Tamm review: Sampling to estimate the frequency distribution of tree diameters or ages across large forest areas

Abstract

A simulation study examined what sampling methods might be appropriate in a forest inventory to estimate the frequency distribution of tree stem diameters or ages across a forest population that covers a large area. It reviews the efficacy of such inventories done in aspen (Populus tremuloides Michx.) forests in southwestern USA and uses them as a particular example. A range of frequency distributions of stem diameters and tree spatial arrangements across the landscape were simulated as an attempt to cover a wide range of possible forest circumstances. Three sampling methods, fixed-area plot, k-tree and individual-tree sampling all yielded unbiased estimates of the frequency distribution of tree diameters and ages. Point sampling also gave unbiased estimates of the diameter distribution, but with increasingly poorer precision of frequency estimates as tree diameter declined. It yielded biased estimates of the frequency distribution of tree ages because it determines the probability of the inclusion of tree in a sample using the square of its stem diameter, rather than its age; methods exist that might sometimes overcome this limitation, but they require rather complex methodology. The results should apply equally to individual-tree characteristics other than tree age, characteristics such as tree height, severity of disease, suitability for bird roosting or level of seed production.