Use of stable isotopes to study nutrient dynamics in forests
This study examined the contribution that nutrients released from decomposing brash (branches and needles) left on restock sites make to the growth of new trees. Stable isotope techniques were developed to trace the movement of nitrogen, potassium, magnesium and calcium within trees and from brash to new trees. Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings were labelled with 15N, 41K, 26Mg and 44Ca in either a generous or poor supply for one growing season. One group of these seedlings was provided with an unlabelled generous or poor nutrient supply for the following season. By varying the amount of nutrient supplied, the internal cycling of K and Mg, like N, was shown to be independent of current uptake. A second group of seedlings was harvested after labelling and used to stimulate brash in a model system. Nutrients released from the brash were traced into newly planted seedling trees during the next growing season. The contribution from decomposing roots was also estimated using this system. This approach is not financially viable in the forest because of the cation isotope cost. An attempt was made to predict K and Ca uptake using rubidium and strontium as surrogates. However, direct comparison of isotypically measured uptakes, indicated only a weak relationship. Instead, fieldwork concentrated on N dynamics. Brash generated from 5-year-old Sitka spruce after stem injection of 15N was used to replace brash from 50-year-old Sitka spruce on a clearfell site. Implications for management decisions during the restocking of Sitka spruce plantations in Britain are discussed.