Forest ecologists often evaluate how well the species composition of saplings in the understory matches that of the canopy: absence of juveniles suggests that a tree species is suffering population decline. Here we offer a theoretical and empirical test of this assertion using data from a 50-ha census plot in Panama. Theory indicates that higher rates of population change, lambda, lead to more steeply declining size distributions (more juveniles relative to adults). But other parameters also affect the size distribution: lower growth rate of juveniles and lower survival at any size produce more steeply declining size distributions as well. Empirical evaluation of 216 tree populations showed that juvenile growth was the strongest predictor of size distribution, in the direction predicted by theory. Size distribution did correlate with population growth, but weakly and only in understory species, not canopy species. Size distribution did not correlate with the growth rate of larger individuals nor with survival. Results suggest that static in formation on the size distribution is not a good predictor of future population trends, while demographic information is. Fast-growing species will have fewer juveniles in the understory than slow growing species, even when population growth is equal.

Background Helmet use reduces the risk and severity of head injury and death due to road traffic crash among motorcyclists. The protective efficacy of different types of helmets varies. Wearing firmly fastened full face helmet termed as effective helmet use provides greatest protection. This study estimates the prevalence and factors associated with effective helmet use among motorcyclists in Mysuru, a tier II city in Southern India. Methodology Cross sectional road side observational study of 3499 motorcyclists (2134 motorcycle riders and 1365 pillion riders) at four traffic intersections was done followed by interview of random sample of 129 of the above riders. Effective helmet use proportion and effective helmet use per 100 person-minutes of observation was calculated. Multivariate logistic regression analysis was done to identify factors associated with effective helmet use. Results Prevalence of effective helmet use was 28 per 100 riders and 19.5 per 100 person-minutes of observation in traffic intersections. Specific prevalence rates was higher in riders (34.5%), female riders (51.3%), male pillion riders (30.5%) . Riders commuting for work and school and those ever stopped by the police in the past 3 months had significantly higher odds of effective helmet use. Conclusion The effective helmet use among the motorcyclists in Mysore is very low. Strict enforcement and frequent checks by the police are necessary to increase the effective helmet use. Key words: helmet, motorcycle, cross sectional study