226 Writing Team 2012), the contribution of urban trees to directly offset this is not insubstantial. Previous work by Nowak and Crane (2002) estimated trees in New York City alone may store as much as 1.2 mil- lion metric tons of carbon, with an annual rate of net carbon sequestration of more than 20,000 tC yr −1 Using a valuation of $20.3 tC−1 . (Fankhauser 1994), Nowak and Crane (2002) estimated that the 700 mil- lion metric tons of carbon stored by urban trees in the US had a value of more than $14 billion in 2001. The 3 small trees in our study gained a total of 52.1 kg of aboveground dry mass, thereby sequestering 26.0 kg of C, or 95.6 kg of CO2 , during the duration of the study (June 2018 to August 2019, the time when the Spruce 1 sensor saturated)(Table 1). Adding 30% to conservatively account for changes in root mass increases the amount of carbon sequestration to 31.3 kg of C, or 115.1 kg of CO2 . Estimates like these, based on precise data from dendrometers in a variety of urban settings, would help refine our estimates of car- bon storage provided by urban trees. As robust esti- mates of carbon storage and cycling become more important in light of climate change, land use change, and urbanization, networks of dendrometers in urban settings can play an important role in providing urban foresters and arborists a reliable tool for monitoring, quantifying, and valuing the urban contribution to biological carbon sequestration. CONCLUSIONS We installed high-resolution dendrometers in an urban setting and streamed the data to a project web page in near-real time. The resulting time series pro- vides insights into the growth of specimen trees growing in a residential area. From this time series we were able to assess the relationship between fluc- tuations in stem diameter and the ambient environ- mental conditions, the phenology of stem growth, and the magnitude of growth and carbon sequestra- tion. Although we had hypothesized that stem growth would respond most strongly to light and tempera- ture, our results demonstrate that these trees have a discernible response to environmental water relations (soil moisture, precipitation, and VPD) despite being irrigated. We also introduce new analytical tech- niques, HMM and CITs, to interpret established ones (multiple linear regression models and the zero- growth model). Overall, we suggest CITs as a useful way to search for important environmental variables ©2021 International Society of Arboriculture Griffin et al: Stem Radius Fluctuations in Urban Trees and thresholds, which can then be used to test more specific hypotheses related to the environmental con- trol of stem growth and hydrology. Furthermore, as compared to the linear models, the decision trees are more sensitive to unique subsets of the time series and thus can identify important relationships that the overall fit of the linear model might miss. We show that 3 modest ornamental trees sequestered 31.3 kg of C (or 115.1 kg of CO2 ) between June 2018 and August 2019. We further demonstrate that the stem expan- sion is primarily a nocturnal phenomenon, while stem shrinkage corresponds to the daylight hours. Although there were some important differences among our 3 trees, there are obvious general instantaneous rela- tionships with temperature, VPD, and precipitation, suggesting water relations control growth. Detailed information like this is rare in urban environments and yet facilitates a deeper understanding of dynam- ics that is useful to professionals involved in urban ecosystem management. Dendrometers can be a powerful tool in urban set- tings and can provide care and maintenance insights for land owners and land managers in urban areas where trees provide important ecosystem services. The data streaming to the web would allow easy access to data by classrooms and nature centers, pro- viding a more proximate connection for urban resi- dents to the natural world (Kitchin 2014; Galle et al. 2019). Fluctuations in stem diameter happening over remarkably short timescales and tied to the local environmental conditions surprise people and bring tree biology to life. Sharing these data with the public at nature centers and during other outreach opportuni- ties, or working with teachers in a K–12 educational setting, has always piqued curiosity and stimulated deeper thinking and inquiry in STEM topics. These real-time data sets also provide an opportunity for teaching the scientific method, beginning with obser- vation, then generating hypotheses, data analysis, hypothesis testing, and finally interpretation, and the ability to begin the process again immediately and with new information. Livestreaming tree-growth data facilitates an even deeper understanding and appreciation of the biological activity of trees and the services they provide in urban environments and thus can be a powerful tool connecting urban social and ecological systems.
September 2021
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