Abstract
Forest ecosystems sequester large amounts of atmospheric CO
2
, and the contribution from seasonally dry tropical forests is not negligible. Thus, the objective of this study was to quantify ...and evaluate the seasonal and annual patterns of CO
2
exchanges in the
Caatinga
biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. In addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (GPP), ecosystem respiration (R
eco
) and net ecosystem CO
2
exchange (NEE). Results showed that the dynamics of the components of the CO
2
fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m
−2
) when compared to 2015 (−145.0 g C m
−2
) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the
Caatinga
acted as an atmospheric carbon sink during the years 2014 and 2015.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Studying the dynamics of gross primary production (GPP) in seasonally dry tropical forests is of fundamental importance to understand the carbon dioxide (CO
2
) balance in this ecosystem, helping ...mitigate its potential impacts at the regional and global levels. Thus, the objective of this work was to evaluate the accuracy of GPP estimated via remote sensing in the Caatinga biome. A set of observed data retrieved from micrometeorological towers equipped with eddy covariance systems were used to validate remote sensing data. The set was measured in a preserved Caatinga fragment. Remotely sensed GPP data was retrieved from the MOD17A2 version 6.0 product of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard the Terra satellite. The validation of MOD17A2 GPP estimates was carried out through the comparison with micrometeorological data measured in situ. In the Caatinga site the comparison between the two GPP data types showed a moderate correlation with Pearson's correlation coefficient (r) = 0.65 and coefficient of determination (R
2
) = 0.43 and the product performed better in representing GPP in the Caatinga during the dry season. Results showed that although the MOD17A2 product represents the annual behaviour of GPP, the algorithm could be improved in order to provide GPP information that is more similar to surface measured data over these land covers.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
Calotropis procera
(Aiton) Dryand (Apocynaceae) is a native species in tropical and subtropical Africa and Asia. However, due to its fast growing and drought-tolerant, it has become an invasive ...species when it was introduced into Central and South America, as well as the Caribbean Islands. Currently,
C. procera
displays a wide distribution in the world. Invasiveness is important, in particular, because many invasive species exert a high reproductive pressure on the invaded communities or are highly productive in their new distributed areas. It has been suggested that a very deep root system and a high capacity to reduce stomatal conductance during water shortage could allow this species to maintain the water status required for a normal function. However, the true mechanism behind the successful distribution of
C. procera
across wet and dry environments is still unknown.
C. procera
leaves were collected from 12 natural populations in Brazil, Colombia and Mexico, ranging from wet to dry environments during 2014–2015. Many traits of morphology and anatomy from these distinct morphotypes were evaluated. We found that
C. procera
leaves had a considerable capacity to adjust their morphological, anatomical and physiological traits to different environments. The magnitude of acclimation responses, i.e., plasticity, had been hypothesized to reflect the specialized adaptation of plant species to a particular environment. However, allometric models for leaf area (LA) estimation cannot be grouped as a single model. Leaves are narrower and thicker with low amounts of air spaces inside the leaf parenchyma in wet environments, while they are broader and thinner with a small number of palisade cell layers in dry environments. Based on these, we argue that broader and thinner leaves of
C. procera
dissipate incident energy at the expense of a higher rate of transpiration to survive in environments in which water is the most limiting factor and to compete in favorable wet environments.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Many issues involving leaf areas (LA) allometry, such as the possibility of adjusting morphometric models based on the leaves of different species and the influence model performance on intraspecific ...variability, are still unclear. In this work, we described allometric equations for 14 different woody tropical species. This study’s novelty is to show how a generalised allometric equation accurately estimates LAs of different species with similar leaf morphologies oblong: Ŷ = 0.6356(LW)1.0260, and elliptical: Ŷ = 0.6552(LW)1.0094. Natural variation that occurs in the leaf development does not represent an obstacle to the implementation of generalised models. In general, simple linear models are considered ineffective to describe the full range of leaf variation. Extrapolation of these equations to other woody species of similar leaf morphology can be compared with models developed for oblong and elliptical leaves, based on the principle that in potential models, the variation of the intercept coefficient (β0) tended to 0.5 for leaves more elongated to 0.7 in less elongated leaves. While the angular coefficient (β1) tended to vary from 0.97 for less elongated leaves, up to 1 when the relationship between LA is remarkably close to a rectangle.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Cultivation of Jatropha curcas in arid and semiarid non-cultivated areas could be a sustainable strategy for stimulating biofuel production without competing with food crops for land and water ...resources. J. curcas is considered a drought-tolerant species; however, the mechanisms that provide tolerance are unknown. Few efforts have been made to understand the connections between stomatal development and environment conditions. Here, we compared changes in stomatal density (SD) and stomatal index (SI) and their influence on gas exchange in J. curcas. Plants were cultivated in both rainy and dry regions. We describe a distinctive distribution of stomata under the adaxial and abaxial leaf epidermises, where higher SD may have caused the increase in stomatal conductance (gs) with positive effects on net photosynthetic rate (PN). However, when rain was excluded, the variation in gs was strongly related to vapour pressures deficit (VPD), and VPD was strongly related to the PN. Thus, our results suggest that J. curcas may also contribute mitigating the effect of CO2 deposition in the atmosphere, given that a remarkable change in SD and other leaf traits was observed in response to seasonal variations. Moreover, multivariate analysis highlights the high sensitivity of J. curcas plants to VPD which in turn induces rapid stomatal closure and consequent reduction of PN for long periods of time which reflect into a change in the pattern of development resulting in higher SI. These results can help us to understand the relationship between stomatal features and gas exchange in response to environment changes.
•J. curcas grows in semiarid areas are more likely to be limited by water availability than semi-humid counterparts.•Dry season increases either stomatal density and stomatal index.•J. curcas has strong potential for acclimating to the altered environment predicted for global warming.•The VPD more than the soil water modulates gas exchange in J. curcas.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Research focusing on energy, water vapor, and CO2 exchanges over the Caatinga Biome is recent. Consequently, there exist research gaps on this topic that limit our ability to understand and project ...interannual and long-term variations in the CO2 cycle of this environment. Thus, the objective of this study was to evaluate the behavior of the energy and CO2 balances of the Caatinga Biome under extreme rainfall conditions (extreme drought and intense rainfall) using simulations of the SITE model (Simple Tropical Ecosystem Model). Meteorological data from the years 2009 (heavy rainfall) and 2012 (extreme drought) obtained from an INMET automatic station were used. Relationships between monthly GPP and NEE values and rainfall were also analyzed. Although the SITE model had been previously calibrated for the region, adjustments to the calibration were necessary for our study due to the extreme climatic values of the selected years. The results highlight the impact of rainfall on energy and mass exchanges over the Caatinga, particularly evident in the partitioning of the energy balance and the CO2 balance. In 2009 (during heavy rainfall), the fraction of Rn converted into LE was nearly 6% higher compared to the values observed in 2012 (during extreme drought). Regarding CO2, it was found that the Caatinga behaved as a sink, even under extreme drought conditions (2012), with annual average values of −1.86 µmol m−2 s−1 (2009) and −0.81 µmol m−2 s−1 (2012). Relationships between monthly GPP and NEE values and rainfall were also investigated, revealing an asymptotic relationship between the components of CO2 balance and rainfall. It was evident that both the monthly values of GPP and NEE tend to stabilize when monthly rainfall volumes exceed 200 mm.
This research aims to provide a brief evaluation of remotely sensed estimates in representing gross primary production (GPP) over pasture in the Northeast Region of Brazil. We used GPP data measured ...in situ during a micrometeorological experiment performed in a pasture area located in the NEB region using the eddy covariance technique. Remotely sensed GPP was obtained through the MOD17A2H product of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Terra satellite. We concluded that the MOD17A2H product moderately represents surface-measured GPP in the studied site, highlighting the need for improvements in the quality of the remotely sensed GPP estimates over this type of ecosystem.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
Although seasonally dry tropical forests are considered invaluable to a greater understanding of global carbon fluxes, they remain as one of the ecosystems with the fewest observations. In this ...context, ecological and ecosystem models can be used as alternative methods to answer questions related to the interactions between the biosphere and the atmosphere in dry forests. The objective of this study was to calibrate the simple tropical ecosystem model (SITE) and evaluate its performance in characterizing the annual and seasonal behavior of the energy and carbon fluxes in a preserved fragment of the Caatinga biome. The SITE model exhibited reasonable applicability to simulate variations in CO2 and energy fluxes (r > 0.7). Results showed that the calibrated set of vegetation parameters adequately simulated gross primary productivity (GPP) and net ecosystem CO2 exchange (NEE). The SITE model was also able to accurately retrieve the time at which daily GPP and NEE peaked. The model was able to simulate the partition of the available energy into sensible and latent heat fluxes and soil heat flux when the calibrated parameters were used. Therefore, changes in the dynamics of dry forests should be taken into consideration in the modeling of ecosystem carbon balances.
•Eddy covariance energy balance closure was investigated in a seasonally dry tropical forest.•Energy balance closure was better during the wet season.•The best energy balance closure occurred under ...very unstable conditions.•Under stable conditions the closure was poor, although the amount of information analyzed under this condition was reduced.•The largest portion of net radiation was converted into sensible heat flux, except during the wet season.
The energy balance closure obtained through the eddy covariance method is a problem which persists, despite advances in the development and improvement of instruments and recent efforts in the description of corrections and in the characterization of measuring uncertainties. In most places the sum of sensible and latent heat fluxes (H and λE) is less than available energy, i.e. the difference between net radiation (Rn) and soil heat flux (G). This study analyzed the annual and seasonal behavior of the energy partitioning and energy balance closure in the Caatinga Biome, which is a seasonally dry tropical forest located in the semiarid lands of Brazil, using the eddy covariance method. Results showed high seasonal variability in the energy partitioning. During the dry season, approximately 70% of Rn was converted into H and less than 5% of it was converted into latent heat flux (λE). During the wet season, the Rn portion converted into H and λE was similar: ˜ 40%. In annual terms, the Rn portion converted into H and λE was of the order of 50% and 20% respectively. The degree of the energy balance closure varied depending on the method used. When the closure was calculated using orthogonal regressions, the slope varied from 0.87 to 0.90 in 2014 and from 0.92 to 1.00 in 2015. However, when the closure was calculated by the energy balance ratio method, values varied from 0.70 to 0.79 in 2014 and from 0.73 to 0.82 in 2015. The closure was better in 2015 if compared to 2014 possibly due to the more intense turbulence observed in 2015 because friction velocity was higher than in 2014. The better closure in 2015 may also be associated with large eddies, which were more frequent in 2014 as evidenced by the correction coefficients for vertical wind velocity and water vapor and vertical wind velocity and sonic temperature. The energy balance closure was also analyzed considering atmospheric instability conditions and the best results were found under very unstable conditions, while the least expressive results were found under stable conditions. Under these conditions negative values of the energy balance ratio were also observed during dry and transition seasons, indicating that fluxes were reversed during these periods.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Seasonality and annual values of the energy balance and CO2 exchange components are closely related to rainfall variability.•Evaporative stress index (ESI) can reach values of extreme water stress ...during the dry season.•Rain pulses caused systematic increases in Gs and higher CO2 absorption.
Several aspects of the hydrological regime influence the CO2 exchanges between the atmosphere and drylands. The objective of this study was to carry out a thorough assessment of the role of rainfall pulses on the energy fluxes, water vapor and CO2 exchanges over the Caatinga Biome using data collected in 2014 and 2015. We conducted an unprecedented analysis on the effects of precipitation pulses on the different components of the CO2 balance between the Caatinga and the atmosphere. Data was collected in a flux tower equipped with an eddy covariance system in a preserved Caatinga area in Northeast Brazil. Results showed a well defined relationship between rainfall variability and energy balance and CO2 exchange. Rain pulses caused systematic increases in surface conductance (Gs), leading to a higher CO2 absorption in the short-term. In 2014, the higher amount of rain pulse events along with higher overall rainfall amounts resulted in 68 % of total absorbed CO2 being assimilated during post-pulse events. In 2015, fewer but more intense pulses occurred, contributing to 39 % of total CO2 absorbed in the wet season. The overall effect of rain pulses on the CO2 balance over the Caatinga site in the Semiarid Brazil depends on the distribution and amount of rainfall in each pulse event.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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