Restoration of a ponderosa pine forest increases soil CO₂ efflux more than either water or nitrogen additions

1. Ecological restoration often involves returning ecosystem structure to some predisturbance reference state, but ecosystem function must also recover if restoration efforts are to be self-sustaining over the long term. In the south-western United States, ponderosa pine forest structure was altered by disruption of the fire regime following Euro-American settlement. Forest structure is now being restored to presettlement conditions through the application of thinning and burning treatments. However, the effects of these treatments on below-ground ecosystem processes remain unclear. 2. We conducted a water and nitrogen (N) addition experiment in adjacent restored and unrestored ponderosa pine stands and compared soil CO₂ efflux in response to these treatments over a 13-month period. Our goals were to (i) quantify water and N limitation to below-ground carbon (C) cycling in contemporary high-density ponderosa pine forests; and (ii) determine if restoration alleviates water and N limitations. 3. Restoration thinning and burning increased soil CO₂ efflux, along with surface soil water content, temperature and herbaceous fine root biomass, while total fine root biomass decreased as a result of restoration. 4. Water and N additions increased C flux from soils to a similar degree in both restored and unrestored ponderosa pine stands, but the increase was relatively small when compared to that stimulated by restoration. 5. Synthesis and applications. An understanding of how ecosystem processes respond to treatments designed to restore ecosystem structure is critical in ensuring the long-term success of restoration efforts. Here we show that, although water and N stimulate C flux from soils in these semi-arid forests, restoration treatments have a much greater effect on soil C balance than increased water and N availability by themselves. This suggests that increased quality of C inputs from a recovering understorey herbaceous community is a key component of restoring ecosystem function (e.g. below-ground C cycling) in south-western ponderosa pine forests.