2017 ASHS Annual Conference

In dense urban settings tree pits are often the only soil area not covered by impervious surface. Current trends, however, favor covering even these limited exposed soil surfaces, which may have consequences for tree growth and resilience, stormwater mitigation, and soil temperature, especially when coupled with climate change. To study tree response to porous pavement, and associated alterations in soil water distribution and temperature, we planted 12 Platanus ×acerifolia ‘Bloodgood’ whips in simulated urban tree pits in two distinct physiographic regions of Virginia: Valley and Ridge (Blacksburg) and Coastal Plain (Virginia Beach). Six tree pits were covered with porous resin-bound gravel pavement, and six were left as bare soil. We monitored root growth (with minirhizotrons), stem diameter, soil water content at 10-, 20-, 30-, 40-, 60- and 100-cm, and temperature at 10-, 30- and 60-cm depths over two years. In Blacksburg, soil water content at 100-cm depth during the second growing season was significantly higher under pavement, suggesting less water withdrawal by roots from deeper soil regions. Roots under pavement grew more quickly and were shallower than in bare soil, possibly due to higher soil water content at 10 cm under pavement. After two years, 28% of minirhizotron frames indicated root presence within the first 10 cm of soil under the porous pavement compared to 17% in bare soil. Both bare and pavement-covered soil had roots in 29% of minirhizotron frames at 37-47 cm depth. In Virginia Beach, trees in pavement initially had fewer minirhizotron frames with roots, but no differences were evident by the end of year two, although root dry weight averaged 81% greater under pavement. Similar to Blacksburg, trees in pavement had shallower roots (21% minirhizotron frames with roots within top 10 cm of soil, while only 3% at 37-47 cm depth), compared to bare soil (17% and 12%, respectively). Pavement may have extended the root growing season as temperature in October averaged 1.1°C (Blacksburg) and 1.2°C (Virginia Beach) higher at 10 cm than in bare soil. Trees in pavement grew larger at both sites (29% greater stem diameter in Blacksburg, and 51% greater in Virginia Beach). Results suggest that covering tree pits with porous resin-bound gravel pavement immediately after planting promotes faster tree establishment and growth, but encourages shallower root systems, which may lead to infrastructure problems, reduce potential for water extraction from deeper soil layers, and compromise drought resilience in trees.