Oikos 120: 862–872, 2011 doi: 10.1111/j.1600-0706.2010.18625.x © 2011 Th e Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Ulrich Brose. Accepted 17 September 2010 Living in the litter: the infl uence of tree leaf litter on wetland communities Aaron B. Stoler and Rick A. Relyea A. B. Stoler ([email protected]) and R. A. Relyea, Dept of Biological Sciences, 101 Clapp Hall, Univ. of Pittsburgh, Pittsburgh, PA 15260, USA. Empirical research in streams has demonstrated that terrestrial subsidies of tree leaf litter infl uence multiple community factors including composition, diversity and growth of individuals. However, little research has examined the importance of tree litter species on wetlands, which are ubiquitous across the landscape and serve as important habitats for a unique and diverse community of organisms. Using outdoor mesocosms, we assessed the impact of 12 litter monocultures and three litter mixtures (from both broadleaf and conifer trees) on pond communities containing gray tree frog tadpoles Hyla versicolor , periphyton, phytoplankton and zooplankton. We found that leaf litter species had substantial and diff erential impacts on all trophic groups in the community including eff ects on algal abundance, zooplankton density and amphibian growth. In many instances, patterns of responses were specifi c to individual litter species yet some responses, including both pH values and periphyton biomass, were generalizable to broad taxonomic groups. In addition, while most responses of litter mixtures were additive, we found evidence for antagonistic eff ects of litter mixing among responses of periphyton and amphibian body mass. Our results highlight the potential impact of human and naturally driven changes in forest composition on wetland communities through associated changes in leaf litter. Linkage between aquatic and terrestrial ecosystems allows for and riverine systems, studies on lentic ecosystems remain frequent exchange of nutrients and organisms which alters scarce, although leaf litter inputs provide an important local factors such as resource abundance and trophic interac- carbon source in these systems as well (Bonner et al. 1997, tions (Polis et al. 1997). In forested aquatic systems, food Wetzel 2001, Rubbo et al. 2006, 2008). Such habitats have webs are generally nutrient-limited (Wilbur 1997), and net species uniquely adapted to lentic water, high seasonal pro- ecosystem productivity is driven by external carbon sources ductivity, and often stronger trophic interactions than lotic (Fisher and Likens 1973, Vannote et al. 1980, Brinson systems (Williams 1996, Wetzel 2001, Shurin et al. 2002). et al. 1981, Polis et al. 1997). For a variety of forested het- Studies examining litter eff ects in lentic environments have erotrophic aquatic systems, the bulk of carbon input is generally focused on relatively small and ephemeral tree hole derived from plant litter (Nelson and Scott 1962, Minshall ecosystems, where mosquito distribution and performance 1967, Fisher and Likens 1973, Webster and Benfi eld 1986) across three or four litter species is usually dependent on which subsidizes aquatic food web resources (Wallace et al. the species of leaf litter present (Fish and Carpenter 1982, 1997). In fact, leaf litter originating from trees can account Yanoviak 1999, Reiskind et al. 2009). Th ere are a few studies for 99% of total dissolved organic carbon input in stream sys- investigating the role of leaf litter in larger, more diverse wet- tems (DOC, Fisher and Likens 1973). Such allochthonous land ecosystems; comparisons of two litter types have found inputs often regulate community productivity by generat- that litter species matters in these systems as well (Rubbo and ing more primary and secondary production than available Kiesecker 2004, Williams et al. 2008). in situ resources can sustain (Polis et al. 1997). Removal of Th e quality of litter input for consumers hinges upon the these inputs or shifts in the timing and magnitude of litter species composition of the litter and the chemistry of constit- decomposition can have strong eff ects on entire ecosystems. uent species (Brinson et al. 1981, Melillo et al. 1982, Webster As a result, the decomposition of plant litter is an essen- and Benfi eld 1986, Wetzel 2001, Swan and Palmer 2006). tial ecosystem process in forested aquatic systems (Fisher and Litter chemistry varies widely among tree species (Ostrofsky Likens 1973, Polis et al. 1997, Wallace et al. 1997, Meyer 1993, 1997) and these chemical diff erences consequently et al. 1998) and the role of leaf litter in stream environments help determine the food web that can be supported (Webster has received increasing attention in the past decade for its and Benfi eld 1986, Facelli and Pickett 1991, Wallace et al. strong impacts on ecosystem properties and services (Lecerf 1997, Werner and Glennemeier 1999). For example, nitro- and Richardson 2010, Kominoski et al. 2010). While gen and lignin content aff ect litter quality and palatability research on the impact of litter is prevalent in lotic stream (Melillo et al. 1982, Ostrofsky 1997), secondary compounds 862 such as phenolics can act as anti-microbial agents (Ard ó n such research is becoming more and more critical as natural and Pringle 2008), and high litter solubility may darken the and anthropogenic disturbance in forests alters tree species water column, consequently imposing limitations on pri- composition and associated litter. mary production (Karlsson et al. 2009). On a broad scale, We investigated how multiple coniferous and broadleaf single-species inputs of tree litter are generally uncommon in litter species from a diverse, forested landscape infl uence forests; yet understanding the eff ects of individual litter spe- forested wetland communities. Using outdoor mesocosms, cies provides a basis to understand local ecosystem processes we created replicated wetland communities treated with a and may off er predictions for consequences of tree species variety of common litter species that represented 12 spe- loss or addition. cies from six families (alone and in mixtures) and spanned While understanding the eff ects single species of litter is a wide range of leaf structure and chemistry. We investi- an important starting point, the more common scenario in gated two hypotheses: 1) leaf litter species and leaf litter nature involves mixtures of litter species. In stream systems, type (broadleaf vs coniferous) will diff erentially aff ect the it is clear that the eff ects of mixing litter inputs are often dif- dynamics of producers and consumers in a wetland (i.e. ferent than those expected based on single-species eff ects (i.e. density, body mass and survival); 2) litter mixtures will have non-additive). Mixtures of litter may produce synergistic non-additive eff ects on the dynamics of the producers and (i.e. greater than additive) or antagonistic eff ects (i.e. lower consumers in a wetland. than additive) in aquatic ecosystems via interactions at the molecular and microbial level (Kominoski et al. 2007, 2010, Lecerf and Richardson 2010, Swan et al. 2009). A single litter Methods species in a mixture of multiple litter species may facilitate or inhibit the breakdown of other species, thus altering nutri- Our experiment was conducted at the Univ. of Pittsburgh ’ s ent release and environmental conditions (Swan and Palmer Pymatuning Lab of Ecology. Th e experiment employed a 2006). For example, a recalcitrant litter species (e.g. Quercus completely randomized design with 15 treatments that were sp. ) may promote growth of consumers within a community replicated four times for 60 total experimental units. Each by providing a nutrient source that outlasts other litter spe- experimental unit was a 100-l polyethylene wading pool, cies (Swan and Palmer 2005), but may also deter growth by 1 m in diameter and approximately 0.2 m in height, topped lowering the overall nutritional quality of the litter mixture with a 60% shade-cloth lid to prevent escape or entry of (Rosemond et al. 2010). any organisms and to provide an intermediate level of Th e species composition of natural litter mixtures varies light penetration relative to the full range of canopy cover widely depending upon climate, geography and succession under which such ephemeral systems occur (Werner and factors such as disturbance and ecosystem age. However, for- Glennemeier 1999). We fi lled each pool with 100 l of well ests are often identifi ed as coniferous, hardwood, or a mix- water on 12 June 2006 and added leaf litter on 21 June. ture of both, and litter chemistry diff ers strongly between Th e 15 litter treatments included eight broadleaf monocul- these forest types. For example, coniferous litter is often tures, four conifer monocultures, a mixture of the eight more refractory than broadleaf litter and is generally lower broadleaf species, a mixture of the four conifer species, and a in nitrogen (Berg and McClaugherty 2008). Such diff erences mixture of all 12 species (Table 1). All mesocosms contained between litter in these forest types is likely to have strong a total of 100 g of litter, with the mixture treatments using impacts on lentic ecosystem processes, yet such eff ects have equal portions of all species. Th is amount is within the range not been investigated. found in nature (Rubbo et al. 2008). By examining diff erences between the eff ects of indi- Although litter is typically collected immediately after vidual litter species and litter from diff erent forest types, it senescence in the autumn (Rubbo and Kiesecker 2004), is possible to understand the eff ects of litter inputs within we collected fallen leaves and needles in mid-June to sim- and among forests. Achieving this goal is necessary to both ulate a scenario where a dry wetland basin was fi lled by enhance our knowledge of ecological processes in forests and spring rains. Litter was collected from mostly mesic areas to bolster conservation eff orts in these systems.