Flower and seed production as affected by axis category and shoot size in two Patagonian Nothofagus species

Flower distribution within the tree crown may affect both pollination dynamics and the costs of flowering on tree growth. For Nothofagus obliqua (Mirb.) Oerst. and Nothofagus nervosa (Phil.) Krasser, the production of flowers and viable seeds were compared among axes of contrasting size and category in the tree crown. For annual shoots arising from three axis categories (main branches, secondary branches, and twigs), the numbers of nodes, staminate flowers, pistillate flowers and axillary buds, and seed viability were evaluated. Most flowering shoots produced both staminate and pistillate flowers. The number of flowers of each type was related positively with the number of nodes of the shoot. Longer shoots had proportionally more pistillate flowers than short shoots. The three axis categories produced both flower types in direct proportion to the size of their shoots, but main branches presented more axillary vegetative buds than secondary branches and twigs. For N. obliqua, the percentage of viable seeds was lower in secondary branches than in main branches and twigs, perhaps owing to a lower probability of cross-pollination in secondary branches. The coexistence of vegetative and reproductive functions may cause interference between them. Massive allocation of meristems to flowering reduces meristem availability for growth and, especially, flowering in the following growing season.