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

Aimee T Classen,Stephen C Hart,Thomas G Whitham,George W Koch,Samantha K Chapman

doi:10.1890/es12-00411.1

Abstract

Although herbivores are well known to alter litter inputs and soil nutrient fluxes, their long‐term 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.

Highlights

Insect herbivores can have variable impacts on nutrient cycling in ecosystems; they can increase (e.g., Chapman et al 2003, Frost and Hunter 2004, Fonte and Schowalter 2005), slow (e.g., Hartley and Jones 2003), have mixed, or no impact on nutrient cycling (e.g., Classen et al 2007a)
While many recent studies have documented the impacts of insect herbivory on ecosystems (e.g., Belovsky and Slade 2000, Frost and Hunter 2004, Madritch et al 2007, Blue et al 2011, Schowalter et al 2011, Zhang et al 2011), it is difficult to assess insect herbivore impacts on soil C and N accumulation in mesic ecosystem soils because the canopies of plants overlap, background pool sizes are high relative to changes in input rates associated with herbivory, and herbivore manipulation experiments may not be sufficiently long to detect a signal
There were no statistical differences in growing season soil C-efflux among susceptible, resistant, and removed trees (F 1⁄4 0.02, P 1⁄4 0.63; Fig. 2C); soil C-efflux did vary between years (F 1⁄4 1.88, P, 0.0001; Fig. 2C)

Summary

Introduction

Insect herbivores can have variable impacts on nutrient cycling in ecosystems; they can increase (e.g., Chapman et al 2003, Frost and Hunter 2004, Fonte and Schowalter 2005), slow (e.g., Hartley and Jones 2003), have mixed, or no impact on nutrient cycling (e.g., Classen et al 2007a). While many recent studies have documented the impacts of insect herbivory on ecosystems (e.g., Belovsky and Slade 2000, Frost and Hunter 2004, Madritch et al 2007, Blue et al 2011, Schowalter et al 2011, Zhang et al 2011), it is difficult to assess insect herbivore impacts on soil C and N accumulation in mesic ecosystem soils because the canopies of plants overlap, background pool sizes are high relative to changes in input rates associated with herbivory, and herbivore manipulation experiments may not be sufficiently long to detect a signal It remains unclear whether non-outbreak insect herbivores influence larger-scale, longer-term processes such as the development of soil total carbon (C) and nitrogen (N) stocks. These changes in primary productivity may yield different soil C and N accumulation rates beneath each tree in these ‘‘islands of fertility.’’

Methods

Results

Discussion

Conclusion