Fremont cottonwood seedlings are vulnerable to water stress from rapid water-table decline during river recession in spring. Water stress interactions with microclimate extremes are usually the reason for reduced seedling recruitment. Here, we assessed photosynthetic responses of Fremont cottonwood seedlings to water, light and heat stresses which commonly co-occur in habitats where seedlings are establishing. The methodology included measuring photosynthesis, gas exchange and temperatures of restrained leaves.
Results/Conclusions
Under moderate temperature and light conditions, water stress alone had no effect on photosynthetic function. However, stomatal closure during water stress predisposed Fremont cottonwood leaves to light and heat stress resulting in greater photoinhibition and lower temperature thresholds for heat damage. Natural shifts in leaf orientation from horizontal to vertical that occur 50-60 days after germination reduce heat and light stress, minimizing photoinhibition during water stress. Thresholds for damage by water, light and heat stress, when compared to naturally occurring microclimate extremes, suggest that photoinhibition and reduced assimilation are common for Fremont cottonwood seedlings on exposed point bars where they establish. These reductions in photosynthesis likely have negative impacts on growth and may predispose young (<90 day old) seedlings to early mortality during rapid water-table declines. Interactions with heat and light stress are more important in these effects than water stress alone.