Information on carbon stock and flux resulting from land-use changes in subtropical, semi-arid ecosystems are important to understand global carbon flux, yet little data is available. In the Tamaulipan thornscrub forests of northeastern Mexico, biomass components of standing vegetation were estimated from 56 quadrats (200 m2 each). Regional land-use changes and present forest cover, as well as estimates of soil organic carbon from chronosequences, were used to predict carbon stocks and fluxes in this ecosystem.

Plant cover modifies throughfall chemistry, and the solute concentration is dependent on the plant species at any given site. The chemistry of gross rainfall and throughfall of four endemic species planted in northeastern Mexico was evaluated from March 1996 to March 1997. Chemical solutes measured included Ca, K, Mg, Na, Fe, Mn, Cu, and Zn. Dry deposition and canopy leaching fluxes were estimated following the canopy budget model. Variance analyses tested the statistical dependence of the total and net fluxes on the species and seasons. Regression analysis tested the dependence of chemical concentrations on rainfall depth and lag time between rains. A total of 52 rainfall events were recorded during the study period summing 523 mm. Significant differences were noted on the total and net fluxes between the plant species. For total flux, average throughfall (37.8 kg ha⁻¹ year⁻¹) almost doubled the flux of solutes compared to rainfall (24.1 kg ha⁻¹ year⁻¹). Pithecellobium ebano (Berland.) C.H. Mull. (43.3 kg ha⁻¹ year⁻¹), Acacia berlandieri Benth. (38.7 kg ha⁻¹ year⁻¹), and Pithecellobium pallens (Bent.) Standl. (38.4 kg ha⁻¹ year⁻¹) recorded the highest total flux of solutes, and Acacia rigidula Benth. (30.9 kg ha⁻¹ year⁻¹) the smallest. Chemical solutes showed significant differences for total and net fluxes. Ca was the dominant cation with 48% and 52% of the total constituent flux for rainfall and throughfall, respectively. However, K, Mg and Cu approximately doubled in throughfall in contrast to gross rainfall. Species with the largest aboveground biomass had lower throughfall volumes (i.e., higher interception rates) but higher chemical solute inputs to the forest floor. Rainfall depth and lag time between rains explained part of the variation for most species, stressing the partial dependence of the washing effect and the amount of dry deposition on canopies. This research discusses the importance and the sources of incoming solutes on the studied plant species.

This research examines the diversity-productivity relationship in a semiarid scrubland, initially under late successional conditions and subsequently under early successional conditions created by experimental clearing, to explore the roles that productivity and stochastic mortality play in species exclusion in this environment. A total of fifteen plots were studied by measuring environmental conditions and biomass components of shrubs and seedlings. These stands were distributed along a productivity gradient across five different landforms. A hypothesis about the stochastic self-thinning mortality model along the gradient was evaluated with the diversity-productivity-environment data. The diversity-productivity relationship was linear and reversed between the early and late succession stages. The hypothesis of stochastic mortality of species exclusion was rejected in the early stages of succession and partially accepted in the mature stage of succession. Species exclusion was negatively related to productivity gradients, suggesting that strong interspecific competition occurs in high productivity plots and that a larger number of species can survive in higher abiotic stress landscapes. Further research is needed to understand the temporal and spatial variations of the ecological interactions that shape this plant community.

This research is aimed at fitting growth and yield models at the stand scale to project top height, breast height diameter, basal area, and standing volume of trees, as well as above-ground biomass and carbon dynamics in reforested sites with pinyon pines in Iturbide, Nuevo Leon (NL), Mexico. Data on top height and breast height diameter of three species: Pinus cembroides Zucc., Pinus pinceana Gordon, and Pinus nelsonii Shaw were measured at two sites from 1983 to 2002. Data analysis suggested that at 20 years of age, their mean (standard deviation) recorded standing volume approached values of 45 (33), 31 (16), and 25 (14) m³ ha⁻¹ for P. cembroides, P. pinceana, and P. nelsonii, respectively. Mean (confidence interval) standing volumes of 48 (14) m³ ha⁻¹ were recorded for the deep, loamy soils in bottomlands at Las Adjuntas sites, which was higher than the mean (confidence interval) values recorded for shallow, clay soils in slopes at site La Loma of 19 (3) m³ ha⁻¹. P. cembroides and P. pinceana recorded standing volume and carbon stocks well above values measured in native forests of these species. Therefore, we recommend these pinyon pine species for extensive reforestation projects on the semi-arid lands of northeastern Mexico.