Analysis of an Agroforest: The Variable Radius Quadrat Method 1 Harley I. Manner 2 Abstract: Procedures and methods used to determine the structure of an agroforest are presented. Simple statistical procedures to present the data in a meaningful form are also discussed. Agroforests are an important vegetation type in Micronesia and the Pacific Basin. Given the many different physical and cultural environments in which agroforestry is practiced, agroforests differ greatly in their composition, productivity, and interaction between species. Even on the same island, no two agroforests are alike. Unlike a tomato or taro field, the agroforest is extremely complex. Many students of agroforestry ask the basic question “How do we analyze an agroforest so that we can get meaningful and comparatively useful results?” Or, “Is there a method that we can use to get some idea as to what is in an agroforest?” Closely related to that question, is “How productive is an agroforest and how do we measure the productivity of the components of the agroforest?” In order to answer the latter question, however, we need to determine the structure (composi- tion, number of species, number of trees, ages of trees, etc.) of the agroforest. Some Initial Considerations Because agroforests are composed of many different spe- cies which vary in age, height, DBH and other characteristics, and are found in different physical and cultural environments, no two agroforests are exactly alike. Thus it is important to use standardized methods and procedures such that comparisons can be made between the agroforests on different islands and areas. However, before a standard method of analysis can be applied, three initial considerations need to be made: 1. The site (quadrat area) selected for study must be repre- sentative of the agroforest under study. In other words, the site chosen must be as similar as possible to the surrounding agroforest. For example, if in a particular agroforest, taro is a commonly found species in the undergrowth, but your quadrat area does not have any taro, then your study site is not representative. It may be best to select another study site within the agroforest, espe- cially if you don't have time to analyze a large number of quadrats. This assessment of representativeness is usually made visu- ally, but is based on a fairly good working knowledge of the range of agroforestry types. In turn, knowledge of the range of agroforestry types can be gained through a reconnaissance of the 1 An abbreviated version of this paper was presented at the Workshop on Research Methodologies and Applications for Pacific Island Agroforestry, July 16-20, 1990, Kolonia, Pohnpei, Federated States of Micronesia. 2 Geographer, College of Arts and Science, University of Guam, Mangilao, Guam 96923. island, field interviews, or informal discussions with landown- ers, to name a few. When analyzing an agroforest for its compo- nents, an agroforester will constantly ask whether the site under analysis is representative, and, if not, should a more appropriate site to study be found. 2. The site must be large enough to contain the range of species found in the agroforest. If the agroforest at the site is too small, it may not be a representative site. It may also contain species commonly found in other ecosystems. For example, the composition of an agroforest near a pathway or roadside will contain somewhat different species than the center of an agroforest. By selecting a large enough site, such effects are minimized and the likelihood of getting good data are greatly increased. 3. The agroforest and the quadrat in particular should be homogenous in terms of the distribution of its components. However, within every agroforest, there are bound to be differ- ences in the pattern of vegetation. As the investigator, you need to decide whether the differences represent a situation on non- homogeneity. If such patterns are common enough, they need to also be analyzed. For example, in the Mwoakillese agroforests at Sokehs, Pohnpei, there are patches of Cyrtosperma chamissionis. Such patches should be described separately as a subunit of that agroforest. Other factors that need to be considered include sampling design (whether random, stratified, or other), availability of time and money for analysis, the number of agroforestry types, and the purposes of your study, to name a few. These topics are beyond the scope of this paper, but there are many references available. The Variable Radius Quadrat The variable radius quadrat is a relatively easy method to use in agroforests. Unlike fixed area quadrats or sampling plots, the variable radius quadrat depends on the number of trees (or other plants) to determine the size of sampling area. This method is called the variable area quadrat method because the area of trees (of a particular number) will vary from place to place. An important characteristic of the agroforest is the density of trees, which can be determined by using this method. The procedures for using this method and the accompanying Form 1 are pre- sented below: 1. Fill in the preliminary information found at the top of Form 1. Other information of your choosing can be added to the sheet. 2. Locate a point (randomly or systematically) in the agroforest. 3. Mentally locate or physically mark the 10 (or 20) closest trees/shrubs that have a d.b.h. (diameter at breast height or 1.3 m above the ground), starting at the center and moving outward. It is better to use 20 trees than 10 trees, particularly if you have USDA Forest Service Gen. Tech. Rep. PSW-GTR-140. 1993. 3