Reviewing the role of plant litter inputs to forested wetland ecosystems: leafing through the literature

Abstract

The input of senescent terrestrial leaf litter into soil and aquatic ecosystems is one of the most massive cyclic subsidies on Earth, particularly within forested ecosystems. For freshwater systems embedded within forests, litter inputs provide a vital source of energy and nutrients that allows greater production than in situ resources can provide. In return, freshwater food webs can provide an enormous amount of material to the terrestrial landscape through biotic respiration, photosynthesis, and organism emergence. Most research concerning this important aquatic-terrestrial link has focused on lotic ecosystems (i.e., streams and rivers); far less attention has been given to its role in lentic systems (i.e., wetlands and lakes). A focus on small forested wetlands is particularly important, as these systems account for a disproportionate amount of global carbon flux relative to their spatial coverage, and the decomposition of leaf litter is a major contributor. Here, we review six themes: (1) the evidence for the role of leaf litter inputs as an ecologically important subsidy in forested wetlands; (2) the bottom-up effects of quantitative and qualitative variation in litter inputs; (3) how diversity in litter mixtures can alter ecological functioning; (4) evidence for top-down consequences of litter inputs through toxic effects on predators and parasites, and the alteration of predator-prey interactions; (5) the relevance of our review to other research fields by considering the role of litter inputs relative to other types of subsidies and environmental gradients (e.g., temperature, canopy cover, and hydrology); and (6) the interaction of litter subsidies with anthropogenic disturbances. We conclude by highlighting several high-priority research questions and providing suggestions for future research on the role of litter subsidies in freshwater ecosystems. 3: Effects of leaf litter mixingInsufficient evidence exists to determine the effects of mixing litter species on total litter decay rates.Higher trophic levels exhibit weak responses to increased litter diversity.Forested wetland food webs exhibit a stronger response to changes in average primary and secondary chemical attributes of litter mixtures relative to changes in litter species or chemical diversity.Sterner and Schulz (1998), Cohen et al. (2014), Stoler et al. (2016b) Consumers exhibit nonlinear responses to changes in litter species evenness.

Pathways for cross-boundary effects of biodiversity on ecosystem functioning

“…Similar relationships exist in aquatic ecosystems between producer diversity and primary production, as well as resource-use efficiency [4], despite large differences in the life forms of the primary producers and other ecosystem properties [5]. Effects of biodiversity on plant decomposition are also well documented, although the importance and prevalence are less clear than for plant production [6][7][8].…”

“…Such effects of riparian plant diversity on litter decomposition have received considerable attention in streams and wetlands. They range from synergistic to neutral and antagonistic [6,8,27].…”

Impacts of invasive Amur honeysuckle, Lonicera maackii, leaf litter on multiple trophic levels of detritus‐based experimental wetlands

“…Freshwater food webs are primarily based on materials and energy derived from endogenous production or from inputs of materials and energy derived outside the system boundaries (Polis et al., 1997). Forested wetlands are net heterotrophic systems supported by allochthonous inputs of organic carbon, predominately leaves, from surrounding forests (Rubbo et al., 2006; Stoler & Relyea, 2020). Leaf litter is subsequently decomposed by bacteria and fungi and consumed by detritivores (Colburn, 2004).…”

“…Nutrients and energy derived from this material support higher trophic levels within detritus‐based food webs, so changes in the quantity or quality of terrestrial inputs (e.g. leaf litter) are predicted to initiate bottom‐up effects (Bukovinszky et al., 2008; Polis, 1994; Rubbo et al., 2006; Stoler & Relyea, 2011, 2020). Assessments of the impact of biological invasions on freshwater food web dynamics are needed for quantitative comparisons among systems and to identify important factors underlying the high variability in system responses.…”

“…Many studies have reported ecological impacts of terrestrial invasive plants in terrestrial systems, aquatic invasive plants in aquatic systems, and riparian invasive plants in lotic ecosystems (Gallardo et al., 2016; McCary et al., 2016; McNeish & McEwan, 2016; Stoler & Relyea, 2020). Relatively few studies have examined impacts associated with terrestrial plants within lentic ecosystems, but such studies have observed important ecological impacts of invasion that cross terrestrial‐lentic aquatic boundaries.…”