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What is Basal Area?

beginners management Oct 09, 2019
When you get a forester to visit your land... your trees... they’re going to take several different types of measurements:

DBH, Height, BA.

But what do these all mean? 

DBH (Diameter At Breast Height): 

This is the diameter of a tree at 4.5 feet above the ground... although there can be some exceptions to the placement of the measurement, that’s another lesson... DBH is measured most often with a "measuring tape" tool. Please be aware though that it is not the ordinary measuring tape in your tool box. A DBH tape's "inches" are spread further apart to take into account that we actually have to measure the circumference since we cannot measure across the tree until AFTER it has been cut down. Why is DBH important? Because it tells you the size of the tree which can be categorized into different product classes for harvest. DBH can also tell you how well a tree is growing (if previous measurements are kept up with). 


Hopefully this is self explanatory to you... it’s the height of the tree. This can also be the height to a DBH mill spec, but more often than not it’s the total height of a tree. As my colleague would always say “up until the green touches the blue.” Tree heights are often measured with a tool called a clinometer (other more advanced tools can also be used). This is important because again, can tell you how well the tree is growing (in conjunction with its age). 

BA (Basal Area):

The ever confusing measurement to try to explain to all non-foresters. The technical definition is the cross-sectional area of a tree at 4.5 feet height, typically only referring to merchantable trees. Basal area numbers are measured using a tool called a prism, which "displaces" trees within a plot. Every displaced tree that overlaps with the "top" and "bottom" of the tree is counted as "IN", and if they do not overlap then it is not counted toward the total BA count. (See photo above). But of course that tells YOU absolutely nothing... sometimes it’s referred to as the “density” of a forest stand, even though that doesn’t quite grasp it either because the same number BA can be quite different looking if you don’t have the FULL picture, if you don’t have the rest of the measurements to accompany your basal area number... 
For example: 
A basal area reading of 50 in a pine stand can mean a young pine stand, a poorly growing stand, or a heavily thinned stand, or a larger, second-thinned, mature stand. Without the DBH and height measurements, along with AGE, I cannot say whether “50” is on tract for your goals or not. 
There are some specific numbers foresters generally use as parameters. For instance, rarely will you hear a number below BA 30 sq. feet OR above 200. BA of 120 sq. feet is often use as an indicator it’s time for thinning, and many times looking to thin down to 70 sq. ft (for traditional forestry). But even still, I would need to know the rest of the picture to honestly give some insight: what is the average diameter of these trees, how tall are they, and finally how old are they... 
Whenever you get recommendations they oftentimes will be provided to you in reference to basal area. "4th row thin your pine stand to an average of 70 sq. feet BA" or "wait until you reach 130 sq feet BA before performing your thinning harvest and aim for a 50 sq. feet BA for optimal deer habitat" or "to have a quality quail plantation, do not exceed 40 sq. feet BA".
But for you the landowner - you may still not fully grasp WHAT basal area is and WHY can't that number be a set number to represent the same picture?
Think about when you’re shopping for pants... (ugh the worst I know!)  

But you know generally what different “sizes” of pants mean... what 2, 4, 8, 12 + means- but then there’s the long, short, petite, tall sizes and of course the styles... curvy, bootcut, skinny, and so forth. You and I both know that saying someone should buy you a “size 6” is not the end of the story. You need to add WHAT brand you need in that size AND what STYLE...  I might be a size 2 for Old Navy “curvy bootleg” and a size 6 for Banana Republic “modern skinny fit”... 

In both cases the number can give you a possible picture, but ultimately a number is just a number without the rest of the information.

Although your management recommendations will often be provided to you in terms of basal area, that doesn't mean the other measurement factors AREN'T important. They are just as important to provide that "full" picture of your landscape.


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