Relationship between primary productivity, its environmental controllers and functional diversity in the caatinga vegetation

Abstract

Background and Relevance: Precipitation, temperature and soils have long been recognized as the main environmental factors determining the global distribution of biomes (eg. Whitakker's diagram). Not only significant increase in temperatures is expected for the near future, but also shifts in the total amount of precipitation and its seasonal distribution (IPCC 2014). Plant communities are particularly susceptible to such changes mostly because primary productivity processes (photosynthesis and respiration) depend on the interaction between plants and its physical environment. Other more immediate aspects of global changes are the loss of biodiversity due to land use changes (habitat loss), invasive species and alterations in atmospheric chemistry. A genuine concern exists were ecosystem decline follows biodiversity loss (Naaem 2002). Species diversity has functional consequences because the numbers and types of species establish the collective pool of traits that modulate ecosystem processes (Tilman et al. 1996). Currently, a crucial topic in Ecology is to understand how functional biodiversity relates to ecosystem functioning and consequently, to primary productivity end ecosystem resilience (Hooper et al. 2005). Achieving such understanding is paramount in correctly prescribing interactions among plants and between plants and the environment in dynamic vegetation models, a necessary tool to provide future scenarios for biomes. The moment is opportune as significant changes are taking place resulting in loss of resilience of Brazilian ecosystems (Asner et al 2010, Davidson et al. 2011) creating a feedback loop that most likely will exacerbate the situation. Objectives: This proposal aims to establish and understand relationships between primary productivity, environmental drivers (temperature and precipitation), diversity of species and functional groups, and variability in plant functional traits (related to primary productivity). We want to answer if the variability in certain plant functional traits is influenced by environmental drivers (eg. "Is stomatal sensitivity higher at the drier Caatinga ecosystem, when compared to Cerrado or Amazonian ecosystems?" or "Do plants at Caatinga operate at a higher temperature range than its counterparts at the Cerrado?"). Methodology: This study will characterize the functional diversity present among tree species in 45 research areas comprising Caatinga formations in Pernambuco, Ceará and Bahia. For some of these sites a history of research with a large variety of information is already available from the literature and project partners.The variables to be investigated within the scope of this project are either directly or indirectly related to processes of photosynthesis and respiration. Response curves to CO2, light, relative humidity, and temperature will be taken from leaves using portable photosynthesis systems. Such curves allow the estimation of key eco-physiological traits that can reveal the extent of the variability of the functional diversity. The leaves used for gas exchange measurements will be collected and analyzed for major nutrients. Associated to the measurements above described, we will also determine the mass to area ratio for leaves, the wood density and the Huber value (ratio between leaf area and xylem area) for branches. We will use the highest possible number of species for this study (depending species richness and evenness). Discriminant analysis will be used to generate a posteriory functional groups. Furthermore, phenology of leaves will be monitored along the year by digital imagery methods, and litter production will be determined by use of litter traps. Field campaigns will be organized every other month (six in total) in order to cover the dynamics of the annual seasonality. Finally, the results obtained by this study will be analyzed in association with current and past determinations of primary productivity obtained by flux towers.