Assessment of ecological effects due to forest harvesting: approaches and statistical issues.
Abstract
Timber harvesting exemplifies many aspects of experimental manipulation in applied ecology. Evaluation of the research approaches used to assess ecological effects of harvesting thus has relevance to a diverse range of ecological questions that address large-scale and human impacts on the environment. We measured the frequency of approaches, and also design and statistical issues, in assessments of harvesting effects on three response variables (tree regeneration, vertebrates and water) in two major native forests of south-east Australia. Our evaluation was based on 124 documents that contain unique, field-based research and that represent the complete written record (published and unpublished) of relevant research prior to mid-2002. The dominant approach was contrasts between manipulated stands, nearly always involving mature and old-growth stands as the reference or control. In studies of tree regeneration, focus on wood-production objectives was apparent in the relatively frequent use of stocking standards, and in the use of between- and within-stand contrasts without a control condition. Comparatively greater use of models in vertebrate and water studies suggested more frequent integration of generic principles and, in water studies, reflected strong collective knowledge based on the repeated use of BACI-type designs (i.e. before-after, control-impact) or long-term data sets. Statistical issues such as poor reporting of experimental designs and of sample statistics were common. Explicit hypotheses were rare, and there was no evidence of definition of critical effect sizes, use of power analysis or use of non-classical statistical approaches (e.g. Bayesian inference). Lack of treatment replication was widespread in between-stand contrasts and less frequent in within-stand contrasts. Inappropriate interpretation of results from non-replicated experimental designs remains an ongoing issue. Synthesis and applications. The amount of information obtainable from stand-level manipulations of forest would substantially increase through greater attention to data quality and experimental protocols. Studies require broader experimental contexts, including their potential place in meta-replication frameworks. Increasingly, constraints of large-scale manipulations necessitate the integration of data and the acknowledgement of uncertainty in formalized systems (e.g. Bayesian methods and adaptive management frameworks). Our evaluation suggests the alternative is a perpetuation of short-term, isolated studies that repeat errors and have limited potential to refine knowledge on the ecological effects of timber harvesting and other similarly scaled disturbances.