Brief History of Site Quality Estimation from a Forest Mensuration Perspective Eric C. Turnblom ESRM 410 - Forest Soils and Site Productivity - Autumn.
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Brief History of Site Quality Estimation from a Forest Mensuration Perspective Eric C. Turnblom ESRM 410 - Forest Soils and Site Productivity - Autumn 2009
Site Quality metrics should be … highly correlated with potential maximum wood volume or biomass for the stand independent from stand density or stocking independent from thinning regime (or other stand manipulation) independent from species composition applicable to other species
Primary focus on primary products Choices in methods … Could borrow the agricultural model: bushels [bales, tons, etc.] ac -1 Historical yield records Average the yields over several rotations Issues ? Current yield Volume at a particular age Issues ?
Choices in methods … Height in relation to age Advantages over ag. model For a given age, high correlation between volume yield and stand height Average height of Dominants is little affected by density (Unlike DBH) Not affected by thinning (except thinning of dominants) Site index (avg. ht of dominants at “base” or “index” age) adopted by SAF in 1923 as the cornerstone for productivity estimation in US
H - Age (McArdle, Meyer 1930) Based on Temporary Sample Plots
H - Age (King 1966) Based on Stem Analysis Offers new opportunity for using curves to predict H development into the future
H - Age (Flewelling, et al 2001) Permanent Sample Plot Method Solid (pink) lines are King’s curves dashed (blue) lines are new curves
H - Age (Flewelling, et al 2001) Permanent Sample Plot Method Solid (red) line tracks 300 TPA stand, dashed (blue) line Tracks 1200 TPA
Advantages of Site Index … Height growth is highly correlated with volume growth so height at index age should be with max potential For most species, dominant height growth is unaffected by density Height growth of species of interest is unaffected by species composition
Disadvantages of Site Index … Most Site Index equations do not perform well in young stands, say < 20 years-old
Site Index in very young stands Periodic stand height growth Growth-intercept (or height-intercept) method estimates site index from height growth measured over a short interval S BH20 = 4.9 + 14.23 (GI) S BH20 = Site index of red pine (m at 20 years breast height age), GI =5-year growth intercept (avg. of 3 doms., meters) starting above 1.5 m
Disadvantages of Site Index … Site Index cannot be determined when no trees of the species of interest can be found on the site Might use so-called Overstory Interspecies Relationships or Site Index Conversion Eq’s
Disadvantages of Site Index … Site Index cannot be determined when no trees are present on the site Might use so-called Soil Site Index Equations
Soil Site Index Equations … Steinbrenner (1979) found the following relationship for Douglas-fir in Western WA S = 78.3 + 0.7649 ED + 2.811 DA – 0.0304 (DA x ED) – 1.018 EL S = King's (1966) site index, ED =Effective soil depth (inches) DA =Depth of A horizon (inches) EL =Elevation (100’s of feet)
Disadvantages of Site Index … Site Index is best used in Even-aged stands, i.e., it is very difficult to find trees in uneven-aged stands that have been “free to grow” their entire lives, without sustaining top damage Might adopt the concept of Site Form as measure of site quality Avg. height of trees at index DBH
Site Form … Uneven-aged Red Spruce forest type in Maine (McLintock & Bickford 1957) Huang & Titus (1993) also found this system to be useful in boreal stands in Alberta, as have other researchers for other species elsewhere
Disadvantages of Site Index … Site Index, usually thought of as constant for a given locale, actually may change over time due to changing climate, to treatments such as fertilization, or with the introduction of genetically improved trees
Genetically improved DF trees … Intermediate, Elite gain trees are 6.5%, 8% taller, resp.
Disadvantages of Site Index … While Site Index itself for a particular species is unaffected by species composition, yield can be greatly affected by composition Site Index may not be correlated with potential yield in heterogeneous stands or stands with stockability problems, i.e., stands with highly variable soils, say, peppered with rocky outcrops, wet areas, dry areas, …
Other “hybrid” approaches … Using habitat classification schemes to augment Site Index estimation Habitat Classification uses potential climax vegetation to group areas into similar site types, presumed then to be similar in most respects Difficult to apply in highly disturbed areas, such as in cases where vegetation has been burnt, plowed areas, roadside edges, etc.
Other “hybrid” approaches … Developing process model-based estimates It has been suggested that a good process (or mechanistic) model might be “fit” to local stand data by adjusting its internal site parameter and using that as an index to productivity 3PG (Physiological Principles Predicting Growth) Forest-BGC (BioGeochemical Cycles)
Productivity … Other issues What exactly is productivity ? Should it be some measure of biomass? Challenges remain in the estimation of tree biomass
Biomass estimation issues Actual vs. predicted 20-year stem wood dry weight and carbon storage by treatment, tonne ha -1
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