Spatial arrangement of canopy structure and land‐use history alter the effect that herbivores have on plant growth

Abstract

The spatial arrangement of past and present human activities could affect the strength of species interactions through changes to environmental conditions. To better understand how land‐use history might mediate the effect of insect herbivory on the growth of four herbaceous plant species at the edges between woodlands and open savannas, we coupled large‐scale manipulations of canopy structure by thinning overstory trees to create savanna habitat in non‐agricultural and post‐agricultural longleaf pine woodlands with local‐scale herbivore exclosures at two distances from habitat edges. Light availability, grasshopper abundance, and herbaceous plant cover all increased in patches that received overstory tree thinning. Land‐use history altered the effect that edges created by the overstory thinning treatments had on grasshopper herbivory for one plant species (Carphephorus bellidifolius). The edge between land‐use histories also altered herbivore effects on three plant species in thinned patches. These edge effects were driven in part by changes in availability of alternative resources for grasshoppers, shady edges, or grasshopper abundance. Canopy manipulation also had important effects on herbivory that were independent of the habitat edge: grasshopper herbivory reduced plant growth of the smallest species (C. bellidifolius) regardless of canopy manipulation, whereas herbivory on two larger species (Solidago nemoralis and So. odora) reduced plant growth mainly in patches with an intact overstory canopy. Collectively, this work suggests that the species‐specific effects of grasshopper herbivory on plant growth were driven by both plant traits (e.g., size or growth rates) and landscape‐mediated changes to grasshopper abundance, light availability, or availability of alternative resources for grasshoppers created by land‐use history and canopy structure. Furthermore, because the heterogeneity in herbivory revealed by our large‐scale experiment is likely linked to plant traits, our results provide a means for understanding the profound, but often idiosyncratic, effects of land‐use legacies and edges on plant populations and communities.

Ontogenetic responses of four plant species to additive and interactive effects of land‐use history, canopy structure and herbivory

“…; Hakes & Cronin ; Burghardt ). Interestingly, we found the opposite patterns on these plants during the first year of growth, where plants exposed to grasshopper herbivory were significantly smaller compared to plants in grasshopper exclosures, most often in the unthinned treatments (Hahn & Orrock ) and these plants subsequently experienced greater mortality (Fig. ).…”

“…). Overstorey thinning increased light levels, herbaceous plant cover, and grasshopper abundance, all by threefold or greater, relative to unthinned patches (Hahn & Orrock ). However, differences in plant community composition between post‐agricultural and non‐agricultural sites persisted even after canopy thinning (Turley & Brudvig ).…”

“…; Barton ). Thus, first‐year plants may not have accumulated enough below‐ground carbon storage to be able to compensate for herbivore damage, resulting in reduced growth (Hahn & Orrock ) and survival (Salgado‐Luarte & Gianoli ; Fig. ) in low‐resource environments.…”

Disentangling fragmentation effects on herbivory in understory plants of longleaf pine savanna

“…Grasshoppers (Orthoptera) are major herbivores in the fragments, accounting for >99% of observed herbivory on at least one of our study species, Solanum americanum, and are frequently flushed from many other species when walking between plots (Evans et al. , Hahn and Orrock ). Common genera include Hippiscus, Melanoplus, Pardalophora, Psinidia, and Schistocerca, Trimerotropis ; Melanoplus and Schistocerca are most abundant and both are mainly composed of dietary generalists (Orrock et al.…”

“…, Evans et al. , Hahn and Orrock ). Grasshopper abundance increases with distance to edge and is highest in rectangular fragments, presumably because rectangular fragments contain more area far from edge than winged and connected fragments (Evans et al.…”