Influences of urban hydrology and channel morphology on the dry-season water status of riparian trees in a Mediterranean climate
Urbanization causes changes in hydrology and stream channel morphology that may alter the water resources available for riparian vegetation, potentially driving shifts in riparian plant communities. Stream channel incision is a common consequence of hydrological changes due to urbanization, and is associated with lowered water tables and a reduction in riparian water availability. Yet in regions with extended dry seasons, urban irrigation and wastewater can also dramatically increase dry-season stream flows, potentially providing a water subsidy for riparian vegetation. This study investigated the effects of channel incision and stream flow on the water status of streamside valley oak (Quercus lobata) trees in the Mediterranean climate of Sacramento, California, where streams generally lose to groundwater. Near the end of the 2014 dry season, midday stem water potential (Ψ) was measured in 48 Q. lobata trees growing along low-order streams that varied by level of channel incision and the presence or absence of dry-season stream flow. For each tree, we recorded diameter at breast height, the presence or absence of water in the adjacent stream reach, and channel incision, measured as the height of the tree’s root crown above the thalweg of the stream.
Stem Ψ for the 48 trees ranged from -1.42 MPa to -3.52 MPa, indicative of moderate to severe water stress. Linear mixed effects modeling showed a significant (p < 0.01) negative effect of channel incision and significant (p < 0.01) positive effects of diameter at breast height and presence of stream water. The results suggest that Q. lobata trees utilize water from dry-season stream flows to improve their water status, and that channel incision inhibits their ability to do so. Urban water management and restoration strategies that influence stream channel form and flow regimes may therefore impact the growth and productivity of riparian trees, potentially contributing to changes in riparian plant community composition. The results also suggest that small trees may be particularly sensitive to changes in channel form and dry-season flow due to urbanization, and that larger trees may be more resilient. Mature riparian tree communities may thus reflect conditions prior to urbanization, and the full effects of hydrologic and channel form alteration may not be seen until these legacy trees have been replaced.