Production ecology of clonal Eucalyptus plantations in northeastern Brazil

Abstract

Although millions of hectares of Eucalyptus are intensively managed for wood production in Brazil, little is known about the physiological processes that control growth and how these processes are regulated by internal and external factors. These uncertainties limit the testing of process-based models, which are necessary to evaluate both production and ecological feedbacks of plantations across spatial and temporal scales.

I used one gradient study and two manipulative studies with clonal E. grandis x urophylla in northeastern Brazil to achieve three objectives: (1) identify the main environmental factors controlling growth and the associated resource use (Chapter I); (2) quantify water and nutrient supply effects on carbon (C) gain and C allocation patterns (Chapter II); and (3) evaluate the ability of a process-based model to simulate Eucalyptus response to changes in fertility and climate (Chapter III).

For the first objective, fourteen stands with aboveground net primary production (ANPP) spanning from 9 to 39 Mg ha-1 yr-1 were characterized in their supply and use of water, light and nitrogen. Water was the most limiting resource, and ANPP increased by 2.3 Mg ha-1 yr-1 for each 100 mm yr-1 increase in rainfall. Indirectly, water supply also increased light and nitrogen use by increasing LAI and N-uptake-efficiency. The most productive sites were the most efficient in using water (3.21 kg of ANPP per m3 of transpired water), light (1.14 kg per GJ of APAR) and nitrogen (381 kg per kg of N taken up). Lower VPD, soil water stress and root-to-aboveground ratio were associated with these high-efficiency stands.

For the second objective, an irrigation-fertilization 2 x 2 factorial design was used in a 3-year-old plantation. Carbon budgets were obtained for two years, including ANPP, total belowground C allocation (TBCA), and estimates of aboveground autotrophic respiration. These estimates were summed to obtain the gross primary production (GPP). Irrigation increased GPP by 38% (from 4.8 to 6.7 kg C m-2 yr-1), primarily by increasing canopy quantum efficiency from 0.034 to 0.052 mol C mol-1 PAR and secondarily by increasing LAI. Despite the increase in TBCA (from 1.6 to 1.9 kg C m-2 yr-1), irrigation decreased the fraction of GPP allocated belowground from 34% to 28%, increasing ANPP by 48% (from 1.5 to 2.3 kg C m-2 yr-1). Wood and coarse root increments were the dominant components of the net ecosystem production, with no change in soil C.

The final objective was met by calibrating the 3-PG model with two tropical Eucalyptus trials that had complete production data. Validation was performed using independent data from forty plots (with and without fertilization) monitored for 2 years. The model captured the influence of water and nutrients on C gain and allocation pattern during calibration. The model also responded well to soil fertility and climate conditions during validation, and was particularly sensitive to LAI estimates. Actual wood production ranged from 2 to 51 Mg ha-1 yr-1, compared with model estimates of 10 to 42 Mg ha-1 yr-1 (r2 = 0.78).

Overall, our study indicates that for fertilized Eucalyptus tropical plantations water is the most influential resource controlling C gain, allocation, resource use and the efficiency in using these resources; and process-based models can play an important role in improving the management of these almost-agricultural forests.