224 framework for the study of urban trees and forests and connect the observations to the underlying phys- iological processes that are themselves responding to environmental variation on similarly short timescales. To distinguish periods of growth from those of hydrologically driven fluctuations in stem radius, we used an existing model for partitioning the dendrom- eter data that “assumes zero growth during periods of stem shrinkage” (ZG model, Zweifel et al. 2016). This model limits its definition of growth to periods of time when new maximum stem radii are achieved and assumes stems cannot both shrink due to water deficit and grow due to cell expansion simultane- ously, which has been shown (Lockhart 1965; Steppe et al. 2006; Zweifel 2016; Zweifel et al. 2016). We applied an HMM to the emitted states of the ZG model, which has the advantage of identifying blocks of time dominated by growth without requiring the dendrometer readings be monotonically increasing, so that even during a growth period, growth need not be observed between every 5-minute increment. The growth periods thus identified by the HMM help avoid pseudoreplication that would result from treat- ing ZG “growth” time steps independently. The ZG and HMM methods are shown for two 7-day periods, one featuring consistent growth and the other show- ing a period of water limitation leading to stem shrinkage and the cessation of growth (Figure 4). During the 7 days in June, the ZG model identified 64 individual growth periods, over 70% of which are 15 minutes or shorter; the HMM extracted 3 periods of growth over the same interval. With measurements being collected so frequently by the dendrometers, the HMM provides an effective way to identify sepa- rate growth periods for comparing growth rates to average environmental conditions, as discussed below. We found both air temperature and soil and atmo- spheric moisture affected growth, contrary to what we expected from irrigated trees. In their native envi- ronment of boreal forests (Abrahamson 2015), white spruce is thought to be commonly limited by both cool temperatures (Achuff and La Roi 1977) and soil moisture (McGuire et al. 2010). Despite daily water- ing, the CIT for Spruce 1 in our experiment nonethe- less identified low soil moisture along with colder soil temperatures as retarding stem growth. Previous studies have found that fluctuations in stem diameter are positively correlated with air (Ortuño et al. 2006; Devine and Harrington 2011; Oberhuber et al. 2015; ©2021 International Society of Arboriculture Griffin et al: Stem Radius Fluctuations in Urban Trees Dong et al. 2019), not soil temperatures. We suspect in our study, soil temperatures were buffered from short-term fluctuations in onshore winds and thus represent a more stable representation of the environ- mental temperature controlling growth of the stems inside these dense coniferous canopies. From the regression analysis, we conclude that spruce growth (the amount of expansion during the growth periods) is best explained by a model that contains informa- tion about air temperature, soil moisture, VPD, and all of the interaction terms (Table 2). All of these vari- ables are directly or indirectly related to environmen- tal control of stem water status and have contributed to previous attempts to interpret dendrometer signals (Dong et al. 2019). While our best fit model can account for a significant amount of variation in the growth rate observations, it may have limited predic- tive capability, as more than 70% of the variance is unaccounted for. The reason for this lack of predic- tive power is unclear but may indicate a need to include information on substrate availability, time lags, or source/sink dynamics (Savidge 1983, 1996, 2001; Dengler 2001; Vaganov et al. 2011). Additional dendrometers spanning urban to rural ecosystems might enable further understanding of whether this decoupling from environmental conditions is a unique product of the urban environment. We observed a similar pattern for cedar as described above for the spruce, yet the individual parameter estimates were quite different and indicate a stronger response to soil moisture and VPD. This difference may reflect the evolutionary history of this species. Cryptomeria japonica is an endemic monoe- cious conifer of Japan (Kimura et al. 2014) thought to have a distribution corresponding to moderate cli- matic conditions, including annual precipitation of at least 1,200 mm and annual temperature > 5.0 °C (Tsu- kada 1982; as cited by Kimura et al. 2014). The CITs suggest low VPDs are the strongest determinant of growth in this tree, with critical thresholds of 0.23 kPa (growth) and 0.31 kPa (growth rate). Previous work on stem shrinkage suggests the response is to an interaction between soil water (the source for stem water) and VPD (the driving force for water loss via transpiration)(Hinckley and Bruckerhoff 1975; Gar- nier and Berger 1986; Grossiord et al. 2017) and is consistent with our findings here for the constraints on stem growth. In general, trees from the Cupressa- ceae are believed to be drought tolerant due to the
September 2021
| Title Name |
Pages |
Delete |
Url |
| Empty |
Ai generated response may be inaccurate.
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success.
Downloading PDF
Generating your PDF, please wait...
This process might take longer please wait
