the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2025
Temperature-driven vapor pressure deficit structures forest bryophyte communities across the landscapes
Abstract. Atmospheric vapor pressure deficit (VPD) controls local plant physiology and global vegetation productivity. However, at ecologically crucial intermediate spatial scales, the processes controlling VPD variability and the role of this variability in forest bryophyte community assembly are little known.
To disentangle processes controlling landscape-scale VPD variability and explore VPD effects on bryophyte community composition and richness, we recorded bryophyte communities and simultaneously measured forest microclimate air temperature and relative humidity across topographically diverse landscape representing bryophyte diversity hotspot in temperate Europe. Based on VPD importance for plant physiology, we hypothesize that VPD can be an important also for bryophyte community assembly and that VPD variability will be jointly driven by saturated and actual vapor pressure across the topographically diverse landscape with contrasting forest types and steep microclimatic gradients.
Contrary to our expectation, VPD variability in the forest understory was dictated by temperature-driven differences in saturated vapor pressure, while actual vapor pressure was surprisingly constant across the landscape. Gradients in bryophyte community composition and species richness followed closely the VPD variability. While mesic forest bryophytes occurred along the whole VPD gradient, azonally occurring and rare species preferred sites with low VPD. In result, low VPD sites represent species-rich microrefugia within the landscape and host regionally abundant mesic bryophytes simultaneously with rare species near their distributional range limits.
Our results showed that VPD variability at ecologically crucial landscape scales is controlled by saturated vapor pressure and consequently by the maximum air temperature. Future climate warming will thus increase evaporative stress and reshuffle VPD-sensitive forest bryophyte communities even in topographically diverse landscapes, which are traditionally considered as microclimatic refugia. Azonally occurring rare bryophyte species concentrated in low VPD sites will be especially vulnerable to the future changes in atmospheric VPD.
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RC1: 'Comment on egusphere-2025-1244', Alain Vanderpoorten, 24 Apr 2025
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I found this paper most original and useful as it addresses the very timely question of microclimates on diversity patterns, with a special emphasis on one key parameter: VPD. To give the paper the impact that it deserves and emphasize the relevance of VPD as an important, ecologically meaningful variable, I have two suggestions to make: (i) define VPD and explain, in the Introduction, what its ecological relevance as compared to other microclimatic variables such as T or RH alone (in other words, why would it be important to integrate VPD in ecological studies) and (ii) show that VPD is indeed a better predictor of species richness and composition than T or RH alone. This could be easily done by re-running the analyses, using T and RH as predictors, and showing that the use of VPD results in a higher percent variance of bryophyte richness and composition community explained.
One of the first results being discussed is the tight relationship between VPD and Tmax-it does not seem to me that this results directly stems out the analyses presented? Could this relationship be actually evidenced based on the data generated?
In the meantime, if VPD is tightly correlated with Tmax, does this not slightly undermine the premises of the study, that is, the potential benefit of an integrative variable such as VPD in ecological studies as compared to a ‘simple’ variable like Tmax?
Since the second main result discussed is that it is possible to estimate VPD from local T measurements with HR measured as nearby weather stations, I suggest moving the content of this appendix into the result section of the main text. Would that mean that one can efficiently characterize microclimates using temperature sensors only, which are much cheaper than sensors combining T+HR?
I was a bit surprised by the relatively limited contribution of VPD (about 11%) to variation in species composition among plots, whereas the introduction rightly emphasizes that in poikilohydric organisms like bryophytes, one could expect VPD to be a prime factor driving community composition. Looking at Fig1, one would think that communities at the top of a cliff would be very different from those in buffered conditions. I wonder whether this could be due to the fact that, as Fig1 suggests, this is a very rugged terrain, and that there is hence a huge (intra-plot) micro-habitat variation that is actually more important than (inter-plot) microclimatic variation. More information on the sampling sites would be welcome to understand the spatial heterogeneity potentially present in the sampling plots.
It would help the reader if it was reminded in the Result section based on which analysis each result was obtained. For example, the variation partitioning and db-RDA are not mentioned in the Result section, and mentioning them would help the reader making a link between the M&Ms and results. For example, I am not sure which analysis was implemented to reach the result described L179-180 (‘ecological relevance of VPD as compared to HR alone’).
In the discussion, it would be interesting to add a section explaining what could be the factors accounting for the spatial variation of VPD reported, and why Pair exhibits such a comparatively narrow range of variation. At present the discussion falls a bit short—especially since the entire §4.2 (from L239 onwards) actually belongs to the Introduction (why bryophytes would be sensitive indicators of VPD variation), and not to the Discussion as it does not help interpreting the results presented.
Citation: https://doi.org/10.5194/egusphere-2025-1244-RC1 -
RC2: 'Comment on egusphere-2025-1244', Anonymous Referee #2, 27 Apr 2025
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Dear authors,
I very much enjoyed reading your manuscript and think you have done a good job in both setting the question, conducting the research and writing the paper. It was easy to read and I think you have many strong points to raise including the strong effects of VPD, that it is the saturation that varies, the strong link to maximum temperature and what that implies for our possibility to monitor and study the effects of VPD. Moreover you have illustrated that well with a data set of bryophytes.
I will try to be constructive to point out a few things that perhaps could improve the clarity of the paper. In general I think you could take a careful look at the flow of the text to avoid repetition and increase clarity.
Design of the study.
There is always a trade-off related to the size of the plot to study. A small plot as yours is good to capture the microclimate at one spot, but you will miss a lot of rare species in the landscape. In your case you have a circular plot of 1 meter radius, right? If wo it could be good to spell out. I lack some information on how you selected sites. Was it done using maps and satellite images and getting a coordinate from there? How did you select them in the field? What if a tree was in the plot? Or a big boulder? Did you make any notes on substrate? Substrate composition is often an important driver of species composition of bryophytes. You have an ambitious approach of covering the whole forest landscapes and then perhaps your sample size of 38 plots is a bit low. But you got very interesting results and have an interesting approach so I am still fine with this.
VPD-variability.
I had a bit difficulty in flowing the text of how you calculated VPD-variability and when you talked about the variability over time and over space. And then what you take an average of. I think you need to carefully revise so that a reader understands all of this. For example how can you have a mean value of the standard deviation of the maximum value? And then you talk about range of plot means in Table 1. I am sure you have done it correctly it is just that it become difficult to follow when you have mean and SD values in a day, between days, between plots etc. Especially rows 122-124 I couldn’t follow entirely, but revise also in other parts of the text and figure legends so that it is crystal clear when VPD variability consider spatial or temporal aspects for example.
Grouping of the bryophytes.
Species could be grouped in many ways and you have three columns in Tablc C1: taxonomy, Major biome and Eastern limit. It seems in the results that you would like to say something on what is characterizing those that are sensitive to high VPD. However, in the results you have not really analysed the results in such a way and you instead talk about “small liverworts”, “hygrophilous bryophytes”, “suboceanic” and “mesic” species. And in several other places you talk about “azonal” species, which is a term not many readers will understand. Yet in other places you say “regionally rare species”. You have so many terms and none of these categories are in Table C1. And you use words such as “in contrast” but these groups are not contrasts to each other in most cases but just different ways of describing them. The number of species you have is not very large so perhaps it can be difficult to divide them into several group for the analysis and it might be just enough to tell the general statistics on the community which you have done and present the results at the species level as in Figure 5. Then if you want you can exemplify species which are less and more sensitive, but perhaps don’t need to put them into a category. Or select one or two “traits” and do a formal test. Substrate is another category that is often useful for describing bryophyte communities.
Detailjed comments:
Sensitivity of bryophytes to high maximum temperatures and high VPD. I think there are more references on this even if they might be more implicit. But for example various studies on forest edge effects on bryophytes could be relevant. Check also Dahlberg et al. 2020 in Environmental and Experimental Botany, who saw some interesting correlations with maximum temperature and distributions.Perhaps you might also be interested in Merinero et al. 2020 in Ecology who used evaporometers to capture the importance of VPD as a driver of bryophtye performans.
Figure B1. Would it be good to indicate the 1:1 line in this graph and discuss a bit more on why your line is deviating. But very interesting that you have such a strong relationship!
Citation: https://doi.org/10.5194/egusphere-2025-1244-RC2
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