Alejandro E. Castellanos

Chemistry and toughness predict leaf litter decomposition rates over a wide spectrum of functional types and taxa in central Argentina.

Litter decomposition, a major determinant of ecosystem functioning, is strongly influenced by the litter quality of different species. We aimed at (1) relating interspecific variation in leaf litter decomposition rate to the functional types different species belong to; and (2) understanding the chemical and/or physical basis for such variation and its robustness to environmental factors. We selected 52 Angiosperms from a climatic gradient in central-western Argentina, representing the widest range of functional types and habitats published so far. Ten litter samples of each species were simultaneously buried for 9 weeks during the 1996 summer in an experimental decomposition bed. Decomposition rate was defined as the percentage of dry mass loss after incubation. Chemical litter quality was measured as carbon (C) content, nitrogen (N) content, and C-to-N ratio. Since tensile strength of litter and living leaves were strongly correlated, the latter was chosen as an indicator of physical litter quality. A subset of 15 species representing different functional types was also incubated in England for 15 weeks, following a similar experimental procedure. Litter C-to-N and leaf tensile strength of the leaves showed the strongest negative associations with decomposition rate, both at the species and at the functional-type level. Decomposition rates of the same species in Argentina and in England were strongly correlated. This reinforces previous evidence that species rankings in terms of litter decomposition rates are robust to methodological and environmental factors. This paper has shown new evidence of plant control over the turnover of organic matter through litter quality, and confirms, over a broad spectrum of functional types, general models of resource allocation. The strong correlations between leaf tensile strength – a trait that is easy and quick to measure in a large number of species – decomposition rate, and C-to-N ratio indicate that leaf tensile strength can be useful in linking plant quality to decomposition patterns at the ecosystem level.

Ecological impacts of wildlife conservation units policy in Mexico

The value of biodiversity is often expressed in an abstract manner, while land-use decisions are typically made at the local level, on an economic basis, to meet the immediate needs of local communities. In Mexico, recent legislation has created new economic incentives for biodiversity conservation by allowing landowners and managers to benefit directly from the exploitation of wildlife. This bold move toward market-based approaches has proven popular, but implementation is hindered by a lack of scientific rigor in planning and monitoring requirements that, in some cases, has led to unintended and undesirable consequences. Because the program is expanding rapidly across Mexico and may serve as a model for similar efforts in other countries, the goal of the workshop described here was to review the policy and its on-the-ground effects, and to offer an initial set of recommendations for improving implementation and enhancing conservation outcomes. El valor de la biodiversidad es frecuentemente expresado de una manera abstracta, mientras que las decisiones de uso del suelo son tipicamente hechas a escala local, sobre una base economica, a menudo considerando las necesidades inmediatas de las comunidades locales. Legislacion reciente en Mexico ha creado nuevos incentivos economicos para la conservacion de la biodiversidad permitiendo a los propietarios de la tierra y a los encargados beneficiarse directamente de la explotacion de la vida silvestre. Este audaz movimiento hacia las estrategias basadas en el mercado han resultado populares, pero la implementacion se ha entorpecido por la falta de rigor cientifico en los requerimientos de planeacion y monitoreo que, en algunos casos, ha conducido a consecuencias involuntarias e indeseables. Debido a que el programa se esta ampliando rapidamente a traves de Mexico, y que puede servir como modelo para los esfuerzos similares en otros paises, el objetivo del taller reportado aqui fue revisar las politicas y sus efectos sobre el terreno, y ofrecer un conjunto inicial de recomendaciones para reforzar la implementacion y aumentar los resultados para la conservacion.

Carbon and Water Fluxes in an Exotic Buffelgrass Savanna

ABSTRACT Buffelgrass savanna is becoming widespread in aridland ecosystems around the world following invasion or deliberate land conversion for cattle forage. There is still a gap of information regarding functional and ecohydrological aspects such as carbon, water, and greenhouse gas exchanges in these highly productive novel ecosystems where buffelgrass is an exotic species. We measured net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), gross primary production (GPP), and evapotranspiration (ET) with eddy covariance techniques over a buffelgrass savanna established for cattle grazing, approximately 30 yr ago within the Sonoran Desert. The savanna was a net carbon sink (NEE -230 g C/m2/yr) during both a year with above average and one with below-average precipitation (NEE -84 g C/m2/yr). Water loss through evapotranspiration (ET) was similar to total annual rainfall input. Up to 62% of the annual fixed carbon and 75% of ET occurred during the summer monsoon season, when 72–86% of annual rainfall occurred and buffelgrass was active. ET from summer months explained 73% of variation in NEE, with an average ET of 50 mm H2O/month needed to turn the ecosystem into a net carbon sink during this season. Other seasons in the year, when buffelgrass was dormant, contributed with up to 48% of annual fixed carbon but with higher water use efficiency (-NEE/ET). Wediscuss the importance of the seasonal variability in Reco, GPP, and ET processes and the phenology of native plant species for the net carbon uptake through the year for this managed novel ecosystem.

Physiological response of Cucurbita pepo var. pepo mycorrhized by Sonoran desert native arbuscular fungi to drought and salinity stresses

Plants response to symbiosis with arbuscular mycorrhizal fungi (AMF) under water stress is important to agriculture. Under abiotic stress conditions native fungi are more effective than exotics in improving plant growth and water status. Mycorrhization efficiency is related to soil fungi development and energy cost-benefit ratio. In this study, we assessed the effect on growth, water status and energy metabolism of Cucurbita pepo var. pepo when inoculated with native AMF from the Sonoran desert Mexico (mixed isolate and field consortium), and compared with an exotic species from a temperate region, under drought, low and high salinity conditions. Dry weights, leaf water content, water and osmotic potentials, construction costs, photochemistry and mycorrhization features were quantified. Under drought and low salinity conditions, the mixed isolate increased plant growth and leaf water content. Leaf water potential was increased only by the field consortium under drought conditions (0.5–0.9 MPa). Under high salinity, the field consortium increased aerial dry weight (more than 1 g) and osmotic potential (0.54 MPa), as compared to non-mycorrhized controls. Plants inoculated with native AMF, which supposedly diminish the effects of stress, exhibited low construction costs, increased photochemical capacity, and grew larger external mycelia in comparison to the exotic inoculum.

Foliar C, N, and P stoichiometry characterize successful plant ecological strategies in the Sonoran Desert

Ecological processes are centered to water availability in drylands; however, less known nutrient stoichiometry can help explain much of their structure and ecological interactions. Here we look to the foliar stoichiometry of carbon (C), nitrogen (N), and phosphorus (P) of 38 dominant plant species from the Sonoran Desert, grouped in four different functional types to describe ecological characteristics and processes. We found that foliar N, C:N, C:P, and N:P stoichiometric ratios, but not P, were higher than those known to most other ecosystems and indicate P but not N limitations in leaves. Biological N fixers (BNF) had even higher leaf N concentrations, but bio-elemental concentrations and stoichiometry ratios were not different to other non-N-fixing legume species which underscores the need to understand the physiological mechanisms for high N, and to how costly BNF can succeed in P-limiting drylands environments. Stoichiometry ratios, and to lesser extent elemental concentrations, were able to characterize BNF and colonizing strategies in the Sonoran Desert, as well as explain leaf attribute differences, ecological processes, and biogeochemical niches in this dryland ecosystem, even when no direct reference is made to other water-limitation strategies.

Functional Diversity in Plants: Implications for Conservation Issues of the Mexican Biodiversity

In this chapter, we explore the functional diversity concept and its importance in several ecological issues, especially maintenance of ecosystem services and conservation. We consider that Mexico’s species megadiversity should be reflected into a high functional diversity. However, our knowledge on this issue is still limited. Interest in the functional diversity approach has just increased in Mexico. Despite that, since the 1970s, ecophysiological research in Mexican ecosystems has had important pioneer contributions to our knowledge on functional traits in plants and its ecological importance. In this chapter, we review some case studies describing our knowledge of plant physiological diversity in different ecosystems, as examples of the high functional diversity in Mexico. Unfortunately anthropogenic disturbance is increasingly affecting species biodiversity, in particular the more vulnerable species and ecosystems. Increasing research on the functional traits of Mexican ecosystems will provide important information about species function at the ecosystem level and species vulnerability in the context of human disturbance and/or climatic change. Studies focused in functional diversity as an important component of biodiversity will provide us a solid base for planning on conservation decisions, restoration programs, and maintenance of ecosystem services.