Silvopastoral systems in northwestern Patagonia. I: growth and photosynthesis of Stipa speciosa under different levels of Pinus ponderosa cover

Combination of native pastures with fast-growing tree species (mainly Pinus spp.), may be an interesting economic and environmental alternative for small and medium land-owners in Patagonia,Argentina. Pasture productivity is usually the single most important factor affecting livestock carrying capacity. Therefore the prediction of the effects of tree cover on understory herbaceous production is of great importance for management. This paper reports the growth response of Stipa speciosa Trin. et Rupr. (tillering, tiller height, number of green leaves per tiller, and relative growth) to Pinus ponderosa Doug. (Laws) canopy cover (0 to 100%). Grasses did not grow when crown cover was >70%. Tiller production and relative growth were negatively correlated with tree cover; tiller height was larger under high tree cover. There was no relationship between leaf production and tree canopy cover. Grass leaves maintained net photosynthetic rates, at similar light levels, at a wide range of plant water potentials (from saturation to at least −2.4 MPa). Decrease in photosynthesis occurred at very low plant water potentials (−4.3 MPa). No differences in photosynthetic rate at similar radiation levels were foundbe tween plants growing in different light environments (determined by tree cover) in the field. Stipa speciosa can maintain positivenet CO2 assimilation and growth under moderate shading and severewater stress conditions. For this reason, it can be used in silvopastoral systems, with a proper light management.