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Forest Mensuration II. Lectures 10 Site Productivity Avery and Burkhart, Chapter 15. Climate and Soil. Definition of Site Quality. Site --The area in which a tree or a stand grows Site Quality -- Sum of all environmental factors affecting the biotic community of an ecosystem.
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Forest Mensuration II Lectures 10 Site Productivity Avery and Burkhart, Chapter 15
Climate and Soil Definition of Site Quality • Site --The area in which a tree or a stand grows • Site Quality -- Sum of all environmental factors affecting the biotic community of an ecosystem Soil nutrients, moisture, and aeration Temperature regimes Available light Ambient CO2 concentration
Measurement of Site Quality for Wood-Producing Purpose • Direct measure -historical records of yield • Historical data do not exist for most sites, and • The actual yield can be affected by genetic composition, stand density, competition, and pests. • The most common indirect method involves measurement of tree heights
Tree Height as a Measure of Site Quality • Tree height is • Sensitive to site quality • Little affected by stand density and composition • Strongly correlated to volume • Tree height in relation to tree age has been found the most practical, consistent, and useful indicator • Site index –the measure of site quality • The average total height of dominant and co-dominant trees in well-stocked, even-aged stands at a specific index age, such as 25, 50, or 100 years
Methods of Measuring Site Index • When the subject tree species is present • Application of site index curves • Growth intercept • When the subject tree species is not present • Interspecies site index relationships • Environmental factors, ecosystem classification, indicators
Field Measurement of Site Index • Select site trees • which are dominant or co-dominant, even-aged, showing no evidence of crown damage, disease, sweep, crook, forking, or prolonged suppression • Measure total height with a hypsometer and breast-height age by extracting an increment core • At least 3 site trees are measured, the number depends on ???
Trembling aspen site index curves Chen et al. 1998. For. Ecol. Manage. 102:157-165 Site Index Estimation from Total Height and Breast-Height Age 25 20 15 10 5
Construction of Site-Index Curves • Early site-index curves were constructed by graphical techniques and anamorphic • Anamorphic curves can be constructed by regression techniques
Let H=S i.e., Substitute b0into original equation ( ) Rearrange Construction of Site-Index Curves • An equation for site index can be constructed
Potential Problems with Anamorphic Curves • Anamorphic curves constructed using paired temporary height vs. age data can be biased because in many timber types young stands are associated with generally better sites • Why? • Anamorphic curves assume that a common shape for all site classes. • For many species, height growth exhibits pronounced sigmoid shapes on higher-quality sites, and “flatter” shape on lower-quality sites
Polymorphic Curves • Polymorphic: family of site index curves display differing shapes for different site-index classes • Additional Reading: • Chen, H.Y.H., and K. Klinka. 2000. Height growth models for high-elevation subalpine fir, Engelmann spruce, and lodgepole pine in interior British Columbia. Western Journal of Applied Forestry 15(2): 62-69
Periodic Height Growth • Growth-Intercept Method – The use of height growth for some relative short period during the life of the stand to assess site quality • Applications in intensively managed stands
Interspecies Site-Index Relationships • Interspecies site index relationship Carmean. 2000. OMNR NEST WP03: 21-39.
Interspecies Site-Index Relationships • Relationships to environmental factors, ecosystem classification, indicators Chen et al. 1998. Can. J. For. Res. 28:1743-1755
Limitations Using Site Index to Determine Site Quality • Exact stand age is often difficult to determine in field situations, and small errors can cause large changes in the site index estimate • Concept of site index is not suitable for uneven-aged stands • Site index alone may not provide a valid estimate of the growing capacity for a particular site • Site index may change due to environmental and climatic variations or management activities • Site index for one species can not be translated into a usable index for a difference species on the same site