Last Updated on July 22, 2018 by Naeem Javid Muhammad Hassani
THE FOREST RESOURCE:
Forest resource means growing stock over certain land, so forest resource constitutes two basic things ie Site and Growing Stock
“An area considered in terms of its environment particularly as this determines the type and quality of the vegetation which the area can carry.”
In other words, a site is the product of environmental and edaphic factors mainly rainfall, temperature, humidity, wind frost, soil, slope, elevation, aspect, etc. There are different productive potentials of sites and a term “Site Quality” is used. However, a good or bad site quality means how the different factors affect the production; this is the productive potential of a site for a given spp (ie the good site quality for chir may or man not be good for Acacia).
Methods of Determination of Site Quality:
There are different methods for the determination of site quality of a certain place. a) Direct Method b) Indirect Method
We can determine site quality directly by following ways.
- It is a rough method. We have records of more or less than 100 yrs for Changa Manga Plantation and Chicha Watni plantation which can predict the site quality.
- All the forest resources are capable of producing different volumes. Comparing one place (on the basis of the plantation, etc) with the other in volume, we can guess site quality. This is not used but can be used; nevertheless, it is problematic. We may or may not find the proper age classes for contrast.
- (From Yield Tables) this is the most common and most valid method for the determination of site quality. We use height because it is least influenced by stocking of the forest. Whereas diameter is greatly affected by stocking eg if we have heights of 36m and 32m against 100 yrs age, the site quality of the former is obviously better but we don’t compare it with one measurement with the other. We rather try to match it with “yield tables” of that place because yield table is also a reliable source to determine site quality.
Yield Tables shows different parameters (dbh, ht, vol, thinned vol, increment, basal area, etc.) against age. It differs from volume table in the respect that volume table is used for a single tree but yield table gives us per hectare figure. Moreover, the yield tables always give information about a fully stocked forest (ie 100% stocking or Density = 1; Normal Forest).
For the determination of site quality, the height f dominant and co-dominant trees are taken for comparison. (eg 10-15 trees)
For example, we have a 15m height for 30 yrs age. Now see this 15m height in YT where SQ has already been mentioned. If this lies in SQ I, we consider it as such; otherwise, it may be SQ II etc. The YT will give us figure per unit area (ie per acre or per hectare.)
After the determination of SQ, we measure the vol. Look at the YT eg it shows 230 m3 against that specific height and age, it is noted. Then multiply it with the total area (basal area). These comparative exercises are done for all the sample trees measured.
Since YTs are always for a fully stocked forest, therefore, the fact can be doubled because we hardly find a fully stocked forest. Then a question arises that how to judge whether the forest is overstocked or understocked. This is possible through the basal area.
Eg the YT shows ‘basal area’ (ie the sum of the cross-sectional area of trees at breast height) as 40 m2 and the crop is measured 20 m2, then the stocking will be.
Now, Stocking = Actual Basal Area / Normal Basal Area
Therefore, Stocking = 20 m2 / 40 m2 è 0.5
(1 stocking means 100% stocking)
Now for the measurement of ‘basal area,’ we do Sampling of plots. There are different methods of laying out sampling.
- Through Circling
- Random Sampling
- Systematic Sampling
- Through Circling: Circling involves the fixing of circles and measuring all the tree falling in it. The area of the circles is known eg stretch the radius (r = 17.98 m) and make a circle. The area of a circle is usu taken as one-tenth of a hectare.
- Relascope: This is also called ‘angle gauge method’. Here the plots are not used. We have a fixed angle which is rotated in the form of a circle and the trees falling in it are entered in it individually. The trees are selected proportion wise. This is a very quick method. We have certain factors for the basal area which we multiply with the trees tallied eg 7 trees. The factor is usu 4 for it.
The Basal Area = Trees tallied * BAF
= 7 * 4
= 28 m2 / ha
- Random Sampling: In this method, the plots are selected at random viz in a haphazard way eg put slips of roll nos of students in a lottery box and pick a few of them for consideration. Same is the case with forest. But random sampling is too difficult, rather impossible.
On the other hand, the tress measured may not represent the respective thing properly. Then the grind of searching each plot for its position and its measurement presses heavily.
- Systematic Sampling: Here we select plots at specific distances. We have a fixed azimuth (direction or distance extending from zenith to horizon) and the inventory, as well as the searching of plots, is done easily.
On poor SQ the individual trees occupy more unit area and greater no of trees can be maintained. In other words more stock per ha due to the vitality of trees. SQ means a different rate of growth; the aim of the stocking is to have more coverage of the stock and full stock.
The following example can help in the determination of SQ.
Chir pine – Age 90 years
|Average dbh (cm)||Height (m)||BA m2/ ha||No of trees||Standing vol||Thinned vol||MAI||CAI|
The number 168 means that the density is less. If we compare our measurement and it stands 28, what should be our SQ, either I or II? This can be found by ‘Interpolation’ or ‘Extrapolation’.
In ‘Interpolation’, we find out the position of certain things in b/w two known limits eg;
|SQ I||30.5||Ht measured is 28|
As we know that, one complete shift = 3.9 (ie 30.5 – 26.6 = 3.9)
If 3.9 is equal to one;
Then, 2.5 (30.5 – 28) will be = 1/ 3.9 * 2.5 = 0.64 (here, 30.5 – 26.6 = 3.9)
So the SQ is (1 + 0.64 = 1.64)
If reverse is the situation and as;
3.9 shift = one
1.4 (26.6 – 28) = 1/ 3.9 * 1.4 = 0.36 (here, 28 – 26.6 = 1.4)
So, SQ is 2 – 0.36 = 1.64 (where 2 is the value of bands of Relascope)
After the SQ, we find out volume (actual)
SQ I = 359.6
SQ II = 270.8
Shift it again not 1 but 0.64 ahead of 1.
So (359.6 – 270.8) è 88.8 * 0.64
Now volume = 359.6 – 56.8
= 302.8 per ha
From SQ II, we get 88.8 * 0.36 = 31.9
And volume = 270.8 + 32
= 302.8 cft
In ‘Extrapolation’ we determine something beyond from the two known values eg we have diameter against age that we do not know beyond the curve but if it is extrapolated (extended) or stretched at some length, we can read the values.
Now for increment has a linear relationship with stocking. If density is reduced to 0.9, there is no reduction in increment but if the reduction is 0.8, it will occur. Consequently, the increment is not proportionate to stocking (reduction in) but after all, it is affected in a way that it is the proportionate type. However, it depends upon the age as well. The younger crop put more growth as compared to the old crop. So reduction is more among the older crops. This fact is obvious from the following example.
|Increment||Degree of Stocking|
Problems with Y Tables:
- YTs are always made for even-aged forests like plantations, chir pine forests or blue pine (kail) forests but not for selection forests. However, they are for a small place, not the entire area.
- YTs are applied for a single spp, not for the mixture of spp otherwise the YT is not valid.
(of Site Determination)
Indirect methods involve two things ie
- By vegetation
- By Edaphic and Climatic factors
- It actually refers to Indicator spp. In Pakistan, we don’t have the studies of this kind. This method is useful for chir pine, Indigofera, Blue pine & Grugaria etc. Here we measure the trees but if the trees are not present it is determined by vegetation present in the whereabouts; as they are indicators of the site or SQ.
Edaphic and Climatic Factors: We know that growth is a function of the site, temperature, elevation, aspect, etc.
So, Growth = f (a + b + c + d + e + . . . . . . . . . .)
By ‘soil’ is meant the depth of soil, its structure, texture, mineral contents, pH values, etc. if we have detailed data about the different kinds of soils, we can compare the site with them. But nevertheless, this is very much difficult, rather impossible for a human mind because the site is the combination of all these factors and slight changes in any factor can alter the situation to a great extent.
“Growing stock is the sum of all trees by number or volume in a forest.”
What is important for mgt are the characteristics of a growing stock. The characteristics of growing stock include forest type, stand type, spp composition, size class distribution, age, stocking, the density of the area of growing stock, quality, basal area, and increment or growth of the growing stock.
Each characteristic is now discussed individually:
According to mgt, forest type is that which is identifiable for different things and needs different treatments eg chir pine; we must know the objectives of mgt as well as the different treatments get to be meted out to this forest type.
It is the smallest area within a forest (or it is the smallest unit) which is given uniform treatment in all aspects of Mgt. Stand type is defined as, “A collection of stands within forest types which are sufficiently similar to require some treatment and planning or produce the same type of produce.”
Stand type is consequently meant by the major spp present in that area eg fir, kail, kail/fir, chir/kail, or kail/ chir or oak/ chir etc. If a spp constitutes more than 10% of the area, we include it into the stand type eg when fir 90%, and kail is 10%, it is fir/kail standoff if 30% kail, 40% deodar, it is still fir/kail stand.
In Pakistan we have compartments and sub-compartments instead of stand type; the circumstances of stand type are a different thing. They are further divided into ‘stands’ for better planning, execution, and control. (Planning = activities to be done; Execution = all the activities mentioned in the plan are done on the spot/ ground; Control = all the activities carried out are examined and decided whether the plan is useful or useless.)
“It is the smallest unit of a forest similar in all characteristics of mgt and requiring the same type of treatment.”
Eg pure chir pine has different stands. This division is based on age, the density of stocking, spp composition, slope, and social pressure. These characteristics of a stand may be still defined at some length.
- For a number of reasons, different spp are suited to different sites. The growth rate of different spp and their composition is important for the different value of products. So it is important to know the preservation of spp eg blue pine 40%, deodar 30%, oak 10%, walnut 10%, Alnus 10% = composition – this composition can be by number, by vol or by basal area. A spp can be small by number but greater in vol (eg a few trees of deodar dia and crowns).
Size Class Distribution:
- Size means thickness r dia. It tells us that whether the crop can be managed on a sustained yield basis or not? In a normal forest, the curve is;
- Age tells us growth (for comparison) of different spp and size; and age rotation of spp (we then make the decision about the rotation.)
- Density means stocking or compactness of the crop. It is the number per unit (ha) area or vol per unit area. Stocking tells us whether thinning is to be done or not. This is also needed for yield calculation and value etc.
Area of Growing Stock:
- More area means more investment and financial applications and less area means vice-versa. One hectare area has no planning etc but 2000 ha land will be having some working plan for it, large labor, nurseries, and sound execution and control of activities.
The quantity of Growing Stock:
- Growing stock may be tinged with both qualities ie good as well as bad, but a forester wishes merits eg resin tapping is effective and is of good nature or not. Whether the thinning has improved the quality of timber or not. Then there are a few factors which affect the qualities of the crop.
- Genetic: Either good genus and sound health or requisite qualities.
- Abiotic or Biotic: Sometimes fire destroys the crop severely and deteriorates the quality.
- Silvicultural Practices: In both the cases either greater no or lesser no, the quality affected. Similarly light thinning and heavy thinning; mixing of spp (either group or separate types), and many other silvicultural practices influence the quality of wood.
Sometimes shade tolerant spp can improve the quality of crop of upper storey eg Mulberry (under shisham) does not let shisham to grow lateral branches up to a considerable height.
The quality of shisham can be improved by a dense crop and then later on by its thinning, but still, we need an understorey for further improvement.
Basal Area of Growing Stock:
BA is needed because with the help of BA, vol can be determined and it is the yield table that gives the vol directly.
Vol = BA * Ht (average) * Form factor
- BA indicates that how much thinning or how much BA should be removed and this thinning (ie on the basis of BA) is most accurate
- BA indicates the area covered by different spp. By number, we cannot compare the crop.
- BA is sometimes helpful in the determination of stocking.
- (see increment)
For correction and improvements please use the comments section below.
NJMH is working as Deputy Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu (techurdu.net) Forestrypedia (forestrypedia.com), Majestic Pakistan (majesticpakistan.pk), All Pak Notifications (allpaknotifications.com), Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.