EFFECTS OF INTRINSIC AND ENVIRONMENTAL-FACTORS ON SEEDLING SURVIVAL OF RUMEX OBTUSIFOLIUS IN MEDITERRANEAN ALFALFA CROPS

This paper focusses on the survivorship pattern of seedlings of Rumex obtusifolius L. in the irrigated alfalfa crops (Medicago sativa L.) in central Catalonia (NE Spain). The relationship between this pattern a nd seasonal environmental change was evaluated by monitoring seedling cohorts that had germinated at different times of the year for 31 mont hs. Paired comparisons of cohort survivorship curves were performed ov er their life-spans and for their coexistence period, using Peto and P eto's log rank test. The significance of intrinsic variables such as a ge and size in determining seedling survival was tested using hierarch ical log-linear analysis applied on a three-way contingency table and by means of Goodman and Kruskal's association coefficients. Results sh owed that seedling cohorts emerging at different times in the year exh ibited a similar Deevey type III survivorship pattern. Therefore, surv ivorship was scarcely affected by seasonal environmental variation. Ho wever, since age had little effect on seedling survival compared with size, this age-specific survivorship pattern was, in fact, a consequen ce of both size-dependent mortality and changes in size distribution w ith age. In the first weeks of life, cohort size distribution was grea tly skewed to the smallest classes, with the highest mortality rates, and therefore the risk of death in the cohort was high. Afterwards, th e frequency of the largest size classes increased with age, thus reduc ing the risk of death of the cohort.