Forest vulnerability to extreme dryness stress
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Date
2023
Publication Type
Doctoral Thesis
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Abstract
Forests are home to one of the most important collections of living beings (i.e., trees) that play a major role in supporting life as we know it. Forests sequester carbon dioxide from the atmosphere through photosynthesis, recycle water largely through transpiration, support large terrestrial biodiversity, and provide trillions of dollars’ worth of ecosystem goods and services to society each year. Furthermore, recent reports from the Intergovernmental Panel on Climate Change and the Paris Agreement recognize the substantial contribution of forest to reaching climate change mitigation goals. However, global forests are facing a major risk from an increase in climatic extreme events, especially droughts which have become hotter (“hotter” droughts) due to climate change in the last few decades. Future forests are projected to face significant risk from these hotter droughts that could trigger carbon cycle feedbacks, resulting in accelerated climate change and ultimately undermining their role in climate mitigation. A major characteristic of hotter droughts that adversely affect forests is the co-occurrence of extreme soil dryness and extreme air dryness, thereby resulting in so-called ‘compound’ extreme dryness conditions. The soil dryness is a result of extremely low soil moisture (SM), whereas the air dryness is due to an extremely high vapor pressure deficit (VPD). Both extreme soil and air dryness result in tree water stress and have been recognized as two major constraints on the water use and carbon uptake by the forest. It is important to understand the forest’s vulnerability to these compound extreme dryness events to advance our capacity to predict future forest’s responses to these reoccurring extreme dryness stress events.
As introduced in Chapter 1, this doctoral thesis aims to quantify forests responses to extreme soil dryness, extreme air dryness, and compound extreme dryness (collectively referred as ‘extreme dryness’) based on multi-site ecosystem-level and tree-level measurements spanning over multiple years (≥ 10 years). Ecosystem-level measurements include eddy covariance (EC)-based measurements of net ecosystem productivity (NEP), gross primary productivity (GPP), ecosystem respiration (Reco), and evapotranspiration (ET), along with remote-sensing-based leaf area index (LAI), in addition to meteorological and soil measurements. Tree-level measurements include sapflux density, stem radius, leaf water potential, and stomatal conductance.
Before diving into forest responses, it is essential to quantify how the occurrence and intensity of these extreme dryness conditions, i.e., extreme soil dryness, extreme air dryness, and compound extreme dryness, have changed and are projected to change in the future. This is the focus of Chapter 2 of this doctoral thesis, with Europe as a study area. Using reanalysis and in-situ measurement of daily mean SM and VPD, I show that compared to a reference period (1950-1990), the occurrence of extreme soil dryness, extreme air dryness, and compound extreme dryness has increased by 1.2 [0.8, 1.6] (median [10th percentile, 90th percentile])-fold, 1.6 [1, 2.3]-fold, and 1.7 [0.9, 2.5]-fold, respectively, across Europe during 1991–2021. This increase in the occurrence of extremes was largely across central and Mediterranean Europe. Regional climate model simulations for Europe indicated a 3.4 [2.0, 6.5]-fold increase in the occurrences of compound dry extremes during the mid-21st century (2030–2065) and a 4.2 [2.0, 10.8]-fold increase during the late-21st century (2066-2100). The results show that increase in occurrences of compound extreme dryness relative to the 1950–1990 period over present European forests was more than 1.5-fold, 3-fold, and 3.5-fold during the 1990–2021 period, mid-21st century, and late-21st century, respectively.
As Chapter 2 highlighted extreme air dryness-driven increase in compound extreme dryness, in Chapter 3 I focused on the long-term (≥10 years) changes in multi-site (60 forest sites) forest response in-terms of NEP resistance and NEP recovery to extreme air dryness. Two hypotheses were tested, first, across site differences in NEP resistance and NEP recovery of forest will positively depend on both the biophysical characteristics (i.e., leaf area index (LAI) and forest type) of the forest as well as on the local meteorological conditions of the site (i.e., mean VPD of the site), and second, forests experiencing an increasing trend in occurrences and intensity of extreme dryness will show an increasing trend in NEP resistance and NEP recovery over time due to emergence of long-term ecological stress memory. Results showed that forest types, LAI, and median local VPD conditions explain over 50% of variance in both NEP resistance and NEP recovery, with drier sites showing higher NEP resistance and NEP recovery compared to sites with less air dryness. The second hypothesis was rejected as no consistent relationship between trends of extreme VPD with trends in NEP resistance and NEP recovery were found across different forest sites meaning that increase in atmospheric dryness as it is predicted, might not increase the resistance or recovery of forests in terms of net ecosystem productivity.
Based on chapter 3, we find that less drier forests shows lower NEP resistance and recovery. Therefore, in Chapter 4, we focused our study on two different forest types in Switzerland: a montane mixed deciduous forest (CH-Lae) and a subalpine coniferous forest (CH-Dav) to quantify and compare the impact of extreme soil dryness, extreme air dryness, and compound extreme dryness. The impact in terms of anomalies of ecosystem-level response variables (NEP, GPP, Reco, and ET) and tree-level response variables (tree growth, transpiration, and tree water deficit) were estimated based on a data-driven approach. The results indicated that the impact of compound extreme dryness was similar to the impact of extreme air dryness at the subalpine coniferous forest, whereas compound extreme dryness showed a higher impact than extreme soil or air dryness at the montane mixed deciduous forest in terms of all response variables. Furthermore, the results also indicated a higher vulnerability of montane mixed deciduous forest (CH-Lae) to dryness stress than the subalpine coniferous forest (CH-Dav).
The mid-summer of 2022 at CH-Lae was characterized by an intense compound extreme dryness (CED) event (14th July–4th August 2022). Since, such CED events have also occurred in the recent years of 2015 and 2018 at CH-Lae, I also did an inter-comparison of their impact on ecosystem-level measurements of CO2 and water vapor fluxes, which is the focus of Chapter 5 of the thesis. The results showed that the NEP, GPP, and ET anomaly was highest during the 2022 CED event compared to the 2015 and 2018. The NEP was limited by soil dryness (low SWC) more than by atmospheric dryness (VPD) in 2022, whereas NEP was limited by VPD more than SWC in 2015 and 2018. The European beech showed the lowest leaf water potential (LWP, mean of -2.2 MPa) during the 2022 CED event and also the lowest LWP recovery six days after (on August 11, 2022) the 2022 CED event. However, the LWP and stomatal conductance of all the trees fully recovered by the end of September 2022, when the climatic conditions returned to normal, implying no immediate legacy effect of the 2022 CDH event on the mixed deciduous forest.
The Chapter 6 delivers concluding remarks of this doctoral thesis. Future will expose forests to levels of dryness stress that they have not experienced yet making them highly vulnerable in future. Although a general vulnerability pattern is challenging to derive due to site-specific factors, our findings indicate generally high vulnerability (≈ exposure x risk) of evergreen needleleaf forests, forest with a low LAI and forest growing in cooler and less dry climate (such as that in Central and Northern Europe) to extreme dryness stress. The results from this thesis advance our understanding of forest vulnerability to dryness stress and inform stakeholders where forests would be more resilient in future.
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Examiner : Buchmann, Nina
Examiner : Gharun, Mana
Examiner : Williams, Christopher A.
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ETH Zurich
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Subject
Eddy covariance; Soil moisture; Vapor pressure deficit; Statistical learning; Ecohydrophysiology; Net primary productivity; Gross primary productivity; Ecosystem respiration
Organisational unit
03648 - Buchmann, Nina / Buchmann, Nina
Notes
Funding
ETH-27 19-1 - Forest Vulnerability to Extreme and Repeated Climatic Stress (FEVER) (ETHZ)
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