Patch size of trees affects its cooling effectiveness: A perspective from shading and transpiration processes

- Smaller patches have greater within-patch variations in air temperature.
- Smaller patches have greater within-patch variations in relative humidity.
- Fragmenting a large patch into smaller ones increases shading.
- A non-linear relationship between the transpiration rate and greenspace patch size.
- An optimal patch size for cooling determined by trade-off between shading and transpiration.

Increasing urban greenspace, particularly trees, has been widely recognized as an effective means for urban heat mitigation. Lots of uncertainty, however, occurs on how spatial configuration of trees affects their cooling effectiveness. A frequently asked question from urban planners is that whether a large greenspace patch has better cooling effects than several smaller ones, or vice versa. Here, we attempted to address this question by investigating the effects of patch size of trees on the two key cooling processes: shading and transpiration. We chose two typical tree species, Ginkgo biloba and Populus tomentosa, with 4 different patch sizes, and conducted the research in Beijing. We integrated field measurements of air temperature, relative humidity and transpiration rate with model simulation, and conducted the analysis at both the patch and within-patch level. We found: (1) Smaller patches had higher temperature, lower humidity and greater within-patch variations in temperature and humidity than larger ones. (2) With a fixed area of tree cover, a number of small patches can provide more shade than a single large patch, suggesting a monotonic increase of shade provision with the division of a large patch into smaller ones. (3) There was a non-linear relationship between patch size and transpiration rate, suggesting a maximum transpiration rate might occur at certain patch size. By considering the joint effects of shading and transpiration, an optimal size of patch might occur, at which the joint effects of shading and transpiration are maximized.