DOI: 10.5061/dryad.3xsj3txfh
title: Climate influences the value of a plant structural defence against browsing

description: The circumstances that select for plant anti-herbivore defences are not
 well understood. In New Zealand, the “divaricate” cage-like architecture
 of many woody plants may have arisen as a defence against avian browsing;
 it also has some ability to deter browsing by introduced deer. Its
 prominence on alluvial soils in frosty and droughty areas led us to
 hypothesize that structural defences are of most value where fertile soils
 coincide with climatic constraints that prevent plants from quickly
 growing out of the browse zone. We tested this hypothesis by growing
 seedlings of three divaricate species and their broadleaved congeners on
 fenced and unfenced plots beneath treefall gaps at three humid frost-free
 sites in the North Island of New Zealand, and at three colder and drier
 sites in the South Island. Soil total phosphorus levels were moderate to
 high at all sites (344 to 961 ppm), and pellet counts indicated similar
 average deer densities at North and South Island sites, enabling us to
 attribute variation in seedling growth and survival primarily to climatic
 differences between our sites. The circumstances that select for plant
 anti-herbivore defences are not well understood. In New Zealand, the
 “divaricate” cage-like architecture of many woody plants may have arisen
 as a defence against avian browsing; it also has some ability to deter
 browsing by introduced deer. Its prominence on alluvial soils in frosty
 and droughty areas led us to hypothesize that structural defences are of
 most value where fertile soils coincide with climatic constraints that
 prevent plants from quickly growing out of the browse zone. We tested this
 hypothesis by growing seedlings of three divaricate species and their
 broadleaved congeners on fenced and unfenced plots beneath treefall gaps
 at three humid frost-free sites in the North Island of New Zealand, and at
 three colder and drier sites in the South Island. Soil total phosphorus
 levels were moderate to high at all sites (344 to 961 ppm), and pellet
 counts indicated similar average deer densities at North and South Island
 sites, enabling us to attribute variation in seedling growth and survival
 primarily to climatic differences between our sites. On fenced plots,
 average relative height growth rates in the North Island exceeded those on
 the southern sites by 64 %, and growth of broadleaved species averaged 132
 % and 99 % greater than that of divaricate congeners at North and South
 Island sites, respectively. On the northern sites, broadleaved species
 also held a large (137 %) net height growth advantage over divaricates
 even on unfenced plots. In contrast, exposure to browsing on the coldest
 sites resulted in complete loss of the broadleaved net height growth
 advantage. Synthesis. Even in the presence of deer, divaricate plants were
 overgrown on humid frost-free sites, but not on colder and drier sites,
 where browsing neutralized the superior growth potential of broadleaved
 species. The cost of structural defences therefore paid off only on sites
 where climatic adversity reduces the ability of plants lacking such
 defences to grow quickly out of the browse zone, in agreement with our
 hypothesis.
description: The data was obtained by a field experiment. We grew seedlings of
 three divaricate species and broadleaved congeners on three warm moist
 sites and three colder, drier sites. Half of our plots were exposed to
 deer browsing, the other half were fenced to exclude deer. After two
 years, we compared net height growth of divaricate and broadleaved
 congeners across our six sites.