Winter climate limits subantarctic low forest growth and establishment

Area of Science:

• Ecology
• Climatology
• Botany

Background:

• Campbell Island, a subantarctic island, exhibits minimal temperature variation.
• Despite climate warming, the forest limit has not ascended.
• Previous studies noted an unexpected lack of upward forest limit movement with warming.

Purpose of the Study:

• To investigate the influence of summer and winter climate on treeline species' growth and age structure.
• To determine the relative importance of seasonal climate variables on *Dracophyllum longifolium* and *Dracophyllum scoparium*.

Main Methods:

• Dendroecological methods were employed to analyze tree growth and age-class structure.
• Local climate data from a meteorological station were utilized.
• Hierarchical regression models assessed climate-growth relationships, accounting for plot-level factors.

Main Results:

• Winter climatic conditions significantly impacted growth and establishment more than summer conditions.
• Optimal establishment occurred during warm, dry winters (mean winter temp >7°C, precipitation <400 mm).
• Growth was negatively affected by wide winter temperature fluctuations and increased rainfall.

Conclusions:

• Winter warmth enhances seedling survival and recruitment but increases metabolic stress in established plants, reducing growth.
• The contrasting effects of winter temperature on growth and establishment suggest different underlying mechanisms.
• Future winter warming may lead to complex, varied responses in plant growth and establishment globally.