Spatially oriented studies have examined the role of competition on plant populations and communities but not the combined effects of microsite heterogeneity and competition. The aim of this study was threefold: first, to apply and test a common geostatistical tool (semivariograms) to disentangle competition and microsite effects; second, to assess the results of this methodology against a generalized early stand development model for tree populations; and third, to examine the role and timing of microsite and competition processes in early population stages. We mapped and measured annual relative growth rates of trees in three different-aged ponderosa pine stands in Patagonia, Chile. We tested the relative support of five a priori semivariogram-based hypotheses and showed that through stand development, many sites followed our expected sequence of semivariogram models. These translated to initial spatially random growth followed by microsite-dominated, mixed microsite and competition, and finally pure competition effects on growth. Our approach will have many and diverse applications wherever processes differ in the type of spatial pattern they exhibit as well as in spatial scale. We emphasize that this methodology works best when there is strong a priori support for the hypotheses being tested but the timing, strength, and occurrence of processes are not known.
Shift work has serious health impacts due to desynchronization of the circadian rhythm; consequently, the workers have increased sleep disturbances. Another impact is working hours, which can contribute to decreased sun exposure and lead to the development of hypovitaminosis D. Vitamin D has been implicated in extraskeletal functions in many physiological mechanisms, including sleep. Therefore, we aimed to verify the association between sleep parameters and hypovitaminosis D in shift workers with cardiovascular risk. We conducted a cross-sectional study of 82 male rotating shift workers (24-57 years old) with at least one cardiovascular risk factor (such as hyperglycemia, dyslipidemia, abdominal obesity, physical inactivity, hypertension, and smoking). Polysomnography was used to evaluate sleep parameters, while vitamin D levels were measured using a chemiluminescence method. Logistic regression was used to model the association between hypovitaminosis D and sleep parameters after adjustment for relevant covariates. Hypovitaminosis D (< 20 ng/mL) was seen in 30.5% of the workers. Shift workers with hypovitaminosis D had lower sleep efficiency, increased microarousal index, and lower arterial oxygen saturation after adjusting for seasonality, age, and body fat. Therefore, we suggest that hypovitaminosis D is associated with greater sleep disturbances in rotating shift workers with cardiovascular risk factors.
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