Transpiration of urban trees and its cooling effect in a high latitude city

Abstract

An important ecosystem service provided by urban trees is the cooling effect caused by their transpiration. The aim of this study was to quantify the magnitude of daytime and night-time transpiration of common urban tree species in a high latitude city (Gothenburg, Sweden), to analyse the influence of weather conditions and surface permeability on the tree transpiration, and to find out whether tree transpiration contributed to daytime or nocturnal cooling. Stomatal conductance and leaf transpiration at day and night were measured on mature street and park trees of seven common tree species in Gothenburg: Tilia europaea, Quercus robur, Betula pendula, Acer platanoides, Aesculus hippocastanum, Fagus sylvatica and Prunus serrulata. Transpiration increased with vapour pressure deficit and photosynthetically active radiation. Midday rates of sunlit leaves ranged from less than 1 mmol m(-2) s(-1) (B. pendula) to over 3 mmol m(-2) s(-1) (Q. robur). Daytime stomatal conductance was positively related to the fraction of permeable surfaces within the vertically projected crown area. A simple estimate of available rainwater, comprising of precipitation sum and fractional surface permeability within the crown area, was found to explain 68% of variation in midday stomatal conductance. Night-time transpiration was observed in all studied species and amounted to 7 and 20% of midday transpiration of sunlit and shaded leaves, respectively. With an estimated night-time latent heat flux of 24 W m(-2), tree transpiration significantly increased the cooling rate around and shortly after sunset, but not later in the night. Despite a strong midday latent heat flux of 206 W m(-2), a cooling effect of tree transpiration was not observed during the day.

Effects of ground surface permeability on the growth of urban linden trees

“…2 Map showing the site and trees selected for the study Our findings of positive effects of surface permeability on stem growth and current-year shoot growth (Fig. 3) are in agreement with earlier reports of positive influences of surface permeability on tree vitality, growth and transpiration (Celestian and Martin 2005;Ferrini and Baietto 2007;Grabosky and Gilman 2004;Iakovoglou et al 2001;Jim 1997;Konarska et al 2015;Quigley 2004;Roberts 1977;Sanders et al 2013;Schröder 2008). This effect was probably in part due to the combined effects of better soil aeration (Wiseman 2004), higher soil microbial activity and nutrient release (Gemtos and Lellis 1997), and a cooler and moister belowground and aerial environment immediately surrounding the trees (Celestian and Martin 2004;Graves 1994;Montague and Kjelgren 2004;Mueller and Day 2005).…”

“…While a large study with 600 trees in New Jersey, USA, failed to detect any significant relationship between the permeable surface cover and tree growth across nine tree species (Vrecenak et al 1989), other investigations reported enhanced growth and gas exchange of trees surrounded by grass or bare soil compared to those planted over paved surfaces (Celestian and Martin 2005;Ferrini and Baietto 2007;Grabosky and Gilman 2004;Konarska et al 2015;Sanders et al 2013). Clearly, further studies are needed to investigate the links between ground surface permeability and tree functioning, and whether such possible effects are gradual or limited to the most severe cases of impermeability only.…”

“…Several studies have shown that impervious surfaces such as pavement in the near vicinity of tree stems can negatively influence the water availability, vitality and growth of urban trees (Roberts 1977;Jim 1997;Iakovoglou et al 2001;Quigley 2004;Schröder 2008;Gillner et al 2014;Konarska et al 2015). In cities, much of the precipitated water is directly lost through surface runoff into storm drains, potentially causing severe water shortage of trees growing in areas where only a small fraction of the ground is permeable (Gillner et al 2014, Konarska et al 2015.…”

“…In cities, much of the precipitated water is directly lost through surface runoff into storm drains, potentially causing severe water shortage of trees growing in areas where only a small fraction of the ground is permeable (Gillner et al 2014, Konarska et al 2015. In addition to negative effects on tree water supply, impervious surfaces may also negatively affect urban trees through enhanced extreme soil and air temperatures.…”

Influence of vegetation and building geometry on the spatial variations of air temperature and cooling rates in a high‐latitude city

“…A similar diurnal pattern of the park cooling effect was found in Gaborone, Botswana (Jonsson, ) and Nagoya, Japan (Hamada and Ohta, ). While the addition of soil moisture can result in higher night‐time air temperature in a park than in a built‐up area (Spronken‐Smith and Oke, ), available soil moisture can also increase cooling due to evaporation and enhanced tree transpiration (Whitlow and Bassuk, ; Kjelgren and Clark, ; Rahman et al , ; Konarska et al , ). In hot, dry climates, urban parks can be up to 7 °C cooler than their surroundings in the evening, while during daytime they can be even warmer unless they are irrigated (Spronken‐Smith and Oke, ; Potchter et al , ; Lindén, ).…”