Review and synthesisA meta-analysis on the effects of changes in the composition of native forests on litter decomposition in streams
Introduction
In small forest streams, autochthonous primary production is limited by low solar irradiation and the decomposition of litter produced by the terrestrial vegetation is a fundamental ecosystem process that fuels aquatic food webs with carbon (C), nutrients and energy (Vannote et al., 1980, Wallace et al., 1997). The decomposition of organic matter is carried out mainly by microbes (mostly aquatic hyphomycetes) and invertebrate detritivores (Hieber and Gessner, 2002, Pascoal and Cássio, 2004) and is highly sensitive to changes in environmental conditions (Gessner and Chauvet, 2002, Young et al., 2008). Given the heterotrophic nature and high aquatic–terrestrial interface of these small streams, aquatic communities and processes are highly vulnerable to changes in the surrounding vegetation.
Forest clearance (e.g. conversion into pasture or agriculture) has strong effects on aquatic communities and ecosystem functioning as the system shifts from heterotrophic to autotrophic in response to the decrease in litter inputs and increase in solar irradiation (Hladyz et al., 2011a). The replacement of native forests by commercial tree plantations, or their invasion by exotic woody species, can also affect aquatic communities and processes despite the fact that the heterotrophic nature of the streams is generally maintained, i.e. shade continues to be provided and foodwebs are based on the decomposition of allochthonous litter (Graça et al., 2002, Kominoski et al., 2013). However, the studies addressing the effects of the replacement of native forests by tree plantations or their invasion by exotic woody species on litter decomposition often show contrasting results. For instance, previous studies reported stimulation (Whiles and Wallace, 1997, Lecerf et al., 2007, Menéndez et al., 2013), inhibition (Abelho and Graça, 1996, Kennedy and Hobbie, 2004, Lecerf and Chauvet, 2008), or no major difference (Bärlocher and Graça, 2002, Lecerf et al., 2005, Riipinen et al., 2010, Hisabae et al., 2011) in litter decomposition rates in streams flowing through altered forests when compared with streams flowing through native forests.
The variability in the reported results suggests that the effects of forest change could be moderated by other factors. For instance, the replacement of native forests by tree plantations with very dissimilar traits (e.g. deciduous vs. evergreen species) is likely to produce stronger effects on streams than the replacement by trees with similar traits (e.g. mixed deciduous broadleaf forest vs. deciduous broadleaf tree plantations) or moderate invasion by exotic species (Kominoski et al., 2013). In addition, stronger effects might be expected when forest changes occur both at the catchment and riparian level than only in the riparian area or at the catchment level. The effects may also differ depending on whether they are driven by changes in litter inputs (stronger when forest change occurs at the riparian level) or by changes in hydrology and water chemistry (stronger when forest change occurs at the catchment level).
The effects of forest change may also depend on physical and chemical litter properties; the decomposition of soft, high nutrient (low C:nutrient ratio) litter is likely to respond to forest change if this affects detritivores, while the decomposition of tough, low nutrient litter is likely to be less sensitive to forest change since this is mostly carried out by microbial decomposers, whose communities are reported to be functionally redundant (Dang et al., 2005, Ferreira et al., 2006, Gulis et al., 2006). High quality litter may also decompose faster in streams flowing through tree plantations if it acts as an island of good quality resource in a streambed of recalcitrant litter (e.g. Alnus glutinosa litter in streams flowing through eucalyptus or conifer plantations); when given a choice, detritivores prefer soft, high quality litter (Canhoto and Graça, 1995).
Total litter decomposition (i.e. carried out by both microbes and invertebrates) is likely to be more sensitive to forest change than microbial-driven litter decomposition if invertebrates are the main players on litter decomposition (Gulis et al., 2006). Also, since invertebrate activity on decomposing litter highly depends on microbial conditioning (Canhoto and Graça, 2008), any negative effects of forest change on microbes may be amplified by invertebrates.
The region where forest change takes place can also moderate the magnitude of its effect on litter decomposition. Streams in drier regions such as in the Mediterranean may dry out during the warmer months and it has been shown that these streams have lower invertebrate richness than permanent streams (Datry et al., 2011). Intermittent streams may thus be more susceptible to further environmental changes as forest change than streams in humid regions (Ferreira et al., 2006).
Changes in dissolved nutrient availability could also moderate the effect of forest change on litter decomposition (Molinero et al., 1996, Díez et al., 2002). If the replacement of native forests by plantations with poorer litter quality is accompanied by an increase in dissolved nutrient availability due to fertilization or atmospheric nitrogen deposition, microbial conditioning of submerged litter might be stimulated and differences in litter decomposition rates between native and altered streams might be limited.
Here we present a systematic review of 24 studies addressing the effect of forest change on litter decomposition in streams and published between 1993 and 2015. Using meta-analysis, we (i) determine the magnitude and direction of the mean effect of forest change on litter decomposition, (ii) assess which characteristics of the incubation conditions influence the magnitude of the effects and (iii) identify research gaps that could guide future studies on the effect of forest change on stream functioning.
Section snippets
Literature search and selection of relevant primary studies
We searched for primary studies that addressed the effect of forest change on litter decomposition in streams published between January 1970 and September 2015 in English, French, Portuguese and Spanish. The search was done using Google Scholar, personal literature databases and reference lists in primary studies and in review papers. The following combinations of search terms and their equivalents in French, Portuguese and Spanish were used in Google Scholar: decomposition or processing or
Overview of studies
The earliest study that satisfied all our inclusion criteria was published in 1993 (Pozo, 1993). Since then, studies comparing litter decomposition in streams flowing through native and altered forests have been accumulating at a rate of ca. 1 per year (1993–2015); however, in the period 2010–2015, this rate has increased to 1.8 studies per year.
The most commonly studied type of forest change was the replacement of native forests by eucalyptus plantations (42% of studies; 59% of cases) followed
Overall effect of forest change on litter decomposition in streams
Our systematic review of the effect of the replacement of native forests by tree plantations or invasion by exotic woody species on litter decomposition in streams showed that forest change significantly inhibited litter decomposition by 18%. Our results are robust to publication bias as indicated by the Rosenberg fail safe numbers. However, they might be conservative as the ‘trim and fill’ method suggests an even stronger negative effect of forest change on liter decomposition (−29%). Although
Conclusion
We found that there is an overall inhibition of litter decomposition with forest change and that the significance of this effect depends on the type of forest change, extent of change, identity of the decomposing litter and type of decomposer community. Thus, predictions of the effects of forest change on stream functioning need to take into account multiple factors besides the identity of the species in plantations.
The significant inhibition of litter decomposition when eucalyptus plantations
Acknowledgements
We thank E. Chauvet, S. Hladyz, M. Inoue, T. Kennedy, I. Laćan, A. Lecerf, A. Martínez and M. Menéndez for providing information that was not available in the primary studies. We also thank two anonymous reviewers for their comments on an early version of the manuscript. This study was supported by the Portuguese Foundation for Science and Technology (FCT) through the strategic project UID/MAR/04292/2013 granted to MARE. Financial support by the FCT to V. Ferreira (SFRH/BPD/76482/2011, program
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2022, Forest EcosystemsCitation Excerpt :However, this variation was not a significant factor in the litter decomposition rate. Especially the replacement of natural forests with Eucalyptus plantations is known to inhibit litter decomposition (Kavvadias et al., 2001; Ferreira et al., 2016; Ferreira and Guérold, 2017). But in this study, both natural (disturbed and undisturbed) and plantation forests have comparable decomposition rates.