Browsing by Subject "leaf area index"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Divergent trends of ecosystem-scale photosynthetic efficiency between arid and humid lands across the globe
    (Wiley, 2022-09) Wei, Fangli; Wang, Shuai; Fu, Bojie; Wang, Lanhui; Zhang, Wenmin; Wang, Lixin; Pan, Ning; Fansholt, Rasmus; Earth and Environmental Sciences, School of Science
    Aim Widespread greening and an increasing global terrestrial carbon sink over recent decades have been reported. However, the spatio-temporal relationships between vegetation greenness and productivity and the factors influencing this relationship remain unclear. We define a new metric of ecosystem-scale photosynthetic efficiency (EPE) to analyse its spatio-temporal pattern and investigate how potential drivers regulate the greenness–productivity relationship. Location Global. Time period From 2001 to 2016. Major taxa studied Global terrestrial ecosystems. Methods This study used global datasets of leaf area index (LAI) and solar-induced fluorescence (SIF) as proxies of vegetation greenness and ecosystem productivity, respectively, to propose a new metric of SIF/LAI, representing ecosystem-scale photosynthetic efficiency (EPE). We identified the spatial pattern and dynamics of EPE and examined factors influencing EPE. Results The results showed a weaker increase in productivity compared with the global greening rate from 2001 to 2016, suggesting a decline in EPE at the global scale. This decline in EPE indicates a disproportionate increase in terrestrial productivity against the widespread greening. When stratified into areas following an aridity gradient, we found that EPE overall showed upward trends in arid and semi-arid areas, and downward trends in dry sub-humid and humid regions. The EPE was controlled primarily by soil moisture, which promoted or constrained the EPE in xeric and mesic ecosystems, respectively. Moreover, the increase in short vegetation cover and atmospheric water demand contributed positively or negatively to EPE changes in xeric and mesic ecosystems, respectively. Main conclusions Our study shows that greening of the Earth is associated with decreasing EPE, revealing that current rates of carbon sequestration do not increase proportionally to greening of the Earth and highlighting that soil moisture is a key controller of EPE. These results help to reduce the uncertainties in future climate change impacts on vegetation dynamics, thus having implications for sustainable ecosystem management and climate change mitigation.
  • Thumbnail Image
    Item
    Quantifying the Controls on Evapotranspiration Partitioning in the Highest Alpine Meadow Ecosystem
    (AGU, 2020-04) Cui, Jiangpeng; Tian, Lide; Wei, Zhongwang; Huntingford, Chris; Wang, Pei; Cui, Zhongyin; Ma, Ning; Wang, Lixin; Earth Sciences, School of Science
    Quantifying the transpiration fraction of evapotranspiration (T/ET) is crucial for understanding plant functionality in ecosystem water cycles, land‐atmosphere interactions, and the global water budget. However, the controls and mechanisms underlying the temporal change of T/ET remain poorly understood in arid and semiarid areas, especially for remote regions with sparse observations such as the Tibetan Plateau (TP). In this study, we used combined high‐frequency laser spectroscopy and chamber methods to constrain estimates of T/ET for an alpine meadow ecosystem in the central TP. The three isotopic end members in ET (δET), soil evaporation (δE), and plant transpiration (δT) were directly determined by three newly customized chambers. Results showed that the seasonal variations of δET, δE, and δT were strongly affected by the precipitation isotope (R2 = 0.53). The δ18O‐based T/ET agreed with that of δ2H. Isotope‐based T/ET ranged from 0.15 to 0.73 during the periods of observation, with an average of 0.43. This mean result was supported by T/ET derived from a two‐source model and eddy covariance observations. Our overarching finding is that at the seasonal timescale, surface soil water content (θ) dominated the change of T/ET, with leaf area index playing only a secondary role. Our study confirms the critical impact of soil water on the temporal change of T/ET in water‐limited regions such as the TP. This knowledge sheds light on diverse land‐surface processes, global hydrological cycles, and their modeling.
  • Thumbnail Image
    Item
    Weakening amplification of grassland greening to transpiration fraction of evapotranspiration over the Tibetan Plateau during 2001-2020
    (Elsevier, 2023-10) Jin, Zheng; You, Qinglong; Zuo, Zhiyan; Li, Mingcai; Sun, Guodong; Pepin, Nick; Wang, Lixin; Earth and Environmental Sciences, School of Science
    Evapotranspiration plays a key role in regulating the water cycle in terrestrial ecosystems. Transpiration fraction to evapotranspiration (TF) characterizes the role of vegetation function in evapotranspiration and is a pivot feature for the interactions between soil, ecosystem, and the atmosphere. Generally, the amount of vegetation has the most direct impact to TF, but the sensitivity of TF to vegetation change is not known, which could be a metric to evaluate the role of greening in land-atmosphere water transportation. To address this key knowledge gap, we used remote sensing observations of leaf area index (LAI) as a proxy for vegetation amount and derived the sensitivity of TF to LAI (θ = ∂TF/∂LAI)) for the grasslands over the Tibetan Plateau (TP). Results showed there is a substantial decline of θ over the TP during 2001–2020, decreasing from over 4% m−2 m2 to below 1.5% m−2 m2. To analyze the drivers of θ, we introduced a diagnosis framework for soil-atmosphere hydraulic stress shifts based on trends in soil moisture and atmospheric vapor pressure deficit. Results implicated the θ decline over the TP is likely not induced by the dry-wet balance shifts by soil and atmosphere, nor by the vegetation's stomal conductance. Instead, we speculate that the θ decline is induced by a marginal damping effect in case the TF rise is approaching to grassland's limit over the TP. Our finding appeals future studies on terrestrial evapotranspiration to consider the role of vegetation in controlling transpiration and emphasize that greening may not lead to higher TF.