Earlier Flowering Phenology and Pollinator Visitation on Urban Green Roofs Compared to Ground-Level Gardens

Round 1

Reviewer 1 Report

This manuscript provides a detailed comparison of the flowering phenology and pollinators between the green roof and ground plant species. It is of great significance to elucidate the ecological function of green roofs.

There are 2 minor revision suggestions:

1. In Study Sites, authors used a larger intensive green roof and a smaller extensive green roof. However, the subsequent observation did not mention whether the plants and pollinators were from the intensive GR or extensive GR. If you did not study the extensive GR, please delete it in the M&M. If you studied extensive GR, it is better to include the comparison between extensive GR and intensive GR.  

2. Fig.5: AG and GR should be arranged in the same order for all plant species. CHNA is opposite to that of other plants.

Author Response

Thank you for your feedback!

In response to your suggestions:

1) The inclusion criteria of the study meant that only a single species (Ipomopsis aggregata) was observed on the extensive green roof. Since there was only one species on the extensive green roof and fourteen different species on the intensive green roof, it does not make sense to compare the extensive and intensive roofs. Lines 377-379 in the discussion section note this, however, it's not clear which species were observed on which roof from the methods section. We added a sentence (lines 154-156) in the methods section to note that Ipomopsis aggregata was observed on the extensive green roof. 

2) Issues with AG/GR data order for CHNA in Figure 5 are fixed.

Reviewer 2 Report

This is an interesting and mostly well-written analysis of differences between roof- and ground-level flowering phenology and pollinator visitation.  I think the appropriate literature is cited, the figures are clear, and the tables are useful. I've made a moderate number of editorial corrections/suggestions on the PDF, and pointed out a citation that needs more information. I think this will be a good contribution to the pollination/phenology literature. 

Comments for author File: Comments.pdf

A moderate number of minor corrections are marked on the PDF. 

Author Response

Thank you for your valuable feedback!

All editorial errors that you identified in the manuscript have been fixed.

In response to your broader questions in the PDF:

• "Maybe an analysis of growing degree days between years would explain much of this variation." This is a great idea. We will implement this strategy in future work (ideally in experimental systems with less inherent variability).
• How different was soil moisture? Irrigation rates were similar. We did not measure soil moisture, unfortunately, so we are unable to quantify soil moisture differences. Given the large aggregate size on the green roof and lower water holding capacity, it seems likely that soil moisture was systematically lower on the green roofs.
• Line 333 "bibliography needs more information about this reference." Thank you for pointing this out. Full reference information was inserted into the bibliography.

Reviewer 3 Report

The matter of plant phenology within urban contexts is of wide concern given the strong influence of urban heat island effects and global warming on plants’ flowering. As novel ecosystems usually experience strong aridity and high temperatures, green roofs provide an ideal system to compare the local effect of warming on plants’ phenology and growing compared with ground level plants. In this regard, the manuscript of Guidi and Bousselot addressed this issue by describing the flowering phenology and temperature variability of 15 plant species growing on two green roofs and at ground level in Denver, Colorado (USA), a location characterized by hot summers. Most interestingly, the authors went a step forward by assessing the plant-pollinator interaction during the same sampling periods (March through October of 2019 and 2020) and locations (ground level, green roofs), with the aim to evaluate green roofs as early flowering reservoirs in the city.

The manuscript is well written, with the objectives clearly stated and the methodology accurately explained to be reproduced. The results are also clear and the authors go to the point by giving relevance to the most important trends. The Tables and Figures are all appropriate. Given the nature of the phonological data the authors did not perform statistical analyses. I would like to emphasize that according to the aims of the study I believe that this was a correct decision and that the graphical representation of the results is accurate enough to interpret the observed trends. For instance, a statistical analysis of mean substrate temperatures (Figure 7) would not change the interpretation of the results, given the marked difference between ground level and green roofs’ temperatures. The conclusions are consistent and based on their own results. The authors are also aware of the descriptive nature of their study and provide a very reasonable explanation on this issue. Overall the manuscript would be of interest for a wide audience not only interested in green roof design but also in urban heat island and plant phenology. Below I indicate some minor observations.

Minor comments

Line 46. Please check for incomplete brackets and an extra space before the reference

Lines 108-116. Please, see Domínguez, M. V. S., González, E., Fabián, D., Salvo, A., & Fenoglio, M. S. (2020). Arthropod diversity and ecological processes on green roofs in a semi-rural area of Argentina: similarity to neighbor ground habitats and landscape effects. Landscape and Urban Planning, 199, 103816.

Line 403. Please, check for space,

Tables and Figures. Please, add the meaning of AG and DR to all legends.

Author Response

Thank you for your valuable feedback!

All of your editorial comments have been addressed.

Thank you for making us aware of the Domínguez (2020) paper. This has been added to bolster background information provided in lines 108-116 of the manuscript.