Sap flow and leaf gas exchange response to drought and heatwave in urban green spaces in a Nordic city
Abstract. Urban vegetation plays an important role in offsetting urban CO2 emissions and mitigating heat through tree transpiration and shading. With frequent heatwave events and the accompanying drought, the functioning of urban trees is severely affected in terms of photosynthesis and transpiration rate. The detailed response is however still unknown despite tree functioning having crucial effects on the ecosystem services they provide. We conducted sap flux density (Js) and leaf gas exchange measurements of trees (Tilia cordata, Tilia × europaea, Betula pendula, Malus spp.) located at four types of urban green areas (Park, Street, Forest, Orchard) in Helsinki, Finland, over two contrasting summers 2020 and 2021. Summer 2021 had a strong heatwave and drought, whereas summer 2020 was more typical for Helsinki. In this study, our aim was to understand the responses of urban tree transpiration and leaf gas exchange to heatwave and drought and examine the main environmental drivers controlling the transpiration rate during these periods in urban green areas. We observed varying responses of tree water use during the heatwave period at the four urban sites. Js was found to be 35–67 % higher during the heatwave as compared to the non-heatwave period at the Park, Forest, and Orchard sites but no significant difference was found at the Street site. Our results showed that Js was higher (31–63 %) at all sites during drought as compared to non-dry periods. The higher Js during the heatwave and dry periods were mainly driven by the high atmospheric demand for evapotranspiration represented by the vapor pressure deficit (VPD), suggesting that the trees were not experiencing severe enough heat or drought stress that stomatal control would have decreased transpiration. Accordingly, maximum assimilation (Amax), stomatal conductance (gs), and transpiration (E) at the leaf level did not change at the four sites during heatwave and drought periods. However, gs was substantially reduced during the drought period at the Park site. VPD explained 55–69 % variations in the daily mean Js during heatwave and drought periods at all sites except at the Forest site where the saturation of Js at high VPD was evident due to low soil water availability. The heat and drought conditions were untypically harsh for the region but not excessive enough to restrict stomatal control and the increased transpiration indicating that ecosystem services such as cooling was not at risk.
Joyson Ahongshangbam et al.
Status: open (until 12 Apr 2023)
- RC1: 'Comment on bg-2023-5', Anonymous Referee #1, 24 Feb 2023 reply
Joyson Ahongshangbam et al.
Datasets of sap flow, meteorological, leaf gas measurements in urban green areas in Helsinki https://doi.org/10.5281/zenodo.7525319
Joyson Ahongshangbam et al.
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The introduction is not tailored to the research question but rather puts in various issues about urban trees, which are unfortunately sometimes wrongly cited (see also specific comments). The ways how cooling and shading might be affected by heat and drought (stomata, leaf senescence, hydraulic failure) are not properly described, nor are direct (heat and drought) and indirect impacts (nutrition, air pollution impacts) or impacts that may mitigate damaging influences (CO2, irrigation) properly differentiated. Also, hypotheses are not clearly specified and H1 and H3 seem to be the same anyway. Transpiration will be increased (vpd) or decreased (stomatal control), or both (in which cases)? Drought decreases stomatal conductance (old news) or photosynthetic capacity (and then impacts stomata)?
The methodology is problematic, trees were of different species at each site, so neither species differences nor site differences can be evaluated. Furthermore, trees varied considerable in height and age, and measurements were taken at different heights and also the stratification within the crown was not homogeneously done. In addition, some sites were irrigated while others were not. It is not clear, how the ‘normalization’ by vpd is actually done and if this is in accordance with theoretical considerations regarding the vpd response. In addition, it is not clear, how much drought stress was actually present at the different sites since only percentages of water are given without indication of how much water is available in absolute or relative (absolute in relation to total water holding capacity) terms. Overall, since vpd is not the only influence, which is necessary to consider in order to compare the different site and species responses, a more complex approach seems to be necessary in order to differentiate between site and species impacts. Perhaps, this means using a model that describes gas exchange based on climatic as well as soil conditions.
Despite the very different boundary conditions and species and the few samples for each condition, the discussion tries to differentiated between heat and drought effects although both influences were simultaneously occurring and can hardly be distinguished. This leads to counter-intuitive results (such as a restricting (significant?) role of soil moisture for forest gas exchange in the wet period) and very disputable conclusions (e.g. “severe weather events did not alter the stomatal action”). The potential benefit of this study, which is the different response of species that a) do not close stomata, b) close stomata, c) are damaged by non-stomatal effects, could not be addressed due to the various degree of stress and different boundary conditions. Therefore, the conclusion is that species and site differences are causing the variation – which is probably true but could not really be shown given that neither species could be compared on different sites nor site influences could be evaluated using the same species. Accordingly, also the impacts that different species might have on their environment under heatwave conditions are highly speculative and are more based on literature than on measurements presented here.
Specific comments (and some technical corrections)
L1: I guess the role in offsetting CO2 emission is actually not important. If anything, consider pollution deposition effects. (see also L27)
L5: how many trees?
L10ff: Do you say that street trees reduced their transpiration during a heatwave (relative to other sites) but not during dry periods? Can you actually differentiate between the two kinds of stress?
L15ff: Here you say that there is no effect on stomata during heatwaves - also for street trees. How does this fit to the indication of street trees having reduced their transpiration?
L17-20: In one sentence you say that drought limited transpiration in forests and in the following it is stated that drought was so mild that it could not affect stomata control. What did I miss here??
L28/29: regulating energy balance and cooling the surrounding is actually the same.
L29: pollution deposition, not ‘infiltration’
L32/33: Sorry, but none of the indicated references are saying that urban trees are important to mitigate the global GHG. In the contrary!
L35: human disturbances? Do you mean damages due to traffic or pruning by urban managers?
L43-45: I cannot derive this conclusion from Bussotti et al.. Did I miss something?
L65: What’s the difference between research question 3 and 1/2? Do you want to address the impacts in combination?
L60ff: Since the research capacity is limited, I think it is logical to assume that not all urban trees but a selection based of the most representative species is targeted. Please elaborate the text to make it more specific.
L71-75: Shift to site description.
L80: sparse tree cover! Roadside single trees (not a plantation)!
L118: do you mean damages by pedestrians?
L134: ‘normalizing’ flux density by dividing by vpd seems strange to me since velocity will likely have an S-shape response of vpd. Is this a common strategy (please cite) or may it be that you first would need to derive the dependency between both and then normalize by applying the respective function? Or did you do that as it seems later on that you fitted curves to this relationship?
L175ff: I don’t understand this. What is the control period to define heatwaves? The long-term (30year) average for the respective season/month/specific days? The heatwave duration was more than one month? Can you provi de explicit temperatures?
L183: What do you mean with a ‘further’ separation of periods … based on soil moisture? What is this give an absolute value that you indicate here – an arbitrary value between field capacity and wilting point? What is the water content relative to field capacity at this point?
L185: Are the hypotheses (if better defined) not better evaluated with transpiration than with sapflux density? Or by Water use efficiency?
L202: I would rather say site climate instead of microclimate (which is normally used for microsites such as canopy layers).
L206: Soil moisture at what time of the year? Or do you mean maximum water content?
L211: Do you mean sapflow rate instead water use? (Water use would be expressed per m-2 ground area).
Figure 3: Strange that there is no increase in soil moisture at the 1st of July despite considerable rainfall. Any explanation for this?
Table 2: Do you mean average soil moisture within the indicated period. Please use average (relative) available soil moisture instead.
L236: Still unclear how a normalization was done (see also comment to L134).
L282: vpd is due to impervious cover? Do you mean it is due to the increased temperatures that are caused by the impervious cover? Or are you assuming that vpd could be lower because of soil evaporation?
L289ff: Again, it is difficult to comprehend, what ‘water use’ means. If it is defined on the sapwood area, stem size indeed plays a major role (as indicated), but is this also the case if the water consumption/transpiration on a ground area basis would be calculated? I guess the latter is more important in the context of comparing site conditions.
L291ff: Similarly, water availability is only meaningful if it is the water storage capacity minus the water bound by the wilting point. The definition is however, not clear.
L298ff: needs references. For Betula isohydry, I would recommend to consider Zapater et a. 2013. Tilia, however, seems also to be fairly isohydric (Leuschner et al. 2019), so I am not sure about the logic of the reasoning here.
L301ff: What do you want to tell here? That Tilia may use more or less water than other trees? Or that site conditions might influence the transpiration rate of Tilia? But this is generally true for any species.
L323: replace ‘cover’ by ‘represent’ or similar (of course, one day does not cover the whole period)
L324ff: This is a conclusion that is contrary to the statement that Js/VPD declined during drought. Please provide an explanation for your conclusion. I can also not see where you see a similarity to Gillner et al. despite the very general fact that transpiration is mostly higher in summer. If this is about species comparison, the whole paragraph needs to be directed towards this.
L333ff: You are probably indicating that the drought was not sufficient to cause damages to photosynthesis (non-stomatal effects, see e.g. Gourlez de la Motte et al. 2020), since you already discussed that a stomatal effect should have taken place. In addition, the stomatal control may be differently strong, depending on the isohydry or anisohydry of the species. The literature references should support your respective message and not only indicate similar results under possibly similar conditions.
L351: delete ‘, suggesting that … at these sites’ (redundant)
L389-391: not shown in this paper
L391-393: this cannot be derived from this study
L393/4: wishful thinking, not a conclusion
Gourlez de la Motte L, Beauclaire Q, Heinesch B, Cuntz M, Foltýnová L, Sigut L, Manca G, Ballarin I, Vincke C, Roland M, et al. 2020. Non-stomatal processes reduce gross primary productivity in temperate forest ecosystems during severe edaphic drought. Philosophical Transactions of The Royal Society B Biological Sciences 375(1810): 20190527.
Leuschner C, Wedde P, Lübbe T. 2019. The relation between pressure–volume curve traits and stomatal regulation of water potential in five temperate broadleaf tree species. Annals of Forest Science 76(2): 60.
Zapater M, Bréda N, Bonal D, Pardonnet S, Granier A. 2013. Differential response to soil drought among co-occurring broad-leaved tree species growing in a 15- to 25-year-old mixed stand. Annals of Forest Science 70(1): 31-39.