Forests can play a significant role both in halting biodiversity loss and in mitigating climate change. A variety of payments for ecosystem services (PES) schemes exists to promote biodiversity conservation in forests. These schemes could be used to strengthen the role of forests as carbon sinks as well. This paper analyzes the implications of supplementing a PES scheme that targets boreal forest biodiversity with a carbon index. We use a site selection framework to examine how the proposed scheme impacts the promotion of both targets. We compare a case where the selection is done solely based on biodiversity values to a case where the selection is done based on both biodiversity and carbon benefits. The carbon index is formulated as current carbon storage or as future carbon sink. Correspondingly, biodiversity is maximized based on either current ecological values or potential ones. We compare equal or differing weights for biodiversity and carbon indexes, and examine trade-offs between biodiversity and CO2 in current and future values. Combined index values increase with the carbon index, but there is a trade-off between biodiversity and CO2 values if the conservation budget is not increased when the carbon index is introduced. There is a temporal trade-off in biodiversity and carbon values between selecting sites based on current or future values. Younger stands are preferred at the expense of old-growth stands with the carbon index. Weights can be used to balance the trade-off between biodiversity and carbon benefits. Overall, risks to losing significant ecological value from the conservation network are negligible, but the limited number of sites decreases the generalizability of the results.