Genotypie and environmental variability in the growth and photosynthesis of Sitka spruce (Picea sitchensis [Bong,] Cam)

Abstract

In 1990, clonal seedlings from twelve full-sib families of Sitka spruce (Picea sitchensis (Bong.) Carr.) were planted at four contrasting sites throughout the UK; Newcastleton and Wauchope in the Scottish Borders; Scootmore m Morayshire; and Llandrindod in Wales. There were large differences in nutrient concentrations between sites, with Newcastleton generally having the highest nitrogen- and phosphorus- based nutrient concentrations and Wauchope the lowest. Both sites had a similar soil structure, with the soil at Scootmore having a lower ability to capture moisture. Scootmore experienced the highest temperatures during the growing season and Wauchope the lowest. There was little difference in precipitation levels between sites but there were large differences between years. After 10 years of growth in the field, height, diameter and wood density (using the Pilodyn® technique) were measured. The resulting data showed that Newcastleton and Wauchope were similar and had greater tree growth, whereas Scootmore and Llandrindod were also similar but had smaller tree growth but higher wood density. However, a high mortality rate at Llandrindod excluded this site from further analysis. The 10-year data were used to classify the clones into clusters, and a clone representative of each cluster was chosen for further analysis. The selected clones showed the same growth patterns between sites; C20177 had largest growth rate; C20211 had smallest growth rate but higher wood density; C20208 was intermediate in terms of growth and wood density. However, large variation was apparent at each site, a result of environmental impacts on the growth rates. Chlorophyll, total nitrogen (N) and phosphorus (P) were extracted from differently aged needles, at different heights in the canopy for each clone at each site. N and р did not vary between clones or heights in the canopy, although did increase with increasing needle age. The distribution of chlorophyll followed light intensity patterns (increasing in shaded older and lower canopy needles) and was significantly higher in C20177. All foliar constituents were higher in Newcastleton trees and lowest in Scootmore. Foliar constituents reflected the nutrient concentration in the soils, although Scootmore had lower foliar concentrations than expected, which may be a result of the reduced moisture availability at this site. The lower uptake of nutrients at Scootmore explains the lower growth rate at Scootmore. Photosynthetic light response curves showed highest photosynthetic and respiration rates at Newcastleton and showed lowest rates at Wauchope. There was little difference in rates between clones or position in the canopy. Various parameters of light efficiency showed little difference between clones, position in the canopy or site. Differences in total non-structural carbohydrates were evident between sites (Wauchope had the highest concentrations) and between clones (C20177 had the lowest). Lower carbohydrate levels in C20177 reflected the greater growth rates by this clone. Destructive biomass sampling of above-ground organs reflected the results of the earlier height, diameter and wood density measurements, indicating that the clones had not changed growth patterns between the years 2000 and 2004. Between sites, only C20177 showed a change in carbon (C) allocation, with a switch from allocating the majority of C to the trunk at favourable sites (Newcastleton) to allocating the majority of с to branches under nutrient or water stress (Wauchope and Scootmore respectively), с allocation below- ground showed no clear pattern between clones, although fine root density was lowest for C20177, suggesting this clone was less effected by nutrient or water stress. Differences in С allocation below-ground between Wauchope and the other two sites also suggested that nutrient concentration had a greater effect on coarse roots. Parameters were calculated from the observed data and used to simulate photosynthetic rates of Sitka spruce in a process-based model of tree evapotranspiration. A comparison between observed and simulated data showed that the model predicted seasonal, site and clonal differences but the absolute values were overestimated at Newcastleton and underestimated at Wauchope and Scootmore. A sensitivity analysis showed that six parameters largely affected the output of the model and, with the majority of these parameters extrapolated from the literature; they would explain the large differences between simulated and observed data.