phosphorus concentrations were significantly higher in the litter of the invasive species, but differences in mean SLA and decomposition rates were non-significant. The directions of the differences in all traits, however, were the same as those reported from Hawaii (Allison & Vitousek 2004; Rothstein et al. 2004; Hughes & Denslow 2005) and many other regions of the world (Ehrenfeld 2003).
Comparisons made within ecological groups revealed strong differences between natives and invasives among pioneer and canopy species, but not among understorey species. Because we used the most common species in our ecosystem, which belonged to several plant families, we cannot exclude the possibility that these comparisons may have a phylogenetic bias. However, the differences for pioneer species fit well with those reported elsewhere for native and invasive species in disturbed and nutrient-rich environments (e.g. Daehler 2003). It may be expected that fast growing, invasive pioneer species would profit most from release from natural enemies, which could explain why despite their particular leaf properties they are also successful in these stressed ecosystems. However, other studies have shown that the differences of herbivore damage sustained by native and invasive pioneer species in the Seychelles were rather small (Dietz, Wirth & Buschmann 2004; Kueffer 2006).
The canopy species showed the biggest differences in litter characteristics between native and invasive trees, with the native canopy species –N. hornei in particular – having very low nutrient concentrations and SLA, and very high polyphenol concentrations. These properties have also been found for endemic canopy species on other oceanic islands (Vitousek 2004); however, it is not certain whether dominant canopy species on oceanic islands differ generally from continental species and, if so, whether such differences are due to geographic isolation or to the very nutrient-poor soils on many islands.
One important conclusion, therefore, is that the tree species invasive in the nutrient-poor soils of upland forests, although few in number, are by no means a uniform group. Some of them possess traits usually associated with rapid resource acquisition and growth, while others resemble many native species in possessing litter traits characteristic of stress tolerant plants adapted to very resource-poor conditions (compare Aerts & Chapin 2000).
In the case of the litter decomposition rates, the small differences between species may also be related to the site quality, which may explain why higher nutrient concentrations among invasive species did not generally translate into higher decomposition rates. At very infertile sites, initial rates of decomposition are often determined more by the carbon quality and polyphenol concentration of litter than by its nutrient concentrations (Hobbie 2000; Bridgham & Richardson 2003). In our study, the rate of litter decomposition was significantly correlated with several parameters, including SLA, tannin concentration, nitrogen concentration and C : N ratio, but none of these could be identified as the single most important factor. However, two other results suggest that the N concentration was of minor importance, and that C quality was the major factor limiting decomposition. First, the litter of F. moluccana and C. verum released nutrients during initial decomposition, despite having high C : N and C : P ratios compared to the values cited as the critical ratios for immobilization (cf. Bridgham & Richardson 2003). Second, we carried out an ancillary laboratory experiment where litter of C. verum and N. hornei was allowed to decompose in soils sampled at the 15 field plots, either with or without fertilisation (N, P or complete fertilizer) (Kueffer 2006). None of the fertilizer treatments influenced decomposition rates. This could not be due to high nutrient availability in soil (e.g. due to the disturbance associated with sampling, mixing and filling the pots) because cotton pieces incubated in the same way decomposed faster with complete fertilizer.
Many other studies have reported striking differences in litter traits between invasive and native tree species. However, most of these compared pioneer invasive species with late-successional (understorey or canopy) native species (e.g. Allison & Vitousek 2004; Rothstein et al. 2004; Hughes & Uowolo 2006), making it impossible to distinguish between effects due to provenance (i.e. whether native or invasive) and those due to ecological group (e.g. early vs. late-successional). In contrast, in another study where invasive and native species were compared within ecological groups (vine, shrub, tree) decomposition rates did not generally differ (Ashton et al. 2005). These results confirm the importance of controlling for ecological group as well as phylogeny when comparing native and invasive species.
is the soil under invasive plants more fertile?
An important assumption of our study is that any differences in soil conditions under the three tree species were caused by the trees themselves. Although we cannot exclude the possibility that pre-existing site differences determined where each trees species grew, we think this is unlikely because all trees are found under a wide variety of environmental conditions (Fleischmann 1997; Kueffer & Vos 2004) and they grow closely interspersed in apparently uniform areas.
Our study revealed three main differences in soil nutrient availability. First, P concentrations (total and ion-resin extractable) were higher in CIN than in NOR stands while C : N and C : P ratios were lower. Second, N was mainly mineralized as nitrate in FAL soils but as ammonium in CIN. While nitrification increases the availability of N for some plants it can also lead to higher leaching losses (Bardgett 2005). Third, P availability was lower in FAL than in CIN stands. The lower soil pH of the FAL stands might have contributed to these differences by enhancing P fixation to Fe and Al hydroxides (Bardgett 2005).
Despite these significant effects, the general weakness of the impacts of C. verum and F. moluccana on nutrient availability was unexpected. In particular, the enhancement of soil N availability by the F. moluccana was negligible compared to the 17- to 121-fold increases in N availability under F. moluccana on young volcanic flows in Hawaii (Hughes & Denslow 2005). Similarly, the increase in P availability in CIN stands was much smaller than might have been expected from the data on litter properties. We can think of two main reasons why nutrient conditions in the three stand types were not more different.
First, N-fixation of F. moluccana may be restricted by phosphorus deficiency (compare e.g. Vitousek et al. 2002). Although we did not measure N-fixation, we observed nodules containing the characteristic pink colour of leghaemoglobin on the roots of both trees and saplings of F. moluccana. However, the measures of soil P availability indicate that P levels were indeed very low below F. moluccana stands. The generally low levels of phosphorus in upland soils may also explain why several N-fixing alien species, some of them invasive in the lowlands (e.g. Leucaena leucocephala) and others used in reforestation schemes (e.g. Albizia lebbeck or Gliricidia sepium), have been unable to invade established upland forest stands (Kueffer & Vos 2004).
Second, the stands we studied were small, so that ‘edge effects’ may have played a role. For example, the litter of F. moluccana probably also contributes significantly to nutrient input through litterfall in the other stands. We measured litterfall under the different stands over a period of several months and estimated that c. 20% of the litterfall was from species other than the one dominating the stand. A patchy distribution may also have allowed a regular exchange of the soil fauna between the stands with fast- and slow-decomposing litters, and this may have reduced stand-level effects on litter decomposition like those found in invasions by F. moluccana over large areas in Hawaii (Hughes & Uowolo 2006). The presence of patches of early-successional invasive species (and of clumps of native species such as N. hornei) in a matrix of secondary forests dominated by late-successional invasive species such as C. verum is the common invasion pattern in the Seychelles. More generally, secondary forests containing mixtures of both alien and native species may be typical of long-term secondary successions on oceanic islands, while the mono-specific alien forests often studied (e.g. Hughes & Denslow 2005) may be a transient early-sucessional stage (compare Lugo 2004).
Future research is needed to clarify other possible explanatory factors specific to very nutrient-poor forests. We suggest that the few species that do invade these forests possess particular adaptations promoting the efficient uptake or use of nutrients (reviewed in Kueffer 2006), and that these reduce nutrient cycling rates or soil nutrient availability despite nutrient-rich litter. For instance, C. verum forms a very dense root mat in the topsoil, making it an aggressive competitor for below-ground resources (Kueffer et al. 2007). Similarly, F. moluccana is known to be very efficient in taking up P and can therefore reduce soil P availability when grown in plantations (Binkley & Ryan 1998). And besides P limitation, co-limitation by micronutrients such as potassium could also have played a role, though little is known about the impact of invasive plants on soil nutrients other than nitrogen and phosphorus (Ehrenfeld 2003). An indication that K availability is very low in the upland forests of the Seychelles is provided by data on litter K concentrations. In the case of N. hornei (0·26 mg g−1), this value was about half the lowest concentration measured in litter of an ecologically similar tree, Metrosideros polymorpha, in Hawaii (0·51 mg g−1) (Hobbie 2000). Such a low level of availability could have an impact upon invasive species with a higher K requirement; indeed, litter K concentration was much higher in the invasive species we studied (C. verum: 2·20 mg g−1; F. moluccana: 1·68 mg g−1) than in the native species (N. hornei: 0·26 mg g−1, M. eleagni: 0·68 mg g−1).
In conclusion, our study indicates that in secondary forests on very nutrient-poor soils invasive plants may have only limited impact upon nutrient dynamics or impacts that are complex and difficult to predict. Many of the most problematic invasive species possess growth traits similar to those of native species, and P limitation may moderate impacts of any differences that do exist between native and invasive species. Future research is needed to clarify the role of these and possibly other factors, including co-limitation by potassium or efficient nutrient-uptake by invasive species such as C. verum or F. moluccana. We expect that the patterns observed in this study are more generally relevant for ecosystems on very nutrient-poor soils.