Tolerance or avoidance: drought frequency determines the response of an N₂ -fixing tree

Area of Science:

• Ecology
• Plant Physiology
• Climate Change Biology

Background:

• Climate change is increasing drought frequency globally.
• Droughts can impact crucial ecosystem processes like symbiotic nitrogen fixation (SNF).
• Understanding plant responses to drought is vital for predicting ecosystem recovery.

Purpose of the Study:

• To investigate the effects of varying drought frequencies on the ecophysiology and SNF rates of *Robinia pseudoacacia*.
• To determine if *Robinia pseudoacacia* can maintain growth and nitrogen acquisition under increased drought frequency.
• To elucidate the role of SNF in plant adaptation to drought stress.

Main Methods:

• *Robinia pseudoacacia* seedlings were subjected to controlled wet-dry cycles simulating different drought frequencies.
• Mean soil moisture was kept constant across treatments.
• Measurements included final biomass, carbon (C) and nitrogen (N) fixation rates during wet and dry phases, and ecophysiological responses.

Main Results:

• Drought frequency did not significantly affect final biomass or overall mean SNF rates.
• Seedlings exhibited differential C and N fixation rates between wet and dry periods, dependent on drought frequency.
• Under high-frequency droughts, C and N fixation were suppressed during dry spells but enhanced during wet periods, indicating a compensatory strategy.

Conclusions:

• *Robinia pseudoacacia* demonstrates resilience to increased drought frequency through a combination of drought tolerance/avoidance and compensatory growth.
• Symbiotic nitrogen fixation (SNF) appears to be a key mechanism supporting leaf acclimation and post-drought recovery.
• SNF plays a significant role in enabling plants and ecosystems to adapt to changing drought patterns.