Assessing costs of reproduction in mountain birch: the importance of considering the modular level

We studied the effects of production of male catkins on growth and the subsequent year's male catkin production in mountain birch, Betula pubescens subsp. czerepanovii at both the shoot and branch level. (Shoot is defined here as the product of a single growing season while branch refers to a structure consisting of several shoots.) Like heterophyllous trees in general, mountain birch canopy expansion takes place via the production of long shoots. We found phenotypic trade-offs between long shoot growth and male reproduction at the shoot level in the year of catkin production. Generative parental long shoots (long shoots with male catkins) were significantly shorter than the vegetative ones (long shoots without a male catkin). In contrast, we found no effects of male reproduction on the subsequent year's male catkin production at the shoot level. Although the mean length of secondary long shoots (long shoots growing from the lateral buds of parental long shoots) did not differ between vegetative and generative parental long shoots, there was considerable between-individual variation in the response of individual trees. In addition, production of male catkins diminishes canopy expansion in mountain birch because the number of secondary long shoots produced by generative parental long shoots was smaller than that of vegetative parental long shoots. At the branch level, the association between total long shoot growth and male catkin production was positive, i.e. no trade-off was found. This may be because the strong sink strength (the ability of a branch to import assimilates from elsewhere in the tree) of branches bearing reproductive long shoots masks possible trade-offs. We emphasize the importance of considering several levels of the modular hierarchy when analysing costs of reproduction in modular organisms.