Conifers May Ameliorate Urban Heat Waves Better Than Broadleaf Trees: Evidence from Vancouver, Canada
Round 1
Reviewer 1 Report
The authors discuss the effectiveness of trees to counteract extreme temperatures in cities. In fact, they evaluate temperatures of Metro Vancouver (Canada, at 49°N) deduced from Landsat 8 data and correlate them with different vegetation and soil characteristics. The study is meant to fill a gap in the understanding of the shade effect of mostly endemic conifers and to deduce an educated recommendation for planting policy.
The astonishing outcome of this research is that native conifers generate a bigger cooling effect than nonnative broadleaf trees.
Despite the title, indicating a global conclusion, this study is made in and for Vancouver with its moderate and relatively humid climate. The authors argue, why endemic trees are superior to neophytes regarding the cooling effect and come up with the explanation that it relates to the transpiration of those trees, adapted to the local climate.
The study appears to be well organized. Introduction, methods and results are understandable.
The graphic presentation of the modelling is not self-explanatory but I do not have a better solution.
I would like to have it a little less pathetic and with less global claim as the research and its results is restricted to urban Vancouver and the climate situation there.
After slight rewording I think the manuscript is ready for publication.
Kind regards,
Norbert Kaul
Author Response
Comments and Suggestions for Authors
Comment: The authors discuss the effectiveness of trees to counteract extreme temperatures in cities. In fact, they evaluate temperatures of Metro Vancouver (Canada, at 49°N) deduced from Landsat 8 data and correlate them with different vegetation and soil characteristics. The study is meant to fill a gap in the understanding of the shade effect of mostly endemic conifers and to deduce an educated recommendation for planting policy.
The astonishing outcome of this research is that native conifers generate a bigger cooling effect than nonnative broadleaf trees.
Despite the title, indicating a global conclusion, this study is made in and for Vancouver with its moderate and relatively humid climate. The authors argue, why endemic trees are superior to neophytes regarding the cooling effect and come up with the explanation that it relates to the transpiration of those trees, adapted to the local climate.
Response: Thank you for your comments! We have changed the title to clarify that we are drawing on evidence only from Vancouver, BC.
Comment: The study appears to be well organized. Introduction, methods and results are understandable.
The graphic presentation of the modelling is not self-explanatory but I do not have a better solution.
Response: We have edited the modeling output graphs to make them clearer, including removing abbreviations from the graphs. (Reviewer 2 also recommended we make these graphs clearer.)
Comment: I would like to have it a little less pathetic and with less global claim as the research and its results is restricted to urban Vancouver and the climate situation there.
Response: We have followed your advice and added “Vancouver” to the title.
Comment: After slight rewording I think the manuscript is ready for publication.
Reviewer 2 Report
The paper aims to understand how different tree and land cover classes might affect summer surface temperature by means of remote sensing data and Bayesian models in Metro Vancouver, Canada
First of all, the idea behind this paper is very interesting, the introduction and discussion sections are well presented. However, the results section is very poor. I think that the paper doesn’t follow a clear scientific rigor as a complete absence of literature review and conclusions section was noticed. Here are some comments related to the paper:
Page 4, line 104-106, “To convert the surface temperature band to Kelvin, we multiplied by 0.00341802 and added 149.0, as described in the Data Format Control Book [28]. We next subtracted 273.15 to achieve the surface temperature in degrees Celsius”. This gives the surface temperature of a heterogeneous area of 30*30 m, in fact that area consists of different classes with different emissivity values. I think, in my point of view, the emissivity (related to wavelength of band 10 of Landsat) should be considered as a variable.
Page 5, line 127-130, do you mean a validation of the extracted temperature from Landsat images was made using in situ temperature observations? If so, where is the results? However, from Landsat you got surface temperature and in situ it is air temperature.
Page 5, line 135-136, “After removing the ’other’ land covers, we excluded all polygons from further analysis that summed to less than 0.5*36=18”. The authors do not justify why? Also, after removing the other classes what about the cooling effect of those omitted classes?
Does the geometric configuration of conifers trees, voids in-between tops of trees, branches and leaves has a cooling effect? As it permits in somehow the air to flow. this air flow with even low evapotranspiration rate will produce a cooling effect greater than that in broadleaf trees cause.
In general, Figures (3, A1, A2) are poor presented and must be improved.
Author Response
Comments and Suggestions for Authors
Comment:
The paper aims to understand how different tree and land cover classes might affect summer surface temperature by means of remote sensing data and Bayesian models in Metro Vancouver, Canada
First of all, the idea behind this paper is very interesting, the introduction and discussion sections are well presented. However, the results section is very poor. I think that the paper doesn’t follow a clear scientific rigor as a complete absence of literature review and conclusions section was noticed. Here are some comments related to the paper:
Response: Thank you for your comments. We have heavily edited the Introduction and sought to make our literature review more clear and logical. We have also edited the results section to make the description of results more clear. We have also re-written the last paragraph of the paper to make our conclusion — that local and global climate mitigation strategies must be linked—more clear.
Comment: Page 4, line 104-106, “To convert the surface temperature band to Kelvin, we multiplied by 0.00341802 and added 149.0, as described in the Data Format Control Book [28]. We next subtracted 273.15 to achieve the surface temperature in degrees Celsius”. This gives the surface temperature of a heterogeneous area of 30*30 m, in fact that area consists of different classes with different emissivity values. I think, in my point of view, the emissivity (related to wavelength of band 10 of Landsat) should be considered as a variable.
Response: Thanks for raising this concern. From our read of the Data Format Control Book (https://www.usgs.gov/media/files/landsat-8-9-olitirs-collection-2-level-2-data-format-control-book), the surface temperature band is the product of an algorithm that takes variable emissivity into account. We have re-written the text to read:
“The surface temperature band accounts for variable emissivity (estimated from ASTER GED) and other corrections [28].”
Comment: Page 5, line 127-130, do you mean a validation of the extracted temperature from Landsat images was made using in situ temperature observations? If so, where is the results? However, from Landsat you got surface temperature and in situ it is air temperature.
Response: This is not meant to be a validation, but a preliminary comparison, since, as you note, one is surface temperature and the other is air temperature. The results are in the last paragraph of the Results section. We have re-written this sentence for clarity:
“To preliminarily understand the differences between Landsat surface temperature and in situ air temperature observations, we extracted the daily high air temperatures from two weather stations on Weather Underground (wunderground.com) for the same day as the Landsat observations.”
Comment: Page 5, line 135-136, “After removing the ’other’ land covers, we excluded all polygons from further analysis that summed to less than 0.5*36=18”. The authors do not justify why? Also, after removing the other classes what about the cooling effect of those omitted classes?
Response: Thanks for noting this lack of justification. We agree that it’s important to include our reasoning here. We have added the following text:
“After removing the 'other' land covers (because there are very few of them and we hypothesized that these classes would not be informative for understanding surface temperature), we excluded all polygons from further analysis that summed to less than 0.5*36=18.”
Comment: Does the geometric configuration of conifers trees, voids in-between tops of trees, branches and leaves has a cooling effect? As it permits in somehow the air to flow. this air flow with even low evapotranspiration rate will produce a cooling effect greater than that in broadleaf trees cause.
Response: We have added a sentence about how airflow could also impact the differential cooling:
“Differences in tree form and structure could also affect the higher cooling rates of conifers (Domingo et al., 1996).”
Comment: In general, Figures (3, A1, A2) are poor presented and must be improved.
Response: We have changed the font size and x-axis labels of Figure 3 and A2, and replaced diagonal labels with abbreviations in Figure A1. We hope this makes the figures clearer.
Reviewer 3 Report
The paper presents an analysis of surface temperature of land cover classes using Landsat imagery. While the result is convincing, the title of the paper is misleading. The authors explained that the broadleaf trees may be warmer because they are often planted as street trees. Please modify the paper title to better reflect the findings. Several other comments are given below.
- To me, Figure A2 also presents quite important findings. Please present it in the main body of the paper and add adquate discussions.
- It seems the title of the x-axis of Fig.A2(a) was missing. Please double check it.
- Lines 242-243, Urban heat island quite often is most severe at night time. Please justify why the analysis was performed to explore day time surface temperature.
- To distinguish the influence of urban/rural environment on surface temperature of conifers and broadleaf trees, was it possible to analyze those conifers and broadleaf trees planted in similar urban or rural areas?
- A recent review titled "urban heat island and its interaction with heatwaves: A review of studies on mesoscale" might be relevant to complement the background of the work.
Author Response
Comments and Suggestions for Authors
Comment: The paper presents an analysis of surface temperature of land cover classes using Landsat imagery. While the result is convincing, the title of the paper is misleading. The authors explained that the broadleaf trees may be warmer because they are often planted as street trees. Please modify the paper title to better reflect the findings. Several other comments are given below.
Response: Thank you for your comments! We believe that more street broadleaf trees may be a contributing factor, but we do not think this is the main mechanism behind the lower temperatures of conifers. After reading your comments, we realize that we had made this insufficiently clear. We have sought to clarify this in the text:
“Previous studies have shown that transpiration was higher in trees planted in grass than those planted in paved cutouts \cite{Rahman2020}. Broadleaf trees are more prevalent along Vancouver's streets due to their root morphology \cite{Melles2000}, the aesthetic preferences of Vancouver's mostly British colonizers \cite{Ley1995}. Thus it is possible that abundance of broadleaf trees near concrete sidewalks and paved roads may have contributed to the higher temperature of broadleaf trees in our study. However, \citet{Pace2020} found that water availability and wind speed were more important for determining transpiration rates than ground surface. Future research might assess how planting substrate mediates the relationship between tree type and surface temperature in Vancouver.”
- Comment: To me, Figure A2 also presents quite important findings. Please present it in the main body of the paper and add adquate discussions.
Response: We have moved Figure A2 to the main text and included discussion of it.
- Comment: It seems the title of the x-axis of Fig.A2(a) was missing. Please double check it.
Response: Subfigure (a) and (b) share an x-axis. We have moved the subfigures closer together to make this more clear.
- Comment: Lines 242-243, Urban heat island quite often is most severe at night time. Please justify why the analysis was performed to explore daytime surface temperature.
Response: Great point! We have added an acknowledgement of the higher nocturnal urban heat island effect, and we include a justification for why we chose the time of day that we did:
“Our findings may not generalize to other times of day. We chose to analyze urban surface temperatures at noon when they are likely to be most severe. However, studies have shown that cooling effects of trees and lawns may reverse at night, since trees retain moisture and disrupt cooling airflow over the ground \cite{SpronkenSmith1998}. Furthermore, urban heat island effects are most severe at night \cite{Oke1975}. Future studies might examine how surface temperature--conifer relationships vary over the course of the day.”
- Comment: To distinguish the influence of urban/rural environment on surface temperature of conifers and broadleaf trees, was it possible to analyze those conifers and broadleaf trees planted in similar urban or rural areas?
Response: This is a great idea for a future direction! We have edited the following text:
“Future work might also analyze the spatial arrangement and clustering of land cover classes (perhaps across similar urban and rural landscapes), which may affect surface temperatures \cite{Myint2015}.”
- Comment: A recent review titled "urban heat island and its interaction with heatwaves: A review of studies on mesoscale" might be relevant to complement the background of the work.
Response: Thanks for pointing us to this very relevant paper. We have added multiple citations to it throughout.
Round 2
Reviewer 2 Report
The paper aims to understand how different tree and land cover classes might affect summer surface temperature by means of remote sensing data and Bayesian models in Metro Vancouver, Canada
The reviewed article has been sufficiently improved. Now it seems to be much more complete. The only minor thing remains is that the following corrections are missed in the attached manuscript
- “The surface temperature band accounts for variable emissivity (estimated from ASTER GED) and other corrections [28].”
- “To preliminarily understand the differences between Landsat surface temperature and in situ air temperature observations, we extracted the daily high air temperatures from two weather stations on Weather Underground (wunderground.com) for the same day as the Landsat observations.”
- “After removing the 'other' land covers (because there are very few of them and we hypothesized that these classes would not be informative for understanding surface temperature), we excluded all polygons from further analysis that summed to less than 0.5*36=18.”
- “Differences in tree form and structure could also affect the higher cooling rates of conifers (Domingo et al., 1996).”
Author Response
Comment:
The reviewed article has been sufficiently improved. Now it seems to be much more complete. The only minor thing remains is that the following corrections are missed in the attached manuscript.
Response: Thanks for catching these inconsistencies! The following text now appear at the following line numbers:
- “The surface temperature band accounts for variable emissivity (estimated from ASTER GED) and other corrections [28].”
- Now appears at line number 96
- “To preliminarily understand the differences between Landsat surface temperature and in situ air temperature observations, we extracted the daily high air temperatures from two weather stations on Weather Underground (wunderground.com) for the same day as the Landsat observations."
- Now appears at line number 127
- “After removing the 'other' land covers (because there are very few of them and we hypothesized that these classes would not be informative for understanding surface temperature), we excluded all polygons from further analysis that summed to less than 0.5*36=18.”
- Now appears at line number 136
- “Differences in tree form and structure could also affect the higher cooling rates of conifers (Domingo et al., 1996).”
- Now appears at line number 203, slightly modified to fit the sentence.
