Decomposition of plant litter is a key process in nutrient and carbon cycling in terrestrial ecosystems. Disentangling the relative effects of different factors, including climate, local environmental factors, and species identities (which determine litter quality), on leaf litter decomposition rate provides insight into forest carbon cycles. In this study, we conducted a three-year decomposition experiment of leaf litter in 26 plots across Northeast China (Changbai, Zhuangguangcai, Xiaoxing'an, and Daxing'an Mountains) to examine the relative effects of climate, forest type, and gap fraction on leaf litter decomposition rate. The leaf litter decomposition rate differed among forest types in the following order: deciduous broadleaf forest > deciduous broadleaf and needle-leaf mixed forest > deciduous needle-leaf forest > evergreen needle-leaf forest > Betula ermanii forest. A multivariate analysis showed that climate factors (energy and water availability) were strong predictors of the leaf decomposition rate, which explained 55.5% and 65.0% of the variation in decomposition coefficient (k) and the time required for decomposition of 95% of the litter (t_95%), respectively. However, the independent effect of climate on decomposition was weak, and most of the variation was attributed to the joint effect of climate variables with forest type and light availability (< 9%). Climate drives changes in forest types (species composition), which in turn affects the leaf decomposition rate. This joint effect explained 46.8% (k) and 56.8% (t_95%) of the total variation in leaf decomposition rate. Moreover, changes in climate and forest types can affect under-canopy light availability (measured using the gap fraction), which declined with increasing temperature and indirectly improved leaf decomposition rate. The joint effects of climate, forest type, and light availability explained 23.9% and 22.3% of the variation in k and t_95%, respectively. Our results indicate that climate affects litter decomposition mainly through its indirect effects on biological factors such as species composition and canopy structure, which influence litter quality and light availability. It is necessary to understand the interactions of climate with these biological factors for predicting changes in litter decomposition in response to future climate change.