Long‐term insect herbivory slows soil development in an arid ecosystem

Abstract

Abstract. Although herbivores are well known to alter litter inputs and soil nutrient fluxes, their longterm influences on soil development are largely unknown because of the difficulty of detecting and attributing changes in carbon and nutrient pools against large background levels. The early phase of primary succession reduces this signal-to-noise problem, particularly in arid systems where individual plants can form islands of fertility. We used natural variation in tree-resistance to herbivory, and a 15 year herbivore-removal experiment in an Arizona piñon-juniper woodland that was established on cinder soils following a volcanic eruption, to quantify how herbivory shapes the development of soil carbon (C) and nitrogen (N) over 36-54 years (i.e., the ages of the trees used in our study). In this semi-arid ecosystem, trees are widely spaced on the landscape, which allows direct examination of herbivore impacts on the nutrient-poor cinder soils. Although chronic insect herbivory increased annual litterfall N per unit area by 50% in this woodland, it slowed annual tree-level soil C and N accumulation by 111% and 96%, respectively. Despite the reduction in soil C accumulation, short-term litterfall-C inputs and soil C-efflux rates per unit soil surface were not impacted by herbivory. Our results demonstrate that the effects of herbivores on soil C and N fluxes and soil C and N accumulation are not necessarily congruent: herbivores can increase N in litterfall, but over time their impact on plant growth and development can slow soil development. In sum, because herbivores slow tree growth, they slow soil development on the landscape.

Feedbacks link ecosystem ecology and evolution across spatial and temporal scales: Empirical evidence and future directions

“…However, studies investigating the ecological importance of genetic variation in plants and herbivores provide compelling evidence that contemporary evolution as a result of plant–herbivore interactions can have ecosystem‐level effects (Figure : ecosystem engineering side of FB2 ). For example, Classen, Chapman, Whitham, Hart, and Koch (), Classen, Chapman, Whitham, Hart, and Koch () demonstrated that piñon pine ( Pinus edulis ) susceptibility to a scale insect herbivore is correlated with plant traits that increased nitrogen (N) cycling through litter decomposition but reduced N and carbon (C) accumulation in soil over decades. Additionally, selective consumption of particular plants (e.g.…”

The importance of plant genotype and contemporary evolution for terrestrial ecosystem processes

“…3). The presence of insect herbivores could directly or indirectly alter the amount of labile C and N entering the soil environment (Classen et al 2013) and the composition of decomposer communities (Classen et al 2006). Insect herbivores, in addition to the genotypic composition of the resident plant population, could contribute in a nonadditive fashion to produce the interactive effect of insect herbivores and genotypic composition on N cycling.…”

Plant genetic effects on soils under climate change

“…For example, in a 2 year study Classen et al (2013) initially found that herbivory on insect resistant and susceptible tree genotypes did not affect litterfall-C inputs and soil C-efflux rates. However, when the study was expanded over 36-54 years, herbivory on susceptible trees was shown to slow soil C and N accumulation by 111 % and 96 %, respectively relative to resistant trees.…”