Dynamics of cohorts of cladodes and related effects on reproduction in theshrub Retama sphaerocarpa in semi-arid south-eastern Spain

Changes in the size and age structure of the canopy of the leguminous shrub Retama sphaerocarpa in semi-arid south-eastern Spain were investigated by monitoring growth and survivorship of cladodes (photosynthetic stems) on ma rked shoots over a period of 26 months. Three basic morphological types of cladodes - long shoots, short shoots, and secondary short shoots - were dis tinguished. The canopy of the shrubs contained three annual cohorts of cladodes in spri ng and summer. The number and size of shoots produced each year was highly variable and was apparently related to the amount of rainfall during the pr eceding cool season. The spring cohort of 1994 produced only 37% of cladode s compared with 1993. Two cohorts of shoots were actually initiated in spri ng and late summer of each year, but the second cohort produced only 2-12% of the number of cladodes compared with the spring cohort. The proportions of the different cohorts in the canopy changed rapidly from April to August , but only slowly during the remainder of the year when only two annual coh orts remained after extensive litterfall in late summer. This late summer l itterfall caused a substantial reduction in green canopy area (40-50%) whic h was achieved mainly be shedding of one year old cladodes. The life expect ancy of cladodes decreased with increasing order of their morphological typ e from 850 +/- 25 days in long shoots to 563 +/- 4 days and 546 +/- 9 days in short shoots and second order short shoots, respectively. Flowering and fruiting took place from May to July, almost exclusively on o ne year old cladodes, and coincided with the maximum development of the can opy. Flowering intensity was high in 1994, when individual shoots supported a mean number of approximately 150 flowers. Shoots produced an average of 12.6 +/- 0.6 and 5.3 +/- 1.0 fruits per shoot in 1993 and 1994, respectivel y. Most of the annual fruit crop (80-90%) was shed during litterfall in lat e summer. A proportion of 10-20% was retained in the canopy for up to 12 mo nths, however, with some fruits persisting for more than 22 months.