Climate influences the value of a plant structural defence against browsing

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 u...