Stand dynamics and tree coexistence in an analytical structured model: the role of recruitment.

Understanding the mechanisms of coexistence and niche partitioning in plant communities is a central question in ecology. Current theories of forest dynamics range between the so-called neutral theories which assume functional equivalence among coexisting species to forest simulators that explain species assemblages as the result of tradeoffs in species individual strategies at several ontogenetic stages. Progress in these questions has been hindered by the inherent difficulties of developing analytical size-structured models of stand dynamics. This precludes examination of the relative importance of each mechanism on tree coexistence. In previous simulation and analytical studies emphasis has been given to interspecific differences at the sapling stage, and less so to interspecific variation in seedling recruitment. In this study we develop a partial differential equation model of stand dynamics in which competition takes place at the recruitment stage. Species differ in their size-dependent growth rates and constant mortality rates. Recruitment is described as proportional to the basal area of conspecifics, to account for fecundity and seed supply per unit of basal area, and is corrected with a decreasing function of species specific basal area to account for competition. We first analyze conditions for population persistence in monospecific stands and second we investigate conditions of coexistence for two species. In the monospecific case we found a stationary stand structure based on an inequality between mortality rate and seed supply. In turn, intra-specific competition does not play any role on the asymptotic extinction or population persistence. In the two-species case we found that coexistence can be attained when the reciprocal negative effect on recruitment follows a given relation with respect to intraspecific competition. Specifically a tradeoff between recruitment potential (i.e. shade tolerance or predation avoidance) and fecundity or growth rate. This is to our knowledge the first study that describes coexistence mechanisms in an analytical size-structured model in terms of competitive differences at the regeneration state.

General characteristics of an early arthritis cohort in Argentina.

OBJECTIVE: The aim of the present study is to describe the general characteristics of a cohort of patients with early arthritis in Argentina. METHODS: CONAART (Consorcio Argentino de Artritis Temprana--Argentine Consortium for Early Arthritis) is an initiative of seven rheumatology centres across Argentina. Patients were included if they had at least one or more swollen joints and <2 years of disease duration. Social, demographic, familiar, hereditary, clinical and laboratory data were recollected. At first visit and every year, X-rays of hands and feet were performed and working characteristics and pharmaco-economic data were re-collected. RESULTS: A total of 413 patients were included. Of them, 327 (79.2%) were women with a median age of 49 years and a median disease duration of 6 months. Of the total, 183 (44.3%) had RA (ACR 1987) and 167 (40.4%) undifferentiated arthritis (UA). Other diagnoses included: 12 crystalics, 11 PsA, 6 uSpA, 6 other CTD, 1 AS and 27 other diagnosis. As 85% of our population had RA and UA, we only compared these two groups of patients. Patients with RA had significantly worse activity parameters of the disease (DAS of 28 joints), functional capacity (HAQ) and quality of life (Rheumatoid Arthritis Quality of Life) than patients with UA. The frequency of RF and anti-CCP, and symmetrical distribution were also significantly higher in patients with RA compared with UA patients. All patients with RA initiated early specific treatment, in a period no longer than 6 months from the beginning of the disease. CONCLUSION: Early arthritis clinics are a useful tool to identify and treat patients with different forms of joint involvement.

An analytical model of stand dynamics as a function of tree growth, mortality and recruitment: the shade tolerance-stand structure hypothesis revisited.

Light competition and interspecific differences in shade tolerance are considered key determinants of forest stand structure and dynamics. Specifically two main stand diameter distribution types as a function of shade tolerance have been proposed based on empirical observations. All-aged stands of shade tolerant species tend to have steeply descending, monotonic diameter distributions (inverse J-shaped curves). Shade intolerant species in contrast typically exhibit normal (unimodal) tree diameter distributions due to high mortality rates of smaller suppressed trees. In this study we explore the generality of this hypothesis which implies a causal relationship between light competition or shade tolerance and stand structure. For this purpose we formulate a partial differential equation system of stand dynamics as a function of individual tree growth, recruitment and mortality which allows us to explore possible individual-based mechanisms--e.g. light competition-underlying observed patterns of stand structure--e.g. unimodal or inverse J-shaped equilibrium diameter curves. We find that contrary to expectations interspecific differences in growth patterns can result alone in any of the two diameter distributions types observed in the field. In particular, slow growing species can present unimodal equilibrium curves even in the absence of light competition. Moreover, light competition and shade intolerance evaluated both at the tree growth and mortality stages did not have a significant impact on stand structure that tended to converge systematically towards an inverse J-shaped curves for most tree growth scenarios. Realistic transient stand dynamics for even aged stands of shade intolerant species (unimodal curves) were only obtained when recruitment was completely suppressed, providing further evidence on the critical role played by juvenile stages of tree development (e.g. the sampling stage) on final forest structure and composition. The results also point out the relevance of partial differential equations systems as a tool for exploring the individual-level mechanisms underpinning forest structure, particularly in relation to more complex forest simulation models that are more difficult to analyze and to interpret from a biological point of view.

Artropatías microcristalinas: rol actual de la ultrasonografía / Crystaline arthropaties: current role of the ultrasonography

En el transcurso de los últimos años, la ultrasonografía ha generado un interés cada vez mayor en el campo de la reumatología debido a su relevante potencialidad y sus numerosas posibilidades de aplicación. La mayor parte de los estudios científicos se han orientado principalmente hacia la evaluación del potencial de la ultrasonografía en la detección precoz del proceso inflamatorio y del daño articular en pacientes con artritis reumatoidea temprana, en la evaluación de los distintos síndromes regionales dolorosos, en el estudio y monitoreo terapéutico en pacientes con artritis inflamatorias crónicas y en su utilidad como guía para la terapéutica locoregional. Son pocos los estudios que se han avocado a determinar el rol y la potencialidad de este método de imágenes en pacientes con artropatías microcristalinas. El progreso continuo de la tecnología en el campo de la ultrasonografía musculoesquelética ha generado instrumentos con la capacidad de obtener imágenes de alta resolución que contribuyen a la detección de mínimos depósitos de cristales de urato monosódico y/o de cristales de pirofosfato de calcio, no detectables en muchas ocasiones mediante la radiografía convencional. La presente revisión tiene como objetivo principal demostrar la utilidad de la ultrasonografía en el estudio de pacientes con artropatías microcristalinas mediante la ilustración de los hallazgos ecográficos más representativos de dichas entidades.