Base cation budgets under residue removal in temperate maritime plantation forests

Abstract

The removal of harvest residues—branches, treetops and needles—has the potential to impact the long-term sustainability of forest soils. In this study we used mass balance calculations to predict the impact of harvesting on ecosystem balances of calcium (Ca), magnesium (Mg), potassium (K) as well as on soil acidification (ANCle) for forests in Ireland. Balances were determined for three harvesting scenarios: stem-only harvest (SOH), stem plus branch harvest (SBH) and whole-tree harvest (WTH). The study was carried out at forty sites—selected on a grid across the country. The sites were plantations of Sitka spruce (Picea sitchensis (Bong.) Carr), Norway spruce (Picea abies (L.) Karst.) and provenances of lodgepole pine (Pinus contorta (Dougl. var. latifolia)). Flux calculations were based on site measurements of standing biomass and soil properties as well as regional maps of atmospheric deposition. Under SOH and SBH, inputs were predicted to be sufficient to meet outputs for Ca, Mg and K budgets. Atmospheric deposition was the most important input to Ca and Mg balances. For K, inputs from mineral weathering were as important as deposition. Under WTH, Ca output was greater than input at 19 of the 40 study sites. However, the difference was small relative to the size of soil Ca pools; at these sites, exchangeable pools could support WTH removal for a median of 220years. Magnesium and K removal under WTH was supported by inputs in deposition and weathering. For soil acidification budgets (ANCle), base cation (BC) removal in harvesting under all scenarios was much greater than that BC generated by weathering, suggesting that soils will become acidified over the long term. There was considerable uncertainty around the calculation of fluxes. For Ca balances, confidence intervals spanned positive and negative values at many sites such that it was not possible to predict the balance of Ca budgets. In addition, uncertainty in flux calculations was particularly important for K budget because soil exchangeable pools were small and could be depleted within one to two rotations.