The sustainability of timber and biomass harvest in perspective of forest nutrient uptake and nutrient stocks

The sustainability of tree harvest is questioned since harvest results in increased nutrient losses which may reduce nutrient stocks in forest soils, particularly in forests on acidified and poor soils with low base saturation. We used a new forest experiment to quantify nutrient stocks and nutrient uptake rates in mature forest stands, and to assess the forest nutrient balance in relation to different forest management scenarios: clearcutting, shelterwood and thinning; and whole-tree harvest (WTH), stem-only harvest (SOH) and wood only harvest (WOH, with on-site bark stripping). Forests were dominated by trees of Fagus sylvatica, Pseudotsuga Menziesii or Pinus sylvestris, all situated on poor, acidified soils. We measured forest biomass and nutrient stocks based on destructive sampling of fifteen mature trees per species and by using new, calibrated allometric relationships. Aboveground stocks of N, P, S, K, Ca, Mg, Mn, Cu, Fe and Zn were calculated for foliage, branches, stem bark and stem wood. Annual forest growth and nutrient uptake were determined using tree ring measures and allometric relationships. Organic layer nutrient stocks and available nutrients in the mineral soil were determined following intensive soil sampling. Stands of beech, Douglas fir and Scots pine differed in aboveground biomass and nutrient stocks, with highest biomass stocks in Douglas fir. However, beech stands had the highest aboveground nutrient stocks, nutrient uptake rates and nutrient losses following harvest, followed by Scots pine. Organic layer nutrient stocks generally exceeded aboveground nutrient stocks, except for the base cations and Mn. Compared to SOH, WTH increased nutrient export between 66% (Douglas fir) up to 100% (Scots pine), while WOH decreased the nutrient export between 23% (beech) up to 41% (Douglas fir). High aboveground base cation and Mn stocks indicate potential long-term threats to forest nutrition if trees are harvested. However, in Douglas fir stands, nutrient losses through SOH may fully recover when using rotation periods of 80 years. Contrary, negative Ca balances are predicted when applying SOH in beech and Scots pine, since Ca stocks are potentially depleted within 2 final fellings. WTH poses, regardless of the species, potential threats for sustainable biomass harvest as nutrients cannot be recovered using common rotation periods. WOH conserves nutrients within the forest posing opportunities for sustainable biomass harvest. For similar temperate forest on acidified, sandy soils, we therefore recommend to limit tree harvest depending on the tree species, and to avoid WTH and consider WOH to better conserve critical nutrients required for long-term forest recovery.