Body mass of moose calves along an altitudinal gradient

We tested whether altitude increased environmental heterogeneity in moose (Ales alces) populations on the landscape scale. Dressed mass from moose calves collected during 10 consecutive years was related to local climate information from 26 different weather stations (located from 7 to 525 in above sea level). Two hypotheses were contrasted: (1) Are moose offspring from higher altitudes heavier because of increased forage quality with increasing altitude? or (2) Are moose offspring from lower altitudes heavier because the growing season is longer there so that the time moose calves can grow is extended at these lower altitudes? During October (when moose calves stop growing), both sexes showed a significant negative relationship with altitude. The dressed body mass of female calves was more affected by increased altitude (-1.2 kg per 100 in increase in altitude) than males (-0.8 kg per 100 in). The mass of both sexes was positively related to the length of the growing season (i.e., days >6degreesC). Female mass showed significant cohort effects during 4 of the 10 years investigated (range -6.3 to 3.4 kg), whereas males showed cohort effects during 2 years (-3.7 to 3.5 kg). We conclude that the annual variation in the length of the growing season together with year-specific effects may introduce considerable spatial and temporal variation in moose population dynamics mediated via offspring body mass. We suggest that this impact on population dynamics may be most pronounced at higher altitudes. Our analysis further suggests that a permanent factor (altitude) may interact with random variations (weather) to influence the length of the growing season and thus complicate the management of moose populations by creating cohorts composed of females of unequal quality. We suggest that managers be alert for sudden changes in ungulate recruitment due to unusually weak or strong cohorts reaching reproductive age.