## 4 Discussion
### Biomass Removal Decision Support Matrix for Nutrient Sustainability  
The results from the nutrient budgets shown in Figures 5-7 are used to generate a simple decision matrix to inform the sustainability of brash removal on five main forest soil types (Table 6). A traffic light approach is adopted for each of three harvesting scenarios, comprising CH, WTHB and WTHG. Green represents a positive nutrient budget and therefore 'go', orange a variable budget with decision subject to site Yield Class, with 'go'if equal or less than the Yield Class threshold and 'no-go' if greater than the Yield Class threshold. The matrix implies that conventional harvesting poses no issues for site nutrient sustainability for first rotation Sitka spruce on all main forest soil types, regardless of Yield Class. However, this relies on the nutrient input from the pre-existing vegetation plus a single rock phosphate fertiliser application on peaty gley and deep peat soils. For the WTHB scenario, issues mainly arise for Ca on podzolic soils (the most nutrient poor), and for K on deep peats, where brown brash can only be harvested for lower yielding stands of YC \(<\) 12 and YC \(<\) 16 respectively (Table 6). This reflects the higher base cation content of needles and branches, with brown brash comprising 15% of total tree Ca, 23% for Mg and 34% for K (Figure 2). There is no limit for WTHB extraction on mineral gleys, brown earths and peaty gleys. As expected, the WTHG option posed the greatest issue, especially on podzols, but also on brown earths and deep peats (Table 6). This practice was not sustainable in terms of Ca balance for the harvesting of green brash from SS crops of YC \(>\) 8, YC \(>\) 20 and YC \(>\) 18 on these three soil types, respectively. WTHG is also not sustainable for Mg on deep peat for crops YC \(>\) 18 and for K for crops YC \(>\) 8. This is not surprising as the total Sitka spruce brash (branches and needles) contains 63%, 70% and 75% of total tree Ca, Mg and K, respectively (Figure 2). The impact of WTHG has been studied at two sites in the UK--one at Kielder in England on peaty gley soils [55], and the other at Beddgelert in Wales on peaty podzols [59]. No significant differences were reported by [55] in soil Ca or Mg between WTHG and CH treatments in the Kielder study, although base cation concentrations were lower under WTHG in the upper mineral soil. This contrasted with the results of [59] at Beddgelert, who found soil Ca to be higher in the Bsg horizon of the WTH plots but lower in the organic horizon compared to CH plots. These mixed results reflect similar studies in temperate and boreal forests [5], with WTHG leading to reduced exchangeable Ca Mg and K in some mineral soils [60; 61; 62] but having no effect in others [63; 64]. A metadata analysis by [12] found WTHG significantly lowered pH and base saturation (BS) in the forest floor, but without apparent changes in CEC. The only significant effect of WTHG in the subsoil were reductions in CEC and BS. WTHG in spruce and pine-dominated stands reduced Ca and K in the forest floor but had no effect on Mg and pH. Such studies are subject to a number of inherent limitations, including the size and spatial variability of the soil base cation pools, which makes detecting a treatment effect very difficult [5]. Our findings and resulting matrix are in line with a similar study in Ireland, which assessed the impact of CH, WTHB and WTHG on base cation balance and soil acidification for Sitka spruce (_Picea sistchenis ([PERSON].) [PERSON]_), Norway spruce (_Picea abies_ ([PERSON]) and