Advances in remote sensing: potential to gain insight into the ecosystem services of eucalypt plantations

Satellite remote sensing is increasingly used on agricultural and forestry sectors for spatial characterization, management and monitoring of large areas at stand or intra-stand scales. This usage will increase in the future with the expansion of the number and quality of sensors and the analysis capacities. This presentation review the main type of earth observation sensors, their physical principles, and their main usage in forestry and agronomy. Then a focus is given on the particular case of remote sensing of Eucalyptus industrial plantations for estimation of plantation area and afforestation dynamics; stand biochemical and structural characteristics; plant water status and other functional variables; cultivation practices and different aspects of stand management. Tropical and subtropical Eucalyptus plantations specificities require special attention while using generic methods or products, and often need the development of dedicated methodologies, advocating for more specific research. Successful use of remote sensing for spatial characterization and monitoring of eucalyptus plantations (e.g. biomass, leaf area index, classification, etc.), but also main issues and knowledge gaps are illustrated with examples from Brazil. In a broader aspect, these remote sensing characterisation could be used for ecosystem services assessments such as water use, carbon sequestration, wood production and environmental impacts. This is discussed through three major development points 1) the necessary synergy between remote sensing and process-based models; 2) the watershed-scale spatialization and modelling of the spatial interactions;. 3) to take advantage of the potentiality offered by current and future satellite mission and analysis method

Using the MAESTRA model to simulate light interactions and photosynthesis in a heterogeneous agroforestry system under alternative density

Coffee-based agroforestry systems display two heterogeneous plant layers (shade tree and coffee) both discontinuous, uneven-aged and subjected to sequential pruning. Many field experiments in coffee agroforestry research have been implemented to evaluate the impact of agricultural practices on coffee production, such as shade level, pruning management or planting density. Modeling the light interception by shade and coffee trees is a promising option to evaluate different scenarios and select some management alternatives. Due to the spatial and temporal complexity of the agroforestry system, we hypothesized that a three-dimensional individual tree process-based model such as MAESTRA was adapted to simulate light interactions and photosynthesis. To our knowledge, MAESTRA has not been tested in agroforestry applications yet. MAESTRA was parameterized here according to field inventories and other structural measurements (leaf area per tree, leaf angles, etc.), and the photosynthesis submodel was parameterized using leaf scale gas exchange measurements. The simulations of light interception (absorbed photosynthetically active radiation, APAR) and photosynthesis (Gross Primary Productivity, GPP) were verified using direct measurements at plant and plot scales. The experiment took place in the Central Valley of Costa Rica within the Coffee-Flux platform located in the Rainforest Alliance certified, 6 km², 1 000 m altitude, Aquiares coffee farm. The climate was tropical humid with no dry season. The studied coffee plantation (Coffea arabica cv. Caturra) was grown under low density, free-growing, Erythrina poepigiana shade trees. Coffee pruning practices was a coppice selection system type which resulted in an uneven-aged population of resprouts (1 to 6 years old). The main results presented in the paper are: 1) a successful validation of APAR and photosynthesis simulated by MAESTRA at the scale of coffee plants with measurements performed in situ using a transient-state whole tree chamber; 2) a successful validation of modeled APAR and GPP at plot scale with in situ eddy-covariance measurements; 3) a comparison of plot APAR and GPP under different realistic scenarios, after varying shade trees densities, and pruning practices for coffee. (Résumé d'auteur

Coppice potential of 16 different species of Eucalyptus after an eight-year rotation

The adoption of coppice is a commonly used management in different regions of Brazil for plantations of Eucalyptus, which requires less investments. However, there are variations in the ability of each genotype to emit sprouts, and higher yields can be obtained by changing the genetic material. The objective of this stndy was to evaluate the coppice potential of 16 Eucalyptus genotypes of different species, (14 clonal and 2 seed origin). The experiment is located in the area of the EUCFLUX-IPEF Cooperative Research Program, in the state of Sao Paulo, Brazil. The following variables were evaluated at 100 days after harvest of an eight-year rotation of the 16 genotypes: number of shoots per stnmp, total height, and basal diameter of the three main shoots. Twelve genetic materials showed one or two shoots per stnmp, and the three clones, with higher development in height and diameter, showed 3 or more shoots per stnmp. The best shoots of the best clones presented total height of approximately 1 m and average base diameter of 8 cm. The genotypes with the highest number of shoots also presented better development in height and basal diameter. The 3 less developed genotypes presented mean basal diameter of less than 3 cm and average height of 40 cm for the three main shoots. The results show the clone/species coppice potential, which is important to move forward with coppicing techniques. Future studies will be conducted to evaluate the productivity physiology of these genetic materials

Modelling forest management within a global vegetation model (ORCHIDEE)

Forests have an essential carbon function, removing each year about 20% of global anthropogenic CO2 emissions. Models have been designed to quantify the distribution of CO2 fluxes and ecosystem carbon dynamics at the global scale, but these global vegetation models usually do not account for management and stand structure. In most cases - and in particular in the biosphere component of the IPSL ESM model (ORCHIDEE) - forests are represented as an average single tree that has reached a near-equilibrium state. This coarse representation has so far impaired the capacity of global models to correctly simulate the interactions between managed forests and the climate system. This presentation describes a new module that explicitly simulates forest management, stand structure, and tree mortality within ORCHIDEE. Forest representation is detailed from an "average tree" to an "average stand", with an explicit distribution in diameter classes that varies with stand age, location and climate. A set of rules are defined to calculate the critical tree density, intensity and frequencies of thinning, etc... Through a sensitivity analysis, the reaction of the model to changes in management and physiological parameters is presented. The model¿s fit to forest inventory data and yield tables is assessed. Eventually, the explicit simulation of forest management is shown to have an important impact on the simulation of biochemical cycles, both in terms of carbon fluxes and stocks. (Résumé d'auteur

Interannual changes in the carbon budget of European forests : detecting hot-spots periods of variability

(1) The interannual variability of carbon fluxes of forest ecosystems is analysed at both site and regional level by locating the seasonal period which explain the annual fluxes variability (hot-spot periods, HSP), and finding their meteorological driving causes. (2) The analysis is first conducted on seven European forest sites representing contrasted species and climatic conditions. Net Ecosystem Exchange (NEE), separated into Gross Primary Production (GPP) and ecosystem respiration (TER), together with ancillary data measured during several years provide useful information about meteorological and physiological controls of ecosystem carbon fluxes variability. HSP and their climatic causes were found on these sites. The ability of the ORCHIDEE process-based model to represent the HSP and their climate drivers is assessed at this scale by comparing the measurements with the simulations. (3) Secondly, up-scaling to European forests was performed with ORCHIDEE simulations at a 0.25° grid. Maps of HSP for GPP, TER and NEE, and their meteorological drivers were analysed. The GPP interannual variability is explained by spring and summer month periods, whereas HSPTER occurred throughout the year. As a result of compensating effects between GPP and TER, the periods of HSPNEE are reduced compared to HSPGPP and HSPTER. It is shown that there is a latitudinal contrasted pattern of HSP for GPP and TER, positively controlled by temperature in northern Europe, and by soil water availability in southern Europe. This latitudinal boundary fluctuates from early spring to late summer for the HSPGPP, whereas it is less clear for HSPTER. HSPNEE and their meteorological drivers mainly follow the GPP ones. (Résumé d'auteur

Competition for light and light use efficiency for Acacia mangium and Eucalyptus grandis trees in mono-specific and mixed-species plantations in Brazil

Mixed plantations with N-fixing species might be an attractive option for limiting the use of fertilizer in highly productive Eucalyptus plantations. A randomized block design was set up in southern Brazil, including a replacement series and an additive series design, as well as a nitrogen fertilization treatment, and conducted during a full 6 years rotation. The gradient of competition between Eucalyptus and Acacia in this design resulted in very different conditions of growth of Acacia, from totally dominated up to dominant canopies. We used the MAESTRA model to estimate the amount of absorbed photosynthetically active radiation (APAR) at tree level. This model requires the description of the scene and distinct structural variables of the two species, and their evolution with time. The competition for light is analysed by comparing the inter-specific values of APAR during a period of 2 years at the end of the rotation. APAR is further compared to the measured increment in stem wood biomass of the tree, and their ratio is an estimation of the light use efficiency for stemwood production at tree-scale. Variability of these LUE are analysed in respect to the species, the size of the tree, and at plot scale (competition level). Stemwood production was 3400, 3900 and 2400 gDM/m2 while APAR was 1640, 2280 and 2900 MJ/y for the pure Eucalyptus, pure Acacia and 50/50 mixed plantation, respectively, for an average LAI of 3.7, 3.3 and 4.5, respectively. Individual LUE for stemwood was estimated at an average value of 1.72 and 1.41 gDM/MJ/tree for Eucalyptus and Acacia, respectively, and at 0.92 and 0.40 gDM/MJ/tree when they were planted in mixed 50/50 plantations. LUE was highly dependant on tree size for both species. At the plot scale, LUE for stemwood were 2.1 gDM/MJ and 1.75 for Eucalyptus and Acacias, respectively, and 0.85 for the mixed 50/50 plantation. These results suggest that the mixed 50/50 plantation, which absorbed a higher amount of light, produce less stemwood since half of the canopy (Acacias) are dominated, and the other half does not benefit much in terms of tree growth compared to absorbed light. The eventual benefit of the nitrogen-fixing species is not visible in the mixture with 50% of each species. More attention has to be paid to introducing acacias in an additive series with the same density of eucalyptus trees as in the monospecific stands

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