Plants change the timing of leaf emergence and other recurring biological events (their phenology) based on environmental cues such as temperature. In wildlands, air temperature may differ among microclimates within a landscape. In urban environments, human-built infrastructure can further diversify microclimates within a smaller area such as a city block. For example, the proportion of pavement, buildings, and parkland can impact local air temperatures due to differences in surface heat retention and shading, however it is not clear how urban land cover features and associated air temperatures may be affecting spring tree growth.
The purpose of our study is to determine how urban microclimate affects the timing of leaf budburst in urban trees. We used 36 clones of Malus x ‘Spring Snow’ crabapple trees across Boulder, Colorado to observe the timing and proportion of budburst and early spring growth. At each site, we measured hourly air temperature adjacent to the Spring Snow tree and characterized the land use type as commercial, high density residential, low density residential, or park. We also made weekly clippings of dormant branches from trees at each site, grew them hydroponically under greenhouse conditions, and observed budburst in 2019.
Results/Conclusions
Over the February-May 2019 study duration, we found time of budburst at our 36 field sites to be positively correlated with air temperature. Mean air temperature and Julian date of budburst were significantly different among commercial, residential and park land cover classes. In the commercial areas, mean air temperature and the Julian date at which crabapple buds had burst was significantly higher and earlier, respectively, than in all other land-use types. Over the eight-week greenhouse study, leaves emerged on branches clipped from commercial sites an average of 1-2 days earlier in commercial sites compared to residential and park sites (commercial = 9.03 days +\- 0.39 SE, park = 10.1 days +\- 0.33 SE, residential = 10.6 days + /- 0.29 SE); and up to 12 days earlier than other land cover classes during certain weeks.
Successful tree bud break has many benefits for city sustainability goals such as sequestering carbon, slowing water runoff, shading to mitigate heat, improving air quality, and provisioning of food and materials. Understanding how features of our built environment affect air temperature and consequently, may accelerate or delay spring tree growth, could influence greenspace management decisions about planting locations, canopy diversity, maintenance, and agricultural schedules.