New users often ask, what biometric methods are supported by YTGEN? In the forest industry most people consider these are functions or equations used to model the shape, volume, and inside bark diameter at any point on the stem. These are used in the log-making process to emulate felling practices like stem breakage, and to calculate stem volumes and allow bucking model to fit log products into their diameter specifications.

**So what does YTGEN consider to be a biometric method? **

In YTGEN these are generalized in the term “Methods” and are stored inside a Method file (*.YTM). Each method type has quite specific purposes. For example a “breakage method” will model the height at which a stem breaks during harvesting and before log-making can start. A “volume method” will calculate stem volume from DBH and height. A “taper method” will calculate the inside bark diameter at any point on the stem or the volume between two points, given stem volume.

**So What Type of Methods are Supported by YTGen? **

You can define your own methods using the following common functional forms shown in table 1 and defined in table 2. Please note this is not a fixed list and just the current accepted method functional definition forms as of August 2013. Silmetra are adding to this list all the time when new forms become useful for users. However even with the forms shown below this provides resource foresters a wide scope for developing and implementing the right function for their own needs.

Table 1 – Method Definitions in YTGEN

Table 2 – Definitions for variables found in Table 1

Term |
Definition |

height |
Stem height (m) |

slope |
absolute value of ground slope in degrees |

ern |
a random number generated from a normal distribution with mean=0.0 and standard deviation=1.0 |

dbh |
stem over-bark diameter at breast height (cm) |

dib |
stem inside-bark diameter (cm) |

bh |
breast height (m). This is a model parameter rather than an input. |

volume |
stem volume inside bark (m^{3}) |

h |
Stem position as distance from ground level (m) along stem |

X |
(distance from tip of tree (m)) / height
= (height – h)/height |

k |
PI / 40 000 |

b1-b9 |
Parameters, the values of which are user-supplied. |

dbhib |
stem diameter inside bark at breast height (cm) |

age |
plot age (years) |

log(x) |
natural logarithm (base e) |

**Find out more? **

You can find out more about using methods in the YTGEN Help file.

*Figure 1 YTGEN Help file on Biometric Methods*

**Example of a Breakage Model Implemented**

One of most commonly used stem breakage models used in P.radiata is what most New Zealand resource foresters call “breakage 1”. This was developed in 1976 from stems felled in a study in Kaingaroa Forest in the Rotorua wood supply district. It uses breakage subtype 1, with coefficient variables b2=0.668399 and b7=0.13376, with all remaining set to 0.

So what does this mean?

Well using the “method test” feature of YTGEN you can quickly set a test stem and complete a simulation of breaking 10,000 trees and get an idea of what the breakage model does. So in Figure 2 below you can see 0.6% trees will remain unbroken, with a mean break height of 0.66% of the tree height and the standard deviation of this mean break height being 13%. So quickly you can see what the effect of the coefficients b2 and b7, these being the mean break height as a % of the tree height (b2*height) and b7 creating a normal distribution of break heights around the mean using a random number generated from a normal distribution with mean=0.0 and standard deviation=1.0 (b7*height*ern). Ofcourse setting most of the other coefficients means they are not used (0*x=0), except b8 which by definition has meant that the probablity of the stem not breaking has been set to 0% (except for influence of the ern = 1.0, which would mean the odd stem does not break, hence 0.6% of stem remain unbroken in the test dataset).

You can graph this up yourself by using the “copy break heights to clipboard” feature available from the right click inside the test stem results below. Doing this enables you to quickly paste these into EXCEL for example and look at break heights as shown in Figure 3.

*Figure 2 – example of the Method File view in YTGEN with a test stem using Breakage function 1*

**What about Growth Models?**

I hope to cover this in another article as these are handled differently inside YTGEN.

If you would like to know more or understand about creating, developing or testing your methods for your own forest estate, talk to the Interpine team.