## 4 Discussion  
### Base Cation Inputs and Outputs in Forest Ecosystems  
Mineral weathering dominated base cation inputs across most sites, especially those on base-rich geologies such as Alice Holt on Gault Clay, Savernake on Cretaceous chalk, Thetford on sands over chalk, and Coalburn and Tummel on carboniferous sediments. The opposite was the case for sites on acidic geologies and soils, such as Rannoch and Rogate overlying sandstone, where deposition accounted for 48% to 95% of base cation inputs (Table 3b). Distance from the west coast exerted a strong influence on Mg deposition, reflecting marine inputs at sites like Llyn Brianne and Grizedale, but to a much lesser degree than in a similar study in Ireland [46]. Notably, canopy leaching was found to be important for K (55-85% of throughfall deposition), Ca (9-35%) and Mg (4-29%), highlighting the need to correct for internal base cation cycling to avoid overestimating deposition inputs. The importance of accounting for canopy nutrient transformation and processing has been previously noted for nitrogen [47] and other nutrients [48].  
Figure 7: Forest K budget for Sitka spruce first rotation, YC 6-24, on five main forest soil types.  
The UK Level II sites have a wide range of cation deposition which follows the range of sulphur (S) and nitrogen (N) deposition across the sites (Table 1, [28; 49; 50]), with high inputs at historically industrially polluted sites (Sherwood, Grizedale, Lyn Brianne) but also sites close to busy road networks (Alice Holt) or intensive farming (Savernake and Theftord) and lower inputs of cations at more pristine sites such as Rannoch and Tummel. This suggests that further pollution abatements [51; 52; 53] may lead to decreases not only to S and N but also in base cation inputs to forest ecosystems.  
Many factors influence the balance between base cation leaching versus tree uptake, including site quality and tree growth. Wide variation in tree uptake rates is evident amongst the sites, reflecting the different tree species and site types. Tree growth rates ranged between 2.3 to 3.5 t ha\({}^{-1}\) y\({}^{-1}\) for oak, 3.3-5.1 t ha\({}^{-1}\) y\({}^{-1}\) for Scots pine and 6.0-8.1 t ha\({}^{-1}\) y\({}^{-1}\) for Sitka spruce, although this was not directly related to base cation uptake. Oak and spruce generally had similar levels of uptake, while pine tended to be the lowest for all four base cations. This was despite the base cation concentrations measured in the different woody compartments being similar between the three species (Table 2, Supplementary Materials Table S1). The low uptake values for pine reflected the nature of the soil, with all three pine sites involving nutrient poor podzols over sandstone (Table 4; Figure 5). Soil and geology also strongly affected Ca and Mg uptake rates across oak and spruce sites, with the highest values measured on sites overlying base-rich geology (e.g., oak at Alice Holt vs. at Grizedale, and spruce at Coalburn vs. Lyn Brianne).  
Base cation leaching varied greatly between sites, especially for Ca, which can be explained mainly by differences in geology and rainfall (Table 1). Highest Ca leaching was measured for pine at Theftord due to the influence of chalk, while it was lowest for spruce on the acidic soils and geology at Lyn Brianne (Table 3, Figure 4). In contrast, leaching of K was highest at Grizedale, reflecting high rainfall and marine deposition (Table 3b). Future changes in rainfall amount and intensity can be expected to have a significant impact on forest base cation budgets.