Poor acclimation to experimental field drought in subalpine forest tree seedlings

Area of Science:

• Ecology
• Climate Change Biology
• Forestry

Background:

• Climate change necessitates understanding tree species' drought tolerance.
• Research has largely ignored seedling responses to drought, focusing instead on mature trees.
• Regeneration dynamics are critical for forest resilience to changing climate conditions.

Purpose of the Study:

• To assess the drought tolerance of subalpine conifer seedlings (Abies lasiocarpa and Picea engelmannii) in the Rocky Mountains.
• To compare morphological and physiological responses to simulated drought conditions based on seedling size.
• To evaluate the implications of seedling drought responses for future forest dynamics.

Main Methods:

• Conducted a 2-year in situ experiment excluding summer precipitation for subalpine conifer seedlings.
• Simulated drought conditions by reducing soil volumetric water content.
• Measured morphological traits (e.g., root biomass) and physiological responses (e.g., net photosynthesis, water use efficiency).

Main Results:

• Drought significantly reduced soil moisture in experimental plots.
• No significant morphological shifts were observed in response to drought for either species.
• Net photosynthesis decreased by 78% for spruce and 37% for fir under drought conditions.
• Larger seedlings exhibited greater water use efficiency due to stomatal control, but this was not significantly different between drought and ambient conditions.

Conclusions:

• Subalpine conifer seedlings demonstrated a lack of responsiveness to water stress, prioritizing carbon gain over drought mitigation.
• Morphological traits did not significantly change, indicating limited acclimation capacity.
• These findings suggest substantial vulnerability of seedling populations, particularly spruce, to recurrent or prolonged droughts under climate change scenarios.