How riparian trees  manage their water balance

Posted on March 26, 2022

In riverine ecosystems, riparian vegetation supports aquatic and terrestrial biodiversity, stabilises river banks and modulates water flow between the stream and the floodplain.  However, stream channelisation has led to the widespread disconnection and loss of floodplain forests.  Successful riparian restoration schemes are those that create the conditions for plants to become established and survive fluctuating groundwater levels.  The roots of most riparian tree species tap the groundwater or the capillary fringe just above it, and the depth to groundwater structures the composition and distribution of floodplain plants.  If groundwater levels are too low, trees can suffer from drought stress, which is evidenced by a drop in leaf and stem water potentials, the closure of stomatal pores, and reduced transpiration.  On the other hand, if groundwater levels are too high the rooting zone can turn anoxic.  Because there’s a lack of detailed information on the effects of short-term groundwater fluctuations on plant growth and water stress, a team of environmental engineers studied the responses of riparian trees to changing groundwater levels and climate in a braided gravel-bed river (the Maggia) in southern Switzerland.  During two growing seasons, they collected data on diurnal changes in transpiration (as inferred from stomatal conductance and stem diameter) in species of willow (Salix), alder (Alnus) and poplar (Populus), which are the most common flood-tolerant trees along central European rivers.  They worked at two sites close to the river: at one site the average depth to groundwater was 4.4 m, while at the other it was much shallower (2.4 m).  The tree data reflected changing patterns of water uptake during the night and transpiration during the day, with stomatal conductance peaking in the morning and then decreasing until late afternoon.  Daily fluctuations in stem diameter were greatest in midsummer, when evapotranspiration was highest.  Regression analyses showed that at the site with a deep groundwater table, transpiration was driven mainly by groundwater depth, which suggested that trees closed their stomata as groundwater levels dropped.  By comparison, at the site where groundwater was closer to the surface and the depth to groundwater less variable, transpiration was driven mainly by microclimate, especially temperature, radiation and air moisture.  (Stomatal pores open in response to radiation and photosynthesis, but close in response to reduced air moisture).  These results show that riparian trees respond to short-term changes in groundwater levels, even when they’re close to the channels of braided rivers.  Groundwater and floodplain morphology together create conditions that allow dense riparian vegetation to establish, which in turn creates a growth-promoting microclimate with more shade, better wind protection, lower temperatures and higher humidity.  The findings highlight the need to consider microclimatic controls when modelling aspects of plant physiology such as transpiration, photosynthesis and growth.

Reference:  Martinetti, S, et al. 2021. Field evidence of riparian vegetation response to groundwater levels in a gravel-bed river. Ecohydrology 14, e2264.