Consequences of the senescence growth phase on the performance of treed bioretention cells

Bioretention cells vegetated with trees were studied to understand the changes in performance for mitigating stormwater quantities and qualities during the senescence period. Stormwater runoff simulations were conducted in a temperate climate for 1.1-, 1.5- and 2-year return period storm events applied to field-based treed bioretention cells planted with a mix of Betula nigra , Betula nana , and Salix lutea trees, and grassed bioretention cells planted with turf grass. Eighteen separate storm events (six for each return period) were applied at various times during the 2020 summer growth period starting with the onset of senescence in late August to early September through to abscission in early October. Changes in water quality and quantity performance were analyzed over the senescence period for several parameters including water volume retention, chemical oxygen demand, total nitrogen (TN), total organic nitrogen (TON), total phosphorus (TP), orthophosphate, and total suspended solids (TSS) using non-parametric statistical tests. These were supported with additional analysis of daily evapotranspiration (ET) and antecedent moisture content (AMC). Correlations with the timing of senescence on the treed bioretention cell's performance were visible and significant (α = 0.05) over the testing period for water retention, TP, TN and orthophosphate. However, these results were not observed, or significant, for the grassed cell. The analysis showed that the treed cell's contribution to contaminant removal is highly correlated with changes in ET and AMC; whereas the grassed cell showed changes correlated only to AMC. This work demonstrates that the senescence period will lead to diminished water quantity retention and changes in nutrient retention and other stormwater contaminants from bioretention cells vegetated with trees. • Senescence significantly impacts the performance of bioretention cells with trees. • Impacts of seasonal changes in turf-grass were not observed. • Water volume and phosphorus retention decline significantly.