In forest ecosystems, the variations of competitive and colonization ability among different species is assumed to be important for the species coexistence. Under global climatic change, the forest succession dynamic and the landscape composition may change through two pathways: 1) the climatic change affect species’ colonization ability and consequently the colonization process (including seed production, seed dispersal and seedling establishment) in the forest succession; and 2) the climatic change affect species’ competitive ability and consequently the competition process for light and nutrient after seedling establishment. In this study, we coupled a forest ecosystem process model (PnET-II) and a spatially dynamic forest landscape model (LANDIS-II) to study the relative importance of the two pathways in a transitional forest landscape (the Boundary Water Canoe Area) response (measured by the forest type composition) to climatic change for the period 2000-2400 AD. In this simulated system, the climatic change effect on the colonization is mainly through the modification of species establishment probability, while the effect on competition is mainly through the modification of growth rate determined by the aboveground net primary production. The two-way analysis of variance (ANOVA) was used to quantify the importance of the two pathways (through competition or colonization), and their interaction in the forest-type-composition response to climatic change. For the two-way ANOVA, we used an ensemble of 27 climate predictions profiles (the monthly values of temperature, precipitation, CO2, and photosynthetically active radiation during the period 2000-2099 AD) to define the possible levels of competitive and colonization ability under future climatic change.

Results/Conclusions
Our results show that for the short-term (<100 years) forest forest-type-composition response to climatic change, the pathway through competition is relatively more important. For the long-term (>100 years) forest forest-type-composition response to climatic change, the pathway through colonization becomes to be relatively more important. The results also show that the interaction between the two pathways has substantial contribution (around 10% of contribution after 2300 AD) in the long-term forest-type-composition response to climatic change.