216 of many urban parks programs (Nowak et al. 2018), and aerial photographs (Spurr and Brown 1946) can provide useful insights as to quality and distribution of urban greenery. More recently, technological solu- tions such as aerial LiDAR (Alonzo et al. 2014), drones (Elliott 2016), and satellites (Palomino et al. 2017; reviewed in Nitoslawski et al. 2019) show great potential to add to the understanding of tree growth in urban environments. All of these approaches, how- ever, require repeat observations, which can be expensive and/or infrequent. As a result of the limited spatial and temporal scales of the tree growth data typically obtained, our understanding of the links between the biogeophysical environment and the physiological underpinnings of tree growth in urban forests is similarly limited. Dendrometers provide an alternative method of monitoring radial growth of trees and could provide much-needed high-resolution data in urban ecosys- tems. When mounted to the non-living sapwood/ heartwood of the tree, these highly responsive tree growth sensors can record when new cells are pro- duced by the adjacent vascular cambium and/or the hydration of the xylem, cambium, phloem, or bark (Zweifel et al. 2014). Not only can these instruments provide detailed records of radial stem growth and water use (Steppe et al. 2006; Zweifel et al. 2006), when used in urban settings, the instruments are eas- ily connected to the Internet and can stream data live in real or near-real time. The result is a time series of stem radius that can be used for research, education, and outreach and provides a much-needed link between growth and the environment. In this study, we installed 3 dendrometers in a sub- urban backyard along with a weather station to study the short time-scale interactions between tree growth and meteorological conditions in urban environ- ments. We use a time series spanning 1.5 years to model tree growth and the relationship to local weather and climate. We define statistical relation- ships between growth and light, temperature, soil moisture, and growing degree days to test the hypoth- esis that, in this urban setting where trees are irri- gated, diameter growth is more sensitive to environmental variables related to energy gain (light and temperature) than to factors related to water use (precipitation, soil moisture, and the atmospheric vapor pressure deficit [VPD]). We also discuss the usefulness of this monitoring approach and the ©2021 International Society of Arboriculture Griffin et al: Stem Radius Fluctuations in Urban Trees research, education, and outreach potential of an expanded dendrometer network in urban forests. MATERIALS AND METHODS Location and Plant Material Three adjacent ornamental trees growing in a back- yard in Southampton, New York, USA, were used for this study: two white spruce (Picea glauca [Moench] Voss) and one Japanese cedar (Cryptomeria japonica [L. f.] D. Don). The experiment was established in June of 2018. The native soils of this location are of the Bridgehampton silt loam association (NRCS 2009) and are both well-drained and inherently low in mineral nutrition. Significant soil improvement, mod- ification, and the addition of organic materials and fertilizers has taken place. The study area is irrigated, and thus the study trees rarely experience prolonged water limitation. The climate of Southampton is mar- itime and strongly seasonal, with historical July (the warmest month) minimum and maximum tempera- tures averaging 18 and 27 °C, respectively, and his- torical January (the coldest month) minimum and maximum temperatures averaging −4.4 and 3.9 °C, respectively (https://prism.oregonstate.edu/explorer). Rainfall is evenly distributed throughout the year, averaging 106.43 mm month−1 and totaling 1.28 m yr−1 . Growing degree days were also calculated as the accu- mulated average temperatures above 5 °C following the coldest day of the year. Diameter Growth and Environmental Monitoring A time series of tree diameter was recorded by a point dendrometer (Hise Scientific Instrumentation, Somers, New York, USA). The dendrometer is attached to the stem of the tree using a mounting bracket that main- tains the potentiometer in a fixed position relative to the non-living xylem with a stainless steel 6.35-mm- diameter rod inserted into the stem. All dendrometers were placed on the north side of the tree (to minimize exposure to direct solar radiation) at a height of 1.37 m from the ground surface. Dendrometers were installed on top of the thin bark (after carefully removing any loose bark with a small rasp while avoiding damage to the living tissues), and thus all processes causing changes in stem diameter below the location of the sensor were recorded. These may include: bark swell- ing and shrinking, cambial activity, changes in phloem, xylogenesis, and stem hydraulics. Every 5 minutes
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