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Forest Thinning With Assisi Manager


Management actions are carried out according to rules in an Assisi Manager forest management plan simulation. There are no limit to the number of rules that can be created. Often a set of management rules are created that reflect your best practices or the approaches that you wish to try. These rules are then used over and over again while exploring different management strategies.

The Management Rules Editor

Assisi makes creating thinning rules easy using the Rules Editor.


The Rules Editor is where forest management rules of any type are created. Later, when designing a management plan, these rules are added to management actions within the plan. Let's take a look at the options for thinning rules in particular.

Thinning Methods, Markers, Targets, Directions and Terms (Oy!)

At its most basic, thinning removes trees in an ordered fashion until a target is met. Targets in Assisi can be set at the stand, species and diameter levels with species or groups of species using different targets within a single rule. This makes for numerous possibilities, but ultimately the rule should reflect how tree thinning occurs in the real world. So, in general it's the definition of thinning targets and the ordering of tree removal at these three levels that a thinning rule is mostly concerned with.

Targets can be BA, TPA, RDI, CCF, volume among others. A target at the stand level applies to the total of all trees in the stand, regardless of species. A target at the species level applies to single species or species groups. Targets at the diameter level apply to diameter classes within tree species. A thinning action ends when at least one of these levels of targets is met.

Thinning Method determines how trees are chosen for thinning. Options are From Above, From Below, Even and Per Tree. When a block of trees is to be thinned, you can select the smallest first, largest first or thin evenly across size classes. When trees are thinned by size classes, the Per Tree method can be used to limit each tree class to a curve of targets by diameters. The units that are used for selection are called Markers and can be either DBH or crown ratio.

Finally, Direction determines whether the target is an amount to remove or an amount to leave. And Term determines whether the target is a percent of current or an actual amount of units.

Enough! How about an Example?

OK. That's a lot of options. But, they are needed. There are so many objectives for thinning that having all these options available is a good thing!

Diameter Based Thin with Hardwood Reserves

Consider the following. We want to promote the growth of Douglas fir in a stand with Grand fir competition but also retain a certain amount of residual hardwood for species diversity. The following is a screen shot of the Rules Editor for a thinning rule that might be used.


First on the thinning rule list is the removal of Grand fir. The Grand fir rule has a Leave Target of 0 TPA. This will remove all Grand fir trees until none remain. Simple enough.

Hardwood is next and is using a Leave Target of 25 ft BA. Note that multiple species of hardwood are listed so all trees of these hardwood species will be grouped and targeted the same for thinning. The Even method will remove hardwood trees in small steps of BA until there is 25 ft of BA remaining. Of course, if there is less than 25 ft of BA to begin with, no hardwood will be removed and thinning will continue on with Douglas fir.

Douglas fir is using a diameter based thin where the target is expressed as a graph of BA versus diameter rather than single target values for all trees of Douglas fir as the Grand fir and Hardwood do. The graph of diameter points are set up using the plot. The Direction is Remove and the Terms are Percent so the graph is expressing the percent of current BA to remove. The Method is set to Per Tree which tells Assisi to consider each tree individually on the graph, not as a group. In other words, the BA to remove for each tree is the percent value where that tree's diameter falls on the graph.

Results

Assisi's Forest Stand Simulator is an ideal way to experiment with thinning options because it can show the time history of species and trees in a variety of ways. Below is the result of running the above thinning rule on a stand from the Assisi sample database.

The first plot shows species BA over a 30 year time horizon of the run. The Grand fir was removed, the hardwoods were reduced and the Douglas fir thinned. The solid lines are BA and the dotted lines are BA increment. The time step, year and age of the stand is shown on the X axis.


But what if you want to see how the rule chose individual trees? The following graph shows the growth of each individual tree over the life of the run. The upper chart shows the amounts of BA for each tree at the end of the run, color coded by species. The lower chart shows the path of BA vs DBH that each tree had for the entire run. Notice the vertical lines. They are tree diameter and BA at the time the tree was thinned. For Douglas fir (green) it is apparent that a larger amount (80%) of trees were removed for smaller diameters while a small amount (down to 0%) were removed at large diameters. Grand fir (light green) was completely removed. Of the hardwoods (brown), the removal was even across diameter sizes.


Results for individual trees can also be browsed. The top plot shows current trse BA. The bottom shows the history BA as a function of DBH. The vertical drops are the removal of BA during the thin.

You see, even with all that complexity, Assisi Manager makes creating and exploring forest management plans easy!

Rich Howard

#Simulation #Management #Rules #Thinning

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