Environmental parameters and tree physiology in two semiarid land use systems in Brazil

To determine if reduced tree densities (number of trees per unit area) lead to changes in micro-meteorological and tree physiological characteristics, two areas with different tree densities were assessed. One was an agrosilvopastoral system (AGP) with low tree density, and the other, a secondary forest (SF) with greater tree numbers, both located in a semiarid region of Brazil. Data were collected simultaneously by two automated weather stations: rainfall, air (T a ) and soil temperatures (T s5cm ), relative humidity (RH air ), photosynthetically active radiation (PAR), soil moisture at two depths (M s30cm and M s50cm ), and wind velocity (V w ). Net photosynthesis in Cordia oncocalyx Allemão trees was measured with an infrared gas analyzer in February, March, May, July, August and September 2011, which encompasses a rainy period followed by a dry period. Average values of T a , T s5cm , V w and PAR were greater and average values of RH air and M s50cm were lesser in the AGP. Photosynthetic rates were greater in plants growing under the AGP at the onset of the dry season when V w , PAR and T s5cm were greater and M s50cm was lesser. Photosynthetic rates correlated strongly with physical parameters during the dry season, especially under SF. Differences in tree numbers between AGP and SF led to differences in physical environmental parameters; however, the latter had less influence on photosynthetic rates in C. oncocalyx during the rainy season. During the dry season, all physical parameters had an impact on net photosynthesis under SF but not under AGP. This indicates a certain independence of plants in AGP.