The architectural design of trees protects them against large herbivores

Tristan Charles‐Dominique,Jean‐Francois Barczi,Elizabeth Le Roux,Simon Chamaillé‐Jammes,Clare Mcarthur

doi:10.1111/1365-2435.12876

Abstract

Summary Where large browsers are abundant, the survival of trees depends on their ability to deploy defences, either chemical or structural. Structural defences include the arrangement of dense and intricate architecture, termed ‘cage’ architecture. Previous studies showed that trees developing in herbivore‐rich environments tend to have more cage architecture but its precise effect on mammalian herbivores remains unknown. In this paper, we experimentally test how cage architecture affects the bite rate of goats, a generalist mammalian herbivore. We selected 11 palatable tree species with contrasting architectures. We described their caginess using an index combining spinescence and woodiness of their stems. Finally, we evaluated how the caginess of trees slows down herbivores when feeding on the inner leaves in tree crowns. We observed that the bite rate of goats on inner leaves of the cagiest trees was so severely reduced that they could not satisfy their daily nutritional requirements. We discuss how this could affect the preference of wild herbivores for less cagy trees, especially at the end of the dry season. A lay summary is available for this article.

Highlights

In many ecosystems large mammalian herbivore populations affect the composition and structure of plant communities (Skarpe 2001; Co^te et al 2004; Greve et al 2012), and can in places, become one of the primary factors determining plant species abundance (Martin et al 2010; Midgley, Lawes & Chamaille-Jammes 2010; Staver et al 2012)
Six of the 11 species (Acacia nilotica, Dichrostachys cinerea, Gymnosporia harveyana, Gymnosporia maranguensis, Scutia myrtina and Ziziphus mucronata) induced a clear ‘cage effect’ on goat bite rate, which was slower inside the crown of the three intact saplings than on isolated branches (Fig. 2; Tables S2 and S3)
The strength of the cage effect varied greatly across this set of species, (Fig. 2): The cage effects induced by Z. mucronata and A. nilotica were at least twice as strong as those induced by S. myrtina and D

Summary

Introduction

In many ecosystems large mammalian herbivore (hereafter, large herbivores) populations affect the composition and structure of plant communities (Skarpe 2001; Co^te et al 2004; Greve et al 2012), and can in places, become one of the primary factors determining plant species abundance (Martin et al 2010; Midgley, Lawes & Chamaille-Jammes 2010; Staver et al 2012). Plants that reduce intake rates, either due to reduced bite size (e.g. small leaves) and/or bite rate (e.g. spinescence), are generally less favoured (Cooper & Owen-Smith 1986; Belovsky et al 1991; Milewski, Young & Madden 1991; Grubb 1992; Gowda 1996; Charles-Dominique, Midgley & Bond 2015). Spinescence has been the main focus of studies that have investigated how plants can physically impair mammalian herbivore foraging (Cooper & Owen-Smith 1986; Hanley et al 2007; Skarpe et al 2012; Perez-Harguindeguy et al 2013; Burns 2014). Several studies point towards an effect of whole-plant structure in limiting browsing, but this hypothesis lacks experimental support

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Discussion

Conclusion