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Vegetation is a highly dynamic component of the Earth surface and substantially alters the water cycle. Particularly the process of oxygenic plant photosynthesis determines vegetation connecting the water and carbon cycle and causing various interactions and feedbacks across Earth spheres. While vegetation impacts the water cycle, it reacts to changing water availability via functional, biochemical and structural responses. Unravelling the resulting complex feedbacks and interactions between the plant-water system and environmental change is essential for any modelling approaches and predictions, but still insufficiently understood due to currently missing observations. We hypothesize that an appropriate cross-scale monitoring of plant-water relations can be achieved by combined observational and modelling approaches. This paper reviews suitable remote sensing approaches to assess plant-water relations ranging from pure observational to combined observational-modelling approaches. We use a combined energy balance and radiative transfer model to assess the explanatory power of pure observational approaches focussing on plant parameters to estimate plant-water relations, followed by an outline for a more effective use of remote sensing by their integration into soil-plant-atmosphere continuum (SPAC) models. We apply a mechanistic model simulating water movement in the SPAC to reveal insight into the complexity of relations between soil, plant and atmospheric parameters, and thus plant-water relations. We conclude that future research should focus on strategies combining observations and mechanistic modelling to advance our knowledge on the interplay between the plant-water system and environmental change, e.g. through plant transpiration. [ABSTRACT FROM AUTHOR]

Forests are structurally complex ecosystems with several canopy layers, each with distinct functional properties contributing differently to the net ecosystem CO 2 exchange. A large number of studies have addressed ecosystem-scale net CO 2 fluxes in forests, but only few studies have investigated the role of the forest understory. The goal of this study was to quantify the below-canopy contribution to ecosystem CO 2 fluxes of a mixed deciduous forest in Switzerland. Below- and above-canopy eddy-covariance (EC) measurements were made continuously over two years, complemented by within canopy wind and CO 2 concentration profile measurements. On an annual basis, the below-canopy fluxes indicated a net carbon source dominated by soil respiration, while the above-canopy fluxes were dominated by tree photosynthesis leading to a net carbon sink. Below-canopy fluxes showed a net carbon sink only in spring, with the early emergence of understory plants before overstory canopy leaf-out. Below-canopy respiration partitioned from the EC measurements agreed well with previous chamber-based soil respiration measurements. However, below- and above-canopy fluxes became decoupled under full canopy closure, thus leading to unaccounted below-canopy fluxes when measured only above the canopy. Wind and CO 2 concentration profile measurements supported this finding. Decoupling was independent of low turbulence conditions and decoupled periods could be identified using the relationship between the below- and above-canopy standard deviation in vertical wind velocity. A decoupling correction was applied to the above-canopy measurements during decoupled periods and corrected annual net ecosystem production (NEP) agreed well with independent estimates from biomass inventory combined with models. Overall, the below-canopy fluxes contributed 79% to annual ecosystem respiration, but only 9% to annual ecosystem photosynthesis. The decoupling correction reduced annual NEP for the site from about 760 to 330 g C m −2 yr −1 . Our results showed that below-canopy EC measurements are essential in this mixed deciduous forest, and likely in many other forests, to fully understand the carbon dynamics within structurally complex ecosystems. [ABSTRACT FROM AUTHOR]

Stand-replacing disturbances, such as harvesting, change former forest net CO 2 sinks into net sources due to significantly reduced photosynthetic uptake and continued respiratory losses. To quantify these effects, this study used data from a Fluxnet-Canada Douglas-fir chronosequence on Vancouver Island, where the most mature site (62-year-old; DF49) was commercially harvested in 2011 creating a 77-ha clearcut (HDF11). Carbon (C) exchange was measured continuously using the eddy-covariance technique for more than a decade pre-harvest and for three years after harvesting. Automated non-steady-state chambers were used to measure soil respiration ( R s ) before and after harvesting. The mature stand transitioned from a moderate C sink (net ecosystem productivity (NEP) = 560 g C m −2 yr −1 ) before harvesting to a strong C source (NEP = −1000 g C m −2 yr −1 ) in the first year after harvesting. Gross ecosystem photosynthesis (GEP) decreased from 1890 g C m −2 yr −1 before harvesting to 130 g C m −2 yr −1 in the first year after harvesting, while ecosystem respiration ( R e ) decreased by just 15% from 1325 to 1130 g C m −2 yr −1 . This small decrease in R e suggests that heterotrophic respiration ( R h ) increased to partially compensate for a significant reduction in autotrophic respiration ( R a ) due to the loss of respiring roots, boles, branches and foliage. The post-harvest C balance results from HDF11 were also compared with those for a previously harvested stand (HDF00) in the same chronosequence. It was harvested in 2000 and was located 3 km away from HDF11. Considerable differences in NEP, GEP and R e were observed in the two clearcuts for the first two comparison years following harvesting, with HDF11 being a stronger source of C (NEP = −780 and −697 g C m −2 yr −1 ) than HDF00 (NEP = −620 and −520 g C m −2 yr −1 ). This was mostly due to greater R e at HDF11 likely due to a greater amount of decomposing organic matter and logging residue. These results show that caution is necessary when drawing conclusions about C fluxes from a single site in an ecozone. [ABSTRACT FROM AUTHOR]

Sun-induced chlorophyll fluorescence ( SIF ) is a radiation flux emitted from chlorophyll molecules and is considered an indicator of the actual functional state of plant photosynthesis. The remote measurement of SIF opens a new perspective to assess actual photosynthesis at larger, ecologically relevant scales and provides an alternative approach to study the terrestrial carbon cycle. Recent studies demonstrated the reliability of measured SIF signals and showed significant relationships between SIF and gross primary production ( GPP ) at ecosystem and global scales. Despite these encouraging results, understanding the complex mechanisms between SIF and GPP remains challenging before SIF can be finally utilized to constrain estimates of GPP . In this study, we present a comprehensive assessment of the relationship between far-red SIF retrieved at 760 nm ( SIF 760 ) and GPP , and its transferability across three structurally and physiologically contrasting ecosystems: perennial grassland, cropland and mixed temperate forest. We use multi-temporal imaging spectroscopy (IS) data acquired with the Airborne Prism EXperiment (APEX) sensor as well as eddy covariance (EC) flux tower data to evaluate the relationship between SIF 760 and GPP EC . We use simulations performed with the coupled photosynthesis–fluorescence model SCOPE to prove trends obtained from our observational data and to assess apparent confounding factors such as physiological and structural interferences or temporal scaling effects. Observed relationships between SIF 760 and GPP EC were asymptotic and ecosystem-specific, i.e., perennial grassland ( R 2 = 0.59, rRMSE = 27.1%), cropland ( R 2 = 0.88, rRMSE = 3.5%) and mixed temperate forest ( R 2 = 0.48, rRMSE = 15.88%). We demonstrate that asymptotic leaf level relationships between SIF 760 and GPP EC became more linear at canopy level and scaled with temporal aggregation. We conclude that remote sensing of SIF provides a new observational approach to decrease uncertainties in estimating GPP across ecosystems but requires dedicated strategies to compensate for the various confounding factors impacting SIF – GPP relationships. Our findings help in bridging the gap between mechanistic understanding at leaf level and ecosystem-specific observations of the relationships between SIF and GPP . [ABSTRACT FROM AUTHOR]