10/4/04W3-Measuring Forests.doc Page 1 of 4Measuring Forests: Field TechniquesForests Through Time and Space Fall/ Winter, 2004/05This workshop will give you a set of field techniques that you can use to quantify some attributes of forests.Most of these methods are standard field methods either from botany or from forestry. Over the quarter, youwill be visiting a number of different forests and taking these same measurements. At the end of the quarter,you will be asked to summarize and reflect on what patterns are discernable in the data.For most of these measurements, we will use feet and acres as units since they are standard in the forestindustry and a number of the instruments we will use are calibrated in these units. Unfortunately, we don’thave enough tapes that are in feet, so you will need to convert from metric to English and vice versa (1 ft =.3048 m, 1 inch= 2.54 cm)DefinitionsThere are a number of terms and conventions that need to be defined.Diameter at breast height (DBH). This the diameter of a cross section of a particular tree at about 4.5 ftabove the ground. It is a measure of tree trunk size.Basal area (BA). In simplest terms, basal area is the cross-sectional area of a tree at breast height. It can becalculated using the formula for the area of a circle = (! x r2). There are tables that give the BA for trees ofvarying DBH.Basal area per acre. This is the sum of the basal areas for each tree in one acre of forest. It is a measure ofstand density and expresses the relative amount of land occupied by trees. Another way to think about it isthat it is the amount of tree vegetation on a unit of land area. BA/acre can also be used to compare therelative dominance of different tree species within a forest.Variable plot sampling. This technique makes use of an angle gauge or a special prism to sample the forest.Through sampling theory and statistical calculations, these tools have been calibrated so that each treecounted in the sample represents a specific basal area per acre (expressed as ft2 per acre). The chances of atree being counted in the sample depend on its DBH and the distance between the viewer and the tree. Smalltrees will be counted if they are close to the viewer, while larger trees can be further away and still counted.The advantage of this sampling method is that it can be done rapidly and it gives a good estimate of the basalarea per acre.Fixed plot sampling. As the name implies, this involves establishing a plot of known dimensions andsampling the vegetation within that plot. We will be identifying the plants within the fixed plots and usingthis information to characterize the understory vegetation.Variable Plot SamplingEach of the sampling devices used for variable plots are used slightly differently. We will be using a varietyof wedge prisms or an angle gauge. Each device will have a basal area factor (BAF). You must note the BAFin your notebook. Multiplying the number of counted stems by the BAF will give you the basal area per acre.The main differences between prisms and angle gauges are as follows:Prisms. It doesn’t matter how far you hold the prism from your eye. However, as you rotate in a circle tosample the forest, the prism should remain over the same point. Mark the center of the plot with a stake.Prisms must be held with the thick edge perpendicular to the ground. The prism will offset an image of thetree trunk. If this offset image intersects with the actual trunk viewed outside the prism, the tree is counted.Trees where the offset image does not intersect the tree are ignored and borderline trees (offset image barelytouches the tree) are counted as half a tree.10/4/04W3-Measuring Forests.doc Page 2 of 4We have prisms with BAFs of 20 and 40, and angle gauges with BAFs of 5, 10, 20, and 40. The higherBAFs are used in stands with larger trees. We will use either 20 or 40 BAF for our surveys. Ideally, thechosen BAF will yield an average of 5 to12 counted trees per sample point.Angle Gauges. The angle gauges wehave consist of a metal sight with threedifferent size openings and a bead chainto measure the distance from your eyeto the metal sight. Unlike a prism,which can be held any distance fromyour eye, the angle gauge sight must beheld the same distance from your eye inorder to function. Also, when using anangle gauge, the viewer’s eye is thecenter of the plot, whereas with a prism,the prism is the center of the plot. Treesthat are wider than the appropriateopening in the angle gauge are counted.Trees narrower are ignored andborderline trees (appear the same widthas the opening) are counted as half atree.Sampling a forest withvariable plot samplingDetermining the center of the plot is usually done arbitrarily prior to arriving in the field to minimizesampling bias. A common method is to layout a grid where a sample plot is located fixed distances alongperpendicular lines. Once the plot center has been located, position either the observer (using an anglegauge) or the prism (using a wedge prism) over the center. Beginning with an obvious landmark tree, slowlyrotate 360° and count all the trees that are “IN”. It is permissible to move slightly to view hidden trees solong as the distance from the hidden tree is kept constant. For each plot, record the number of counted trees.Once the process becomes familiar, sampling a variable plot is quite rapid. To accurately assess the forest, anumber of variable plots should be sampled and the counts averaged (compute the average out to threedecimal places). The average count is then multiplied by the BAF to determine the basal area per acre for theforest. This lumps all the tree species together. Data collected in the fixed plots can then be used to portionthe total basal area of the forest into basal area per acre for each of the overstory tree species.Fixed plot samplingFixed plots are used to sample additionalattributes of the forest, such as overstory treespecies composition, density and species ofshrub and herbaceous layers, canopycoverage. Plot size varies depending on thenumber of trees per acre. Small plots are usedin forests with many trees per acre. We willuse primarily 0.1 and 0.05 acre plots.BreastHeight4.5 ftPlot Size (acres)Plot Size (ft2)Plot Size (m2)Circular Radius (ft)Circular Radius (m)Square side (ft)Square side (m)1 43,560.0 4,046.7 117.8 35.9 208.7 63.60.5 21,780.0 2,023.4 83.3 25.4 147.6 45.00.25 10,890.0 1,011.7 58.9 17.9 104.4 31.80.1 4,356.0 404.7 37.2 11.3 66.0