Water and nutrient outflow following the ecological restoration of a ponderosa pine-bunchgrass ecosystem

In the late 1800s, fire suppression, livestock grazing, and a wet and warm climate led to an irruption of pine regeneration in Pinus ponderosa Laws. ( ponderosa pine) forests of the southwestern United States. Pines invaded bu nchgrass openings, causing stand structure changes that increased the numbe r of stand-replacing fires. Ecological restoration, via thinning and prescr ibed burning, is being used to decrease the risk of stand-replacing fires a nd ameliorate other effects of pine invasion. The effects of aboveground re storation on belowground processes are poorly understood. We used a hydrolo gic model and soil water nutrient concentrations, measured monthly below th e rooting zone, to estimate restoration effects on nutrient losses by leach ing from a mature ponderosa pine forest near Flagstaff, Arizona. Replicated restoration treatments included thinning to pre-1880 stand densities (part ial restoration), thinning plus forest floor fuel reduction followed by a p rescribed burn (complete restoration), and an untreated control. Water outf low occurred only between January and May and was lowest from the control ( 47 and 28 mm in 1995 and 1996) and highest from the partial restoration tre atment (67 and 59 mm in 1995 and 1996). The concentrations (typically <0.10 mg/L) and estimated annual losses (<0.02 kg/ha) of NH4+-N, PO43--P, and or ganic P were similar among treatments. Nitrate and organic N concentrations were as high as 0.80 mg N/L; however, these concentrations and estimated a nnual losses (<0.13 kg N/ha) were similar among treatments. Our results sug gest that restoration will not enhance nutrient loss by leaching or alter s tream chemistry in ponderosa pine forests.