Natural Range of Variability

The overall goal of the Natural Range of Variability (NROV) approach is to match human disturbance regimes (i.e. logging) with those of natural processes.

The ‘Natural Range of Variability’ approach refers to past variation in undisturbed* ecological conditions.  In general, those using the NROV concept ask:

1.  What were ecological conditions like before?
2.  How different are ecological conditions now?
3.  What can we do to make ecological conditions of now more similar to what they were before?

*For North America, ‘undisturbed’ usually refers to conditions prior to European settlement.

A fundamental assumption of the NROV concept is that because some organisms are adapted to the range of variability in certain ecological conditions, approximating this range through management actions will result in their persistence and the persistence of the natural range of biodiversity across harvested landscapes.

The NROV can be measured for a wide range of variables such as stream temperatures, or the amount of wetlands, old growth or coarse woody debris across a landscape.  The NROV in characteristics of natural disturbances, such as the frequency or size of fires, is called the ‘natural disturbance regime.1  The use of the NROV concept to manage forests by approximating the effects of natural disturbances is referred to as the ‘natural disturbance model’ approach.2

The NROV concept has been applied in two ways:

1.  To guide management decisions.  For example, attempts to bound the effects of logging within the NROV can give a broad estimate of how much forest to cut, how often to cut, how to cut (the pattern across the landscape), and what to leave behind (e.g. patches of live trees).2

2.  To make predictions about the effects of management practices that can be used to prioritize management actions.  The conditions defined by the NROV can act like benchmarks against which the risks involved in current or proposed future management actions can be assessed.

The NROV should not be used to define an optimal state of the forest to achieve some theoretical maximum in biological integrity or diversity.  Instead, techniques such as snag retention and ‘leave’ areas (uncut patches), which mimic unburned patches, result in more heterogeneous vegetative patterns more similar to conditions to which native species have adapted.  Management actions also need to consider socioeconomic considerations.  For example, emulating the average fire size in the boreal forest of northern British Columbia would result in much larger clearcuts than are socially acceptable.  More importantly, past ecological conditions are highly variable.3  Thus, there is no one value that best captures the range of natural ecological conditions.4-5

Natural disturbances can be used to guide decisions regarding:

1.  Harvest unit size;
2.  Harvest unit pattern;
3.  Rate of harvest (rotation time);
4.  Biological legacies – amount, type, pattern;
5.  Rate of re-planting;
6.  Tree species composition for re-planting.

Forestry practices that emulate natural disturbances are more likely to maintain the full range of species native to a particular region.  However, though a more beneficial approach to logging, no forestry operations will have the same impact on biodiversity as natural disturbances.  Logging removes trees – up to 95% of wood volume – while natural disturbances like fires kill trees but usually result in a loss of only about 10% of the wood volume.6  Logging will likely result in habitat loss for some species, regardless of attempts to emulate natural disturbances with techniques like retention and post-harvest burning.7  A second major challenge is the quantification of a baseline regime to emulate.  A ‘natural’ baseline may be difficult to describe in areas previously modified by first nations people.8  Further, natural disturbances are highly variable through time, and regimes will likely be altered by future climate change.9  Variability is also higher at smaller spatial scales.  For example, the proportion of old growth Douglas-fir forest in the Pacific Northwest varies from 0 to 100% at a scale of 100 to 1 000 hectares, but 30-75% in the entire area.6  Thus, the description of a regime may too narrowly define a range of conditions, such as patch size, resulting in too little heterogeneity across landscapes.

The largest barrier to the application of the NROV concept to forest management in British Columbia is a lack of quantitative data.  Fire is the best described of all disturbances, since it is generally more difficult to gather data on disturbances from wind, insects or tree disease.  However, only fire frequency is adequately described – there are few data on other attributes such as fire size and severity.  Quantitative data that can be used to describe the NROV in ecological conditions are sufficient for on average only 31% of BEC subzones.10

1Agee, J.K. 2003. Historical range of variability in eastern Cascades forests, Washington, USA. Landscape Ecology 18:725-740.
2Hunter, M.L. 1993. Natural fire regimes as spatial models for managing boreal forests. Biological Conservation 65:115-120.
3Armstrong, G.W. 1999. A stochastic characterization of the natural disturbance regime of the boreal mixed wood forest with implications for sustainable forest management. Canadian Journal of Forest Research 29:424-433.
4Landres, P.B., P. Morgan and F.J. Swanson. 1999. Overview of the use of natural variability concepts in managing ecological systems. Ecological Applications 9:1189-1206.
5Kuuluvainen, T. 2002. Disturbance dynamics in boreal forests: Defining the ecological basis of restoration and management of biodiversity. Silva Fennica 36:5-11.
6Lindenmayer, F.B. and J.F. Franklin. 2002. Conserving forest biodiversity: a comprehensive multiscale approach. Island Press, Washington, DC.
7Stuart-Smith, K. 2002. Songbird communities is a pyrogenic habitat mosaic. International Journal of Wildland Fire 11: 75-84.
8Hunter, M.L. 1996. Benchmarking for managing natural ecosystems: are human activities natural? Conservation Biology 10: 695-697.
9Bergeron, Y., P.J.H. Richard, C. Carcaillet, S. Gauthier, M. Flannigan, and Y.T. Prairie. 1998. Variability in fire frequency and forest composition in Canada’s south-eastern boreal forest: a challenge for sustainable forest management. Conservation Ecology.  
10Wong, C., H. Sandmann and B. Dorner. 2002. Estimating historical variability of natural disturbances in British Columbia. Report to the Ministry of Forests.