for MYRLIN stand tables
MYRLIN can accommodate one level of sub-sampling on a simple fixed area plot within the normal options specification.
However, there are many other ways in which sample plots may be designed. These include:
To accommodate these, an additional column can be specified on the Tree data sheet, which must contain the number of trees per ha each observed tree is equivalent to. This is referred to as the tree area weight. This column can usually be filled in using an appropriate Excel formula, based on either tree diameter, or a sub-plot ID column. This column number must be filled in on the Options sheet in cell B16. The plot size on the Options sheet must be set to -1. Examples are given in the table below:
| Two or more sampling levels within each plot | Use an IF() function in Excel together with the diameter column to give the area weight according to the tree diameter. For example, if trees of 40 cm plus are sampled on 1 ha, 20 cm plus on 0.1 ha, and 10 cm plus on 0.05 ha, and diameters are in column D, then the formula: =IF(D2>=40,1,IF(D2>=20,10, 20)) will give area weights of 1, 10 and 20 trees/ha for each tree sampled, based on its size, and hence the size of sub-plot in which it occurs. Generally, area weight is derived from plot or sub-plot size as: Area weight = 1/(Plot size in ha) |
| Sub-sampling on every nth plot | This can be accomodated by a similar method to that above, by treating groups of n plots as a cluster. If the frequency of sub sampling is n, the threshold diameter is d, and the plot area is p, then the general area weight formula would be: =IF(D2>=d,1/p,n/p) For example, suppose normal plots are sampled to 20 cm,and one in 10 are also sampled down to 5 cm dbh. Plot size is 0.1 ha. Then the required IF statement should be: =IF(D2>=20, 10 , 100) In other words, trees on the normal plots would have an area weight of 10, and those below 20 cm dbh on the sub-sample plots would have area weights of 100 trees/ha. |
| Point sampling | With point sampling, the area weight depends on tree diameter and the basal area factor F used. If tree basal area is g m2/ha, then effective plot area for each tree is g/F. Area weight is the reciprocal of this. Hence the general formula is w = F/(k.d2) where w is the area weight and d is diameter in cm, and k is the constant 0.00007854. The basal area factor F must be metric, in m2/ha. This equation can be put directly into an Excel formula. For example, with diameter in column D, and a basal area factor of 4 m2/ha, the formula would be: =(4/(0.00007854*D2^2) |
| Plot sizes or basal area factors that vary between sample unit | It sometimes happens that different plot sizes or basal area factors are used on different sampling areas, to reflect large changes in forest type or density. This could be combined with any of the above schemes (sub-sampling, clustering,point sampling). In this case an additional column is required to give the plot size as well as the plot ID. This is then used to construct the above formulae, instead of using a constant value. For example, suppose different basal area factors were used in different stages of an inventory, and are entered in column C of the data, with diameter as before in column D. Then the area weight formula from the above case would become: =(C2/(0.00007854*D2^2) |