## 4 Discussion
### Biomass Removal Decision Support Matrix for Nutrient Sustainability
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**Nutrient Budgets**} & & \\  & & **Ca** & & & **Mg** & & **K** & \\ \hline
**Harvesting regime** & **CH** & WTHB & WTHG & CH & WTHB & WTHG & CH & WTHB & WTHG \\ Soil type & & & & & & & & & \\ Gleys (surface and ground water) & & & & & & & & \\ Brown Earths & & & & & & & & \\ Podzols/Inonpans & & & & & & & & \\ Peaty gleys on Ca-rich geology & & & & & & & & \\ Peaty gleys on acid geology & & & & & & & & \\ Deep peats & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Biomass removal decision matrix. The numbers represent forest Yield Class. 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’ and orange a variable budget with the 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. Lodgepole pine (_Pinus contorta (Dougl. var. latifolia_)) forests across 40 sites [65]. Under CH and WTHB, inputs were predicted to be sufficient to sustain outputs for Ca, Mg and K, while under WTHG, Ca output was greater than input at 19 of the 40 study sites. However, the difference between inputs and outputs was small relative to the size of soil Ca pools, which could support WTHG removal for a median of 220 years [65]. This contrasts with the much lower exchangeable pools in the podzolic sandy soils of some of the sites such as Rogate, Sherwood and Rannoch in our study (Table 2) and a significant positive relationship between base cation (Ca + Mg + K) balance and soil CEC, with clustering of sites with very low CEC having the lowest base cation balance (Figure 4). The base cation balance dependence on site productivity and tree uptake was also clear (Figures 5-7). Our results support the restriction of brash removal from better growing stands over that of poor stands on vulnerable soils, which is simply a reflection of the greater base cation content and removal in higher yielding, harvested crops. In reality, nutrient poor soils would not support a high YC and therefore naturally constrain the scope for negative base cation budgets. However, rising CO2 and climate change have the potential to increase tree growth and exacerbate nutrient issues. The brash harvesting matrix is based on the best dataset available for evaluating base cation budgets for UK forests, drawing on the Intensive Forest Monitoring Level II Network and biomass partition modelling for Sitka spruce. However, there are a few important caveats of note:  
1. The base cation budgets are limited to first rotation Sitka spruce and will change for subsequent rotations, most notably in terms of the loss of the base cation input from the pre-forest vegetation. This will make the budgets less positive or more negative, although the deficit could be at least partly offset by greater soil weathering and/or organic matter mineralisation due to restock cultivation, provided these rates are sustainable. 2. The calculated base cation budgets are imprecise and subject to significant uncertainty, especially when applied to individual sites. Consequently, the decision matrix should be used with care and it is recommended that its application is monitored and reviewed to inform future assessments and refinement of the practice. One issue is the estimation of soil mineral weathering, which tends to dominate base cation inputs. Although several methods were used to estimate this key component, they all have high uncertainties [35]. Others found variability in weathering estimates to be too high to set thresholds for harvesting practice since the increase in base cation export due to residue harvesting was considerably smaller than the uncertainties associated with mineral weathering estimates [66].