Abstract
The grassland/savannas in the South American Semiarid Chaco have been massively transformed into shrubland. However, the ecology of this phenomenon is little known and restoration initiatives almost non-existent. We provide information about 1) the differences in plant structure and diversity along a woody encroachment gradient, and 2) the association among plant structural and functional attributes with environmental factors. We utilized 38 sample units and obtained tree and shrub cover, herbaceous biomass, species richness and composition of grasses and woody plants and several herbaceous functional types (mainly grasses). The environmental factors were historic grazing intensity and fire frequency. Related to item 1, we found: a) shrub cover was significant and progressively greater along the woody encroachment gradient and largely exceeded the tree cover, b) the herbaceous biomass was significantly smaller, with a high woody cover (x̄=89.7%), c) the grass species richness was maximum at intermediate levels of woody cover (x̄=39.3% and 64%), d) the dissimilarity in grass species composition was maximum between very low and high levels of woody cover (x̄=15.8% and 89.7%), and e) woody species richness and composition were uniform along the woody encroachment gradient. In relation to item 2, we observed: 1) the increase in historic grazing intensity was significant and strongly associated with the reduction of fire frequency, which, in turn, was significantly associated with the increase in tree and shrub cover and the decrease in herbaceous biomass, suggesting that overgrazing reduces or eliminates the grassland fires and promotes the woody encroachment, and 2) several grass functional types were significantly associated with the historic grazing intensity and fire frequency. Our results are useful both to anticipate the grassland/savannas responses to woody encroachment processes and to plan restoration and management strategies.
Highlights
En los últimos 150 años, muchos pastizales/ sabanas áridos y semiáridos del mundo sufrieron un proceso de lignificación a partir del establecimiento y propagación de plantas leñosas (D’Odorico et al 2012)
La lignificación ejerce una influencia fuerte en la diversidad y los tipos funcionales de plantas (Bestelmeyer et al 2003; Archer et al 2017)
Flora chaqueña-Argentina-(Formosa, Chaco y Santiago del Estero)
Summary
A escala cartográfica 1: 500000, el área de estudio es una planicie suave (i.e., pendientes generales entre 0.5 y 2%) cubierta por sedimentos loessicos, subdividida a su vez en tres unidades geomorfológicas (i.e., Planicie Ondulada Norte, Planicie Sur y Estribaciones Serranas) (Angueira and Vargas Gil 1993; Kunst et al 2006) (Figura 2). Los sitios ecológicos medias lomas originalmente estuvieron ocupados por ecotonos entre bosques altos y pastizales/ sabanas. B) Vista del área de estudio, con las unidades geomorfológicas delimitadas y los pueblos de Lavalle, Choya y Villa la Punta como referencias geográficas. Donde este estudio hizo foco, estuvieron ocupados en sus orígenes por pastizales/sabanas, con árboles dispersos de S. lorentzii y A. quebracho-blanco, arbustos como V. aroma, G. decorticans y Aloysia sp. Ello configura ambientes más húmedos y favorables para la productividad de las pasturas nativas (Morello and Adamoli 1974)
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