IoF_M_Sc_Forest_Management_Site_Quality.pptx

Forestry Department students really note

1. M.Sc. Forestry UNIT 7: SUSTAINED YIELD AND SITE QUALITY Ram Bichari Thakur Ph.D. Scholar Institute of Forestry The Office of Dean, Kirtipur , Kathmandu [email protected], 9841555567/ 9851195551 Monday, July 09, 2020

2. UNIT 7: SUSTAINED YIELD AND SITE QUALITY Importance and determination of site quality 7.2.1 Site assessment for potential production of a site 7.2.2 Reasons for assessment (land allocation and development planning, choice of species, growth of species, forecasting of growth and yield) 7.2.3 Site assessment method 7.2.4 Maximum mean annual increment class

3. Glossory Site Site factors Site quality Site index Site tree Site assessment

4. Site in general, refers to a piece of ground that is used for a particular purpose (Collins English Dictionary) in NRM, refers to various conditions present at a particular geographic location ( Bettinger et al. 2017) in forestry, refers to the area in which a tree or a stand grows (Burkhart et al. 2019) in summary, refers to a complex of physical & biological factors of an area that determines the type & quality of the vegetation that the area can carry

5. Site factors effective climatic, edaphic, physiographic & biotic conditions of a site, which influence the vegetation of the site also called habitat factors or locality factors can be categorized into 4 groups: Climatic Edaphic Physiographic Biotic factors

6. Site quality the inherent productive capacity of a specific location (site) in the forest (as defined in Forestry Glossary by Massachusetts Executive Office of Energy and Environmental Affairs) jointly determined by site factors

7. Site index (SI) a measurement commonly used by foresters to describe the productivity of a site (i.e. a quantitative measure of site quality )

8. Site tree a tree in a stand that gives a good representation of the average dominant or codominant tree in the stand Site trees are used to calculate the SI of the site in reference to a particular tree species Site trees should meet certain specifications: be dominant or codominant & even-aged , show no evidence of crown damage, disease, deformation & prolonged suppression

9. Site assessment process of assessing productivity (productive capacity or site index) of a site used to determine productive potential of a site

10. Reasons for Site Quality Assessment Knowledge of productive potential of a site is crucial for Land allocation decision making & development planning Choosing species for plantation Forecasting growth & yield Investment & production planning especially in plantations

11. An example of yield table

12. Site Assessment Method Ideally, site quality should be measured directly in terms of yield Historical yield records provides direct measure of site quality This method works well in agriculture (e.g. annual crops) However, it is not easily applicable in forestry where the crops are grown on long rotations Thus, site quality in forestry is usually measured indirectly as a function of site characteristics &/or vegetation characteristics

13. Site quality is expressed as ordinal ranks, e.g. Site Quality I, Site Quality II, & so on designating comparative productive capacities in descending order Site quality can be assessed in 2 general ways: Measurement of site factors/Physical factors approach Measurement of vegetation characteristics

14. Forest Site Quality Forest site quality is defined as the physical and biological factors that characterize a site's ability to support tree growth ( Skovsgaard and Vanclay , 2008). Forest site quality is influence by a complex array of factors and is defined both qualitatively and quantitatively. Two primary measures used in forest site evaluation are phytocentric and geocentric.

15. Forest Site Quality Cont'd.   Phytocentric measures are tree-based metrics that attempt to characterise a site based on attributes of the stand or components of individual trees. Geocentric measures rely on physical site properties such as climate and soils to assess forest sites. For example, soil texture and nutrient availability are direct geocentric measures of forest site, while total volume is a direct phytocentric measure ( Skovsgaard and Vanclay , 2008).

16. Potential and realized measures of forest site evaluation Phytocentric measures are often realized measures of a forest site evaluation because they rely on tree-based attributes , which are influenced by a variety of factors that are not easily assessed , such as past natural disturbances. Potential productivity can be expressed as the site's maximum ability to support total biomass of a particular species or any species. Potential productivity is independent of current stand structure. Site productivity in forestry refers to the ability of a stand to produce aboveground wood volume .

17. Table (1): Methods used for assessing forest site productivity (Adopted from Skovsgaard and Vanclay , 2008)   Direct Indirect Phytocentric Volume measurements Site index; habitat type Geocentric Soil texture; soil moisture; available radiation Aspect; elevation; latitude; longitude

18. Phytocentric measures of site quality Site index One of the most common phytocentric measures of forest site quality is site index, which is the expected height at a certain reference age . Among the reasons for its widespread adoption are:- relative ease its proven efficacy in predicting volume growth and yield, strength of the relationship between tree height and age in even-aged forests height growth of certain species is often unaffected by stand species composition if the tree is in a dominant position.

19. Site index Site index is the most widely used measure of forest site productivity, particularly in statistical growth models.

20. Site Index Cont'd. Some key assumptions in the use of site index as a measure of site quality are that :- tree height is independent of stand conditions ; dominant trees that have not experienced any suppression or other damage are easily identified; and tree height is an effective integrator of the key biological determinants of growth.

21. Site Index Cont'd . Limitations of site index as a productivity measure:- several studies found tree height to be significantly influenced by stand density (Lynch, 1958; Cieszewski and Bella, 1993; MacFarlane et al., 2000; Flewelling et al., 2001 ). is difficult to apply in mixed species or uneven-aged stands ; can change over time for a given stand;

22. Site Index Cont'd . Limitations of site index as a productivity measure:- is highly sensitive to measurement errors; is rather imprecise in very young or older stands ; cannot be applied to areas that have no trees ; cannot be used to compare productivity potential between species ; and requires a well-constructed dominant height equation.

23. Site Index Cont'd. For use in growth models, site index estimates must be available for each species in a stand . An ideal measure of forest site productivity is consistent over time. For a given stand, site index often varies significantly due to changes in genetics, climate, and influence of management practices . Determining site index requires the measurement of the height and age of individual trees, both of which are subject to measurement error . Not only does measurement error bias the resulting site index estimates, it can also result in biased site index equations themselves ( Goelz and Burk, 1996).

24. Site Index Cont'd. Site index estimates are based on a reference age, which is typically set at some age less than the anticipated rotation age ( Goelz and Burk, 1996). The accuracy of site index estimates decreases significantly as stands diverge from the site index reference age.

25. Site Index Cont'd. Phytocentric measures of site productivity rely on trees to assess site quality. Conse - quently , trees must be present to determine site quality. Site index estimates are only applicable to the species present. Estimating site index requires a species-specific height–age equation.

26. Anamorphic and Polymorphic Site Index Models Polymorphic site equation model: differences between any two curves are not proportional to the ratio of their site indices. Anamorphic ( proportional) site equation model: differences between any two curves are proportional to the ratio of their site indices. Polymorphic equations are often preferred because they are more flexible and can often behave like an anamorphic model form if appropriate.

27. Base age is the reference age, in years, for determining site index . The base age is often set near average rotation length for a species.

28. Example of Site Index SI of 25 at index age 50 years (SI 50 = 25) means that the average height of the dominant & codominant trees on the site will be 25 m when they are 50 years old lower the index poorer the site, higher the index better the site

29. Field measurement of site index To calculate SI, identify dominant & co-dominant trees representative of forest site or stand i.e., site trees number of site trees to measure depends on variability of height & age in the stand being evaluated quantify total height & age of site trees define index age (age at which MAI culminated) construct SI curves

30. Construction of site index curves if one measures a stand that is at an index age, the average height of site trees is the SI consequently , a set of curves or an equation is needed to project the dominant stand height to the standard index age, called SI curves.

31. Construction of site index curves... once height & age data are compiled & index age is defined, two techniques can be used to construct SI curves Graphical technique Regression technique

32. Graphical technique Data on heights & ages are collected from a variety of stands on different site quality land & of different ages. These paired height-age values are then plotted on graph paper & a guide curve is drawn to depict the general trend in the data. To avoid bias in guide curve, it is important to represent all SI classes of interest approximately equally at all ages.

33. Graphical technique... once the guide curve is drawn, then other SI curves are drawn proportional to the guide curve for instance, if guide curve passes through height 100m at index age (the “SI 100 curve”), then the “SI 90 curve” is drawn by multiplying values on the guide curve by 0.9, the “SI 110 curve” by multiplying by 1.1, & so on this results into SI curves of the same shape called anamorphic SI curves a subjective technique

34. Regression technique Anamorphic SI curves are now constructed using regression technique Prepare candidate regression equations or models Fit the observed data into the models Select the best fitted model (equation) out of the candidate models Construct the guide curve using the selected model Pick an index age & get the height at that index age Construct curves for other site indexes that are proportional to the guide curve

35. Common growth models used for constructing site index curves where, Ht = height at index age t a, b, & c are constants

36. Plant indicators The presence of certain overstory and/or understory plant species to indicate productivity Like site index , the utilization of this approach for evaluating forest productivity has a long history.

37. Plant indicators The use of plant indicators is purported to have several advantages: ( 1) vegetation reflects the sum of all environmental factors important to plants; ( 2) several vegetation characteristics have potential significance as ecological indicators beyond just productivity; and

38. Plant indicators The approach is based on several premises including :- ( 1) the species with the highest competitive powers are the best indicators; ( 2) understory vegetation stabilizes more quickly than other strata; and ( 3) key indicator plants are the same for all sites that have similar growing environments within a region ( Pregitzer et al., 2001).

39. Other phytocentric measures A variety of other phytocentric measures of forest site productivity are used to varying degrees. Maximum mean annual increment (MAI) is generally considered a suitable measure of site productivity ( Skovsgaard and Vanclay , 2008), but it is difficult to determine without the use of a simulation model . Like site index, the use of maximum MAI is still strongly influenced by past management , and varies significantly from species to species.

40. Site assessment by Maximum MAI Maximum MAI is used as a measure of forest productivity in both the United States (Hanson et al., 2002) and Scotland (Tyler et al., 1996 ). Yield-based measures of site productivity were proposed as a practical alternative to site index ( Schmoldt et al., 1985), but have seen relatively little application in practice. Growth of individual trees also is an effective measure of site productivity.

41. Site assessment by maximum mean annual increment

42. Growth Intercept Method The growth intercept method uses a measure of periodic height increment near breast height for dominant trees to indicate productivity (e.g. Powers and Oliver, 1978). The periodic height increment is determined by measuring the distance between a specified number of whorls (or, the length of a specified number of internodes ). Therefore , the method is only applicable to species with distinct whorls.

43. Limitations of Phytocentric Measures All phytocentric measures of forest site productivity assume that trees are effective representatives of site productivity. S ite and tree genotype interactions are complex and suggest that a variety of factors can influence a tree’s productivity . Despite these drawbacks, these measures are the most commonly used in forestry.

44. Geocentric measures of site productivity Geocentric measures of forest site productivity rely on different earth-based measures of site conditions to assess productivity. The most common geocentric measures are classified into three main groups: physiographic, climatic, and soils-based attributes . Although geocentric measures attempt to avoid the limitations of phytocentric measures, they are often species specific since species can respond differently to environmental factors .

45. Physiographic M easures The most common physiographic measures of forest site productivity are slope, aspect, elevation , latitude, and longitude . Slope, aspect, and elevation are the primary physiographic measures of site productivity commonly used in the Forest Vegetation Simulator (FVS; Crookston and Dixon, 2005 ). Topographic and landform measures also are used to assess site productivity.

46. Physiographic Measures Other topographic variables found to be useful predictors are distance to a ridge, vertical angle to the top of a sun-blocking ridge, concavity of slope, and slope position. These variables are relatively easy to derive using digital elevation models (DEMs).

47. Climatic measures Regardless of climate data availability , climate was found to be an effective predictor of site productivity in several studies. Rather than relate climate to site index, Pokharel and Froese (2009) reported that mean annual temperature was just as effective as site index in an individual-tree basal area growth equation. Ung et al. (2001) developed a biophysical site index based primarily on climatic variables , which was just as effective as traditional site index for growth and yield predictions in the boreal forest of eastern Canada.

48. Disadvantages of using climatic variables as measures of forest site productivity Climatic variables are often derived from latitude, longitude and elevation, which can result in imprecise local estimates; Short-term weather events can have more of an influence on estimated productivity than long-term climate “ normals ”; Climate varies strongly from year to year and decade to decade; Variables are often highly correlated, which can make it difficult to find the most influential variable ; and Climate is influential at larger geographic scales , while other factors may control productivity at the local scale.

49. Soil measures Most of the studies have predicted site index as a function of soil attributes ( Carmean , 1975 ). Some of the most important soil factors that influence productivity are parent material, depth, texture, and nutrient availability. The primary disadvantage of using soils as a basis for productivity is that regional maps of soil attributes are often unavailable, incomplete, or at resolutions that are not useful for forest management planning.

50. Summary Measures of forest site productivity are necessary for development of effective growth and yield models . A wide variety of approaches to assessing forest site productivity are used, but each has their own advantages and disadvantages. Site index is the most common means for quantifying site productivity . Other important measures of site productivity used in growth and yield models are habitat type , and physiographic and climatic variables.

51. References Weiskittel , A. R ., Hann , D. W ., Kershaw, J. A . & Vanclay , J. K . (2011). Forest Growth and Yield Modeling . John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK