Positional effects on fruit production and filling in two anemophilous Nothofagus species

Abstract

Within-tree variation in fruit and seed production is little understood for anemophilous trees. This study contributes to identifying this variation for Nothofagus obliqua and N. nervosa by assessing the relative fruit production of reproductive shoots and patterns of filled fruit production in relation to flower position in the tree crown and along flowering shoots. Trees from a natural population (Lanín National Park, Argentina) and a cultivated orchard (Río Azul Nature Reserve, El Bolsón, Argentina) were included in the study. Flowering shoots of high branches (6 m from the ground) and low branches (2.5 m) were sampled, and shoot morphometric attributes (cupule production relative to basal stem diameter and number of leaves) and fruit filling were then evaluated. The proportion of filled fruits arising from different nodes along flowering shoots and for branches at contrasting heights in the crown were quantified. Morphometric attributes were similar for flowering shoots from low and high branches. In N. obliqua, fruit filling was greater in low than high branches, and did not vary with flower position along the flowering shoot. In N. nervosa, fruit filling was unaffected by branch height and tended to decrease as cupule position approached the distal end of the flowering shoot. Pollen limitation could be the main factor affecting fruit filling in N. obliqua, since the lack of relationship between shoot morphological traits and fruit filling does not suggest resource limitations. In N. nervosa, the decline of fruit filling from proximal to distal cupules within the flowering shoots could reflect resource pre-emption by proximal cupules and/or better conditions for pollination in this position. The greater distance between staminate and pistillate flowers along flowering shoots in N. nervosa than in N. obliqua is probably a major factor in explaining the between-species differences found in this study.