... Eight-month-old cork oak (Quercus suber L.) saplings were trans-planted in July 1996 into 7.7... more ... Eight-month-old cork oak (Quercus suber L.) saplings were trans-planted in July 1996 into 7.7 dm3 pots filled exclusively with washed sand. ... [7] Ferrier RC, Alexander IJ, Internal redistribution of N in Sitka spruce seedlings with partly droughted root systems, For. Sci. ...
This study analyses the influence of vegetation structure (i.e. leaf area index and canopy cover)... more This study analyses the influence of vegetation structure (i.e. leaf area index and canopy cover) and seasonal background changes on moderate-resolution imaging spectrometer (MODIS)-simulated reflectance data in open woodland. Approximately monthly spectral reflectance and transmittance field measurements (May 2011 to October 2013) of cork oak tree leaves (Quercus suber) and of the herbaceous understorey were recorded in the region of Ribatejo, Portugal. The geometric-optical and radiative transfer (GORT) model was used to simulate MODIS response (red, near-infrared) and to calculate vegetation indices, investigating their response to changes in the structure of the overstorey vegetation and to seasonal changes in the understorey using scenarios corresponding to contrasting phenological status (dry season vs. wet season). The performance of normalized difference vegetation index (NDVI), soil-adjusted vegetation index (SAVI), and enhanced vegetation index (EVI) is discussed. Results showed that SAVI and EVI were very sensitive to the emergence of background vegetation in the wet season compared to NDVI and that shading effects lead to an opposing trend in the vegetation indices. The information provided by this research can be useful to improve our understanding of the temporal dynamic of vegetation, monitored by vegetation indices.
In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physi... more In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO 2 efflux from stems under shaded and unshaded conditions, as well as the concentration of CO 2 dissolved in sap [CO 2 *], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO 2 release from stems of E. globulus. CO 2 efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mmol m 22 s 21 , respectively. Temperature and [CO 2 *] both had important, and similar, influences on the rate of CO 2 efflux from the stems, but neither explained the difference in the magnitude of CO 2 efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO 2 from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO 2 efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO 2 efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylemtransported CO 2 .
Background/Question/Methods In the last decades, the number of occurrences of extreme fluctuation... more Background/Question/Methods In the last decades, the number of occurrences of extreme fluctuations in temperatures has been increasing. The extent to which plants are able to acclimate to short term temperature changes needs further investigation. The objective of this experiment was to test whether fluctuating temperatures affect plant growth and to what extent plants are able to acclimate to short term changes in temperature. For this purpose, poplar saplings (Populus deltoides x nigra) were grown in a growth chamber under constant PAR (300mmolm-2s-1) and RH (50%) but under three different temperature regimes: control (C), medium fluctuations (M) and extreme fluctuations (E). In the C treatment, the day/night temperature was constant throughout the experiment. In the other two treatments, temperature increased for five days, by 5ºC (M) or 10ºC (E) over the control and then decreased for five days by the same amount. The cycles of high and low temperature were continuously repeated...
Background/Question/Methods Biodiverse permanent pastures rich in legumes have been implemented i... more Background/Question/Methods Biodiverse permanent pastures rich in legumes have been implemented in Portugal as a management tool to increase soil fertility, grassland productivity and animal population carrying capacity, and were later selected as a national land-use change strategy for increasing carbon sequestration and climate change mitigation. Sown, legume-rich improved pastures are commonly found in the understory of Mediterranean-type agroforestry systems – the Montados - with evergreen oaks, such as cork oak (Quercus suber L.), as the dominant tree species. However, little is known about the effects of these pastures on the physiology and productivity of co-occurring trees. Understanding the impact of sown, biodiverse pastures on cork oak, in particular assessing potential benefits from increased legume-nitrogen-fixation, can provide important information for future land-management and policy decisions. We examined a suite of leaf-level physiological and morphological traits...
Neste estudo utilizaram-se dados espectrais medidos em campo e provenientes de detecção remota pa... more Neste estudo utilizaram-se dados espectrais medidos em campo e provenientes de detecção remota para monitorizar a dinâmica temporal da actividade fotossintética e para parametrizar um modelo de estimativa da PPB numa área de montado de sobro, em que os fluxos de água e carbono são medidos continuamente pelo método da covariância turbulenta. Obteve-se uma relação linear significativa entre o NDVI, calculado com os dados espectrais, e a FRFA, medida com o ceptómetro, para as árvores e os arbustos. Observou-se uma melhoria significativa das estimativas da produtividade primária bruta quando o modelo é forçado com valores de NDVI (sensor MODIS) em comparação com a utilização de apenas dados meteorológicos. A inclusão da contribuição para a produtividade primária bruta de cada tipo de vegetação, integrada com NDVI e PRI obtidos de medições espectrais de campo, compara-se melhor com as observações. Estes resultados corroboram perspectivas para melhorar as estimativas da sazonalidade da PP...
ABSTRACT The effect of the interaction between water stress and growth at elevated CO2 on the lea... more ABSTRACT The effect of the interaction between water stress and growth at elevated CO2 on the leaf photochemistry were studied in Quercus ilex trees, growing in a naturally CO2 enriched site (the Bossoleto, Italy). Those plants were compared with plants growing nearby in similar soils but at ambient CO2 concentration. Our data indicate that down-regulation of photosynthesis is lower in non-watered trees from the enriched site than in non-watered trees from the ambient-CO2 site This may be associated to the higher carbon availability for the consumption of the excess absorbed light energy or to an increased protection of the photochemical apparatus against oxidative stress in plants grown under elevated CO2.
ABSTRACT It is nowadays clear that the inclusion of spectral indexes into biogeochemical models c... more ABSTRACT It is nowadays clear that the inclusion of spectral indexes into biogeochemical models can greatly improve actual estimates of gross primary productivity (GPP) at local and global scale. Several vegetation indexes can be obtained by the reflectance of light at specific wavelengths. Among them, the Normalized Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI) and the Photochemical Reflectance Index (PRI) were found suitable to represent different characteristics of ecosystems strictly related with GPP, such as biomass and photosynthetic capacity (NDVI, EVI) or radiation use efficiency (PRI). In Mediterranean cork oak savannas, characterized by high heterogeneity, the application of spectral indexes derived from coarse spatial resolution remotely sensed data (e.g. MODIS imagery) to represent the performance of the whole ecosystem is complex. A better knowledge of the variability of vegetation indexes for specific vegetation types, assessed in fieldwork, is fundamental to the interpretation of the same indexes obtained with satellite data and a key step through the integration of such indexes into biogeochemical models. We consider three different vegetation types: trees, grasses and shrubs, concurring to the overall ecosystem carbon budget in Mediterranean cork oak savannas. Since April 2011, reflectance measurements were performed in the range of 300-2500nm by the use of a handheld hyperspectral spectroradiometer (FieldSpec3, ASD Inc. CO, USA) in several species of the three vegetation types in a cork oak savanna eddy covariance site located in central Portugal. Measurements were always performed around solar noon and repeated approximately every two weeks. Several vegetation indexes were calculated. All indexes showed clear differences among vegetation types and among species. Marked seasonal trends were identified for grasses and shrubs, clearly related with the onset of dry summer conditions. Both NDVI and EVI decreased in grasses from April to the second half to July when they reached the same value of litter and bare soil. The recovery was only observed after the first rains in October. The occurrence of rain in October also reestablished the values of NDVI and EVI for evergreen shrubs that showed a marked decrease in September. On the contrary, both indexes remained stable in cork oak along the whole measurement period (April-December). In the perspective of the use of spectral indexes derived from satellite imagery, we compared the PRI values calculated with the wavelengths of 531 and 570nm proposed originally for this index (PRI570) and the wavelengths available on MODIS data: 531 and 550nm (PRI550). Both the indexes revealed different trends for the three vegetation types. Anyway, measurements of cork oak leaf reflectance performed with a leaf clip and compared with leaf chlorophyll concentration, showed that PRI570 is better correlated with leaf chlorophyll concentration than PRI550. Our results suggest that established vegetation indexes are well suited to represent the seasonal variability of the three vegetation types. In further studies, we intent to use the temporal variability of vegetation indexes to estimate the contribution of each vegetation type to the total GPP and its seasonal variability.
Most investigations of plant responses to changes in temperature have focused on a constant incre... more Most investigations of plant responses to changes in temperature have focused on a constant increase in mean day/night temperature without considering how differences in temperature cycles can affect physiological processes and growth. To test the effects of changes in growth temperature on foliar carbon balance and plant growth, we repeatedly exposed poplar saplings (Populus deltoides × nigra) to temperature cycles consisting of 5 days of a moderate (M, +5 °C) or extreme (E, +10 °C) increase in temperature followed by 5 days of a moderate (M, -5 °C) or extreme (E, -10 °C) decrease in temperature, with respect to a control treatment (C, 23.4 °C). The temperature treatments had the same mean temperature over each warm and cool cycle and over the entire study. Our goal was to examine the influence of recurring temperature shifts on growth. Net photosynthesis (A) was relatively insensitive to changes in growth temperature (from 20 to 35 °C), suggesting a broad range of optimum temperat...
At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings ... more At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15 N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO 2 exchange rate in newly developed leaves compared with pre-existing leaves.
Daily variations in photoprotective mechanisms were studied in sun and shade leaves of 40-year-ol... more Daily variations in photoprotective mechanisms were studied in sun and shade leaves of 40-year-old cork oak (Quercus suber L.) trees during early summer in Portugal. Although trees were not severely water stressed because predawn leaf water potentials remained high, photosynthesis and stomatal conductance decreased at midday. The midday depression in gas exchange was not reversed by short-term exposure to ''optimal'' conditions of temperature, light and vapor pressure deficit. Chlorophyll a fluorescence, maximum photochemical yield of photosystem II and the quantum yield of noncyclic electron transport showed midday depressions, but recovered by the evening. Both short-term changes in the components of the xanthophyll cycle (reversible de-epoxidation of violaxanthin during the day) as well as long-term changes (higher xanthophyll content in sun compared with shade leaves) were detected and may play a role in the dissipation of excess energy at midday. Because the activities of enzymes of the antioxidant system, superoxide dismutase and ascorbate peroxidase, were high enough to cope with the increase in oxygen reactive species likely to arise under the stressful conditions of midday, we conclude that these enzymes may provide an additional mechanism for energy dissipation.
In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physi... more In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO 2 efflux from stems under shaded and unshaded conditions, as well as the concentration of CO 2 dissolved in sap [CO 2 *], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO 2 release from stems of E. globulus. CO 2 efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mmol m 22 s 21 , respectively. Temperature and [CO 2 *] both had important, and similar, influences on the rate of CO 2 efflux from the stems, but neither explained the difference in the magnitude of CO 2 efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO 2 from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO 2 efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO 2 efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylemtransported CO 2 .
Respiration of stems and branches of trees (R S ) has typically been estimated by measuring radia... more Respiration of stems and branches of trees (R S ) has typically been estimated by measuring radial CO 2 efflux from woody tissue (E A ) and rates of efflux are often scaled temporally using a temperature relationship (Q 10 ). High concentrations of CO 2 in xylem sap ([CO Ã 2 ]) have been shown to affect E A , and the transport of CO 2 in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in E A . Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, E A and [CO Ã 2 ] were greater at low sap velocity, while the amount of respired CO 2 transported in sap (transport flux, F T ) was greater at high sap velocity. E A was linearly correlated with [CO Ã 2 ]. In branches of three diameter classes (1, 2, and 3 cm), volume-based E A , F T , and R S did not differ, but surfacearea based CO 2 fluxes increased with diameter class. Regardless of diameter, E A accounted for only 30% of respired CO 2 at high sap velocity, while at low sap velocity, E A accounted for 71% of respired CO 2 . E A , F T , and R S measured at 5, 20, and 35°C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, E A accounted for only 18% of the CO 2 released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO 2 in water. These results directly demonstrate the transport of respired CO 2 in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.
ABSTRACT In the context of global climate change, the quantification of carbon fluxes in forest e... more ABSTRACT In the context of global climate change, the quantification of carbon fluxes in forest ecosystems and how they vary inter-annually are important issues. Process-based models are flexible tools that permit assessing ecosystem productivity at different spatial and temporal scales and for different management and environmental conditions. On the other hand, carbon and water fluxes at the ecosystem scale may be measured using eddy covariance techniques, thus providing useful data for testing and validation of models. The principal aim of the work was to calibrate and evaluate two versions of a process-based model that differ in the autotrophic respiration (RA) modelling. The original version (3PGN) is based on a constant ratio between the net primary production (PN) and the gross primary production (PG), while, in a new version (3PGN*), developed by the authors, RA was modelled as a function of temperature and biomass. The two model versions were calibrated and evaluated using a comprehensive dataset consisting of forest growth experimental data and eddy-covariance measurements. The two model versions were calibrated and evaluated under a Bayesian framework consisting in model calibration, model comparison and analysis of model-data mismatch. Sensitivity and uncertainty analyses of 3PGN and 3PGN* were also carried out. The BC showed that the data were informative for almost 70% of the parameters. BC also allowed identification of the parameters to which the models were most sensitive and to assess parameter correlations. Key parameters were those for carbon allocation, some of the parameters related to water stress and site fertility. Bayesian model comparison showed that the 3PGN*, with the new autotrophic respiration model based on maintenance and growth respiration, has higher conditional probability of being correct than the original 3PGN, based on the simple NPP vs. GPP ratio.
This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspi... more This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspiration in the eucalypt site of Espirra in Southern Portugal (38 • 38 N, 8 • 36 W). This site was included in the "Carboeurope" European network and is part of a 300 ha eucalypt forest, with about 1100 trees ha −1 , intensively managed as a coppice for pulp production and characterized by a 12-month annual growing period. The climate is of Mediterranean type with a long term annual average precipitation of 709 mm and an annual average air temperature of 15.90 • C. During the measurement period (2002-2009) two main events took place, which changed the annual sink pattern of the forest: a drought period of two years (2004-2005) and a tree felling (October and November 2006). We analyzed the daily, seasonal and inter-annual variation of carbon uptake and evapotranspiration, and their relationships with the events and the variability of the main meteorological variables. Before the felling, annual net ecosystem exchange (NEE) increased from −865.56 g C m −2 in 2002 to −356.64 g C m −2 in 2005 together with a deep decrease in rainfall from 748 mm in 2002 to 378.58 mm and 396.64 mm in 2004 and 2005, respectively. For the same period, seasonal patterns of carbon uptake showed maximum values in April and decreased in July-August. The eucalypt stand recovered its carbon sink ability since June 2007 and had a NEE of −209.01 g C m −2 in 2009.
ABSTRACT Process-based models are powerful tools for sustainable and adaptive forest management. ... more ABSTRACT Process-based models are powerful tools for sustainable and adaptive forest management. Bayesian statistics and global sensitivity analysis allow to reduce uncertainties in parameters and outputs, and they provide better insight of model behaviour. In this work two versions of a process-based model that differed in the autotrophic respiration modelling were analysed. The original version (3PGN) was based on a constant ratio between net and gross primary production, while in a new version (3PGN∗) the autotrophic respiration was modelled as a function of temperature and biomass. A Bayesian framework, and a global sensitivity analysis (Morris method) were used to reduce parametric uncertainty, to highlight strengths and weaknesses of the models and to evaluate their performances. The Bayesian approach allowed also to identify the weaknesses and strengths of the dataset used for the analyses. The Morris method in combination with the Bayesian framework helped to identify key parameters and gave a deeper understanding of model behaviour. Both model versions reliably predicted average stand diameter at breast height, average stand height, stand volume and stem biomass. On the contrary, the models were not able to accurately predict net ecosystem production. Bayesian model comparison showed that 3PGN∗, with the new autotrophic respiration model, has a higher conditional probability of being correct than the original 3PGN model.
... Eight-month-old cork oak (Quercus suber L.) saplings were trans-planted in July 1996 into 7.7... more ... Eight-month-old cork oak (Quercus suber L.) saplings were trans-planted in July 1996 into 7.7 dm3 pots filled exclusively with washed sand. ... [7] Ferrier RC, Alexander IJ, Internal redistribution of N in Sitka spruce seedlings with partly droughted root systems, For. Sci. ...
This study analyses the influence of vegetation structure (i.e. leaf area index and canopy cover)... more This study analyses the influence of vegetation structure (i.e. leaf area index and canopy cover) and seasonal background changes on moderate-resolution imaging spectrometer (MODIS)-simulated reflectance data in open woodland. Approximately monthly spectral reflectance and transmittance field measurements (May 2011 to October 2013) of cork oak tree leaves (Quercus suber) and of the herbaceous understorey were recorded in the region of Ribatejo, Portugal. The geometric-optical and radiative transfer (GORT) model was used to simulate MODIS response (red, near-infrared) and to calculate vegetation indices, investigating their response to changes in the structure of the overstorey vegetation and to seasonal changes in the understorey using scenarios corresponding to contrasting phenological status (dry season vs. wet season). The performance of normalized difference vegetation index (NDVI), soil-adjusted vegetation index (SAVI), and enhanced vegetation index (EVI) is discussed. Results showed that SAVI and EVI were very sensitive to the emergence of background vegetation in the wet season compared to NDVI and that shading effects lead to an opposing trend in the vegetation indices. The information provided by this research can be useful to improve our understanding of the temporal dynamic of vegetation, monitored by vegetation indices.
In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physi... more In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO 2 efflux from stems under shaded and unshaded conditions, as well as the concentration of CO 2 dissolved in sap [CO 2 *], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO 2 release from stems of E. globulus. CO 2 efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mmol m 22 s 21 , respectively. Temperature and [CO 2 *] both had important, and similar, influences on the rate of CO 2 efflux from the stems, but neither explained the difference in the magnitude of CO 2 efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO 2 from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO 2 efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO 2 efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylemtransported CO 2 .
Background/Question/Methods In the last decades, the number of occurrences of extreme fluctuation... more Background/Question/Methods In the last decades, the number of occurrences of extreme fluctuations in temperatures has been increasing. The extent to which plants are able to acclimate to short term temperature changes needs further investigation. The objective of this experiment was to test whether fluctuating temperatures affect plant growth and to what extent plants are able to acclimate to short term changes in temperature. For this purpose, poplar saplings (Populus deltoides x nigra) were grown in a growth chamber under constant PAR (300mmolm-2s-1) and RH (50%) but under three different temperature regimes: control (C), medium fluctuations (M) and extreme fluctuations (E). In the C treatment, the day/night temperature was constant throughout the experiment. In the other two treatments, temperature increased for five days, by 5ºC (M) or 10ºC (E) over the control and then decreased for five days by the same amount. The cycles of high and low temperature were continuously repeated...
Background/Question/Methods Biodiverse permanent pastures rich in legumes have been implemented i... more Background/Question/Methods Biodiverse permanent pastures rich in legumes have been implemented in Portugal as a management tool to increase soil fertility, grassland productivity and animal population carrying capacity, and were later selected as a national land-use change strategy for increasing carbon sequestration and climate change mitigation. Sown, legume-rich improved pastures are commonly found in the understory of Mediterranean-type agroforestry systems – the Montados - with evergreen oaks, such as cork oak (Quercus suber L.), as the dominant tree species. However, little is known about the effects of these pastures on the physiology and productivity of co-occurring trees. Understanding the impact of sown, biodiverse pastures on cork oak, in particular assessing potential benefits from increased legume-nitrogen-fixation, can provide important information for future land-management and policy decisions. We examined a suite of leaf-level physiological and morphological traits...
Neste estudo utilizaram-se dados espectrais medidos em campo e provenientes de detecção remota pa... more Neste estudo utilizaram-se dados espectrais medidos em campo e provenientes de detecção remota para monitorizar a dinâmica temporal da actividade fotossintética e para parametrizar um modelo de estimativa da PPB numa área de montado de sobro, em que os fluxos de água e carbono são medidos continuamente pelo método da covariância turbulenta. Obteve-se uma relação linear significativa entre o NDVI, calculado com os dados espectrais, e a FRFA, medida com o ceptómetro, para as árvores e os arbustos. Observou-se uma melhoria significativa das estimativas da produtividade primária bruta quando o modelo é forçado com valores de NDVI (sensor MODIS) em comparação com a utilização de apenas dados meteorológicos. A inclusão da contribuição para a produtividade primária bruta de cada tipo de vegetação, integrada com NDVI e PRI obtidos de medições espectrais de campo, compara-se melhor com as observações. Estes resultados corroboram perspectivas para melhorar as estimativas da sazonalidade da PP...
ABSTRACT The effect of the interaction between water stress and growth at elevated CO2 on the lea... more ABSTRACT The effect of the interaction between water stress and growth at elevated CO2 on the leaf photochemistry were studied in Quercus ilex trees, growing in a naturally CO2 enriched site (the Bossoleto, Italy). Those plants were compared with plants growing nearby in similar soils but at ambient CO2 concentration. Our data indicate that down-regulation of photosynthesis is lower in non-watered trees from the enriched site than in non-watered trees from the ambient-CO2 site This may be associated to the higher carbon availability for the consumption of the excess absorbed light energy or to an increased protection of the photochemical apparatus against oxidative stress in plants grown under elevated CO2.
ABSTRACT It is nowadays clear that the inclusion of spectral indexes into biogeochemical models c... more ABSTRACT It is nowadays clear that the inclusion of spectral indexes into biogeochemical models can greatly improve actual estimates of gross primary productivity (GPP) at local and global scale. Several vegetation indexes can be obtained by the reflectance of light at specific wavelengths. Among them, the Normalized Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI) and the Photochemical Reflectance Index (PRI) were found suitable to represent different characteristics of ecosystems strictly related with GPP, such as biomass and photosynthetic capacity (NDVI, EVI) or radiation use efficiency (PRI). In Mediterranean cork oak savannas, characterized by high heterogeneity, the application of spectral indexes derived from coarse spatial resolution remotely sensed data (e.g. MODIS imagery) to represent the performance of the whole ecosystem is complex. A better knowledge of the variability of vegetation indexes for specific vegetation types, assessed in fieldwork, is fundamental to the interpretation of the same indexes obtained with satellite data and a key step through the integration of such indexes into biogeochemical models. We consider three different vegetation types: trees, grasses and shrubs, concurring to the overall ecosystem carbon budget in Mediterranean cork oak savannas. Since April 2011, reflectance measurements were performed in the range of 300-2500nm by the use of a handheld hyperspectral spectroradiometer (FieldSpec3, ASD Inc. CO, USA) in several species of the three vegetation types in a cork oak savanna eddy covariance site located in central Portugal. Measurements were always performed around solar noon and repeated approximately every two weeks. Several vegetation indexes were calculated. All indexes showed clear differences among vegetation types and among species. Marked seasonal trends were identified for grasses and shrubs, clearly related with the onset of dry summer conditions. Both NDVI and EVI decreased in grasses from April to the second half to July when they reached the same value of litter and bare soil. The recovery was only observed after the first rains in October. The occurrence of rain in October also reestablished the values of NDVI and EVI for evergreen shrubs that showed a marked decrease in September. On the contrary, both indexes remained stable in cork oak along the whole measurement period (April-December). In the perspective of the use of spectral indexes derived from satellite imagery, we compared the PRI values calculated with the wavelengths of 531 and 570nm proposed originally for this index (PRI570) and the wavelengths available on MODIS data: 531 and 550nm (PRI550). Both the indexes revealed different trends for the three vegetation types. Anyway, measurements of cork oak leaf reflectance performed with a leaf clip and compared with leaf chlorophyll concentration, showed that PRI570 is better correlated with leaf chlorophyll concentration than PRI550. Our results suggest that established vegetation indexes are well suited to represent the seasonal variability of the three vegetation types. In further studies, we intent to use the temporal variability of vegetation indexes to estimate the contribution of each vegetation type to the total GPP and its seasonal variability.
Most investigations of plant responses to changes in temperature have focused on a constant incre... more Most investigations of plant responses to changes in temperature have focused on a constant increase in mean day/night temperature without considering how differences in temperature cycles can affect physiological processes and growth. To test the effects of changes in growth temperature on foliar carbon balance and plant growth, we repeatedly exposed poplar saplings (Populus deltoides × nigra) to temperature cycles consisting of 5 days of a moderate (M, +5 °C) or extreme (E, +10 °C) increase in temperature followed by 5 days of a moderate (M, -5 °C) or extreme (E, -10 °C) decrease in temperature, with respect to a control treatment (C, 23.4 °C). The temperature treatments had the same mean temperature over each warm and cool cycle and over the entire study. Our goal was to examine the influence of recurring temperature shifts on growth. Net photosynthesis (A) was relatively insensitive to changes in growth temperature (from 20 to 35 °C), suggesting a broad range of optimum temperat...
At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings ... more At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15 N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO 2 exchange rate in newly developed leaves compared with pre-existing leaves.
Daily variations in photoprotective mechanisms were studied in sun and shade leaves of 40-year-ol... more Daily variations in photoprotective mechanisms were studied in sun and shade leaves of 40-year-old cork oak (Quercus suber L.) trees during early summer in Portugal. Although trees were not severely water stressed because predawn leaf water potentials remained high, photosynthesis and stomatal conductance decreased at midday. The midday depression in gas exchange was not reversed by short-term exposure to ''optimal'' conditions of temperature, light and vapor pressure deficit. Chlorophyll a fluorescence, maximum photochemical yield of photosystem II and the quantum yield of noncyclic electron transport showed midday depressions, but recovered by the evening. Both short-term changes in the components of the xanthophyll cycle (reversible de-epoxidation of violaxanthin during the day) as well as long-term changes (higher xanthophyll content in sun compared with shade leaves) were detected and may play a role in the dissipation of excess energy at midday. Because the activities of enzymes of the antioxidant system, superoxide dismutase and ascorbate peroxidase, were high enough to cope with the increase in oxygen reactive species likely to arise under the stressful conditions of midday, we conclude that these enzymes may provide an additional mechanism for energy dissipation.
In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physi... more In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO 2 efflux from stems under shaded and unshaded conditions, as well as the concentration of CO 2 dissolved in sap [CO 2 *], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO 2 release from stems of E. globulus. CO 2 efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mmol m 22 s 21 , respectively. Temperature and [CO 2 *] both had important, and similar, influences on the rate of CO 2 efflux from the stems, but neither explained the difference in the magnitude of CO 2 efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO 2 from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO 2 efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO 2 efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylemtransported CO 2 .
Respiration of stems and branches of trees (R S ) has typically been estimated by measuring radia... more Respiration of stems and branches of trees (R S ) has typically been estimated by measuring radial CO 2 efflux from woody tissue (E A ) and rates of efflux are often scaled temporally using a temperature relationship (Q 10 ). High concentrations of CO 2 in xylem sap ([CO Ã 2 ]) have been shown to affect E A , and the transport of CO 2 in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in E A . Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, E A and [CO Ã 2 ] were greater at low sap velocity, while the amount of respired CO 2 transported in sap (transport flux, F T ) was greater at high sap velocity. E A was linearly correlated with [CO Ã 2 ]. In branches of three diameter classes (1, 2, and 3 cm), volume-based E A , F T , and R S did not differ, but surfacearea based CO 2 fluxes increased with diameter class. Regardless of diameter, E A accounted for only 30% of respired CO 2 at high sap velocity, while at low sap velocity, E A accounted for 71% of respired CO 2 . E A , F T , and R S measured at 5, 20, and 35°C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, E A accounted for only 18% of the CO 2 released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO 2 in water. These results directly demonstrate the transport of respired CO 2 in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.
ABSTRACT In the context of global climate change, the quantification of carbon fluxes in forest e... more ABSTRACT In the context of global climate change, the quantification of carbon fluxes in forest ecosystems and how they vary inter-annually are important issues. Process-based models are flexible tools that permit assessing ecosystem productivity at different spatial and temporal scales and for different management and environmental conditions. On the other hand, carbon and water fluxes at the ecosystem scale may be measured using eddy covariance techniques, thus providing useful data for testing and validation of models. The principal aim of the work was to calibrate and evaluate two versions of a process-based model that differ in the autotrophic respiration (RA) modelling. The original version (3PGN) is based on a constant ratio between the net primary production (PN) and the gross primary production (PG), while, in a new version (3PGN*), developed by the authors, RA was modelled as a function of temperature and biomass. The two model versions were calibrated and evaluated using a comprehensive dataset consisting of forest growth experimental data and eddy-covariance measurements. The two model versions were calibrated and evaluated under a Bayesian framework consisting in model calibration, model comparison and analysis of model-data mismatch. Sensitivity and uncertainty analyses of 3PGN and 3PGN* were also carried out. The BC showed that the data were informative for almost 70% of the parameters. BC also allowed identification of the parameters to which the models were most sensitive and to assess parameter correlations. Key parameters were those for carbon allocation, some of the parameters related to water stress and site fertility. Bayesian model comparison showed that the 3PGN*, with the new autotrophic respiration model based on maintenance and growth respiration, has higher conditional probability of being correct than the original 3PGN, based on the simple NPP vs. GPP ratio.
This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspi... more This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspiration in the eucalypt site of Espirra in Southern Portugal (38 • 38 N, 8 • 36 W). This site was included in the "Carboeurope" European network and is part of a 300 ha eucalypt forest, with about 1100 trees ha −1 , intensively managed as a coppice for pulp production and characterized by a 12-month annual growing period. The climate is of Mediterranean type with a long term annual average precipitation of 709 mm and an annual average air temperature of 15.90 • C. During the measurement period (2002-2009) two main events took place, which changed the annual sink pattern of the forest: a drought period of two years (2004-2005) and a tree felling (October and November 2006). We analyzed the daily, seasonal and inter-annual variation of carbon uptake and evapotranspiration, and their relationships with the events and the variability of the main meteorological variables. Before the felling, annual net ecosystem exchange (NEE) increased from −865.56 g C m −2 in 2002 to −356.64 g C m −2 in 2005 together with a deep decrease in rainfall from 748 mm in 2002 to 378.58 mm and 396.64 mm in 2004 and 2005, respectively. For the same period, seasonal patterns of carbon uptake showed maximum values in April and decreased in July-August. The eucalypt stand recovered its carbon sink ability since June 2007 and had a NEE of −209.01 g C m −2 in 2009.
ABSTRACT Process-based models are powerful tools for sustainable and adaptive forest management. ... more ABSTRACT Process-based models are powerful tools for sustainable and adaptive forest management. Bayesian statistics and global sensitivity analysis allow to reduce uncertainties in parameters and outputs, and they provide better insight of model behaviour. In this work two versions of a process-based model that differed in the autotrophic respiration modelling were analysed. The original version (3PGN) was based on a constant ratio between net and gross primary production, while in a new version (3PGN∗) the autotrophic respiration was modelled as a function of temperature and biomass. A Bayesian framework, and a global sensitivity analysis (Morris method) were used to reduce parametric uncertainty, to highlight strengths and weaknesses of the models and to evaluate their performances. The Bayesian approach allowed also to identify the weaknesses and strengths of the dataset used for the analyses. The Morris method in combination with the Bayesian framework helped to identify key parameters and gave a deeper understanding of model behaviour. Both model versions reliably predicted average stand diameter at breast height, average stand height, stand volume and stem biomass. On the contrary, the models were not able to accurately predict net ecosystem production. Bayesian model comparison showed that 3PGN∗, with the new autotrophic respiration model, has a higher conditional probability of being correct than the original 3PGN model.
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Papers by Sofia Cerasoli