## 4 Discussion
### Long Term Changes in Base Cation Budgets  
Rising trends in base cation balances over the 10+ years of monitoring were recorded at 7 of the 10 sites for one or more cations (Table 5). Significant rising trends were most consistent for K (four sites), where they were mainly driven by increasing K deposition (r\({}^{2}\) = 0.275, \(p\) < 0.1, Savernake; r\({}^{2}\) = 0.341, \(p\) < 0.05, Tummel; r\({}^{2}\) = 0.473, \(p\) < 0.01, Llyn Brianne). This was thought to be related to repeated insect infestations at the canopy level in the latter part of the record, either by caterpillars on the oak or aphids on the spruce. Insect outbreaks were reported by [48] and would have increased K leaching in canopy throughfall and thus deposition input. A decline in K leaching also contributed to the rising trend and was the dominant driver at Rogate (r\({}^{2}\) = 0.521, \(p\) < 0.05). Drainage volume did not change over the time period and since weathering is held constant, the decline in leaching is likely to be related to increased tree uptake. This could not be confirmed by the data as tree uptake estimates are based on the average tree growth rate at the sites and measured cation concentrations in the different parts of the trees (stems, branches, leaves) at one point in time (see Supplementary Materials Table S1).  
Rising trends for Ca and Mg were strongest for Sherwood and Coalburn, where they were associated with significant declines in leaching outputs over the measurement period. This was thought to be mainly due to declining acid deposition, with the Sherwood site showing a steep recovery from historic air pollution and acidification [28,50,54]. The rising base cation balance at this site is in line with long term predictions by SAFE (1800-2100 year) [54] and MAGIC models (1990-2020 year) [48]. At Coalburn, the decline in leaching outputs may also have been due to increased tree uptake. Tree basal area increased by 35% over the monitoring period due to the strong growth of the Sitka spruce crop (although the method did not allow this to be incorporated into the tree uptake estimate).  
Alice Holt was the only site to show a consistent declining trend in Ca and Mg base cation budget. This was mainly driven by declining deposition inputs (Ca, r\({}^{2}\) = 0.562, \(p\) < 0.01; Mg, r\({}^{2}\) = 0.412, \(p\) < 0.01), probably associated with emission control of acidic pollutants, but unlike at Sherwood and Coalburn did not result in reduced leaching losses. The declining trend is not in line with MAGIC model predictions (1980-2020) [50] and is perhaps associated with changing weather patterns affecting southern England.  
Overall, soil type, geology, and tree species were the predominant factors influencing site base cation balance, while temporal changes were driven by ongoing recovery due to emission control and the effects of pest outbreaks on nutrient cycling within forest canopies. The latter are becoming increasingly common and need to be factored into model predictions. As weathering is one of the balance elements with the highest uncertainties in its estimation, sites on clay soils will have high uncertainties associated with their cation balance compared to sites on sandy podzols.