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 subs equent year's male catkin production in mountain birch, Betula pubescens su bsp. czerepanovii at both the shoot and branch level. (Shoot is defined her e as the product of a single growing season while branch refers to a struct ure consisting of several shoots.) Like heterophyllous trees in general, mo untain birch canopy expansion takes place via the production of long shoots . We found phenotypic trade-offs between long shoot growth and male reprodu ction at the shoot level in the year of catkin production. Generative paren tal 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 catk in production at the shoot level. Although the mean length of secondary lon g shoots (long shoots growing from the lateral buds of parental long shoots ) did not differ between vegetative and generative parental long shoots, th ere was considerable between-individual variation in the response of indivi dual trees. In addition, production of male catkins diminishes canopy expan sion in mountain birch because the number of secondary long shoots produced by generative parental long shoots was smaller than that of vegetative par ental 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 bran ch to import assimilates from elsewhere in the tree) of branches bearing re productive long shoots masks possible trade-offs. We emphasize the importan ce of considering several levels of the modular hierarchy when analysing co sts of reproduction in modular organisms. (C) 2000 Annals of Botany Company .