Can trees record impacts of urban growth on watersheds?

Expansion of urban areas into natural habitats often compromises the health of watersheds and ecosystems. One impact of urbanization is the leakage of water from the municipal infrastructure into natural streams. This leakage may have an unintended positive impact on riparian ecosystems by buffering them against the effects of drought and other climatic stresses. In central Texas, bald cypress (Taxodium distichum) trees are typically found in streamside ecosystems, and their growth is strongly limited by available water. As such, bald-cypress tree-ring chronologies may serve as indicators of how riparian systems respond to urbanization and associated water leakage.Here, such a buffering effect is evaluated for the Waller Creek watershed in downtown Austin, Texas. This watershed has become highly urbanized over the 20^th century and substantial amounts of its flow are due to municipal water leakage, even in times of intense drought. Two tree cores were taken from each of twenty bald cypress along Waller Creek. The cores were measured for ring thickness, crossdated, and used to generate a growth-increment chronology. This chronology spanned 1944-2017 and was compared to two existing chronologies generated from rural bald cypress records elsewhere in the region. Waller Creek trees were characterized by anomalously low synchrony within and among cores, suggesting that environmental constraints, notably drought, did not limit growth. The chronology correlated with the summer (Jun-Aug) Palmer Drought Severity Index, but this relationship was unstable over time, declining from an r of 0.6 in the 1940's to 0.0 in the late 1980's. Furthermore, the correlation between Waller Creek and a rural bald cypress chronology (which better captured regional drought signals) also declined dramatically from an r of 0.5 in the 1940's to 0.0 by the 1980's. These results indicate that in urban ecosystems, resident trees are decoupled from climatic stress, with the unintended effect of buffering these ecosystems from drought, thereby enhancing resilience to climate change.