Arboriculture & Urban Forestry 47(5): September 2021 evolution of drought-resistant xylem and a general increase in the carbon investment in xylem tissue (Pittermann et al. 2012), yet drought sensitive since the tree rings of many members of this family faith- fully record the occurrence of droughts (e.g., Sano et al. 2009; Buckley et al. 2017, 2018; B. Buckley per- sonal communication). Previous work on cedar (Abe and Nakai 1999; Abe et al. 2003) shows a detailed response of xylem development to stem water status, suggesting cedar is highly sensitive to water avail- ability. Just as with spruce, the linear model for cedar had a greater capacity to account for variation in the rate of growth than in the magnitude of growth, yet even the rate model only accounted for 19% of the variance. Clearly, our ability to predict short-term fluctuations in the rates and amounts of growth requires further study, particularly in the urban envi- ronment, and a more complete understanding of the physiological controls of cambial activity. We also note the potential influence of lag times and time accumulation effects of climate on tree growth, which could decouple the high-resolution measurements of growth from the environmental drivers on these short timescales. Thus on the whole, we find only partial support for our hypothesis that growth in urban irri- gated trees would be more strongly controlled by environmental factors related to energy gain (tem- perature and solar radiation) than by factors related to water use (precipitation, VPD, and soil moisture). Seasonal Growth Trends Tracking the phenology and climatic response of trees in urban settings is critical to understanding both the ecosystem services urban trees provide and how global climate change might affect these services. Vegetation phenology, and particularly tree growth, is often studied using remote sensing surveys of spec- tral reflectance (e.g., Ren et al. 2018), yet these stud- ies can be challenging in urban settings due to the highly heterogeneous urban landscape and the rela- tively large spatial scales and infrequent flyover times of satellites (Melaas et al. 2016). Dendrometers can quantify both the short- and long-term phenology of tree growth, allowing urban arborists to gain insights into the specific environmental conditions stimulat- ing and retarding tree growth, affording the opportu- nity to initiate tree care when it will be most effective. The onset of growth is likely triggered by warming temperatures (Begum et al. 2013) and changes in day 225 length (Oribe and Kubo 1997; Chang et al. 2020). In the urban setting studied, we find evergreen tree growth begins in late April and continues through the spring, summer, and fall, ending in late September (spruce) or mid-November (cedar). Spruce growth is coincident with a day length of 13 hours, 20 minutes and an accumulated growing degree day of 69.7 °C, whereas the initiation of radial expansion in the cedar is associated with longer days and much higher ther- mal accumulation (13 hours, 45 minutes and 160.6 °C), possibly due to bark thickness, which may affect the relative rate of warming and the breaking of cambial dormancy (Oribe and Kubo 1997). As urban environ- ments are typically warmer than their rural counter- parts, they may have an earlier start to the growing season (Yang et al. 2020). A monitoring system along an urban-to-rural gradient could provide additional insights into how urban environmental conditions affect tree growth. Detecting the onset of radial growth may go unno- ticed, as it begins well in advance of leaf growth of the more common deciduous trees native to our study region and even before leaf and stem elongation in many evergreen species (Oribe et al. 1993; as cited in Oribe and Kubo 1997). In temperate ecosystems, radial growth continues throughout the spring, sum- mer, and early fall and demonstrates a clear seasonal pattern, related to cold hardiness and surviving the winter (Catesson 1994). Cold temperatures, a decreas- ing temperature trend, photoperiod, and changes in light quality (Chang et al. 2020) all cue the end of growth and cold hardening. After a brief resting phase when cell division is not possible, the cambium will remain in a quiescent stage (Catesson 1994), capable of cell division once environmentally favorable con- ditions return. Dendrometers provide the opportunity to observe and even quantify these various stages of activity; placing dendrometers in urban settings enables the study of how the built environment might alter the growth and cold-response of trees. Without real-time dendrometer data, the evergreen growth form of our study trees would have made the pheno- logical timeline very difficult to detect, and thus we suggest that networks of dendrometers in urban areas would also provide an effective tool for arborists to monitor growth and precisely time pruning, irriga- tion, and nutrient supply. While urban areas are responsible for more than 70% of CO2 emissions (Global Energy Assessment ©2021 International Society of Arboriculture
September 2021
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