Winter Climate Limits Subantarctic Low Forest Growth and Establishment

<div><p>Campbell Island, an isolated island 600 km south of New Zealand mainland (52°S, 169°E) is oceanic (Conrad Index of Continentality  = −5) with small differences between mean summer and winter temperatures. Previous work established the unexpected result that a mean annual climate warming of <i>c</i>. 0.6°C since the 1940's has not led to upward movement of the forest limit. Here we explore the relative importance of summer and winter climatic conditions on growth and age-class structure of the treeline forming species, <i>Dracophyllum longifolium</i> and <i>Dracophyllum scoparium</i> over the second half of the 20^th century. The relationship between climate and growth and establishment were evaluated using standard dendroecological methods and local climate data from a meteorological station on the island. Growth and establishment were correlated against climate variables and further evaluated within hierarchical regression models to take into account the effect of plot level variables. Winter climatic conditions exerted a greater effect on growth and establishment than summer climatic conditions. Establishment is maximized under warm (mean winter temperatures >7 °C), dry winters (total winter precipitation <400 mm). Growth, on the other hand, is adversely affected by wide winter temperature ranges and increased rainfall. The contrasting effect of winter warmth on growth and establishment suggests that winter temperature affects growth and establishment through differing mechanisms. We propose that milder winters enhance survival of seedlings and, therefore, recruitment, but increases metabolic stress on established plants, resulting in lower growth rates. Future winter warming may therefore have complex effects on plant growth and establishment globally.</p></div

The relationship between the average site-wide mean annual growth rate and a) the symmetry of the maximum temperature around the mean (skew) and b) the amplitude of the scattering (standard deviation).

<p>The relationship between the average site-wide mean annual growth rate and a) the symmetry of the maximum temperature around the mean (skew) and b) the amplitude of the scattering (standard deviation).</p

Results from the a) linear regression model for growth and b) logistic regression model for age-class structure fit within a Bayesian framework.

<p>The mean (filled dots) and 95% credible intervals (solid lines) for the parameter estimates describe the effect of each explanatory variable on plot-level annual growth rates. Categorical variables are relative to a reference class (good soil drainage, eutrophic soil type, <i>D. scoparium</i> species) as described in the Methods section. Credible intervals crossing the zero line (dashed) are not significant.</p

Pearson's product-moment correlations between climate variables and mean site-level annual growth and establishment between 1941 and 1999.

<p>Relationships with a correlation score >0.25 are indicated by bold typeface. Growth and establishment values are the average values across the five plots. GGD 5 °C refers to growing degree days above 5 °C during the summer months and wFDD refers to freezing degree days calculated as the summed difference in the mean daily temperature from 0 °C over the winter months. A negative FDD value indicates temperatures above freezing (0 °C).</p

Age-class structure across all plots according to a) establishment year and b) climatic conditions.

<p>A) Frequency distributions of the year of establishment (in 5 year bins) for <i>Dracophyllum</i> plants across all plots. Dashed vertical lines represent the period in which farming occurred on the island. Livestock number and distribution were limited after 1931 but were not eradicated until 1991. This figure is modified from Bestic et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093241#pone.0093241-Bestic1" target="_blank">[5]</a>. B) The percent of total establishment across all plots in four winter climate categories between 1941 and 1993 according to winter precipitation and maximum temperature. Winter climate at the year of establishment is classed as whether winters were wet (>400 mm total precipitation) or dry (<400 mm total precipitation) and warm (>7°C on average) or cool (<7°C on average).</p

The five plots used to assess growth-climate relationships were characterized by <i>Dracophyllum</i> species (<i>Dracophyllum scoparium</i>, <i>Dracophyllum longifolium</i>, hybrid of the two species), soil type, soil drainage, soil fertility, and elevation (masl).

<p>Soil type is based on soil drainage and fertility as defined by Bestic <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093241#pone.0093241-Bestic2" target="_blank">[21]</a>.</p

Map of plot and weather station locations on Campbell Island.

<p>Map of plot and weather station locations on Campbell Island.</p

Seeds found in coprolites from An avian seed dispersal paradox: New Zealand's extinct megafaunal birds did not disperse large seeds

S3. Table listing plant species with seeds identified from coprolite

Final germination of E. dentatus and P. ferruginea from An avian seed dispersal paradox: New Zealand's extinct megafaunal birds did not disperse large seeds

S4. Final germination percentages of Elaeocarpus dentatus (blue, left) and Prumnopitys ferruginea (green, right) seeds treated with acid, mechanical scarification, handcleaning, and left whole (untreated). Error bars are bootstrapped standard error of the mean

Description of putative moa dispersed fruits from An avian seed dispersal paradox: New Zealand's extinct megafaunal birds did not disperse large seeds

S1. Description of putative megafaunal fruit