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We investigated biomass allocation patterns in sporophyte-bearing and non-sporophytic plants of the perennial moss Dicranum polysetum Sw. from southern Sweden by comparing dry masses of various reproductive and vegetative plant modules. Assessing parental vegetative biomass is critical in bryophytes due to continuous decay of basal shoot portions. We propose a method to estimate the photosynthetically active gametophyte biomass at the onset of the current sporophyte cycle retrospectively. With this measure for vegetative plant size, the proportional investment in reproductive structures, reproductive effort, was 16% when sporophytes were produced, and 1.3% when perichaetia remained unfertilized. In sporophytic plants, vegetative apical growth and reproductive output (measured both as dry mass and sporophyte number) were positively correlated, and reproductive effort was inversely related to the mass of the annual segment preceding sporophyte initiation. The development of mature sporophytes was negatively associated with annual shoot segment and innovation size. Moreover, sporophyte development reduced the probability of future perichaetial initiation and the mass of new perichaetia. In sporophytic individuals, investments in innovations and reproductive structures were negatively correlated. Mean mass per sporophyte + perichaetium decreased with increasing number of mature sporophytes per perichaetium. These patterns strongly suggest that D. polysetum experiences a cost of sexual reproduction, that sporophyte maturation is resource limited, and that apical sinks compete.