José Návar

The causes of stemflow variation in three semi-arid growing species of northeastern Mexico

Abstract This study was conducted to determine the role of some plant parameters on stemflow generation. Stemflow measurements in individual shrubs of three semi-arid growing species (Diospyrus texana, Acacia farnesiana and Prosopis laevigata) were carried out under natural and simulated rainfall conditions in northeastern Mexico. Stemflow coefficients for individual shrubs were developed. The analysis of variance showed a significant difference among the species for both natural and simulated rainfall conditions. The multiple linear models suggested that the number of branches and position on the canopy, instead of total projected branch area, controls stemflow. There were also suggestions that bark roughness, leaf and twig position may also explain some of the interspecific stemflow variation. This contribution was suggested by an introduced funnelling ratio.

Biomass component equations for Latin American species and groups of species

Abstract• Studies dealing with the estimation of biomass, site productivity and the contribution of forests to the global carbon balance require the use of allometric equations. There have been a great number of equations developed to estimate biomass components of trees and shrubs in various ecosystems. However, there are less literature compilations that address the calculations of biomass components.• I report a total of 229 sets of allometric equations to estimate biomass components for 102 species in 72 different forest communities of arid, semi-arid, subtropical, tropical and temperate Latin-American ecosystems.• The selection of the appropriate allometric model is a key element in the accurate estimation of biomass, stand productivity, carbon stocks and fluxes, and as a consequence, it is important to apply special effort to the selection and estimation of biomass equations.• I also discuss statistical methods of parameter estimation and recommend the dissection of two conventional allometric equations when biomass studies are conducted on a wide range of diameters. In order to use nondestructive procedures of biomass estimation such as the fractal theory, the null hypothesis that the mean slope b value is equal to 2.67 was rejected for Latin American biomass species.• This information is critical for the establishment of environmental projects that aim to estimate conventional parameters (i.e., productivity, habitat quality and fuel wood) as well as environmental features (i.e., stocks and fluxes of carbon and nitrogen).Résumé• Les études portant sur l’estimation de la biomasse, la productivité des stations et la contribution des forêts à l’équilibre mondial du carbone nécessitent l’emploi d’équations allométriques. Il y a eu un grand nombre d’équations qui ont été développées pour estimer la biomasse des composantes des arbres et des arbustes dans différents écosystèmes. Cependant, il existe beaucoup moins de compilations de la littérature scientifique qui portent sur les calculs des composantes de la biomasse.• Je rapporte un total de 229 séries d’équations allométriques pour estimer les composantes de la biomasse de 102 espèces différentes dans 72 communautés forestières des zones arides, semi-arides, sub-tropicales, tempérées des écosystèmes latino-américains.• La sélection du modèle allométrique est un élément clé pour l’estimation précise de la biomasse, de la productivité des stations, des stocks de carbone et des flux et, en conséquence, il est important de bien sélectionner les équations permettant l’estimation de la biomasse.• J’ai également discuté les méthodes statistiques d’estimation des paramètres et recommandé la dissection des deux équations allométriques classiques lorsque les études sur la biomasse sont conduites sur une large gamme de diamètres. Afin d’utiliser les procédures non destructives d’estimation de la biomasse telles que la théorie des fractales, l’hypothèse nulle que la pente moyenne b a une valeur égale à 2,67 a été rejetée pour la biomasse des espèces de l’Amérique latine.• Cette information est critique pour la mise en place de projets environnementaux qui visent à estimer des paramètres classiques (c’est-à-dire, la productivité, la qualité du site, le bois de chauffage), ainsi que des caractéristiques environnementales (c’est-à-dire, les stocks et les flux de carbone et d’azote).

Spatial variations of interception loss components by Tamaulipan thornscrub in northeastern Mexico

Measurements of gross rainfall, throughfall and stemflow fluxes were conducted from April 1997 through April 1998 within a Tamaulipan thornscrub community in northeastern Mexico. Throughfall and stemflow fluxes represented 78.1% and 3.0% of gross rainfall, implying that interception loss from this community was 18.9%. Canopy water fluxes were often statistically different between study plots and these differences were largely explained by variations in plant dasometric characteristics. The revised Gash analytical model of rainfall interception loss was employed and simulated interception loss values were close to observed values. However, results from this study suggest that the parameters employed within the Gash analytical model are sensitive to spatially variable plant characteristics such as stem diameter, stem height and weighted leaf area index. This suggests that the transferability of the Gash model may be limited even within the same ecosystem unless the spatial variability of plant dasometric characteristics are taken into account.

Fitting the analytical model of rainfall interception of Gash to individual shrubs of semi-arid vegetation in northeastern México

Abstract This study was conducted to estimate the components of interception loss and to fit the analytical model of rainfall interception of Gash to a semi-arid plant community in northeastern Mexico. The interception and components of interception loss were estimated from measurements of gross rainfall, throughfall and stemflow in individual shrubs of Diospyrus texana, Acacia farnesiana and Prosopis laevigata . The components of the interception loss did not deviate from other estimates in other ecosystems, except for evaporation rate during storms. In general, the analytical model performed well, although estimated total interception loss was highly sensitive to changes in both the magnitude of evaporation rate during storms and the proportion of rainfall that is stemflow.

Seal formation and interrill erosion on a smectite-rich Kastanozem from NE-Mexico

Abstract On-site and off-site effects of interrill erosion can be highly significant for river and reservoir sedimentation and can cause serious water contamination. Interrill erosion changes during storms, reflecting the effect of seal formation on soil resistance to interrill erosion. Swelling clay soils appear to be particularly complex in their response to rainfall. Assessment of interrill erosion hazards on such soils requires effective quantitative description of the effects of seal formation on interrill erosion. However, short-term changes of seal properties are not reflected well in current event-based erosion models, leading to poor prediction quality. The objective of this study was to identify the changes in seal condition relevant to temporal change in interrill erosion on a smectite-rich Kastanozem from NE-Mexico. Formation and removal of a non-coherent washed-out layer, consisting of stable aggregates, was the most apparent change in seal condition associated with interrill erosion dynamics. The progressive exposure of a more cohesive washed-in layer resulted in reduced interrill erosion, which is attributed to lower erodibility of the washed-in layer. The results demonstrate that seal formation and interrill erosion dynamics of a freshly tilled Kastanozem are strongly linked. The amount of rainfall required for the removal of the washed-out layer (180 mm) shows that seal formation lasts for a significant part of the rainy season. The changes of interrill erosion caused by seal formation on the Kastanozem are not represented adequately in current methods of erodibility assessment and improving the quality of event-based erosion prediction remains difficult for two reasons. First, relationships between soil properties, seal formation and erosion dynamics are not well understood. Second, the quantitative description of seal property dynamics relevant for interrill erosion is characterized by strong spatial and temporal variability. Therefore, it is likely that even with best modeling efforts, the quality of event-based erosion prediction will be limited by the high uncertainty about erodibility.

The effect of prescribed burning on surface runoff in a pine forest stand of Chihuahua, Mexico

During the period from October 1989 to December 1992, the influence of prescribed burning on runoff volumes was investigated within a Pinus arizonica Engelm.-dominated stand in Chihuahua, Mexico. Prescribed burning increased surface runoff, especially in plots treated with prescribed burns applied over two consecutive years. Surface runoff continued to be large one year after fire application, stressing the temporal effect of prescribed burns. The increment of surface runoff was statistically associated with the reduction of forest fuels and litter-layer depth. Suggestions regarding the application of prescribed burning are given to better safeguard against negative impacts associated with this silvicultural practice.

The contribution of shrinkage cracks to bypass flow during simulated and natural rainfall experiments in northeastern Mexico

Understanding the spatial and temporal dynamics of desiccation cracks and related hydro-geomorphologic processes is a key component for the sustainable management of water resources in Vertisols. The contribution of shrinkage cracks to infiltration of runoff and sediments was studied during natural and simulated rainfall experiments in Vertisols of the semi-arid plains of northeastern Mexico during the summer of 1998. Surface runoff amounted to 10.7% of the total applied rainfall, but it increased from 2.3 t o 18.3% during the first and last applied rainfall. Soil cracks partially filled with transported sediments in surface runoff, but never sealed by swelling or by sedimentation during the sprinkling of 180 mm of rainfall applied in three installments during 1 wk. However, cracks observed in 1999 closed during natural rainfalls with a total depth of 450 mm, distributed in 10 storms during 3 mo. The development of soil cracks and their contribution to promote short-circuit flow are highly variable proces...

Seedling establishment under native tamaulipan thornscrub and Leucaena leucocephala plantation

Abstract In order to investigate the influence that plantations and clearing of native vegetation might have on plant regeneration in northeastern Mexico, seedling survival and germination (i.e., seedling emergence from the soil surface) were measured monthly in native fragments and an exotic plantation over a one-year period. Seedlings were measured in 1-m 2 plots, using 10 replicates under each of three different treatments: (i) dense thornscrub, (ii) semi-cleared thornscrub, and (iii) a dense L. leucocephala plantation. Seedling number peaked during summer and early autumn, lowest number of seedlings occurred during early spring. Excluding grasses, 36 species were present during the study, and only 14 were present in L. leucocephala plantation. Thornscrub and open thornscrub had similar number of grass seedlings, whereas more forb seedlings were present in open thornscrub. Seedlings of woody species were similar for all treatments. Under L. leucocephala germination was low for most species other than itself, most seedlings, however, died during the winter months.

Diversity-Productivity Relationship in the Northeastern Tamaulipan Thornscrub Forest of Mexico

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.

Hydro-climatic variability and forest fires in Mexico’s northern temperate forests

Resumen Los cambios globales en las temperaturas pueden estar modificando el ciclo hidrologico de las cuencas forestales. Este reporte tuvo como objetivos estimar las variables del ciclo hidrologico interceptacion, I, la evapotranspiracion actual, E, y potencial, Et, la escorrentia, Q, y los cambios en el contenido del agua del suelo, θ; asi como evaluar si las variables del ciclo hidrologico estan presentando tendencias u oscilaciones que pu-dieran estar asociadas al calentamiento global en cuencas forestales del centro de Durango, Mexico. El procedimiento del balance de masas sirvio para calcular las variables diarias del ciclo hidrologico con el uso de los sub-modelos de I y Et para evaluar finalmente la Q y θ. Los datos se sujetaron a un analisis de regresion y a tecnicas auto-regresivas y de promedios moviles, ARIMA, para evaluar la significancia estadistica de las tendencias. El valor acumulado del valor estandar de z magnifico y los modelos ARIMA proyectaron estadisticamente bien las oscilaciones mensuales y anuales de las variables del ciclo hidrologico. Las ecuaciones de regresion y las tendencias de los modelos ARIMA mostraron que las variables mensuales y anuales de P, I, E y Et, Q, y θ no siguen tendencias cla-ras en tiempo con significancia estadistica; estas a su vez, siguen oscilaciones que pudieron ser predichas adecuadamente con modelos ARIMA. Se encontro una asociacion consistente (p ≤ 0.05) entre θ y el numero de incendios y la superficie forestal incendiada a pesar de las diferentes escalas en las cuales se evaluaron estas variables. El analisis muestra que el calentamiento global esta contribuyendo a incrementar la variabilidad hidrologica estacional al aumentar los pulsos de sequia y humedad en tiempo. Sin embargo, se requiere de investigaciones adicionales para probar esta hipotesis con el uso de series de tiempo mas largas, con otro tipo de tecnicas estadisticas y con la incorporacion de otras variables en el analisis.Palabras clave: analisis de regresion, modelos auto-regresivos con promedios moviles, precipitacion, interceptacion, evapotranspiracion, escorrentia y contenido del agua del suelo, incendios forestales, plagas y enfermedades.