As trees transpire, they cool their surrounds. In urban areas, where the heat island effect may at times be quite significant, such transpirational activity is necessary for reducing both day-time and night-time temperatures. However, transpiration is not constant, and trees therefore may provide different ‘levels’ of cooling at different parts of the day. Efficacy may in fact be weather-related, or even impacted by the permeability of the ground surrounding the tree’s rooting system – this is because such factors influence how much water is available to the tree. In turn, transpiration rates are affected, which has a knock-on effect of impacting upon ambient temperatures. In this study, the authors investigate seven of the most common urban tree species (Acer platanoides, Aesculus hippocastanum, Betula pendula, Fagus sylvatica, Prunus serrulata, Quercus robur, and Tilia europaea) in Gothernburg, and ascertain their benefits with regards to cooling via transpiration.
From (most of) the study trees (which were ‘avenues’ or groups of a single species), the authors measured stomatal conductance, transpiration rates, and photosynthetic rates both during the day- (solar noon) and night-time (1-4 hours after sunset) on warm summer days; one day where no clouds were present, and one where clouds were moderately present. Certain tree species, including Acer platanoides, only had their measurements taken on one day. The reason for some tree species only having one reading was because of their locations (the map below highlights all locations across the city), and because the equipment used to take the readings was malfunctioning. Data relating to leaf area, solar radiation, and energy loss (heat) beneath the tree canopies was also collected, amongst other things (the table below the map shows some of the collected data relating to the trees surveyed).
In addition to the tree data captured, meteorological data was also taken from the sites. Air temperatures, relative humidity levels, atmospheric pressure, recent rainfall, and so on, were captured, and the article itself contains a very large table outlining the weather conditions at each site.
Results
The authors found that day-time stomatal conductance of leaves was two-times greater in sun leaves than in shade leaves. The greatest differences between stomatal conductances during day and night were observed in Quercus robur and Prunus serrulata along streets and in Tilia europaea within parkland, whilst the lowest difference was observed in Aesculus hippocastanum and Tilia europaea along streets.
Transpiration rates were also found to be higher in sun leaves; on average, three-times higher. However, there was marked variation between species, with street-based Quercus robur and Betula pendula sun leaves having the greatest transpiration rates of the entire study on the sunny, warm, dry day. In addition, park-based Tilia europaea sun leaves were found to transpire at almost twice the rate of its street tree counterparts. Not only this, but individuals growing on streets with wide lawns transpired at higher rates than ones growing on narrow lawns. Transpiration may therefore be location-specific, even when assessing the same species. Such transpirational differences were not found within shade leaves.
Despite the above, and induced by the falling solar radiation levels and air temperatures, the transpiration rates of all individuals begun to drop 2-3 hours before sunset. However, transpiration did persist following sunset (at a rate of up to 20% of daytime rates) – for at least four hours (transpiration may have persisted after, though the data collection stopped 4 hours after sunset). Such night-time rates were also different between species, with Prunus serrulata transpiring at a much greater rate than Betula pendula. The other species all sat within the range these two species established, though day-time transpiration rates were somewhat correlated to night-time transpiration rates – as in, trees that transpired more during the day also did so at night.
Both stomatal conductance and transpiration rates were also shown to be markedly impacted by recent rainfall episodes – particularly over the 5-30 days prior to the data collection. Where there had been less rainfall, stomatal conductance and transpiration was reduced. However, impermeable surface to permeable surface ratios in the immediate surrounds to the study trees was also determined to be a strongly-influencing factor, in terms of how much of the rainfall actually was made available to the trees. Trees that sat in parks or along wide street verges had higher stomatal conductance rates than those on narrow verges – particularly in sun leaves. Across all species, Quercus robur and Prunus serrulata had higher transpiration rates compared to the other tree species, at similar levels of ground permeability.
Such foliar activity led to between 9-64% of incoming short-wave solar radiation being ‘reflected’ back as latent heat energy (associated with transpirational cooling). The lower end of the spectrum was occupied by Tilia europaea residing within narrow-verged and heavy-traffic streets, whilst Quercus robur occupied the upper echelons. Such latent heat ‘reflection’ lead to the reduction in heat energy beneath the tree canopies, and the data collected at each site is shown below. However, it was found that day-time cooling effects were somewhat – or fully – negated by the ‘mixing’ of air in the vicinity of the trees (the air during the day was more volatile, perhaps induced by higher temperatures), meaning that transpirational cooling was actually more effective later during the daylight hours and soon after sunset.
What does this data mean?
In essence, it can be asserted that both the species of a tree, and its location, will influence upon the cooling effects associated with its presence. For cooling effects to be most significant, not only should species selection be carefully undertaken, but urban trees should be provided with the necessary levels of moisture required to sustain ‘healthy’ operations (irrigation may be necessary, in periods of drought – even for larger trees), and surrounding surfaces should be permeable in nature (and if not, irrigation should be strongly considered). For the best cooling effects, trees should be situated within parks. However, as the benefits of tree cooling are most desired in streets where there is a greater accumulation of heat energy, it may have to be accepted that constituent trees will not operate to maximum beneficial capacity – though not utilising trees should not even be considered, as all trees provide some benefit. It may indeed be a case of selecting species that are more effective at cooling the street localities, though also tolerate urban conditions and will not cause damage or other adverse impact in their environment (for small streets with narrow verges, Prunus serrulata may be a good choice of tree – assuming its roots do not cause damage to the pavements).
Because this study suggested that transpirational cooling during the day might not be overly significant (or even non-existent), and as such a conclusion is in-line with other studies, there must be a degree of acceptance that trees are not miracle-workers. Of course, the shading (shadowing) trees provide will lower ambient temperatures (at which point, larger and denser-canopy trees will be preferable), though it is during the later parts of the day (and the early night hours – at a point, transpiration no longer impacts upon cooling during the night) when trees will cool the air, through transpiration, most significantly. This is still nonetheless highly important, as the removal of heat during the evening (and even during the night) can be of marked benefit to residents – not only is it difficult to sleep when it is warm (and costly to cool a property), but increased temperatures have also been attributed to increased restlessness and crime.
Source: Konarska, J., Uddling, J., Holmer, B., Lutz, M., Lindberg, F., Pleijel, H., & Thorsson, S. (2016) Transpiration of urban trees and its cooling effect in a high latitude city. International Journal of Biometeorology. 60 (1). p159-172.
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