Light use efficiency and productivity of 16 genotypes of Eucalyptus along a 6-year rotation in Brazil

Forest stemwood productivity depends on the amount of absorbed light and on the Light Use Efficiency (LUE), i.e. the amount of stemwood produced per amount of absorbed light. Other growth limiting factors than absorbed light are for instance water and nutrients, carbohydrates allocation processes, and management practices. In fertilized eucalyptus plantations in Brazil, a shift in the main factor limiting growth is expected, from light at the beginning of the rotation (not limited by water because soil has been recharged during the interval between rotation) to other factors such as water in the subsequent years. Changes of allocation patterns and foliar traits also occur along the rotation. These trends may differ between genotypes. These hypotheses were tested along a 6-year rotation, with 16 contrasted genotypes planted in 10 randomized repetitions in São Paulo Region, Brazil. Absorbed light was estimated using the MAESTRA 3D model precisely parameterized at tree scale for each plot. Stand growth was computed based on allometric relationships calibrated on regular destructive biomass measurements. Results at stand scale showed that 1) LUE increased with stand age for all genotypes, from 0.15 to 0.70 gdry_matter/MJ on average; 2) light was the main limiting factor during the first year of growth (R2 between 0.5 and 0.95). Subsequently, the variability of wood production explained by light was variable among genotypes (R2 of 0.25 on average; 3) The effect of genotype on stemwood production remains high and significant along the rotation. These results and their implications for plantation management are discussed

Combining ecosystem modelling and remote-sensing to establish the spatial and temporal dynamics of the carbon budget of fast-growing tropical Eucalyptus plantations

In the aim of estimating regional-scale carbon budgets of Eucalyptus plantations, the G'Day ecosystem model was combined with remotely-sensed estimates of the daily fraction of absorbed photosynthetically active radiation (fAPAR). (Résumé d'auteur

Calibration of home-made heat dissipation probes for a full rotation of Eucalyptus grandis trees in Brazil

With the aim of proposing an appropriate calibration equation that could be used for Eucalyptus grandis of any size over a rotation of seven years, we carried out direct measurements of water consumption for 3 trees at 19, 45, 54 and 72 months after planting, and measured values of tree transpiration were compared with estimations from HDPs installed on the trunks. The trees used for direct measurements were cut and kept in situ, standing with the bottom inserted in a water tank. The accuracy of the calibration was checked by two independent methods: 1) directs measurements for other individual trees, and 2) through Eddy covariance measurements. We compared direct measurements for 3 trees of different size at the ages of 29 and 65 months with HDP. At the stand level, 15 trees selected to cover the range of sizes of one commercial plantation at the end of the rotation were monitored by HDP over 8 months. The comparison with eddy-covariance measurements was carried out during dry periods. Resulted showed that the calibration developed for these home-made probes can be used with great reliability at the tree and stand scales

Water uptake in deep soil layers by tropical eucalypt plantations: consequences for water resources under climate change. [P117]

Climate models predict that the frequency, intensity and duration of drought events will increase in tropical regions. Questions such as how planted forests will adapt to future constraints on water availability have broad implications for the supply of wood at low cost that contributes to decreasing the pressure on native forests. Although water uptake by deep roots is generally considered as an efficient adaptation to drought in tropical planted forests, the role of very deep roots to supply the water requirements of trees is still poorly known. Fine roots have been observed in Eucalyptus plantation down to a depth of 16m 5 years after planting in Brazil. The contribution of water stored in deep soil layers to stand evapotranspiration has been quantified using a process-based model over 5 years after planting. Daily simulations of latent heat fluxes and soil water contents down to a depth of 10m satisfactorily matched with measurements over the study period. Our results show that deep roots play a major role in supplying tree water requirements during extended dry periods in Eucalyptus plantations. The fast exploration of deep soil layers by roots provides access to large amounts of water stored in the soil after clear cutting. The water table is no more recharged after canopy closure and the primary production is highly dependent on rainfall amounts as well as on the ability of trees to withdraw water in the water table. Water uptake by tree roots progressed towards deep soil layers during dry periods. On average 20% of tree transpiration was withdrawn below a depth of 10m during the dry seasons. The withdrawal of water in the water table, between the depths of 12 and 18m, occurred during dry periods from age 2 years onwards. Although the amounts of water withdrawn in the water table were small (~1-3% of total transpiration over the study period), this process can be of paramount importance for tree survival in tropical regions in a context of climate changes. (Texte intégral

Using process-based modelling to better understand the impact of mineral (N,P,K) cycles and climate change on stand growth and resource-use in Eucalypt plantations

In audition to wood production services, forest ecosystems play an important role in the mitigation of anthropogenic climate change. However the role of forests as future C sinks is being discussed since there is increasing evidence or a progressive shift from carbon-limited growth to nutrient-limited growth as demonstrated in FACE experiments. The development of mechanistic models. including the main nutrient balance and their relationship with the carbon and water balances. is necessary to evaluate the future response or forests to climate change. In this contribution, we present the first joint evaluation of a coupled C-water-N-P-K model on a large number of biogeochemical measurements collected in fast-growing eucalyptus plantations in Brazil along entire rotations, including trials with variable levels or fertilisation and water availability. We have first adapted the CASTANEA ecophysiological model, primarily designed for temperate and boreal forests, to the simulation of carbon and water fluxes in tropical Eucalypt. Then, the N,P,K fluxes and stocks within the plant were modelled based on process from the literature and calibrated on specific measurements, including the interaction between nutrient content of organs and carbon and water related processes. The final objective is to quantify the relative importance ,if single nutrients and water limitation on stand growth and carbon productivity, and evaluate the impact of climate change on these different cycles through a change in mean temperature. CO2 concentration and precipitation regime

Full-rotation carbon, water and energy fluxes in a tropical eucalypt plantation

Eucalyptus plantations in Brazil are among the most productive forests of the world, reaching mean annual increments of about 50 m3 ha-1 yr-1 over short (6 - 7 yrs) rotations. We continuously monitored water vapor, CO2 and energy flux through the eddy-covariance method over a 9 year-period in southeastern Brazil. Tree growth, Leaf Area Index (LAI), water table depth, and soil water content (SWC) down to 10 m depth were also monitored. Rooting depth and vertical fine root distribution were assessed at various ages. Mean annual evapotranspiration (AET; 1383 mm yr-1) represented 90% of the annual precipitations (P; 1539 mm yr-1). AET reached maximum values (1598 mm yr-1) about 2-3 years after planting (a.p) when LAI peaked and when deep rooting (about 15 m deep 2.5 yrs a.p) provided access to the large amount of water stored in deep soil layers during the first months after clear-cutting and replanting. Most (88%) of the available energy (3852 MJ yr-1) was partitioned to the evaporation process (latent heat fluxes), with very low sensible heat fluxes over the rotation, except after harvesting and replanting when LAI was low, and later in the rotation during dry events. Deep drainage after harvest of the first stand allowed the water table to rise from -18 to -12 m over the first 2 years after replanting. Both measurements of SWC and model simulations showed that deep water storage and subsequent uptake played a major role in supporting the very high wood production and dampening seasonal droughts

Water withdrawal from deep soil layers: a key strategy to sustain growth during dry seasons in tropical Eucalyptus plantations

Little is known about the role of very deep roots to supply the water requirements of tropical forests. Clonal Eucalyptus plantations managed in short rotation on very deep Ferralsols are simple forest ecosystems (only 1 plant genotype growing on a relatively homogeneous soil) likely to provide an insight into tree water use strategies in tropical forests. Fine roots have been observed down to a depth of 6 m at age 1 year in Brazilian eucalypt plantations. However, the contribution of water stored in very deep soil layers to stand evapotranspiration over tree growth has been poorly quantified. An eco-physiological model, MAESPA, has been used to simulate half-hourly stand water balance over the first three years of growth in a clonal Eucalyptus grandis plantation in southern Brazil (Eucflux project, State of São Paulo). The water balance model in MAESPA is an equilibrium-type model between soil and leaf water potentials for individual trees aboveground, and at the stand scale belowground. The dynamics of the vertical fine root distribution have been taken into account empirically from linear interpolations between successive measurements. The simulations were compared to time series of soil water contents measured every meter down to 10m deep and to daily latent heat fluxes measured by eddy covariance. Simulations of volumetric soil water contents matched satisfactorily with measurements (RMSE = 0.01) over the three-year period. Good agreement was also observed between simulated and measured latent heat fluxes. In the rainy season, more than 75 % of tree transpiration was supplied by water withdrawn in the upper 1 m of soil, but water uptake progressed to deeper soil layers during dry periods, down to a depth of 6 m, 12 m and 15 m the first, second and third year after planting, respectively. During the second growing season, 15% of water was withdrawn below a depth of 6 m, and 5% below 10m. Most of the soil down to 12m deep was dried out the second year after planting and deep drainage was negligible after 2 years. As a consequence, during the third year after planting only 4% of water was taken up below 6m. However, during the dry season, this deep water still supplied 50% of water requirements. Our results show that deep fine roots of E. grandis play a major role in supplying tree water requirements during extended dry periods. Large amounts of water are stored in the whole soil profile after clear cutting and the fast exploration of deep soil layers by roots make it available for tree growth. After canopy closure, precipitation becomes the key limitation for the productivity of these plantations grown in deep sandy soils. Our results suggest that a territorial strategy leading to a fast exploration of very deep soil layers might provide a strong competitive advantage in regions prone to drought