Abstract

Northern hemisphere evergreen needleleaf forest (ENF) contributes a significant fraction of global water exchange but regional transpiration (T) observation in ENF ecosystems is still challenging. Traditional remote sensing techniques and terrestrial biosphere models reproduce the transpiration seasonality with difficulty, and with large uncertainties. Solar-induced chlorophyll fluorescence (SIF) emission from vegetation correlates to photosynthesis at multiple spatial and temporal scales. However, how SIF links to transpiration of evergreen forest during seasonal transition is unclear. Here, we explored the relationship between canopy SIF and T retrieved from ground observation towers in ENF. We also examined the role of meteorological and soil factors on the relationship between SIF and T. A slow decrease of SIF and T with a fast reduction in photosynthetically active radiation (PAR), air temperature, vapor pressure deficit (VPD), soil temperature and soil water content (SWC) were found in the ENF during the fall transition. The correlation between SIF and T at hourly and daily scales varied significantly among different months (Pearson correlation coefficient = 0.29–0.68, p < 0.01). SIF and T were significantly linearly correlated at hourly (R2 = 0.53, p < 0.001) and daily (R2 = 0.67, p < 0.001) timescales in the October. Air temperature and PAR were the major moderating factors for the relationship between SIF and T in the fall transition. Soil water content (SWC) influenced the SIF-T relationship at an hourly scale. Soil temperature and VPD’s effect on the SIF-T relationship was evident at a daily scale. This study can help extend the possibility of constraining ecosystem T by SIF at an unprecedented spatiotemporal resolution during season transitions.

Highlights

  • Evergreen needleleaf forest (ENF) accounts for one third of the world’s forest [1], and is an important and highly dynamic component of the terrestrial ecosystem water cycle [2,3]

  • We observed a significant reduction in photosynthetically active radiation (PAR), air temperature, vapor pressure deficit (VPD), soil temperature and Soil water content (SWC) (Figure 1E–I)

  • We observed a significant decrease in Solar-induced chlorophyll fluorescence (SIF), ET, T and SWC from DOY 244 to 254, which could have been due to water transport blocking and stomatal closing caused by soil water reduction [27]

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Summary

Introduction

Evergreen needleleaf forest (ENF) accounts for one third of the world’s forest [1], and is an important and highly dynamic component of the terrestrial ecosystem water cycle [2,3]. ENF transpiration (T) estimation is still imprecise with the currently used empirical/semi-empirical models (e.g., Penman Monteith (PM) equation) and ecohydrological remote sensing models which rely on meteorological factors and remote-sensing vegetation parameters (e.g., NDVI and LAI) [4,5]. This is because ENF canopy structure and green needle leaf area show no significant change during season transitions [6], but photosynthetic activity and water exchange undergo seasonal shifts in evergreen ecosystems [7,8].

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