Will urban fruit trees be first to stop bearing fruit under future climate conditions?

Background/Question/Methods

Human-induced increases in air temperature due to global climate change and urban densification have impacted plant phenology in different ways, with some species advancing and others delaying events such as leaf or flower budburst. Urban plant phenology has been successfully used to detect urban heat islands and to assess climate change impacts on biological systems; however determining the chilling and forcing temperatures that control budburst is notoriously difficult. Furthermore, it is challenging to obtain fine-scale temperature measurements across an urban environment, which contributes to a poor understanding of how budburst in species is affected by microenvironments. Using the city as a living laboratory, we determined how urban microclimate affects the timing of spring leaf and flower budburst in urban trees. We used 40 clones of Malus x ‘Spring Snow’ crabapple trees across Boulder, Colorado to observe the timing and proportion of spring budburst in 2019, 2020, and 2021. At each site, we also measured hourly air temperature, characterized site level variables. To tease out the chilling requirement, we made clippings of dormant branches from Spring Snow trees at each site, grew them hydroponically under greenhouse conditions and observed budburst after various chilling and forcing accumulation in the field.

Results/Conclusions

We found budburst to be positively correlated with site minimum air temperatures for our three study years. Land use zone and impervious surface cover at 150-500 meters from the tree was predictive of air temperature. From the clipped branches observed in the greenhouse, we learned that chilling requirements were met earlier for business districts compared to park land use zones. Because Spring Snow is a low-chill crabapple variety, forcing temperatures were the main driver of budburst variation in the field.Our data suggests urban centers may be the first place to observe instances of no seasonal tree flush and blooms and parks may be the last for urban fruit trees on the edge of their geographic ranges. Areas with the highest impervious surface cover, and consequently earlier bud break, may also have increased risk of frost damage under warm “false springs”. While Spring Snow trees in Colorado are not currently at risk of insufficient chilling, trees in the southern ranges may not overcome dormancy without at least 474 chilling hours. Lack of chilling in the southern ranges for other temperate clonal fruit trees with higher chilling requirements may pose even greater consequences for agriculture operations such as urban fruit orchards.