| 08 May 2026
Linking hydrological responses in forest ecosystems to atmospheric forcing
Abstract. Across ecosystems, soil water is replenished by precipitation events and depleted by evapotranspiration. Evapotranspiration is driven by solar radiation and the aerodynamic evaporative demand of the atmospheric boundary layer (Eaero). However, vegetation regulates the rate of transpiration through species-specific stomatal closure mechanisms that depend on tree water status, which in turn depends on the tree’s water supply and the atmospheric water demand. Therefore, quantifying the effects of precipitation, solar radiation and Eaero on tree-mediated water fluxes is challenging. Here we use ERRA, a framework for de-mixing and de-convolving non-stationary system responses to multiple inputs, to quantify how atmospheric forcing affects ecosystem water fluxes and water content dynamics in a mixed beech and spruce forest. The resulting impulse-response functions describe how soil and tree water fluxes respond to three atmospheric forcing (precipitation, solar radiation, Eaero). Water contents of soils and trees responded positively and rapidly to precipitation pulses, indicating fast infiltration of precipitation into the soil and net increases in tree water contents. Tree water contents responded more clearly to precipitation inputs than sapflow rates did, suggesting that precipitation primarily reduced transpiration rather than enhancing tree water uptake. Trees responded quickly and strongly to impulses of solar radiation, but their responses to Eaero were less distinct, potentially reflecting stomatal closure effects on transpiration. The impulse-response functions reflected species-specific water use strategies and differences in hydraulic capacitance of trees, which buffered root water uptake during periods of high transpiration demand and thus prolonged the refilling of tree water storage after precipitation events. Impulse responses to solar radiation and Eaero were much less distinct in the soils than in the trees, illustrating how forest canopies shield the underlying soils from atmospheric forcing. Our study highlights how impulse-response functions can help to identify soil-plant-atmosphere relations, complementing our understanding of forest ecosystem functioning in response to atmospheric forcing.
I found this a fascinating and exceptional assessment/discussion of the drivers and explanatory factors behind evapotranspiration in spruce and beech stands in Switzerland (greater Zurich area). Most of my "larger" comments are comparatively minor and amount to suggestions for potential tweaks/improvements to the text. In general, though, I would not want to change that much. But perhaps my comments will nonetheless prove useful for sharpening the final draft. For more detail, please see the attached, commented submission pdf.
I do, however, have two-three larger questions that may prove interesting either for this paper, or perhaps for future work.
First, in reading this, I found myself frequently reflecting on the concept of "residence times". The paper never really comes out directly to address this concept. But nonetheless, it seems very prominently interested in and curious about the residence times of water in different tree species, stem, branch, soil and perhaps landscape contexts. Of course, to some extent, these residence times are likely to be partly driven by variation in the intensity and/or magnitude of the precipitation pulse. But still, it seems interesting to think about these questions in this larger context of the various residence times, and how these are affected by climatic variations, tree species type, and the like... I also wonder a bit how such residence times might be graphically displayed...
Second, I find myself wondering a lot about how the tree growth cycle is affected by the various water related fluxes described herein. Though the carbon flux is not assessed here in this particular paper, all the assessed variations in things like sap flow, tree water content, and the ET flux are presumably all tightly linked to variations in growth-related outcomes... Thus, it would be interesting to learn more about how these phenomena are linked.
Together, these points seem to suggest that the current draft of these paper leaves some open questions about the larger take-home messages being crafted here. One of the more important strengths of this paper is the willingness of the Authors to wallow in the most rudimentary of details. But I wonder if this draft, at least, is perhaps a bit lost in the details while missing the bigger picture. What are they really trying to say, perhaps, about possible tipping points and/or the survival limits of different tree species types relative to the changing climatic conditions they currently face?
I feel like there is a larger picture here that is not quite made transparent, but is looming in the background. The Authors seem to make a convincing case for the ERRA analysis framework they deploy. But what are the ultimate implications of this analysis? And what are the principal goals of this paper?
All-in-all, I am not necessarily expecting the Authors to integrate these factors into the current paper. These comments are mentioned more as food for thought, all points provoked by what is generally a fascinating and very detailed discussion of tree water fluxes under varied conditions. Still, I hope there is something useful here.