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Effects of afforestation on water yield: a global synthesis with implications for policy

Farley, KA, EG Jobbágy, RB Jackson
Global Change Biology
Journal Volume/Pages: 
11: 1565-1576

Carbon sequestration programs, including afforestation and reforestation, are gaining attention globally and will alter many ecosystem processes, including water yield. Some previous analyses have addressed deforestation and water yield, while the effects of afforestation on water yield have been considered for some regions. However, to our knowledge no systematic global analysis of the effects of afforestation on water yield has been undertaken. To assess and predict these effects globally, we analyzed 26 catchment data sets with 504 observations, including annual runoff and low flow. We examined changes in the context of several variables, including original vegetation type, plantation species, plantation age, and mean annual precipitation (MAP). All of these variables should be useful for understanding and modeling the effects of afforestation on water yield. We found that annual runoff was reduced on average by 44% ( ± 3%) and 31% ( ± 2%) when grasslands and shrublands were afforested, respectively. Eucalypts had a larger impact than other tree species in afforested grasslands (P=0.002), reducing runoff (90) by 75% ( ± 10%), compared with a 40% ( ± 3%) average decrease with pines. Runoff losses increased significantly with plantation age for at least 20 years after planting, whether expressed as absolute changes (mm) or as a proportion of predicted runoff (%) (P

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