Constraints to transpiration in a simple (but not too simple) model of transpiration

In our collaborative work with Concetta D'Amato  for the WATERSTEM project, we encountered the initial constraint of transpiration imposed by the hydraulic conductance of the stem-root system. Through our research, inspired by Manzoni et al. [2013], we discovered that the sigmoidal form of conductivity leads to an optimum for transpiration. We attempted to reproduce this phenomenon using the data provided by Kroeber et al. [2-13]. After considerable effort, we successfully generated the gray curve in the Figure, which exhibits a peak just before -4 MPa and enables too high transpiration.

However, we realized that the soil resistance was missing from our analysis. To address this, we incorporated the conductivity of a Silt Loam soil using the van Genuchten Mualem parameterization. The resulting brownish curves serve as evidence that the soil plays a crucial role, as anticipated by Carminati and Javaux [2020]. It is important to note that these curves depict the limits imposed by the soil and stem, which determine the potential sapflow rates, but do not reflect the constraints imposed by plant physiology. To account for plant physiology, we introduced the stomatal resistance, represented by the three dashed curves under different working hypotheses whose parameterization was taken from Daly et al. [2004]. The red points in the Figure represent the plant's working points (although the coupling with the atmospheric boundary layer is not depicted). One notable aspect of the Figure is that at typical soil suctions, the sapflow curves appear relatively flat, and the working points result in relatively constant sapflow despite variations in xylem/leaves pressure. The complete story will soon be available in Concetta's Ph.D. thesis, and the detailed process of creating the Figure can be found in its supplemental material notebooks.

References

Carminati, Andrea, and Mathieu Javaux. 2020. “Soil Rather Than Xylem Vulnerability Controls Stomatal Response to Drought.” Trends in Plant Science 25 (9): 868–80. https://doi.org/10.1016/j.tplants.2020.04.003.

Daly, Edoardo, Amilcare Porporato, and Ignacio Rodriguez-Iturbe. 2004. “Coupled Dynamics of Photosynthesis, Transpiration, and Soil Water Balance. Part I: Upscaling from Hourly to Daily Level.” Journal of Hydrometeorology 5 (3): 546–58. https://doi.org/10.1175/1525-7541(2004)005<0546:cdopta>2.0.co;2.

Kröber, Wenzel, Shouren Zhang, Merten Ehmig, and Helge Bruelheide. 2014. “Linking Xylem Hydraulic Conductivity and Vulnerability to the Leaf Economics Spectrum—A Cross-Species Study of 39 Evergreen and Deciduous Broadleaved Subtropical Tree Species.” PloS One 9 (11): e109211. https://doi.org/10.1371/journal.pone.0109211.

Manzoni, Stefano, Giulia Vico, Gabriel Katul, Sari Palmroth, Robert B. Jackson, and Amilcare Porporato. 2013. “Hydraulic Limits on Maximum Plant Transpiration and the Emergence of the Safety-Efficiency Trade-Off.” The New Phytologist 198 (1): 169–78. https://doi.org/10.1111/nph.12126.

Some papers that discuss tree allometry to obtain biomass and sapwood cross sectional area

In order to accurately determine the water budget of trees (see also yesterday's post), it is crucial to establish a connection between the quantity of sapwood and the transpiration rate from the leaves. One essential factor in this process is obtaining accurate measurements of the sapwood cross-sectional areas (CSA). However, it is important to note that these CSA measurements can vary significantly from one plant to another. Acquiring this data can be challenging, and as a result, researchers have conducted studies aiming to establish allometric relationships as a means to estimate these measurements. To assist me in finding relevant literature on this topic, I reached out to my colleague involved in the WATERSTEM project. Below, you will find the literature they recommended.

References

Berry, Z. Carter, Nathaniel Looker, Friso Holwerda, León Rodrigo Gómez Aguilar, Perla Ortiz Colin, Teresa González Martínez, and Heidi Asbjornsen. 2018. “Why Size Matters: The Interactive Influences of Tree Diameter Distribution and Sap Flow Parameters on Upscaled Transpiration.” Tree Physiology 38 (2): 263–75. https://doi.org/10.1093/treephys/tpx124.

Kubota, Mitsumasa, John Tenhunen, Reiner Zimmermann, Markus Schmidt, Samuel Adiku, and Yoshitaka Kakubari. n.d. “Influences of Environmental Factors on the Radial Profile of Sap Flux Density in Fagus Crenata Growing at Different Elevations in the Naeba.” https://academic.oup.com/treephys/article/25/5/545/1712832.

Lüttschwager, Dietmar, and Hubert Jochheim. 2020. “Drought Primarily Reduces Canopy Transpiration of Exposed Beech Trees and Decreases the Share of Water Uptake from Deeper Soil Layers.” Forests, Trees and Livelihoods 11 (5): 537. https://doi.org/10.3390/f11050537.

Lüttschwager, Dietmar, and Rainer Remus. 2007. “Radial Distribution of Sap Flux Density in Trunks of a Mature Beech Stand.” Annals of Forest Science 64 (4): 431–38. https://doi.org/10.1051/forest:2007020.

Niccoli, Francesco, Arturo Pacheco-Solana, Sylvain Delzon, Jerzy Piotr Kabala, Shahla Asgharinia, Simona Castaldi, Riccardo Valentini, and Giovanna Battipaglia. 2023. “Effects of Wildfire on Growth, Transpiration and Hydraulic Properties of Pinus Pinaster Aiton Forest.” Dendrochronologia 79 (126086): 126086. https://doi.org/10.1016/j.dendro.2023.126086.

Petrík, Peter, Ina Zavadilová, Ladislav Šigut, Natalia Kowalska, Anja Petek-Petrik, Justyna Szatniewska, Georg Jocher, and Marian Pavelka. 2022. “Impact of Environmental Conditions and Seasonality on Ecosystem Transpiration and Evapotranspiration Partitioning (T/ET Ratio) of Pure European Beech Forest.” WATER 14 (19): 3015. https://doi.org/10.3390/w14193015.

Thurner, Martin, Christian Beer, Thomas Crowther, Daniel Falster, Stefano Manzoni, Anatoly Prokushkin, and Ernst-Detlef Schulze. 2019. “Sapwood Biomass Carbon in Northern Boreal and Temperate Forests.” Global Ecology and Biogeography: A Journal of Macroecology 28 (5): 640–60. https://doi.org/10.1111/geb.12883.

WATERSTEM project

 It was a pleasure to work with the young guys who setup the PRIN proposal WATERSTEM. Please find below the abstract of the proposal and cross the fingers for the project having success.

Mediterranean mountainous basins provide critical water supply and ecosystem services, yet these environments are increasingly at risk due to anthropogenic stressors and competition for water across urban, agricultural and environmental demands. On the top of this, future climate projections suggest a drier and warmer Mediterranean with large increases in the frequency, duration, and severity of hydrological droughts (i.e., runoff and groundwater levels below than normal) with serious consequences for the management of water resources and natural ecosystems. In spite of the recent progress in land surface monitoring, current drought estimation in widely used operational products still largely relies on poorly parameterized potential evapotranspiration, in combination with simple hydrological bucket models (e.g., drought indices) which have shown to lead to questionable results. As hydrological systems are intrinsically intertwined with climatological and ecological systems, the propagation of meteorological droughts (i.e., precipitation below than normal and higher temperatures) through them is modulated by a variety of mechanisms which are linked to carbon and water cycle interactions and specifically to how different plant species i) access subsurface water storages and ii) respond to water stress, high CO2 and high evaporative demand. Ignoring the parameterization of these mechanisms is often the norm in state-of-the-art land surface and hydrological models and impacts water balance closure via incorrect representation of transpiration leading to uncertainties in hydrological drought prediction. The ultimate goal of WATERSTEM is to unravel the interactions between carbon and water cycles as to understand the modulating effect of the vegetation on water-supply deficit (as opposed to the more frequently addressed meteorological drought) and its impact on water resources and natural ecosystems in Mediterranean climates. The work plan will focus on six Mediterranean mountainous basins and will employ a novel combination of field monitoring (water stable isotopes, tree ring analysis and geophysical measurements), remote sensing, data assimilation and ecohydrological models. WATERSTEM will develop a multidisciplinary and novel conceptual framework that will be used to translate the acquired scientific knowledge into practices to support water resources and silvopasture management across a variety of Mediterranean climates and physiographic settings. This is the core research of WATERSTEM, which is intimately coupled to the concept of Critical Zone (CZ, the Earth's permeable near-surface layer from the top of the trees to the bottom of the groundwater, where rock, soil, water, air, and vegetation interact and sustain life) that is fundamental for understanding and predict: i) hydrological droughts, ii) episodes of vegetation mortality (e.g., forest dieback) and iii) whether natural ecosystems may become a source or sink of carbon during drought.