University of Colorado Denver Denver, Colorado, United States
Abstract: Urban trees provide ecological and societal benefits that make our cities livable. Yet trees are coming under increasing stress from climate change, especially in semi-arid regions of the western US. Predicting how urban tree species with a range of stress tolerances will fare under future conditions requires empirical interrogation of the multiple, interacting factors that shape tree performance. We are leveraging a large-scale tree-planting campaign along an urban canal and recreational trail to assess how initial planting conditions (e.g., water availability, light, topographic position, competition) affect tree survival and performance in the first three years post-planting (the time of highest attrition and economic loss). We assessed three riparian trees native to the study region of Denver, Colorado (Acer negundo, Populus deltoides, and Populus sargentii) and five introduced “street trees” that tolerate drier conditions (Catalpa speciosa, Gleditsia triacanthos, Gymnocladus dioicus, Quercus macrocarpa, and Quercus muehlenbergia). “Thirsty” native riparian trees were planted on bank bottoms near the canal bed while street trees were planted on bank tops near the trail. Preliminary analysis of first-year data shows that survival was near 100% for all species, with the main source of tree death or severe damage being vandalism. Canopy dieback was also low, averaging < 1% ± SE 0.14 in Gleditsia to 13% ± SE 5 in Gymnocladus. Riparian Populus deltoides and Populus sargentii were the only species to dramatically respond to planting conditions in the first year, with height and diameter at breast height of both species increasing with greater distance from a water source (the canal) and decreasing with greater soil moisture availability. This outcome is directly counter to expectations and suggests that managers have more flexibility in where to plant native riparian trees than current heuristic knowledge suggests.
