Linking tree genetics and stream consumers: isotopic tracers elucidate controls on carbon and nitrogen assimilation

Abstract

Leaf litter provides an important nutrient subsidy to headwater streams, but little is known about how tree genetics influence energy pathways from litter to higher trophic levels. Despite the charge to quantify carbon (C) and nitrogen (N) pathways from decomposing litter, the relationship between litter decomposition and aquatic consumers remains unresolved. We measured litter preference (attachments to litter), C and N assimilation rates, and growth rates of a shredding caddisfly (Hesperophylax magnus, Limnephilidae) in response to leaf litter of different chemical and physical phenotypes using Populus cross types (P. fremontii, P. angustifolia, and F hybrids) and genotypes within P. angustifolia. We combined laboratory mesocosm studies using litter from a common garden with a field study using doubly labeled litter ( C and N) grown in a greenhouse and incubated in Oak Creek, Arizona, USA. We found that, in the lab, shredders initially chose relatively labile (low lignin and condensed tannin concentrations, rapidly decomposing) cross type litter, but preference changed within 4 d to relatively recalcitrant (high lignin and condensed tannin concentrations, slowly decomposing) litter types. Additionally, in the lab, shredder growth rates were higher on relatively recalcitrant compared to labile cross type litter. Over the course of a three-week field experiment, shredders also assimilated more C and N from relatively recalcitrant compared to labile cross type litter. Finally, among P. angustifolia genotypes, N assimilation by shredders was positively related to litter lignin and C:N, but negatively related to condensed tannins and decomposition rate. C assimilation was likewise positively related to litter C:N, and also to litter %N. C assimilation was not associated with condensed tannins or lignin. Collectively, these findings suggest that relatively recalcitrant litter of Populus cross types provides more nutritional benefit, in terms of N fluxes and growth, than labile litter, but among P. angustifolia genotypes the specific trait of litter recalcitrance (lignin or tannins) determines effects on C or N assimilation. As shredders provide nutrients and energy to higher trophic levels, the influence of these genetically based plant decomposition pathways on shredder preference and performance may affect community and food web structure.

Litter identity affects assimilation of carbon and nitrogen by a shredding caddisfly

“…These results are consistent with patterns seen among genetically distinct Populus leaves, where lignin concentrations were positively correlated with C and N assimilation, but tannin concentrations were negatively correlated (Compson et al. ). Although both aromatics and lignin slow decomposition (Gessner and Chauvet , Almendros et al.…”

“…Longer experiments (3–5 weeks) have shown similar trends, indicating that these results are not specific to early stages of decomposition (Compson et al. ; Siders et al., in prep ).…”

“…Because slowly decomposing litter can persist in rivers into the spring and summer months when many insects emerge, it may be a particularly important resource. Longer experiments (3-5 weeks) have shown similar trends, indicating that these results are not specific to early stages of decomposition (Compson et al 2018;Siders et al, in prep).…”

“…In contrast, high decomposition rates of P. fremontii were correlated with higher microbial biomass and leaching of dissolved compounds (Pastor et al 2014, Wymore et al 2015. Although microbes living on litter can be an important food source for leaf-shredding insects, some compounds that retard microbial decomposition might increase element transfer to invertebrates (Compson et al 2018). In addition to the Populus research, recent studies of detritus in temperate stream ecosystems also show higher invertebrate growth on slowly decomposing litter (Fuller et al 2015, Halvorson et al 2015, suggesting that decomposition rate alone may not be a good predictor of food quality for detritivores.…”

“…Although microbes living on litter can be an important food source for leaf‐shredding insects, some compounds that retard microbial decomposition might increase element transfer to invertebrates (Compson et al. ). In addition to the Populus research, recent studies of detritus in temperate stream ecosystems also show higher invertebrate growth on slowly decomposing litter (Fuller et al.…”