Plant invasions may alter the soil system by changing litter quality and quantity, thereby affecting soil community and ecosystem processes. We investigated the effect of
Saltcedar (
Saltcedar invasions are associated with several negative effects that can alter species composition and ecosystems processes. Several impacts have been attributed to saltcedar, such as displacement of native species [
Plant communities and the belowground ecosystem are inextricably linked together via carbon input and nutrient flow [
Litter decomposition is mainly controlled by climate, litter quality, and decomposer organisms [
Much of the previous research on effect of saltcedar invasion has focused on water use relative to native plants species, displacement of native vegetation, and the use of saltcedar by native fauna. However, very little attention has been paid to the impacts of saltcedar on the ecosystem processes and the decomposer community. Ashton et al. [
The study sites were located in Las Perdices, central-southern Córdoba, Argentina (32°36′47′′S and 63°39′18′′W) (Figure
Location of the study area in Las Perdices, Córdoba, Argentina.
In this area, we selected two patches of each forest type which we considered replicates: monospecific saltcedar patches (1.6 and 2.1 ha) and native forest patches (1.2 and 2.8 ha). The four sites have the same soil series (according to Soil Taxonomy classifications) and have similar geomorphological characteristics. Slope (1–3%) and elevation (approximately 250 m a.s.l.) were also very similar in the four sites. The distance between the two saltcedar patches was 250 m, and the distance between the two native forest patches was 860 m. Soil organic matter content and soil moisture were higher in the native forest than in the saltcedar forest (organic matter: 7.9% and 0.9%; moisture: 31.5% and 16.5%, resp.) whereas pH values were very similar between saltcedar and native forest (7.89 and 7.83) [
Sampling sites were carefully selected to avoid potentially spurious differences among sites other than invasion impact. Until approximately 1960 the four plots had the same land-use history, because they were natural forests [
We examined decomposition of leaf litter by the litterbag method [
Litterbags were placed in November 2007 and harvested in May 2008, to ensure that they were in the field during the season of highest activity of decomposer fauna in the region [
To account for the possible differences in mesofaunal abundances between litter types (saltcedar and native) and for each litter type between forests, a generalized linear mixed model (GLMM) was performed and Akaike’s information criterion was used to determine the best predictive model. According to the distribution of abundance data, Poisson error distribution and log link function were used. In the selected model, the abundance of fauna (Collembola, Acari, or Mesofauna) was the dependent variables; forest (saltcedar or native) and litter type (local and control) were the fixed factors, whereas the site (replicate of the forest) and the litterbags (replicate in each site) were the random factors. A posteriori tests were performed by the DGC test [
Besides,
Mites (Acari) were the most abundant members of the invertebrate decomposer community in all litter samples, comprising 62% of total microinvertebrates in local litter and 69% in alfalfa (control) litter in the native forest and 64% and 65% of the local and alfalfa litter, respectively, in the saltcedar forest.
Generalized linear mixed model (GLMM) showed that abundances of all decomposer faunal groups were different between forests (Table
Generalized linear mixed model showing the overall effects of forest type and litter type on the abundance of Collembola, Acari, and total mesofauna.
Collembola | Acari | Mesofauna | |||||||
---|---|---|---|---|---|---|---|---|---|
d.f. | Chi-square |
|
d.f. | Chi-square |
|
d.f. | Chi-square |
|
|
Forest | 1 | 1375.15 |
|
1 | 1157.55 |
|
1 | 2298.57 |
|
Litter | 1 | 3.30 | 0.069 | 1 | 143.72 |
|
1 | 132.71 |
|
Forest * litter | 1 | 36.31 |
|
1 | 0.96 | 0.326 | 1 | 5.99 |
|
Abundance of Collembola in litterbags of local and control litter in the native (NF) and saltcedar (SF, white bars) forests. Error bars represent SD. Bars with the same letter are not statistically different.
Abundance of Acari in litterbags of local and control litter in the native (NF) and saltcedar (SF) forests. Error bars represent SD. Bars with the same letter are not statistically different.
Abundance of total mesofauna in litterbags of local and control litter in the native (NF) and saltcedar (SF) forests. Error bars represent SD. Bars with the same letter are not statistically different.
Trophic structure of decomposer fauna in native (NF) and saltcedar (SF) forests for the local and control litter.
Decomposition of local litter was not significantly different between saltcedar and native forests, whereas alfalfa decomposition was significantly higher in the native forest than in the saltcedar forest (Table
Local and control litter decomposition within native and saltcedar forests (grams of initial mass remaining at the end of the experiment, with standard deviation given in brackets). Different letters indicate significant differences (
Forest | Litter | Remaining litter (g) | |
---|---|---|---|
Native | Local | 3.12 (0.48) | a |
Control | 3.67 (0.32) | a | |
Saltcedar | Local | 3.05 (0.34) | a |
Control | 4.37 (0.67) | b |
The analysis of litter chemical composition revealed differences in the quality of the litter of the native and the invaded forests. In the native forest, three of the dominant species (
We investigated the effects of saltcedar invasion on litter decomposition process in four sites with the same soil type and land-use history until approximately 45 years before sampling; therefore, we assume that the principal differences between sites are attributable mainly to saltcedar invasion.
The invasion of saltcedar produced an increase in abundance of Collembola, Acari, and total mesofauna, irrespective of litter type. The increase in collembolan abundance could be attributed to the higher quality of saltcedar litter than that of the native forest litter, which fosters fungal and microbial biomass growth [
We consider that the higher densities of mites in the invaded forest than in the native forest, in both litters, would be due to the absence of competition. According to the intermediate disturbance hypothesis [
The relative proportion of trophic groups was not markedly affected by saltcedar invasion. In this forest, microbe-detritivores showed a slight increase in their relative proportions in both litter types. This shows that trophic group structure is relatively stable and is not strongly altered by saltcedar invasion or by the introduction of a new resource of higher quality than that present in the native forest. This pattern was also observed in grass plots where detritivore numbers showed little difference between invaded and uninvaded communities, despite plant litter changes induced by invasion [
Saltcedar invasion is expected to affect plant litter decomposition because it can influence decomposer community and litter quality, the two factors that determine litter decomposition rates under a given climatic condition. In the present study, to test the influence of soil fauna on decomposition, we placed the same litter type (control litter) in both forests to exclude the effect of litter quality; we found an inverse relationship between abundance of mites, collembolans, and total mesofauna and decomposition. Decomposition was lower in the saltcedar forest, where all taxa showed the highest abundances, than in the native forest. This shows that the abundance of decomposer organisms
Decomposition of the local litter was similar in native and saltcedar forests, showing that both ecosystems have a decomposer fauna adapted to efficiently decompose the autochthonous litter. However, we observed that saltcedar litter had lower lignin and tannin content and higher cellulose content than the litter of the five dominant native species. The relatively higher litter quality of saltcedar in comparison with the native species is in agreement with previous studies [
It has been suggested that invasive species often maintain higher concentrations of leaf nitrogen [
The greatest differences in decomposition were reported to occur between invaded and uninvaded sites rather than between native and exotic species [
In the invaded forest, the decomposition rate of the control litter was much lower than that of the saltcedar litter, which highlights the high specialization of the decomposer fauna in the use of saltcedar litter and its low capacity to adapt to a new resource, even to a high quality one. In the native forest, control litter also decomposed at a slower rate than native litter (without significant differences) but the rate was higher than that of the control in the invaded forest. The introduction of a resource of higher quality than the local one would have a negative effect on decomposition in both ecosystems, but much pronounced in the invaded forest. This finding also supports the conclusion that decomposer fauna would be an important driver of litter decomposition in forested ecosystems, together with litter quality and heterogeneity.
Considering the low efficiency of the soil fauna to decompose a new resource observed in the invaded forest, in a scenario of native forest restoration, in which new species are incorporated, nutrient cycling could be affected, impoverishing the soil system and ultimately reducing nutrient availability. Accordingly, the question arises as to how much time is needed to allow the restoration of the native mesofaunal community and ecosystem functioning. It is therefore important to develop strategies that allow the restoration of the whole ecosystem, considering not only plants but also soil fauna and ecosystems processes.
Conservation managers are increasingly unable to remove all nonnative species from ecosystems and, indeed, such species have become important components of many systems, providing habitat or resources for other species [
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by Secretaría de Ciencia y Técnica of the National University of Río Cuarto, Agencia Nacional de Promoción Científica y Técnológica (PICTb-N°2175), and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). J. Brasca corrected the English style.