Pablo E. Villagra

Effects of Prosopis flexuosa on soil properties and the spatial pattern of understorey species in arid Argentina

Abstract In arid zones dominant woody plants are capable of causing changes in microclimate and soil properties likely to affect species composition, as well as the establishment and spatial distribution of plant species. In North American and European deserts species richness appears to be higher under the canopy of shrubs and trees, in contrast with Chilean deserts where it seems to be lower. Since Prosopis flexuosa (Fabaceae, Mimosoideae) is the most conspicuous tree in the central Monte desert, Argentina, we analysed the effect of this species on the composition and abundance of the shrub and herbaceous layers and on soil properties. We considered two mesohabitats: ‘under P. flexuosa canopy’ and ‘intercanopy areas’. In addition, we analysed the differences between two microhabitats under canopies: ‘northern part of the canopy’ and ‘southern part of the canopy’. Results indicate that species composition and soil properties are affected by both mesohabitats and microhabitats. We found a higher number of shrubs under canopies, whereas that of grasses and perennial forbs increased in intercanopy areas. Concentrations of organic matter, nitrogen, potassium and phosphorus, factors limiting biological productivity in Monte desert soils, were significantly higher under than outside P. flexuosa canopies. Electrical conductivity and concentrations of Na+, Ca++, Mg++ were higher in the northern than in the southern microhabitats. No differences in species richness, evenness or diversity were found between mesohabitats or between microhabitats. We conclude that P. flexuosa modifies the spatial pattern of plant species in the shrub and herbaceous layers and the chemical conditions of the soil, generating spatial heterogeneity on different scales. Abbreviations: EC = Electrical conductivity; SAR = Sodium absorption ratio.

Water subsidies from mountains to deserts: their role in sustaining groundwater-fed oases in a sandy landscape

In arid regions throughout the world, shallow phreatic aquifers feed natural oases of much higher productivity than would be expected solely from local rainfall. In South America, the presence of well-developed Prosopis flexuosa woodlands in the Monte Desert region east of the Andes has puzzled scientists for decades. Today these woodlands provide crucial subsistence to local populations, including descendants of the indigenous Huarpes. We explore the vulnerability and importance of phreatic groundwater for the productivity of the region, comparing the contributions of local rainfall to that of remote mountain recharge that is increasingly being diverted for irrigated agriculture before it reaches the desert. We combined deep soil coring, plant measurements, direct water-table observations, and stable-isotopic analyses (2H and 18O) of meteoric, surface, and ground waters at three study sites across the region, comparing woodland stands, bare dunes, and surrounding shrublands. The isotopic composition of phreatic groundwaters (delta2H: -137 per thousand +/- 5 per thousand) closely matched the signature of water brought to the region by the Mendoza River (-137 per thousand +/- 6 per thousand), suggestin that mountain-river infiltration rather than in situ rainfall deep drainage (-39 per thousand +/- 19 per thousand) was the dominant mechanism of recharge. Similarly, chloride mass balances determined from deep soil profiles (> 6 m) suggested very low recharge rates. Vegetation in woodland ecosystems, where significant groundwater discharge losses, likely >100 mm/yr occurred, relied on regionally derived groundwater located from 6.5 to 9.5 m underground. At these locations, daily water-table fluctuations of 10 mm, and stable-isotopic measurements of plant water, indicated groundwater uptake rates of 200-300 mm/yr. Regional scaling suggests that groundwater evapotranspiration reaches 18-42 mm/yr across the landscape, accounting for 7 17% of the Mendoza River flow regionally. Our study highlights the reliance of ecosystem productivity in natural oases on Andean snowmelt, which is increasingly being diverted to one of the largest irrigated regions of the continent. Understanding the ecohydrological coupling of mountain and desert ecosystems here and elsewhere should help managers balance production agriculture and conservation of unique woodland ecosystems and the rural communities that rely on them.

Estructura y estado de conservación de los bosques de Prosopis flexuosa D.C. (Fabaceae, subfamilia: Mimosoideae) en el noreste de Mendoza (Argentina)

La recomendacion de normas de manejo que regulen el uso de los recursos forestales debe estar basada en el conocimiento de la estructura y dinamica de los mismos. El objetivo de este trabajo fue conocer la estructura poblacional de las principales unidades boscosas del bosque de Prosopis flexuosa del noreste de Mendoza, sus condiciones sanitarias y su potencial productivo. Se muestrearon un total de 1.471 algarrobos en las cuatro unidades boscosas mas representativas. La densidad total de algarrobos fue la siguiente: bosque semicerrado de P. flexuosa con Atriplex lampa y Lycium tenuispinosum en valles intermedanos (Bosque 1): 181 arboles ha-1, bosque abierto de P. flexuosa con Trichomaria usillo y Suaeda divaricata en ondulaciones (Bosque 2): 155 arboles ha-1, bosque abierto de P. flexuosa con T. usillo (Bosque 3): 233 arboles ha-1 y bosque abierto de P. flexuosa con A. lampa en ondulaciones suaves (Bosque 4): 215 arboles ha-1. El analisis de componentes principales de la estructura diametrica agrupo los sitios relevados en las distintas unidades boscosas segun la proporcion de arboles de diametro basal mayor a 25 cm. Los sitios del Bosque 1 (mayor proporcion de arboles grandes), se separaron de los sitios con mayor proporcion arboles pequenos (bosques 2 y 4). Debido al habito de crecimiento de los algarrobos, la cantidad de productos maderables de estos bosques es baja. Ademas, P. flexuosa presenta en el area un porcentaje alto de individuos con mas de dos fustes, la forma en muchos casos es decumbente y la altura de los fustes es menor a un metro. Por lo tanto, el potencial forestal del bosque es bajo y el posible aprovechamiento deberia realizarse a escala local, considerando la inclusion de otras actividades complementarias en zonas establecidas para tal fin

Ecophysiology of Prosopis Species From the Arid Lands of Argentina: What Do We Know About Adaptation to Stressful Environments?

The expansion of the Prosopis genus from the sub-humid Chaco towards colder and drier zones such as Monte, Prepuna and Patagonia biogeographical regions would have implied the acquisition and/or adjustment of morphological and physiological adaptations to stressful environments. In this chapter, we discuss the phenological, morphological and physiological features of seven Prosopis species native to Argentinean arid regions that allow them to avoid or tolerate water stress, salinity, and other environmental stress factors in arid lands. Some of these adaptations appear to be spread over the genus and should confer the capability to deal with the most common stressful factor of arid lands (i.e. water availability); however, other morphological or physiological adaptations appear to be specific to each species, and should be the cause of niche differentiation among species and the occupation of particular environments within arid lands (e.g. sand dunes, saline environments). Finally, we discuss some consequences of these adaptations for the management of Prosopis species. The inter- and intra-specific variability observed in their adaptation to stressful factors suggest that some Prosopis species may be a good option to be used in the restoration of degraded areas or in afforestation projects with productive objectives.

Utilización de documentos históricos en la reconstrucción de la vegetación de la Llanura de la Travesía (Argentina) a principios del siglo XIX

La Llanura de la Travesia se extiende en las provincias de Mendoza y San Luis (Argentina) entre el piedemonte de los Andes y las Sierras Pampeanas. El objetivo de este trabajo fue reconstruir las condiciones ambientales de la Llanura de la Travesia a principios del siglo XIX combinando la informacion de diversas fuentes documentales. Se utilizo como base un testimonio del Comandante Don Faustino Ansay, levantado en 1802 ante la necesidad de verificar el estado del camino de la Travesia o camino Real que unia Corocorto (actual Villa de La Paz) con San Luis. Esta informacion se complemento con el testimonio de otros viajeros que recorrieron el camino. Este analisis revelo un gradiente similar al actual, en terminos de un aumento de la complejidad de la vegetacion en el sentido oeste-este. Sin embargo, la ausencia de Aspidosperma quebracho-blanco sugiere cambios en la distribucion de algunas especies que deberan ponerse a prueba con nuevas observaciones. Se comprueba la validez de la utilizacion de informacion documental para reconstruir la vegetacion del pasado y detectar cambio ambientales

Use of differential water sources by Prosopis flexuosa DC: a dendroecological study

In central-western Argentina, there is a pronounced water deficit gradient, from semiarid climate conditions with 500-mm rainfall/year to arid climate conditions with 80-mm rainfall/year. This climatic transition, governed by the rainfall gradient, occurs between the Arid Chaco and Monte phytogeographic regions and is evidenced by differences in vegetation type, structure, dynamics and tree growth. In turn, the availability of soil moisture, particularly access to the water table, modifies water use strategies by trees along this gradient. We analyzed how water availability, expressed as differences in accessibility to the water table, influences Prosopis flexuosa tree rings along a precipitation gradient. In this manner, we try to interpret the growth of species according to the use of differential water sources. P. flexuosa showed highly varying growth reactions (tree-ring width and hydraulic anatomic parameters) with climate, depending on the ecology of the site. Along the Arid Chaco-Monte gradient, the growth of P. flexuosa is more dependent on variations in rainfall in those areas where water depth is greater than root spread. The climate signal was hidden in those regions where the water table is accessible to the root system.