Effects of intrinsic and environmental factors on seedling survival ofRumex obtusifoliusin Mediterranean alfalfa crops

摘要

This paper focusses on the survivorship pattern of seedlings ofRumex obtusifoliusL. in the irrigated alfalfa crops (Medicago sativaL.) in central Catalonia (NE Spain). The relationship between this pattern and seasonal environmental change was evaluated by monitoring seedling cohorts that had germinated at different times of the year for 31 months. Paired comparisons of cohort survivorship curves were performed over their life-spans and for their coexistence period, using Peto and Peto's log rank test. The significance of intrinsic variables such as age and size in determining seedling survival was tested using hierarchical log-linear analysis applied on a three-way contingency table and by means of Goodman and Kruskal's association coefficients. Results showed that seedling cohorts emerging at different times in the year exhibited a similar Deevey type III survivorship pattern. Therefore, survivorship was scarcely affected by seasonal environmental variation. However, since age had little effect on seedling survival compared with size, this age-specific survivorship pattern was, in fact, a consequence of both size-dependent mortality and changes in size distribution with age. In the first weeks of life, cohort size distribution was greatly skewed to the smallest classes, with the highest mortality rates, and therefore the risk of death in the cohort was high. Afterwards, the frequency of the largest size classes increased with age, thus reducing the risk of death of the cohort.Key words:Rumex obtusifolius, seedling survivorship pattern, size versus age dependency, survival analysis.