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Dennis D. Baldocchi - One of the best experts on this subject based on the ideXlab platform.
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on the inter and intra annual variability of ecosystem evapotranspiration and water use efficiency of an oak savanna and annual grassland subjected to booms and busts in rainfall
Global Change Biology, 2021Co-Authors: Dennis D. Baldocchi, Joe VerfaillieAbstract:Whether annual evapotranspiration of native ecosystems is increasing or decreasing with time as CO2 concentrations are rising, the climate is warming and rainfall experiences booms and busts, remains an unanswered question in the field of global change biology. To answer this question, we measured evapotranspiration and Carbon Dioxide Exchange over and under an oak savanna and over an annual grassland in the Mediterranean climate of California, USA, from 2001 through 2019 with the eddy covariance method; during this 19-year period, CO2 rose 40 ppm, air temperature increased by 1°C and annual rainfall ranged between 133 and 890 mm/year. No temporal trend in evapotranspiration or water use efficiency was observed over this time duration. Many competing positive and negative feedbacks among stomatal sensitivity to Carbon Dioxide concentrations, soil moisture, and vapor pressure deficit, the impact of temperature on saturation vapor pressure and access to groundwater muted the response of evapotranspiration to its changing world when integrated to the ecosystem scale and annual time steps. At the intra-annual time scale, we found that plants transmit information on soil moisture status through their influence on the vapor pressure deficit of the atmospheric boundary layer. The inter-annual variations in evaporative water use by the savanna and annual grassland were relatively decoupled from the booms and busts in rainfall. Instead, variations in length of growing season and access to groundwater explained much of this year-to-year variation in annual evapotranspiration. The access of groundwater by the oak savanna may make these ecosystems more robust in a warmer world, than was previously thought. This is a scale emergent property that needs better consideration in coupled climate-ecosystem models.
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using a regional cluster of ameriflux sites in central california to advance our knowledge on decadal scale ecosystem atmosphere Carbon Dioxide Exchange
2015Co-Authors: Dennis D. BaldocchiAbstract:Continuous eddy convariance measurements of Carbon Dioxide, water vapor and heat were measured continuously between an oak savanna and an annual grassland in California over a 4 year period. These systems serve as representative sites for biomes in Mediterranean climates and experience much seasonal and inter-annual variability in temperature and precipitation. These sites hence serve as natural laboratories for how whole ecosystem will respond to warmer and drier conditions. The savanna proved to be a moderate sink of Carbon, taking up about 150 gC m-2y-1 compared to the annual grassland, which tended to be Carbon neutral and often a source during drier years. But this Carbon sink by the savanna came at a cost. This ecosystem used about 100 mm more water per year than the grassland. And because the savanna was darker and rougher its air temperature was about 0.5 C warmer. In addition to our flux measurements, we collected vast amounts of ancillary data to interpret the site and fluxes, making this site a key site for model validation and parameterization. Datasets consist of terrestrial and airborne lidar for determining canopy structure, ground penetrating radar data on root distribution, phenology cameras monitoring leaf area index and its seasonality,more » predawn water potential, soil moisture, stem diameter and physiological capacity of photosynthesis.« less
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Carbon Dioxide Exchange of a pepperweed lepidium latifolium l infestation how do flowering and mowing affect canopy photosynthesis and autotrophic respiration
Journal of Geophysical Research, 2011Co-Authors: Oliver Sonnentag, Matteo Detto, Benjamin R K Runkle, Yit Arn Teh, Whendee L Silver, Maggi Kelly, Dennis D. BaldocchiAbstract:[1] The net ecosystem Carbon Dioxide (CO2) Exchange of invasive plant infestations, such as perennial pepperweed (Lepidium latifolium L.), is not well understood. A characteristic feature of pepperweed's phenological cycle is its small white flowers during secondary inflorescence. Pepperweed flowering causes uniform reflectance over the visible range of the electromagnetic spectrum, thus decreasing the amount of energy absorbed by the canopy and available for photosynthesis. Little is known about how pepperweed flowering and control measures such as mowing affect canopy photosynthesis and autotrophic respiration (FAR) and thus ecosystem respiration. To examine this question, we analyzed CO2 flux measurements made with eddy covariance over a pepperweed infestation in California, covering three growing seasons. Unmowed pepperweed caused the site to be almost CO2 neutral (2007: −28 g C m−2 period−1) or a net source (2009: 129 g C m−2 period−1), mostly because of reduced maximum photosynthetic capacity by 13 (2007) and 17 μmol m−2 s−1 (2009) due to flowering during the plant's prime photosynthetic period. Reference FAR at 10°C was reduced by 2 μmol m−2 s−1 in 2007 and 2009. Mowing during early flowering reversed the attenuating effects of pepperweed flowering, causing the site to act as a net CO2 sink (2008: −174 g C m−2 period−1) mainly due to prolonged photosynthetic CO2 uptake over the plant's early vegetative growth phase. Our results highlight the tight link between pepperweed's prominent key phenological phase and applied control measures, which together exert dominant control over the infestation's CO2 source-sink strength.
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inter annual variability in Carbon Dioxide Exchange of an oak grass savanna and open grassland in california
Agricultural and Forest Meteorology, 2007Co-Authors: Dennis D. Baldocchi, Ted HehnAbstract:To understand the dynamics of ecosystem Carbon cycling, CO2 fluxes were measured over and under an oak‐grass savanna and over a proximate grassland in California. The measurements were made from 2000 to 2006 using the eddy covariance technique. Annual net Carbon Exchange (NEE) ranged from � 155 to � 56 gC m � 2 year � 1 and from � 88 to 141 gC m � 2 year � 1 at the savanna and nearby grassland, respectively. Inter-annual variability in NEE was significantly related to length of growing season for the savanna, grassland, and tree canopy. We partitioned the NEE into two separate terms—primary productivity (GPP) and respiration (ecosystem respiration, Reco)—and found that the GPP and Reco of the savanna and grassland depended primarily on the amount of seasonal precipitation that occurred while grass and tree canopies were simultaneously active rather than depending on annual precipitation. We also found that NEE was greatly constrained by both Reco and GPP and Recowas significantly constrained by GPP. Reco increased by 79 gC m � 2 year � 1 with each 100 gC m � 2 year � 1 increase in GPP. In addition, measuring CO2 Exchange separately in the overstory and understory of the savanna over multiple years enabled us to partition Reco into heterotrophic and autotrophic respiration terms in a new and direct way. The sensitivity of Reco to GPP (dReco/dGPP) and the baseline of respiration terms both provide useful tools for understanding the dynamics of ecosystem CO2 uptake under current conditions of climate and ecosystem succession stage. # 2007 Elsevier B.V. All rights reserved.
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On the Separation of Net Ecosystem Exchange into Assimilation and Ecosystem Respiration: Review and Improved Algorithm
Global Change Biology, 2005Co-Authors: M. Reichstein, Dennis D. Baldocchi, Marc Aubinet, Eva Falge, Dario Papale, Paul Berbigier, Christian Bernhofer, Nina Buchmann, Tagir G. Gilmanov, A. GranierAbstract:This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem Exchange (NEE) into its major components, gross ecosystem Carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of Carbon Dioxide Exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long
Timo Vesala - One of the best experts on this subject based on the ideXlab platform.
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inter and intra annual dynamics of photosynthesis differ between forest floor vegetation and tree canopy in a subarctic scots pine stand
Agricultural and Forest Meteorology, 2019Co-Authors: Pasi Kolari, Jukka Pumpanen, Liisa Kulmala, Sigrid Dengel, Frank Berninger, Kajar Koster, Laura Matkala, Anni Vanhatalo, Timo VesalaAbstract:Abstract We studied the inter- and intra-annual dynamics of the photosynthesis of forest floor vegetation and tree canopy in a subarctic Scots pine stand at the northern timberline in Finland. We tackled the issue using three different approaches: 1) measuring Carbon Dioxide Exchange above and below canopy with the eddy covariance technique, 2) modelling the photosynthesis of the tree canopy based on shoot chamber measurements, and 3) upscaling the forest floor photosynthesis using biomass estimates and available information on the annual cycle of photosynthetic capacity of those species. The studied ecosystem was generally a weak sink of Carbon but the sink strength showed notable year-to-year variation. Total ecosystem respiration and photosynthesis indicated a clear temperature limitation for the Carbon Exchange. However, the increase in photosynthetic production was steeper than the increase in respiration with temperature, indicating that warm temperatures increase the sink strength and do not stimulate the total ecosystem respiration as much in the 4-year window studied. The interannual variation in the photosynthetic production of the forest stand mainly resulted from the forest floor vegetation, whereas the photosynthesis of the tree canopy seemed to be more stable from year to year. Tree canopy photosynthesis increased earlier in the spring, whereas that of the forest floor increased after snowmelt, highlighting that models for photosynthesis in the northern area should also include snow cover in order to accurately estimate the seasonal dynamics of photosynthesis in these forests.
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temporal variation of ecosystem scale methane emission from a boreal fen in relation to temperature water table position and Carbon Dioxide fluxes
Global Biogeochemical Cycles, 2018Co-Authors: Janne Rinne, Eevastiina Tuittila, Olli Peltola, Maarit Raivonen, Pavel Alekseychik, Sami Haapanala, Mari Pihlatie, Mika Aurela, Ivan Mammarella, Timo VesalaAbstract:We have analyzed decade-long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and Carbon Dioxide Exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable-slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process-based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10-year period methane emission was 18% of net ecosystem Exchange as Carbon. The weak relation of methane emission to water table position indicates that space-to-time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales. (Less)
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simulation and scaling of temporal variation in gross primary production for coniferous and deciduous temperate forests
Global Change Biology, 2004Co-Authors: Quan Wang, Eva Falge, Christian Bernhofer, John Tenhunen, Andre Granier, Timo VesalaAbstract:Observations of ecosystem net Carbon Dioxide Exchange obtained with eddy covariance techniques over a 4-year period at spruce, beech and pine forest sites were used to derive time series data for gross primary production (GPP) and ecosystem respiration (Reco). A detailed canopy gas Exchange model (PROXELNEE) was inverted at half-hour time step to estimate seasonal changes in carboxylation capacity and light utilization efficiency of the forest canopies. The parameter estimates were then used further to develop a time-dependent modifier of physiological activity in the daily time step gas Exchange model of Chen et al. (1999), previously used for regional simulations in BOREAS. The daily model was run under a variety of assumptions and the results emphasize the need in future analyses: (1) to focus on time-dependent seasonal changes in canopy physiology as well as in leaf area index, (2) to compare time courses of physiological change in different habitats in terms of recognizable cardinal points in the seasonal course, and (3) to develop methods for utilizing information on seasonal changes in physiology in regional and continental Carbon budget simulations. The results suggest that the daily model with appropriate seasonal adjustments for physiological process regulation should be an efficient tool for use in conjunction with remote sensing for regional evaluation of global change scenarios.
Russell L Scott - One of the best experts on this subject based on the ideXlab platform.
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Carbon Dioxide Exchange in a semidesert grassland through drought induced vegetation change
Journal of Geophysical Research, 2010Co-Authors: Russell L Scott, Erik P Hamerlynck, Darrel G Jenerette, Susan M Moran, Greg A BarrongaffordAbstract:[1] Global warming may intensify the hydrological cycle and lead to increased drought severity and duration, which could alter plant community structure and subsequent ecosystem water and Carbon Dioxide cycling. We report on the net ecosystem Exchange of Carbon Dioxide (NEE) of a semidesert grassland through a severe drought which drove succession from native bunchgrasses to forbs and to eventual dominance by an exotic bunchgrass. We monitored NEE and energy fluxes using eddy covariance coupled with meteorological and soil moisture variables for 6 years at a grassland site in southeastern Arizona, USA. Seasonal NEE typically showed a springtime Carbon uptake after winter-spring periods of average rainfall followed by much stronger sink activity during the summer rainy season. The two severe drought years (2004 and 2005) resulted in a net release of Carbon Dioxide (25 g C m−2) and widespread mortality of native perennial bunchgrasses. Above average summer rains in 2006 alleviated drought conditions, resulting in a large flush of broad-leaved forbs and negative total NEE (−55 g C m−2 year−1). Starting in 2007 and continuing through 2009, the ecosystem became increasingly dominated by the exotic grass, Eragrostis lehmanniana, and was a net Carbon sink (−47 to −98 g C m−2 year−1) but with distinct annual patterns in NEE. Rainfall mediated by soils was the key driver to water and Carbon fluxes. Seasonal respiration and photosynthesis were strongly dependent on precipitation, but photosynthesis was more sensitive to rainfall variation. Respiration normalized by evapotranspiration showed no interannual variation, while normalized gross ecosystem production (i.e., water use efficiency) was low during drought years and then increased as the rains returned and the E. lehmanniana invasion progressed. Thus, when dry summer conditions returned in 2009, the potential for ecosystem Carbon accumulation was increased and the ecosystem remained a net sink unlike similar dry years when native grasses dominated ecosystem structure.
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effects of seasonal drought on net Carbon Dioxide Exchange from a woody plant encroached semiarid grassland
Journal of Geophysical Research, 2009Co-Authors: Russell L Scott, Darrel G Jenerette, Daniel Potts, Travis E HuxmanAbstract:[1] Annual precipitation in the central and southern warm-desert region of North America is distributed climatologically between summer and winter periods with two prominent dry periods between them. We used energy and Carbon Dioxide (CO2) fluxes from eddy covariance along with standard meteorological and soil moisture measurements at a semiarid savanna in southern Arizona, United States, to better understand the consequences of warm or cool season drought on ecosystem CO2 Exchange in these bimodally forced water-limited regions. Over the last 100 years, this historic grassland has converted to a savanna by the encroachment of the native mesquite tree (Prosopis velutina Woot.). During each of the 4 years of observation (2004–2007), annual precipitation (P) was below average, but monsoon (July–September) P was both above and below average while cool-season (December–March) P was always less than average by varying degrees. The ecosystem was a net source of CO2 to the atmosphere, ranging from 14 to 95 g C m � 2 yr � 1 with the strength of the source increasing with decreasing precipitation. When the rainfall was closest to the long-term average in its distribution and amount, the ecosystem was essentially Carbon neutral. Summer drought resulted in increased Carbon losses due mainly to a shortening of the growing season and the length of time later in the season when photosynthetic gain exceeds respiration loss. Severe cool season drought led to decreased spring Carbon uptake and seemingly enhanced summer respiration, resulting in conditions that led to the greatest annual net Carbon loss.
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partitioning of evapotranspiration and its relation to Carbon Dioxide Exchange in a chihuahuan desert shrubland
Hydrological Processes, 2006Co-Authors: Russell L Scott, Travis E Huxman, William L Cable, William E EmmerichAbstract:Key to evaluating the consequences of woody plant encroachment on water and Carbon cycling in semiarid ecosystems is a mechanistic understanding of how biological and non-biological processes influence water loss to the atmosphere. To better understand how precipitation is partitioned into the components of evapotranspiration (bare-soil evaporation and plant transpiration) and their relationship to plant uptake of Carbon Dioxide (CO2) as well as ecosystem respiratory efflux, we measured whole plant transpiration, evapotranspiration, and CO2 fluxes over the course of a growing season at a semiarid Chihuahuan Desert shrubland site in south-eastern Arizona. Whole plant transpiration was measured using the heat balance sap-flow method, while evapotranspiration and net ecosystem Exchange (NEE) of CO2 were quantified using the Bowen ratio technique. Before the summer rainy season began, all water and CO2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO2 fluxes were dominated by respiration as it took approximately 10 days for the shrubs to respond to the higher soil moisture content. During the growing season, periods immediately following rain events (<2 days) were dominated by evaporation and respiration while transpiration and CO2 uptake peaked during the interstorm periods. The surface of the coarse, well-drained soils dried quickly, rapidly reducing evaporation. Overall, the ratio of total transpiration to evapotranspiration was 58%, but it was around 70% during the months when the plants were active. Peak respiration responses following rain events generally lagged after the evaporation peak by a couple of days and were better correlated with transpiration. Transpiration and CO2 uptake also decayed rather quickly during interstorm periods, indicating that optimal plant soil moisture conditions were rarely encountered. NEE of CO2 was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO2 and greater water use efficiency due mainly to decreases in respiration. Copyright 2006 John Wiley & Sons, Ltd.
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ecohydrological impacts of woody plant encroachment seasonal patterns of water and Carbon Dioxide Exchange within a semiarid riparian environment
Global Change Biology, 2006Co-Authors: Russell L Scott, Travis E Huxman, David G Williams, David C GoodrichAbstract:Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody-plant species. In this paper, we compare ecosystem water and Carbon Dioxide (CO2) fluxes over a grassland, a grassland‐shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody-plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near-surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem Exchange of Carbon Dioxide (NEE) increased with increasing woody-plant dominance. Growing season ET totals were 407, 450, and 639mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep-rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO2 gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March‐December (primary growing season) totals of NEE were � 63, � 212, and � 233gCm � 2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO2 sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced Carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody-plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and Carbon balances in dryland areas.
Travis E Huxman - One of the best experts on this subject based on the ideXlab platform.
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effects of seasonal drought on net Carbon Dioxide Exchange from a woody plant encroached semiarid grassland
Journal of Geophysical Research, 2009Co-Authors: Russell L Scott, Darrel G Jenerette, Daniel Potts, Travis E HuxmanAbstract:[1] Annual precipitation in the central and southern warm-desert region of North America is distributed climatologically between summer and winter periods with two prominent dry periods between them. We used energy and Carbon Dioxide (CO2) fluxes from eddy covariance along with standard meteorological and soil moisture measurements at a semiarid savanna in southern Arizona, United States, to better understand the consequences of warm or cool season drought on ecosystem CO2 Exchange in these bimodally forced water-limited regions. Over the last 100 years, this historic grassland has converted to a savanna by the encroachment of the native mesquite tree (Prosopis velutina Woot.). During each of the 4 years of observation (2004–2007), annual precipitation (P) was below average, but monsoon (July–September) P was both above and below average while cool-season (December–March) P was always less than average by varying degrees. The ecosystem was a net source of CO2 to the atmosphere, ranging from 14 to 95 g C m � 2 yr � 1 with the strength of the source increasing with decreasing precipitation. When the rainfall was closest to the long-term average in its distribution and amount, the ecosystem was essentially Carbon neutral. Summer drought resulted in increased Carbon losses due mainly to a shortening of the growing season and the length of time later in the season when photosynthetic gain exceeds respiration loss. Severe cool season drought led to decreased spring Carbon uptake and seemingly enhanced summer respiration, resulting in conditions that led to the greatest annual net Carbon loss.
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partitioning of evapotranspiration and its relation to Carbon Dioxide Exchange in a chihuahuan desert shrubland
Hydrological Processes, 2006Co-Authors: Russell L Scott, Travis E Huxman, William L Cable, William E EmmerichAbstract:Key to evaluating the consequences of woody plant encroachment on water and Carbon cycling in semiarid ecosystems is a mechanistic understanding of how biological and non-biological processes influence water loss to the atmosphere. To better understand how precipitation is partitioned into the components of evapotranspiration (bare-soil evaporation and plant transpiration) and their relationship to plant uptake of Carbon Dioxide (CO2) as well as ecosystem respiratory efflux, we measured whole plant transpiration, evapotranspiration, and CO2 fluxes over the course of a growing season at a semiarid Chihuahuan Desert shrubland site in south-eastern Arizona. Whole plant transpiration was measured using the heat balance sap-flow method, while evapotranspiration and net ecosystem Exchange (NEE) of CO2 were quantified using the Bowen ratio technique. Before the summer rainy season began, all water and CO2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO2 fluxes were dominated by respiration as it took approximately 10 days for the shrubs to respond to the higher soil moisture content. During the growing season, periods immediately following rain events (<2 days) were dominated by evaporation and respiration while transpiration and CO2 uptake peaked during the interstorm periods. The surface of the coarse, well-drained soils dried quickly, rapidly reducing evaporation. Overall, the ratio of total transpiration to evapotranspiration was 58%, but it was around 70% during the months when the plants were active. Peak respiration responses following rain events generally lagged after the evaporation peak by a couple of days and were better correlated with transpiration. Transpiration and CO2 uptake also decayed rather quickly during interstorm periods, indicating that optimal plant soil moisture conditions were rarely encountered. NEE of CO2 was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO2 and greater water use efficiency due mainly to decreases in respiration. Copyright 2006 John Wiley & Sons, Ltd.
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ecohydrological impacts of woody plant encroachment seasonal patterns of water and Carbon Dioxide Exchange within a semiarid riparian environment
Global Change Biology, 2006Co-Authors: Russell L Scott, Travis E Huxman, David G Williams, David C GoodrichAbstract:Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody-plant species. In this paper, we compare ecosystem water and Carbon Dioxide (CO2) fluxes over a grassland, a grassland‐shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody-plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near-surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem Exchange of Carbon Dioxide (NEE) increased with increasing woody-plant dominance. Growing season ET totals were 407, 450, and 639mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep-rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO2 gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March‐December (primary growing season) totals of NEE were � 63, � 212, and � 233gCm � 2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO2 sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced Carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody-plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and Carbon balances in dryland areas.
Liisa Kulmala - One of the best experts on this subject based on the ideXlab platform.
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inter and intra annual dynamics of photosynthesis differ between forest floor vegetation and tree canopy in a subarctic scots pine stand
Agricultural and Forest Meteorology, 2019Co-Authors: Pasi Kolari, Jukka Pumpanen, Liisa Kulmala, Sigrid Dengel, Frank Berninger, Kajar Koster, Laura Matkala, Anni Vanhatalo, Timo VesalaAbstract:Abstract We studied the inter- and intra-annual dynamics of the photosynthesis of forest floor vegetation and tree canopy in a subarctic Scots pine stand at the northern timberline in Finland. We tackled the issue using three different approaches: 1) measuring Carbon Dioxide Exchange above and below canopy with the eddy covariance technique, 2) modelling the photosynthesis of the tree canopy based on shoot chamber measurements, and 3) upscaling the forest floor photosynthesis using biomass estimates and available information on the annual cycle of photosynthetic capacity of those species. The studied ecosystem was generally a weak sink of Carbon but the sink strength showed notable year-to-year variation. Total ecosystem respiration and photosynthesis indicated a clear temperature limitation for the Carbon Exchange. However, the increase in photosynthetic production was steeper than the increase in respiration with temperature, indicating that warm temperatures increase the sink strength and do not stimulate the total ecosystem respiration as much in the 4-year window studied. The interannual variation in the photosynthetic production of the forest stand mainly resulted from the forest floor vegetation, whereas the photosynthesis of the tree canopy seemed to be more stable from year to year. Tree canopy photosynthesis increased earlier in the spring, whereas that of the forest floor increased after snowmelt, highlighting that models for photosynthesis in the northern area should also include snow cover in order to accurately estimate the seasonal dynamics of photosynthesis in these forests.