No abstract

The integrated relationship in a simple mechanistic model between the critical environmental factors controlling leaf photosynthesis of understory species would be a useful tool to optimize the management of the silvopastoral systems. Individual effect of leaf temperature, water stress and light environment over net maximum photosynthetic rate (Pmax) was evaluated on Festuca pallescens leaves grown in a silvopastoral system of two Pinus ponderosa canopy covers (350 and 500 trees ha -1 ) and natural grassland. The aim was to integrate individual functions for Pmax against these environmental factors into a multiplicative model. We measured pre-dawn water potential (w pd ), leaf temperature and net photosynthetic rate (Pn), stomatal conductance (gs) and intercellular CO 2 concentration (Ci) as a function of photosynthetic photon flux density (PPFD). The highest Pmax under non-limiting conditions was 20.4 lmol CO 2 m -2 s -1 and was defined as standardized dimensionless Pmax s = 1 for comparison of environmental factors. The leaf temperature function showed an optimum range between 20.2 and 21.8°C where Pmax s = 1. Then, Pmax s declined by an average 1 lmol CO 2 m -2 s -1 C -1 from the optimum to 4.7 and 38.5°C. Pmax s decreased at a rate of 9.49 lmol CO 2 m -2 s -1 MPa -1 as water potential reaches -1.9 MPa and showed a lower slope as water potential decreased down to -4.3 MPa. The light environment was estimated from hemispherical photograph analysis. Pmax s was 20% higher in leaves of open control plants than under the maximum tree canopy cover. The simple multiplicative model accounted for 0.82 of the variation in Pmax. Such a simple mechanistic model is the first step towards a more effective decision support tool.

Previous studies of afforestation in Patagonia indicate that 30-50 % tree cover produces positive effects on the pasture. This coverage level is achieved by applying pruning and strong thinning to reduce the volume of timber production per area unit. From an economic standpoint, in order to not reduce the income level of the system, it is necessary to find tree species that could replace ponderosa pine (Pinus ponderosa) and maximize income per volume unit of wood. We evaluated the feasibility of implementation of five broadleaved tree species (native and exotic) with higher intrinsic wood quality than ponderosa pine. We tested the influence of tree cover generated by a framework of silvopastoral plantation on the increase in survival, regrowth and absolute increase in height, and compared ecophysiological variables (net photosynthetic activity, stomatal conductance, intrinsic water use efficiency) of different species to the status of a traditional plantation without tree cover. Additionally values of air temperature and relative humidity were registered under both cover conditions. Preliminary results support the conclusion that three of these five species could be considered as alternatives to ponderosa pine species for establishing silvopastoral systems with higher timber value. Our Based on the results, the use of tree cover generated under the current plantations of P. ponderosa could increase the success of the establishment of these species.

Los sistemas silvopastoriles están sujetos a las interacciones que se generan entre sus componentes. Para asegurar el éxito productivo de estos sistemas el balance neto delas interacciones árbol-pasto debe ser neutro o positivo, de prevalecer la competencia el sistema no es sustentable en el tiempo.