Due to rising demand for renewable energy coupled with efforts to reduce net C
emissions the harvesting intensity in forests is increasing. This may compromise
forest nutrition and productivity. We set out to measure how harvesting intensity
affects the productivity of Sitka spruce in Wales and to test the suitability of wood ash recycling as a means of maintaining soil nutrient levels and forest productivity without causing undue additional pollution hazards.
We compared the effects after 23 years of whole-tree (WTH) and conventional (CH) harvesting on the growth of second rotation Sitka spruce in Beddgelert forest, North Wales. WTH significantly reduced diameter at breast height by 10.9% (p = 0.017) and there was some evidence that it reduced height (by 8.2%; p = 0.164) and stand basal area (by 15.3%; p = 0.101). These effects can be attributed to the much greater removal of N, P, K and Ca by WTH of the first rotation crop, combined with greater competition with tree natural regeneration and other vegetation in WTH plots during early growth of the second rotation of planted trees.
To test the potential of wood ash recycling to forests as a means of alleviating
nutrient shortages following WTH, wood ash in three forms (loose, pellets and co-composted with green waste) and at three doses (2, 4 and 8 t ha1
) was· applied to two second rotation upland Sitka spruce forests. All the applications complied with regulatory limits, e.g. in the soil concentrations of heavy metals after 12 months, and they reduced soil acidity and increased soil concentrations of base cations. The loose and co-composted wood ash caused the greatest soil pollution hazards. In contrast, wood ash pellets showed good promise as a low impact, slow-release (N-free) fertiliser which can be applied using conventional machinery.
A bioassay experiment with 16 different wood ashes showed that the growth of 1 y-old Sitka spruce seedlings is generally greatly increased by the application of wood ash at a single application rate of 8 t ha-1. Mean height increment and total biomass were significantly increased in both an organic horizon soil (+82%, p = 0.019; +19%, p= 0.005 respectively) and a mineral horizon soil (+314%, p < 0.001; + 26%, p < 0.001) following wood ash application. Seedling growth in the mineral horizon soil was most strongly correlated with the base cation concentrations of the applied wood ashes, and in the organic horizon soil with wood ash boron concentration. Wood ash application caused changes in measures of soil quality which, in turn, were strongly correlated with seedling growth, most notably in the mineral horizon soil. Application
of high-carbon wood ashes with loss on ignition > 2% has minimal benefits for soil quality and tree seedling growth. When wood ash is applied in pellet form, rather than in loose form, short-term growth benefits are smaller, but long-term growth benefits are likely to be greater due to the prolonged nutrient release rates from pellets. The use of pellets also reduces pollution risks and prevents the occurrence of rapid, substantial changes to forest soil and flora which are observed following loose wood ash applications.
In conclusion, whole-tree harvesting in Sitka spruce is likely to reduce second rotation productivity and wood ash recycling is a promising means of addressing such declines.