Implications of albedo changes following afforestation on the benefits of forests as carbon sinks

摘要

Increased carbon storage with afforestation leads to adecrease in atmospheric carbon dioxide concentration and thus decreasesradiative forcing and cools the Earth. However, afforestation also changesthe reflective properties of the surface vegetation from more reflectivepasture to relatively less reflective forest cover. This increase inradiation absorption by the forest constitutes an increase in radiativeforcing, with a warming effect. The net effect of decreased albedo andcarbon storage on radiative forcing depends on the relative magnitude ofthese two opposing processes.We used data from an intensively studied site in New Zealand's Central NorthIsland that has long-term, ground-based measurements of albedo over the fullshort-wave spectrum from a developing Pinus radiata forest. Data from this site weresupplemented with satellite-derived albedo estimates from New Zealandpastures. The albedo of a well-established forest was measured as 13 % andpasture albedo as 20 %. We used these data to calculate the directradiative forcing effect of changing albedo as the forest grew.We calculated the radiative forcing resulting from the removal of carbonfrom the atmosphere as a decrease in radiative forcing of?104 GJ tC?1 yr?1. We also showed that the observed change inalbedo constituted a direct radiative forcing of2759 GJ ha?1 yr?1. Thus, following afforestation,26.5 tC ha?1 needs to be stored in a growing forest to balance theincrease in radiative forcing resulting from the observed albedo change.Measurements of tree biomass and albedo were used to estimate the net changein radiative forcing as the newly planted forest grew. Albedo andcarbon-storage effects were of similar magnitude for the first four to fiveyears after tree planting, but as the stand grew older, the carbon storageeffect increasingly dominated. Averaged over the whole length of therotation, the changes in albedo negated the benefits from increased carbonstorage by 17–24 %.