Abstract
Grassland primary productivity is the function that underpins the majority of the fodder production in cattle-rearing silvopastoral farms. Hence, understanding the factors that determine grassland productivity is critical for the design and management of silvpastoral systems. We studied the effect of two factors with documented impact on grassland productivity in seasonally dry silvopastures of Nicaragua, rainfall and trees. We assessed the effects of three species that differed in crown size and phenology, one evergreen, Cassia grandis, and two deciduous species, Guazuma ulmifolia and Tabebuia rosea. Overall, grassland ANPP had a quadratic response to rainfall, with a decline at high rainfall that coincided with peak standing biomass and grassland cover. Trees had a predominately negative effect on grassland productivity, and the effect was concentrated in the rainy season at peak productivity. The effect of the trees corresponded with the tree crown area, but not with crown density. Trees reduced the standing biomass of graminoids and increased forb biomass; thus, the effect of trees on grassland ANPP appears in part to respond to changes in grassland composition. We also found higher levels of soil moisture content below the tree canopy, particularly at the peak of the rainy season when soils tend to become waterlogged. The evergreen species, C. grandis, affected grassland ANPP more strongly than the deciduous species.
Highlights
Grassland primary productivity is the function that underpins the majority of the fodder production in cattle-rearing silvopastoral farms
In seasonally-dry tropical climates, monthly rainfall is an important determinant of net primary productivity (Ospina et al 2012), but studies that have evaluated this feature in silvopasture grasslands of Central America are few, and it is not well known how rainfall interacts with the tree canopy in determining levels of grassland productivity in these systems
The effect of the tree was most evident at the peak of the production period, in June, and in this case, ANPP below C. grandis was lower than ANPP under the two deciduous species (LSD Fisher test P \ 0.05, Fig. 2)
Summary
Grassland primary productivity is the function that underpins the majority of the fodder production in cattle-rearing silvopastoral farms. In addition to the impact on soil nutrient status, trees can decrease water stress and reduce evapotranspiration (Joffre and Rambal 1993), improve the water balance in the soil (Espeleta et al 2004), and may further increase water availability for grassland plants through hydraulic lift (Ludwig et al 2004). Despite these positive effects of trees on the belowcanopy environment, competition between trees and the herbaceous vegetation can be severe and outweigh the facilitation effects (Ludwig et al 2004)
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