Models for estimating biomass and carbon in biomass and soils in "Pinus radiata D. Don", Eucalyptus globulus (Labill) and Eucalyptus nitens (Deane & Maiden) Maiden plantations established in former agricultura lands in Northwestern Spain

Abstract

Climate change is one of the most serious environmental problems nowadays, and are the energy supply and land use change the two most important factors causing this problem. International agreements on global C emissions allow the countries involved to compensate for the release of CO2 by providing C sinks. However, there is a large degree of uncertainty associated with the methods used to estimate the amounts of carbon sequestered, mainly when evaluating some compartments of C sinks, even in steady state systems. The main objective of this thesis is to discuss about modelization of carbon sequestration in forest systems. The experimental design and methods used to collect data are summarised in the following points: i) a network of 120 paired plots established as a chronosequence, where C density was evaluated on living biomass and soil, ii) 40 trees were felled and completely fresh weighed, and intensive sampling was carried out to evaluate the effect of sampling intensity on dry mass estimation, iii) development of static growth models, by considering the threshold density limit derived from self-thinning and harvesting dimensional constrains, iv) evaluation of calorimetry and thermal analysis as novel techniques to elucidate its ability in the study of the nature of SOM and soil microbial metabolism, and v) parameterization of an mechanistic model and evaluation of the effects of two different management alternatives in relation to climate change mitigation. The mean rates of C sequestration (biomass and soil) estimated throughout the rotation ranged between 11.5-14.6 MgCha-1yr-1, and the contribution of the soil ranged from 8-18% Losses of SOC from the uppermost mineral soils during the 10 years following afforestation were observed, derived from the large content of SOM in the pasture soils in the region. The higher losses of SOC from the pine soil were attributed to the lower transfer of organic C to the mineral soil, as a result of the lower litter decomposition rate and the lower belowground litter input from associated vegetation. The intensive sampling considered in biomass equations developing enabled estimation of the effect of sampling intensity and initial sampling point in the stem on the accuracy and bias in estimation of stem wood dry mass. The increases in moisture content and basic density along the stem explained the serious risk of dry mass or weight overestimation when systematic subsamples were considered. The results clearly show the trends in relative errors derived from measurement of the bottom disk or the bottom log, considered by default as the first section that should be measured. Development of stand level models for estimating biomass yield, total energy and carbon sequestration in E. globulus and E. nitens plantations was carried out for first rotation stands. Two silvicultural options for short rotation forestry, one destined for bioenergy production and the other consisting of the standard silviculture regime was evaluated for each specie considered. It was found that 50% increments in the initial density result in only marginal increments in biomass and usable cellulose yields. The application of calorimetry and thermal analysis as novel techniques were tested to evaluate the nature of SOM and compared with results of a well contrasted technique (NMR). The techniques were able to distinguish differences in SOM dynamics in chronosequences of two species, attributable to the different development of understory vegetation and litter composition. A comprehensive study was carried out considering the C sink effect in biomass, soil and wood products, the substitutive effect of bioenergy, and particular conditions of climate and wood market in southern Europe for evaluating several silvicultural alternatives. Harvesting clearcutting and thinning slash for bioenergy use led to a slight decrease in the soil carbon equilibrium, but significantly increased the mitigating effect through bionergy use.