Assessing the Transpiration Rates of Urban Trees in Different Management Contexts

By Laura ThomsonDec 9 2021

Urbanization has led to drastic changes to hydrologic systems due to the replacement of vegetated areas with impervious surfaces, modification of overall water and sediment budgets, and a decrease in stream complexity.

Image Credit: Aleksandrs Muiznieks/Shutterstock.com

An increase in impervious surfaces causes more stormwater runoff, carrying pollutants (such as sediments, metals, and nutrients) from built surfaces into adjacent water bodies and eventually to larger watersheds.

Stormwater runoff causes major issues such as flooding, impaired water quality, and eroded stream banks. Managing stormwater runoff is crucial to improve water quality, ensure healthier communities, and avoid urban flooding. Although the mitigation of urban impacts on hydrology has always depended on engineered solutions, recent strategies have led to innovations combining nature-based solutions and ecological design in cities.

Trees and forests exhibit hydrological functions that minimize stormwater runoff, lower flooding risk, and enhance water quality in developed areas. Standard types of green stormwater infrastructure include rain gardens, green roofs, rainwater harvesting systems, bioretention systems, bioswales, and permeable pavement.

Green stormwater infrastructure is engineered to mitigate urban water quantity and quality issues by groundwater recharge, flood control, and bioretention. However, it can also support secondary ecosystem services such as biodiversity enhancement, carbon sequestration, optimization of human well-being, climate change adaptation, and optimized socio-ecological connectivity.

Urban forests and trees can serve several of these ecosystem services and manage stormwater by various ecohydrological mechanisms such as evaporation, transpiration, stormwater storage in the soil, interception of runoff, stemflow, and throughfall.

Ponte, S. et al.'s study identifies whether transpiration rates of urban trees of the same species differ based on various management contexts and whether the relationship between transpiration and environmental drivers varies in these contexts. Management context was used to mean different urban tree configurations in relation to ground cover, canopy structure, stem density, and location in the built environment that would impact sap flux density.

Results

Microclimate data were collected from June to November 2018. As shown in Figures 1a and 1b, for the study sites, the main everyday air temperature varied from 6 °C to 30 °C and the average temperature across the entire period was 22 °C. Figures 1c and 1d show that for the closed canopy site, the average relative humidity was 78.1%, and for the single and cluster sites, it was 78.4%.

The estimated VPD varied from 0.03 to 1.9 kPa, where the average estimated VPD was 0.67 kPa for the study sites (see Figure 1e,f). During the study period, the total amount of rainfall for the closed canopy site was 284 mm, and for the single and cluster sites, it was 251 mm.

For the single, cluster, and closed canopy sites, the average daily soil moisture varied from 16%–45%, 19%–39%, and 14%–40%, respectively (Figure 1i,j). Table 1 illustrates the average soil moisture content for the single, cluster, and closed canopy sites, which were 30.6%, 27.7%, and 29.8%, respectively.

Figure 1. Microclimate in Baltimore, MD and Gaithersburg, MD from June to December of 2018. Mean daily air temperature for closed canopy site (a) and single and cluster sites (b); mean daily relative humidity for closed canopy site (c) and single and cluster sites (d); mean daily vapor-pressure deficit (VPD) for closed canopy site (e) and single and cluster sites (f); daily total precipitation for closed canopy site (g) and single and cluster sites (h); mean daily soil moisture in the closed canopy (i), single and cluster sites (j). Due to the later date of soil moisture sensors installation, there was a gap in the data for the closed canopy site. Image Credit: Ponte, et al., 2021

Table 1. Characteristics of study trees, including tree size, canopy width north–south (N–S) and east–west (E–W), daily sum of sap flux (J_S), and daily volumetric water content (VWC). Source: Ponte, et al., 2021

Values are mean ± SE.

The management context had a considerable impact on the daily sum of sap flux density (J_S; see Figure 2, repeated-measures ANOVA, p-value < 0.0001). The median J_S for the single site was more than three times that for the closed canopy site (257.2 as against 78.0 g H₂0 cm⁻² day⁻¹; see Figure 2).

Figure 2. Box plots of the daily sum of sap flux density (J_S) across the three management contexts. Unique letters above boxes indicate a significant difference among treatments based on post hoc analysis. (Figure created using R software version 4.0.5). Image Credit: Ponte, et al., 2021

As shown in Figure 3, the single site was found to have the highest J_S of all the study sites. From Table 1, it is clear that the mean J_S at the single site was almost three times that for the closed canopy site.

Figure 3. Time-series of the mean daily sum of sap flux density (J_S) in the outer 2 cm measured in 2018 for the three management contexts. Image Credit: Ponte, et al., 2021

The seasonal variation of J_S in relation to VPD and soil moisture was analyzed by splitting the sap flux data into three seven-week time periods (see Tables 2 and 3). There was no significant relation between J_S and soil moisture, besides early and late summer at the closed canopy and single tree sites (see Table 3). From the R² values of these relationships, soil moisture accounts for very little of the J_S variability (R² values varied from 0.001 to 0.17, see Table 3).

Table 2. Average parameters and R² of daily sum of sap flux density (J_S) versus daily average vapor pressure deficit (VPD) relationships of the form y = a * ln(VPD) + b. Source: Ponte, et al., 2021

Values are mean ± SE. Unique letters indicate cross site differences within each time period based on post hoc analysis.

Table 3. Regression parameters and R² of daily sum of sap flux density (J_S) vs. daily average soil moisture content relationships of the form y = a * (soil moisture) + b. Source: Ponte, et al., 2021

Significant relationships are indicated in bold font.

From the repeated-measures ANOVA of the J_S-VPD regression parameters, it was evident that the slope and intercept were considerably different between closed canopy and single sites over all time periods (see Table 2 and Figure 4).

Figure 4. Mean daily sum of sap flux density (J_S) as a function of VPD during early summer (a), late summer (b), and fall (c). Values are mean ± SE. Image Credit: Ponte, et al., 2021

Discussion

This study analyzed the effects of management contexts on the transpiration rates of red maple and the connection between transpiration and environmental drivers to identify the effects of various urban tree configurations on J_S.

The results of this study are consistent with an earlier study in which California sycamore street trees exhibited the highest sap flux density than trees in unmanaged and irrigated forest sites. The regression analyses in this study showed that the single and cluster sites exhibited a steeper slope compared to the closed canopy site over all time periods (see Table 2).

This study offers an understanding of how various management contexts and microclimate factors impact the ecohydrological fluxes of a regular urban tree species. This is a critical consideration for creating stormwater crediting programs as urban trees can be more precisely combined into planning efforts, enhancing the effectiveness of green stormwater infrastructure networks.

Methodology

This study was performed in 2018 at two cities in Maryland, USA—Baltimore and Gaithersburg —reflecting three urban forest management contexts: single trees found over turfgrass, cluster of trees found over turfgrass, and trees found in a closed canopy forest, including a leaf litter layer (see Figure 5).

Figure 5. Study site locations in the State of Maryland, USA. Illustrations represent the management contexts in each site. Image Credit: Ponte, et al., 2021

Data collection started in June 2018 and all sites were situated in the Piedmont Plateau physiographic province. The soils in this province are moderately sloping, deep, well-drained upland underlain by semi-basic, acidic, and mixed basic rocks. The dominant piedmont soils in this province are Ultic Hapludalfs.

Maximum evapotranspiration is experienced in the month of July, and groundwater recharge takes place from mid-September to March every year. The field sites were chosen depending upon safety, accessibility of sites, and the potential to provide security for the monitoring equipment.

A weather station was set up in the open at each field location to characterize prospective climate drivers of sap flux density, such as precipitation, air temperature, and relative humidity.

Thermal dissipation Granier sensors developed using two cylindrical probes of 2-mm diameter were inserted 2 cm into the sapwood of the bole of 18 monitored red maple trees at a height of 1.4 m. The probes were located about 15 cm apart.

The sensors were covered with aluminum shielding to avoid any interference from rainwater or thermal heating. They were attached to double-shielded cable wires and linked to a CR 1000 data logger with AM 416 and AM 16/32B multiplexers. The voltage differential (∆V) between the ambient temperature of the below reference probe and the upper heated probe was calculated every 30 seconds and recorded as half-hour averages.

Repeated-measures analysis of variance (ANOVA) with linear mixed-effects models helped compare J_S across different management contexts as well as the effect of day of the year. Assumptions of homoscedasticity and normality were checked with the help of QQ and residuals diagnostics plots.

Relationships between VPD and J_S were fit for each tree and for each seven-week time period using natural logarithm functions. Repeated-measures ANOVA was used to test the differences across time for the slope and intercept for significance (p-value ≤ 0.05). A linear regression model was used to fit the relationship between J_S and soil moisture content (%) for each seven-week time period and investigated for overall significance (p-value ≤ 0.05).

Journal Reference:

Ponte, S., Sonti, N. F., Phillips, T. H., Pavao-Zuckerman, M. A. (2021) Transpiration rates of red maple (Acer rubrum L.) differ between management contexts in urban forests of Maryland, USA. Scientific Reports, 11, p. 22538. https://www.nature.com/articles/s41598-021-01804-3.

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