In seasonal tropical forests, evergreen–deciduous mixtures are more productive than monocultures because they intercept more light throughout the year, reflecting complementary resource use by functional groups possessing different traits. This suggests that temperate and boreal forests may also exhibit overyielding, due to the difference in phenology between gymnosperms and angiosperms. However, complementarity could also arise from differences in morphology between needle leaves and broad leaves, or facilitation by N-fixing species. Alternatively, mixtures may be more productive simply because interspecific competition is less intense than intraspecific competition. We used forest inventory data to assess the complementarity of the main functional groups in Switzerland. We employed a trait-based analysis of competition to determine whether: 1) trait differences reduce the intensity of competition between complementary functional groups, 2) complementarity is observed along a broad altitudinal temperature gradient. N-fixing species facilitated the growth of non-fixing species, such that 50/50 mixtures were 50% more productive than monocultures, though half the overyielding was due to the alleviation of intraspecific competition. In contrast, we found no evidence of complementarity between evergreen and deciduous species. For example, in stands where larch was mixed with other gymnosperms, there was no reduction in heterospecific competition among evergreen species, even though evergreens cast shade on one another throughout the year. In cold montane forests, broadleaf species reduced the suppression of needleleaf species, and vice versa. Thus, 50/50 mixtures of needleleaf and broadleaf species were 15% more productive than needleleaf monocultures. However, in warm lowland forests, broadleaf species exacerbated the suppression of needleleaf species, completely offsetting the positive effect that needleleaf species had on broadleaf species. In summary, we found no evidence of complementarity between evergreen and deciduous species, but needleleaf–broadleaf mixtures exhibited overyielding in cold montane forests, which is consistent with the stress gradient hypothesis, though the underlying mechanisms remain uncertain.