Evapotranspiration of subtropical forests and tree plantations: A comparative analysis at different temporal and spatial scales
Introduction
One of the major processes regulating water exchange between terrestrial ecosystems and the atmosphere, particularly in forests, is evapotranspiration (ET), which has two different components: evaporation and transpiration. Evaporation in dense forests refers to the exchange of water from the liquid to the gaseous phase mostly from the canopy, while transpiration indicates the process of water vaporization from leaves of trees. Both processes are driven by the available energy and the drying potential of the surrounding air, but transpiration also depends on the capacity of plants to transport water from the roots and internal water storages to the leaves as well as the stomatal control of water losses (Bucci et al., 2008, Giambelluca et al., 2009).
Information on the effects of tree plantations on regional hydrological cycle processes is extremely important in assessing the impact of land use changes and in developing strategies related to sustainable use of water resources. Some information is available about the hydrological impacts of converting grasslands or shrublands to tree plantations (e.g., Jobbágy et al., 2006). In humid regions where forests are the dominant vegetation type, such as in Northeastern (NE) Argentina, the impact of tree plantations on hydrological processes, particularly evapotranspiration, has not been assessed.
These moist subtropical forests maintain high photosynthetic rates during most time of the year because many tree species are evergreen and the winter period during which the deciduous canopy species drop their leaves is short and with infrequent subzero air temperatures (Gatti et al., 2008, Tan et al., 2012, Cristiano et al., 2014). Changes in land use, particularly the replacement of native forests by tree plantations may have an impact not only on the water balance but also on the carbon balance at a regional level. Tree plantations are expanding at a rate of 5000 km2 per year in South America (Jobbágy and Jackson, 2004, Jobbágy et al., 2006). In NE Argentina, the area dedicated to tree plantations has recently increased five folds (Izquierdo et al., 2008). A paradigm that has not been assessed for humid forest ecosystems is that tree plantations have high productivity with great annual evapotranspiration (Jackson et al., 2005). However, this trade-off between carbon sequestration and water utilization may not be valid for humid subtropical forests (Zhang et al., 2013, Cristiano et al., 2014).
The use of information provided by remote sensing has emerged as a useful tool in studying spatial-temporal dynamics of ecosystem processes. For example, some models allow the estimation of ET from satellite data. The most widely used ET model is the MOD16A2 product from the MODIS-Terra sensor (Mu et al., 2011). This product has been validated using 46 sites with eddy-covariance towers which are mostly located in North America while only two sites are in tropical rain forests close to the equator in Brazil. Currently there is no validation of this model for subtropical Argentinean forests neither using eddy covariance methods nor using sap flow measurements from individual trees. The current study investigates physiological mechanisms regulating transpirational water losses at different scales for native subtropical forests and high yield tree plantations in Northeast (NE) Argentina. The objectives of this study were (1) to understand mechanisms controlling the ET at tree and ecosystem levels including environmental factors such as evaporative demand and incoming solar radiation, and the interaction between canopy structure and the physical environment, described by canopy conductance (gc), and (2) to determine whether ET and gc from high yield tree plantations of Eucalyptus grandis W. Hill ex Maiden, Pinus taeda L. and Araucaria angustifolia (Bert) O. Kuntze were comparable to native subtropical forests in the same region. In this regard it was assessed whether the results of scaling up sap flow measurements to ecosystem level ET were consistent with ET estimated from remote sensing. Research on tree and ecosystem level determinants of evaporative fluxes should improve our understanding of how subtropical trees and tree plantations regulate water fluxes. This information will also help to predict the impact of land use changes on ET at a regional scale.
Section snippets
Study area
Field measurements of stand structure and transpiration were made in tree plantations of P. taeda, E. grandis and A. angustifolia, and in a native subtropical forest stand in the Atlantic Forest within the Iguazú National Park, Misiones Province, NE Argentina (26°25′ S, 54°37′ W). Mean annual rainfall in the area is about 2000 mm and is evenly distributed throughout the year. Mean annual temperature is 21 °C, and frost seldom occurs in winter, thus, temperatures are favorable for growth during
Tree water consumption and stem size across species
Species-specific water consumption (total daily sap flow, SF) was influenced by the average tree size and by species identity. A significant exponential relationship was observed between stem diameter at breast height (DBH) and SF with increasing SF as DBH increases, when all species (both native and cultivated trees) were included in the regression analysis (Fig. 1, p < 0.0001). Annual leaf area index (LAI) calculated by field measurements and by the MODIS sensor were similar and ranged from 5.2
Water consumption at individual level
At individual level, water consumption (daily sap flow) was positively related to tree size regardless of species. This pattern was similar to that found by Meinzer et al. (2001) across 107 tropical tree species. In our study, sap flow was measured in 26 individuals of 13 species in the native forest and in 5–6 individuals per species in each plantation. The diurnal patterns of sap flow of the native forest trees and trees in plantations were similar, but the native forest trees showed lower
Funding
This work was partially supported by ANPCyT through a PITEC project (Consolidación del Aglomerado Productivo Forestal Misiones y Norte de Corrientes) and a PICT 2011-01860, and MAGyP-UCAR (PIA 10,101). Field measurements were conducted in tree plantations of Pindó SA Company.
Acknowledgements
We thank Patricio Mac Donagh, Hugo Reis and APN (CIES, DTNEA and park rangers). We also thank the Instituto de Clima y Agua of INTA Castelar for providing satellite images and particularly Patricio Oricchio for his ongoing advice.
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