Results of the study are organised in three sections: first an analysis to interpolate to higher resolution the eleven climate change variants; second a description of the linkages from the six UK NEA scenarios through four forest management trajectories to each of five FMAs; third a presentation of changes in ecosystem service indicators for each of the forest management trajectories in turn. Additional information describing detail of these results is available in Appendix C (supplementary material).
Interpolating climate data
The predictive power of the regression technique for interpolating climate data to a higher resolution was very good for accumulated temperature (r2 values ~0.9) and good for moisture deficit (r
2 values ~0.6). Residuals were analysed and mapped to understand spatial variation and check for bias and error. For each variable and each of the 11-RCM data sets, the linear regression was used to create 250-m gridded data for the UK by applying the equation to 250-m data sets of mean altitude and centroid latitude and longitude (see Appendix C, section C1, supplementary material).
The 250-m resolution 11-RCM accumulated temperature and moisture deficit data were used in the ESC model to assess the suitability of tree species through time and through a changing climate. To achieve this, the ESC accumulated temperature and moisture deficit values for each climate projection variant were calibrated by adjusting values so that the means for the 30-year climatic baseline period (1961–1990) matched mean values from climatic baseline observations. Calibration was performed using a linear regression analysis between 1000 randomly sampled paired points from both 250-m UK grids, with a very high r
2 value (~0.975). Residuals were mapped to look for bias and error. This produced a final set of 216 climate data sets representing the baseline (1961–1990) and each mean 30-year decadal period from the 2010s to 2080s for each 11-RCM variant for both the accumulated temperature and moisture deficit variables, including a no-change scenario.
Simulation of the management trajectories for Clocaenog and Gwydyr forests
The models in the simulation report results on a decadal basis, but the simulation is executed each year, aggregated for each decade, and presented on a per hectare basis to allow comparison between the two study sites, using units shown in Table 2A (supplementary material, Appendix A). In studying indicator results and in comparing the two study forests, it is important to keep in mind the aggregate age structure of sub-compartments, as well as FMA proportions, which differ for each forest (Tables 4A; Figure 2A—supplementary material, Appendix A). The forest age structure has a direct effect on the temporal distribution of goods and services, and this causes indicators: production, biomass, small wood, and carbon stocks, to show a steep dip in out-turn between 2040s and 2060s before recovering towards the end of the century, as does the wind damage risk score and operations to a lesser extent. The full results of the simulation (supplementary material, Appendix C, Section C2) show indicator values for short-rotation forestry, species diversity, and low-impact silviculture systems relative to the business as usual trajectory (with business as usual in absolute values). The relative values remove the effect of the sub-compartment age structure, and this helps show changes in the goods and services indicators.
Tree species suitability indicator
The current forests of Clocaenog and Gwydyr contain Sitka spruce (Picea sitchensis) as the dominant species, which grows well in mild and moist climatic conditions. Given the warmer climate simulated by all the variants with a much drier climate simulated for five variants, there was a decline in the forest suitability index with no-change in tree species. For some variants (e.g. 3Q3), the impact was predicted severe for all species with a low-tolerance to high-moisture deficits. Many conifer species exhibit stem cracks when the moisture deficits reach levels predicted in the variants studied. Using ESC, it was possible to test for a suitable replacement species for each variant in the simulation, when a site was restocked using the species diversity trajectory. Table 4A (online supplementary material—Appendix A) shows species changes for each of the climate variants. Spruce proportions decreased, while pines, broadleaves, and other conifer species not yet common in UK forestry (e.g. coast redwood) increased. For Clocaenog the range of site types was not as great as at Gwydyr, so the availability of sites suited to fir and broadleaves was reduced, compared to Gwydyr, where more sites were transformed to pines.
Welsh forest policy favours increasing species diversity, and particularly the choice of species that are well suited to site conditions (soils and climate). The species diversity trajectory allowed ESC to choose the most suitable species from a candidate list, to replace a sub-compartment at felling. The impact of this trajectory compared to business as usual on the mean area weighted suitability scores was to maintain the forest area with species suited to site conditions for the whole-rotation length. Take for example the climate variant 3Q3 (Fig. 2) which showed the greatest decline in suitability in business as usual. By 2080, 3Q3 would render much of the forest under business as usual as unsuitable for forest production, as the suitability score approached 0.2. The species diversity trajectory was able to replace the business as usual species with a more drought-tolerant alternative, and for 3Q3 at Gwydyr under species diversity, this resulted in an increase in suitability of 0.3 over business as usual bringing the species suitability for the forest as a whole back up to 0.5 (suitable for timber production), and 0.35 at Clocaenog (marginal for timber production).
Diversification of species across all climate variants indicated an increased opportunity for pines (Scots pine—Pinus sylvestris, Macedonian pine—Pinus peuce), firs (noble fir—Abies procera; grand fir—Abies grandis; Douglas fir—Pseudotsuga menziesii), and other conifers (usually coast redwood—Sequoia sempervirens and western red cedar—Thuja plicata). The algorithm selected species according to ecological suitability and volume production but did not consider the mechanical properties of the timber.
Carbon stocks, timber production, and biomass indicators
The Clocaenog carbon indicator increase was less pronounced as only 6 % of FMAs were transformed to a low-impact silviculture system. The extent of the transformation at Gwydyr was also seen in the steep rise of wind damage risk scores between 2040s and 2070s, as stands with a DAMS of less than or equal to 14 were heavily thinned to promote regeneration and were therefore exposed suddenly to potential wind damage for several years before canopy gaps closed.
The business as usual trajectory shows the effect of the existing age structure of each forest, which produced a peak in timber production in the 2050s at Clocaenog and double peaks from Gwydyr in the 2040s and 2060s.
Biomass peaked in the decade prior to the increase in felling—2030s at Clocaenog and 2020s at Gwydyr. The business as usual trajectory at Clocaenog, and to a lesser extent at Gwydyr, showed that the early peaks were higher for timber production, biomass production, and carbon stocks (prior to felling) for all climate projection variants compared to the baseline simulation. Conversely, small-wood production (<0.17-m-diameter class) projections reduced for all climate variants compared to the baseline climate simulation. However, later in the century, for each of timber production, biomass production, and carbon, the projected range of values from different climate variants increased, and indeed some remained above the baseline climate projection (3Q0, 3Q6, 3Q11, and 3Q13), while others (3Q8,3Q14, 3QK, 3Q16, and 3Q3) indicated lower forest production than was simulated by the models for no-climate change. The timber production, biomass, and carbon projections for the species diversity, and low-impact silviculture systems trajectories, maintained similar or slightly lower levels to the business as usual. The main effect at Clocaenog was that by the end of the century (Fig. 3), compared to business as usual, species diversity reduced the uncertainty of biomass production but at a maximum trade-off loss of 70 t ha−1 to reduce an uncertain biomass production range of 150 t ha−1 from the forest under a business as usual trajectory. At Gwydyr there was no trade-off in biomass under a species diversity trajectory and the range of uncertainty in biomass production was also removed. For each of these indicators, the short-rotation forestry trajectory showed declining levels from mid-century compared to business as usual. This was because short-rotation forestry rotations led to the felling of trees at age 25, whereas the peak in maximum mean annual increment often occurs beyond the age of 50 (i.e. two 25-year rotations produce less extractable material than one 50-year rotation). However, short-rotation forestry would afford many opportunities to adapt species to a changing climate. The low-impact silviculture systems trajectory for Gwydyr showed a gradual increase in biomass carbon stocks, but a reduction in timber production, in response to a 13 % increase in sub-compartments under transformation and remaining un-felled as the FMA4 (intensive even age) sub-compartments were re-allocated to FMA2 (close to nature) and thinned to half the number of stems at age 50. At Clocaenog, the relatively small area available for transformation to low-impact silviculture systems exerted a smaller increase in carbon stocks and biomass with a smaller reduction in timber production through the century.
Wind risk indicator
By the 2040s, wind damage risk score increased by a small amount for the business as usual trajectory in both forests and then declined as more mature sub-compartments were replaced. Compared to business as usual, the species diversity trajectory showed the wind damage risk score to increase very marginally from mid-century as the alternative drought-tolerant species selected in place of Sitka spruce would grow faster in the warmer climate and would therefore become more susceptible to wind damage compared to slower growth in the business as usual trajectory. Similarly, wind damage risk score for the low-impact silviculture systems trajectory increased and peaked in mid-century at Gwydyr with 13 % of the forest transformed to low-impact silviculture systems, whereas at Clocaenog only 6 % could be transformed to low-impact silviculture systems, and this would not have a big effect on the wind damage risk score for the whole forest.
Biodiversity and recreation indicators
Biodiversity and recreation indicators have some similarities as they are based largely on stand age and FMA. For both forests, the biodiversity and recreation indicators remained more or less constant through the century for the business as usual trajectory. Since stand age is a factor of both, the short-rotation forestry trajectory showed a gradual decline through the century as the proportion of stands under the 25-year rotation length increased. The species diversity trajectory had little effect on biodiversity or recreation as this trajectory is concerned only with productive species selection, and not FMA. However, there was a gradual increase in the biodiversity indicator in the low-impact silviculture systems trajectory at Gwydyr, as up to 13 % of the forest was under transformation to a shelterwood system.
Operations (employment) indicator
The operations indicator provides an index of employment, through the number of interventions required per decade. At Clocaenog this remained constant for all trajectories throughout the simulation at a level just below one per decade. Since the DAMS score was higher at Clocaenog and thinning less widely practised, transformation from ‘intensive even age’ (with mainly no-thinning) to FMA5 with no-thinning had little impact on operations frequency. At Gwydyr the operations indicator for the business as usual trajectory maintained a value just below 2 per decade, and this declined to slightly under 1 per decade by the 2080s for the short-rotation forestry trajectory, as thinning stopped in sub-compartments transformed from FMA4 (thinning every 5 years) to FMA5 (with no-thinning).