Climate predicts the proportion of divaricate plant species in New Zealand arborescent assemblages

Abstract

Aim Puzzling features of plants are sometimes explained as legacies of coevolution with extinct herbivores. One example is the convergent evolution of a small-leaved, twiggy 'divaricate' form in > 50 woody species in New Zealand. This growth form was first interpreted as a response to the PlioPleistocene onset of frosty, droughty environments, but opinion now favours the hypothesis that it arose as a defence against large herbivorous birds (moa). It has been argued that the extinction of moa during the last millennium has left divaricate plants at risk of being out-competed by faster growing broadleaved competitors, yet their current abundance in some habitats suggests this growth form might confer other advantages besides protection against avian browsing. We aimed to clarify the adaptive significance of the divaricate growth form by identifying environmental correlates of its geographic distribution in New Zealand.Location The main islands of the New Zealand archipelago, spanning latitude c. 34.5-47°S.Methods We used generalized additive models to examine the environmental correlates of the proportion of local tree and shrub assemblages contributed by divaricate species, on 1152 0.04 ha plots distributed across a wide range of climate and landforms.Results A model with four climatic variables explained 37% of the observed variation. Mean minimum temperature of the coldest month was the single best predictor of the proportion of divaricate species in arborescent assemblages, with annual solar radiation, vapour pressure deficit and annual water deficit also contributing to the final model. Divaricates made their largest contributions to assemblages in the rain shadow of the Southern Alps, on sites with cold winters, low solar radiation and significant vapour pressure deficits, where potential evapotranspiration exceeds precipitation by > 130 mm. No physiographic variables (e.g. slope) contributed to the final model. Main conclusionsThe distribution of divaricate plants suggests they cope with frosty, droughty environments better than most other elements of New Zealand's arborescent flora. This may reflect selection for both (1) physiological resistance to frost and drought and (2) deterrence of herbivores, as climatic constraints on plant growth rates probably leave seedlings exposed for longer to ground-dwelling browsers. However, the widespread persistence of divaricate plants during the c. 450-year hiatus between the extinction of moa and the onset of widespread browsing by introduced ungulates suggests that even in the absence of large herbivores, divaricates are well adapted to frosty, droughty environments.

Frost and leaf‐size gradients in forests: global patterns and experimental evidence

“…However, the latter two of these phenomena are of most relevance to the continental climates of the North Temperate Zonethey are less common in oceanic temperate climates, and rare on tropical mountains. At inland sites in oceanic temperate regions such as New Zealand, southern South America and south-eastern Australia, carbon gain opportunities are interspersed with frost events throughout much of the year; small evergreen leaves enable plants to exploit these unpredictable opportunities while minimising vulnerability to frost damage (Lusk et al, 2016;McGlone et al, 2016), and possibly reducing the need for investment in specialised frost protection mechanisms. Similarly, small evergreen leaves predominate at high elevations on tropical mountains (Grubb, 1977), where frost can occur at any time of year.…”

Macroclimate and Topography Interact to Influence the Abundance of Divaricate Plants in New Zealand

“…McGlone and Webb (1981), however, noted that terraces are also generally the most frost-prone parts of landscapes (Laughlin and Kalma, 1987). A recent modeling study failed to detect any effect of topographic position on the representation of divaricate species in arborescent assemblages, which was predicted best by macroclimatic variables (Lusk et al, 2016). Here we use a larger dataset and a new modeling approach to test the effects of macroclimate and topographic position on divaricate abundance.…”

“…More than 120 years after the first treatment of New Zealand's divaricate plants in the scientific literature (e.g., Diels, 1897), scientists continue to debate the causes of the local abundance of this distinctive growth form (Bond et al, 2004;Lusk et al, 2016;Wood and Wilmshurst, 2017). In New Zealand, the term "divaricate" has been applied to 50-60 woody species from at least 17 different families, with small leaves, long internodes and wide branching angles, often resulting in an interlacing crown (Greenwood and Atkinson, 1977;Kelly, 1994).…”

“…Empirical tests of the climatic hypothesis have been largely inconclusive (Kelly and Ogle, 1990;Darrow et al, 2002;Howell et al, 2002;Lusk and Clearwater, 2015). However, recent modeling confirms an association with frosty and droughty habitats: divaricate species contribute most to arborescent assemblages in the eastern South Island, on sites with cold winters and where evapotranspiration exceeds precipitation; they are also well represented in the central North Island, where the frost-free period is <4 months on many sites (Lusk et al, 2016). Similarly, congeneric contrasts show that divaricate plants are consistently associated with environments that are on average frostier than those of their closest broadleaved relatives (Garrity and Lusk, 2017).…”

“…Although debate about divaricate plants has often been polarized, it has also been suggested that a synergy of climatic and browsing pressures might ultimately be implicated (Wardle, 1985;Cooper et al, 1993). Here we use patterns of divaricate plant abundance across present-day landscapes to address a novel synthetic hypothesis: that the divaricate form (like other putative physical defenses against browsers) is of most value to plants on fertile sites that attract herbivores, and where climatic constraints prevent plants from quickly growing out of the browse zone (Lusk et al, 2016). Greenwood and Atkinson (1977) stated that large concentrations of divaricate plants often occur on the fertile alluvial soils of terraces, and argued this reflected selection for defense against the high browsing pressure resulting from the attraction of herbivores to nutrient-rich environments (e.g., Grubb, 1992;Kanowski et al, 2001).…”