Evaluation of Alnus subcordata for urban environments through assessment of drought and flooding tolerance

The urban environment is stressful and trees experience multiple stresses, including drought, flooding, and extreme heat, all of which are likely to increase under future climate warming and increasing urbanisation. In the selection of tree species to maximise ecosystem services, tolerance to site characteristics such as flooding and severe drought is of critical importance. This study evaluated the suitability of a rare species, Alnus subcordata C.A. Mey (Caucasian alder) from the Hyrcanian forests of southern Azerbaijan,for its functionality as an urban tree. A total of 48 pot-grown, two-year-old saplings of A. subcordata were tested in a greenhouse experiment using a complete randomised block design. Each block contained four replicates of three treatments (waterlogging, drought, control), with 16 plants per treatment. Height differences between treatments were measured, and water status was estimated by determination of midday leaf water potential (ΨL) and stomatal conductance (gs). To estimate drought tolerance reaction in the treatments, leaf water potential at turgor loss (ΨP0) was used together with broken-stick modelling of water status over time. There was a significant difference in tree height between the different treatments. In the drought treatment, A. subcordata plants showed no height increase, while plants in both the waterlogged and control treatments increased in height during the nine-week experiment. Over 63 days of flooding, plant water status was slightly more negative in the waterlogging treatment, but did not deviate essentially from the control. In the drought treatment, plant water status rapidly deviated from the control. There was a significant differencein ΨP0 between treatments, with drought-treated plants showing the lowest value (−2.31 MPa).This study demonstrated that A. subcordata has limited tolerance to drought and seems to rely more on waterloss-avoiding strategies. However, the species may be usable at periodically waterlogged sites, due to its high tolerance to flooding. It could therefore be recommended for wet urban environments and stormwater management facilities, for which reliable guidance on suitable trees is currently lacking.