No abstract

Soil erosion, as a result of livestock grazing, has been widely reported for arid and semiarid ecosystems, but information is lacking in more mesic ecosystems where erosion is generally studied in relation to agriculture. To test the hypothesis that, in the high-mountain rangelands of Córdoba (Argentina), grazing by livestock can drive the system into a rocky desert, 200 4 × 4 m plots under different livestock stocking rates and timings of grazing were monitored for 5 years. Four indicators of soil erosion: change rate of rock surface and of total bare surface, advance rate of erosion edges, and their activity persistence were estimated for each plot. Erosion edges are steps with a vertical bare soil surface, whose advance usually leaves behind an exposed rock area. For each plot, the average annual stocking rate for the 5-year period, and an index of seasonality, were calculated. Multiple regressions were used to analyse the data. Under high stocking rates, rock and bare surface increased, edges advanced faster and persisted more actively, while under low or nil stocking rates, rock and bare surface decreased and edges tended to stabilise. From these results, it was estimated that under high stocking rates, 18% of the whole area could be transformed into rocky surface in 400 years. As fire is a usual tool for this rangeland management, surface soil loss during 1 year in 77 burned and unburned plots, with and without post-fire livestock grazing, were compared. Burned plots lost 0.6 cm of surface soil when grazed, and 0.4 cm when ungrazed, while unburned plots lost less than 0.05 cm when grazed, and gained 0.07 cm when ungrazed. It was concluded that the present-day combination of livestock and fire management has the potential to convert this rangeland into a rocky desert. It is suggested that commercial livestock production, as it is carried on at present, is not sustainable, and some suggestions on changes necessary for a future sustainable grazing industry are made.

Fire frequency has been highlighted as an important component of fire effects on ecosystems; nevertheless, there is scarce information about how fire modulates changes in ecosystem properties, particularly for subtropical dry forests. A long‐term natural experiment and a multiscale approach were used to analyze changes in ecosystem properties in a subtropical dry forest subjected to contrasting fire regimes. Measurements were taken in two adjacent sites that had contrasting fire regimes (low/high fire frequency), and vegetation and soil analyses were combined with dendrochronological methods. Ten plots (20 × 20 m) were established in each site and soil (depth, chemical, and biological properties), and vegetation (species composition and growth of the dominant tree, Lithraea molleoides) variables were measured. Fire scars in L. molleoides were useful in determining fire history. Shrubs and grasses were more abundant in the high fire frequency site, with a high cover of the exotic grass Melinis repens, whereas trees and vines were more abundant in the low fire frequency site. Soil of the high fire frequency site tended to have lower C and nutrient content as well a significant lower depth than those of the low fire frequency site. Growth of L. molleoides individuals was lower in the high fire frequency site. Our results suggest that the long‐term effects of high fire frequency probably increase both soil loss and fire frequency in the future due to changes in plant composition, indicating that frequent burning is not a sustainable management option for this seasonal subtropical dry forest.

Ligustrum lucidum is a highly invasive East Asian tree that successfully colonizes several subtropical and temperate areas around the world. Its invasion capacity results from a widespread human use mostly in urban and periurban settings, very abundant fruit and seed production, small bird-dispersed fruits, high germination rates, resprouting capacity, fast growth rates, low herbivory levels and tolerance to a wide range of light, temperature and soil. All these traits contribute to its ability to rapidly increase in abundance, alter biodiversity, landscape ecology and limit its management. This paper reviews the current knowledge on L. lucidum with particular focus on its uses, distribution, invasiveness, ecological and economic impacts and control measures. Most relevant aspect of the review highlight the negative ecological impacts of L. lucidum, its potential to continue expanding its range of distribution and the need of further studies on the eco-physiology of the species, economic impact and social perception of its invasion and early warning systems.

Aim Existing global models to predict standing biomass are based on trees characterized by a single principal stem, well developed in height. However, their use in open woodlands and shrublands, characterized by multistemmed species with substantial crown development, generates a high level of uncertainty in biomass estimates. This limitation led us to (a) develop global models of shrub individual aboveground biomass based on simple allometric variables, (b) to compare the fit of these models with existing global biomass models, and (c) to assess whether models fit change when bioclimatic variables are considered. Location Global. Time period Present. Major taxa studied 118 species of shrubs. Methods We compile a database of 3,243 individuals across 49 sites distributed worldwide. Including  stem basal diameter, height and crown diameter as predictor variables, we built potential models and compared their fit using generalized least squares. We used mixed effects models to determine if bioclimatic variables improved the accuracy of biomass models. Results Although the most important variable in terms of predictive capacity was stem basal diameter, crown diameter significantly improved the models’ fit, followed by height. Four models were finally chosen, with the best model combining all these variables in the same equation [R2 = 0.930, root mean square error (RMSE) = 0.476]. Selected models performed as well as established global biomass models. Including the individual bioform significantly improved the models’ fit. Main conclusions Stem basal diameter, crown diameter and height measures could be combined to provide robust aboveground biomass (AGB) estimates of individual shrub species. Our study supplements well‐established models developed for trees, allowing more accurate biomass estimation of multistemmed woody individuals. We further provide tools for a methodological standardization of individual biomass quantification in these species. We expect these results contribute to improve the quality of biomass estimates across ecosystems, but also to generate methodological consensus on field biomass assessments in shrubs.

No abstract

Rainfall partitioning into interception loss, throughfall and stemflow affects the amount and the spatial heterogeneity of water entering into the soil at the patch scale, strongly controlling net primary productivity of drylands. In this paper, we explored rainfall partitioning and its biophysical controls in Larrea divaricata (jarilla), one of the most abundant shrubs in the Dry Chaco rangelands (Argentina). On average, interception loss, throughfall and stemflow accounted for 9.4, 78.6 and 12.0% of total rainfall, respectively. Interception loss proportion decreased with the increment of rainfall event size and intensity, whereas throughfall proportion showed the opposite pattern. Stemflow proportion increased with the increment of rainfall event size but presented different relations with rainfall event intensity. The increment of rainfall event intensity increased the stemflow in small events (<20 mm), but decreased it in large events (>20 mm). Stemflow increased in plants with higher angles of insertion of stems (measured at 50 and 100 cm from soil surface; p < .05 and p < .01, respectively), but decreased in plants with larger canopy areas (p = .01). Spatial distribution of throughfall (coefficient of variation) decreased with the increment of rainfall event size and intensity. L. divaricata presented more stemflow generation and fewer interception losses than other similar woody species. Our findings help to understand the key role of vegetation canopy affecting the amount of water entering into the soil in drylands.

R������. La capacidad del suelo de almacenar y regular el flujo de agua depende en gran medida de su su tasa de infiltración y profundidad. Los disturbios como el pastoreo y el fuego son moduladores fuertes del sistema vegetación-suelo, ya que son capaces de alterar la tasa de infiltración y la profundidad del suelo. En sistemas montanos estacionales, esto repercute sobre la capacidad de liberar lentamente el agua almacenada hacia el caudal de los arroyos. Nos propusimos indagar si las variaciones en la estructura de la vegetación, debidas en su mayoría al pastoreo y al fuego, de los bosques de altura de Polylepis australis Bi�er de las sierras de Córdoba se asocian a variaciones en las propiedades del suelo en general y, en particular, a aquellas que regulan la capacidad de almacenar agua. Para abarcar la variabilidad completa de estructura de la vegetación seleccionamos 28 sitios distribuidos en tres establecimientos con distintos manejos, y restringimos las variaciones topográficas. No detectamos asociaciones entre la estructura de la vegetación y la topografía; es decir, en general, logramos restringir el efecto topográfico. Por otro lado, aquellos sitios con vegetación más estructurada presentaron suelos con menor densidad aparente y mayor contenido de materia orgánica y capacidad de campo. En relación a la capacidad de almacenamiento de agua, los suelos más profundos y con mayor tasa de infiltración fueron los menos densos, con mayor contenido de materia orgánica y mayor capacidad de campo, asociados a una vegetación más estructurada. Es decir, mientras más conservado se encuentra el sistema vegetación-suelo de los bosques de P. australis, mayor cantidad de agua puede ingresar al suelo y ser almacenada. En consecuencia, las alteraciones del sistema vegetación-suelo, dadas principalmente por el pastoreo y el fuego, reducen la capacidad de almacenar agua de los bosques de altura del centro de la Argentina.[Palabras clave: Polylepis australis, tasa de infiltración, profundidad del suelo, estructura de la vegetación, suelo, montañas, pastoreo, fuego, disturbio, ecohidrología] A�������. The degradation of highland woodlands of central Argentina reduces their soil water storage capacity. Soil water storage capacity and flow regulation relies mostly upon infiltration rate and soil depth. Disturbs such as grazing and fire strongly modulate the vegetation-soil system, and are capable of altering the infiltration rate and soil depth, as well. In mountain seasonal ecosystems, this impacts on soil capacity to slowly release water into streams. We aimed at analyzing if changes in vegetation structure and soil properties, mainly as a consequence of grazing and fire, of highland Polylepis australis Bi�er woodlands of Córdoba mountains, also include alterations in general soil properties and, in particular, in those related to soil water storage capacity. In order to encompass the complete variability in vegetation structure we selected 28 sites in paddocks with different managements, restricting topographic variations. W...