[Biodiversity Guidebook Table of Contents]
Estimation of natural seral stage distributions
With an estimate of the long-term average return interval of stand-initiating disturbance, the expected age-class distribution can be readily calculated. An assumption of the method is that the probability of disturbance is mostly independent of forest age. We applied disturbance return interval estimates for biogeoclimatic zones originally developed by the Protection Branch, Ministry of Forests, 1992 and modified by expert opinion of the regional Ministry of Forests research ecologists. These estimates are based on large areas over long periods of time, because disturbance patterns are highly variable.
The example in Table A4.1 is the cumulative distribution (% of landscape above or below the indicated age) for an average disturbance return interval of 200 years. Table A4.2 provides the summaries for age categories and return intervals used in calculating the percentage.
The method used for establishing seral stage definitions and distributions is as follows:
- Define three seral stage categories of basic importance to biodiversity: early/young, mature, and old. Early/young forests are defined as generally being less than 40 years old (except 20 years for deciduous stands). Mature forests are defined as 80 years or older for productive coastal forests, and 100–120 years or older for the less productive high elevation forests. The development of mature forest characteristics is most rapid in low elevation coastal forests and slowest in high elevation interior forests. Old forests are defined as 140 years or older for zones with more frequent disturbance, and 250 years or older for less frequently disturbed zones. The age categories for mature and old are based on the estimated minimum age for developing structural attributes in even-aged management. These attributes may be achieved at earlier ages through structural retention or partial cutting strategies where appropriate.
- Determine the approximate natural seral stage distribution based on estimates of the long-term average interval between stand-destroying events (Appendix 3).
- Calculate a seral stage distribution with up to twice the estimated natural proportion of the early/young seral stage and at least half the natural proportion of mature and old seral stages. For the old forest seral stage, the objectives are adjusted for the percentage of old forest in the biogeoclimatic zone (by Forest Region) already in protected areas (assumed to be 12% for these guidebook calculations, however it could be adjusted for actual percentage). For example: If the natural level of old growth is 38%, then the recommended minimum in each landscape unit is calculated as follows:
38% minus 12% in protected areas = 26%
then, 50% of 26% = 13% old seral stage.
Table A4.1. Cumulative age distribution using negative exponential equation with return interval of 200 years. The percentage greater than age t = exp(-[t/b]), where b is the average return interval.
Table A4.2. Landscape percentage based on disturbance return interval
The “natural” proportions in Table A4.2, when applied to subzones, was compared to the best available knowledge of natural age distributions by subzone to confirm this as a reasonable, objective (albeit simplified) ecologically based approach. Measured reconstructions of the natural proportions averaged across all landscapes within a zone or subzone would be the ideal starting point.
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