An experimental approach to explain the southern Andes elevational treeline

Area of Science:

• Ecology
• Plant Physiology
• Biogeography

Background:

• The growth limitation hypothesis (GLH) is widely accepted for treeline formation, but its applicability across different plant taxa remains unclear.
• The presence of exotic Pinus contorta above the Nothofagus treeline in New Zealand suggests alternative mechanisms may be at play.
• Understanding treeline dynamics is crucial for predicting forest responses to climate change.

Purpose of the Study:

• To test the applicability of the growth limitation hypothesis (GLH) in Nothofagus pumilio and Pinus contorta.
• To compare seedling performance and carbon balance of these species under simulated treeline conditions.
• To investigate taxon-specific physiological responses to low temperatures at treelines.

Main Methods:

• Seedlings of Nothofagus pumilio and Pinus contorta were grown at a southern Chilean treeline at two heights: 2m above ground and at ground level.
• Environmental variables, including soil and air temperatures, were monitored over three years.
• Seedling survival, biomass accumulation, and nonstructural carbohydrate (NSC) content were measured.

Main Results:

• Both Nothofagus and Pinus exhibited higher survival rates at ground level compared to the 2m height.
• Nothofagus pumilio showed significant decreases in biomass and NSC at the 2m height, indicating carbon limitation.
• Pinus contorta did not display significant reductions in biomass or NSC at the higher elevation, suggesting no carbon limitation.

Conclusions:

• While Nothofagus pumilio demonstrated physiological responses consistent with carbon limitation at treeline, Pinus contorta did not.
• These findings suggest that the mechanisms driving treeline formation and species survival can be taxon-dependent.
• The study highlights the need to consider species-specific traits when applying general ecological hypotheses like the GLH.