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The reference unit technique was compared with the dimensional analysis approach for estimating large shrub foliige biomass in Northeast Brazil. The techniques were tested on coppicing jurema (Mhosa acutistipula Benth.) and pau branco (Auxemma oncocalyx [Fr. Alem.] Taub.). Both methods provided good estimates of foliage weight. The coeffkients of determination for the reference unit approach ranged from .890 to .985. The P values obtained in applying the dimensional analysis method were .937 and .948. Improvements in estimates with the reference unit method were obtained when (1) a branch unit of 19% of total plant foliage was used versus a unit of only 7%, (2) the branch unit resembled the appearance of the branching of the plant being estimated, and (3) estimations of 3 judges were averaged. Total foliage production on shrubs and trees is one of the most difficult parameters to measure or estimate on native rangelands. The woody material and variable growth form renders most traditional sampling methods, largely derived for agronomic conditions, impractical for shrubs. As a consequence, vegetation sampling technique manuals ha,ve made only passing reference to or given insignificant information on biomass estimation of shrub foliage or current growth (Brown 1954, NAS 1962, Newbold 1967, Pieper 1973, t’Mannetje 1978). Efficient estimations of browse availability are required for research on brush control and grazing animal diet studies. Shrub measurement techniques used in other semiarid areas of the world were examined with the prospect of adapting a suitable method for the caatinga species. The criteria for selection were that the method be nondestructive, time efficient, and relatively precise. Traditional ‘clip-and-weigh’ methods (e.g., Whittaker 1961) were eliminated from consideration because of the labor and cost required (Gimingham and Miller 1968, Rutherford 1979), and because the associated research design did not permit destructive sampling. The twig count method of Shafer (1963) and methods relating foliage weight to the dimensions of individual branches (Whittaker 1962, Ovington et al. 1963, Provenza and Urness 1981) were considered inappropriate because of the high density of coppicing branches sprouting from the stumps of the test species. Two sampling techniques were selected for testing on large shrubs of Northeast Brazil. The first of these employs the principle of matching standards against samples (Hutchinson et al. 1972, Andrew et al. 1979), such as estimating the number of multiples of the reference unit (e.g., leafy branch) present in the entire plant. This technique was tested by Andrew et al. (1981) on 2 small Australian shrubs (Atriplex vesicaria Heward ex Benth. and Maireana sedifolia F. Muell.) and compared favorably with other techniques for estimating shrub biomass. The second technique, dimensional analysis, requires establishing a relationship between Authors are research assistant and associate professor. Department of Range Science, Utah State Universitv. Logan 84322. This research was carried oiti as iart of the United States Agency for International Development Title X11 Small Ruminants Collaborative Research Support Program under Grant No. AID/ DSAN/ XWGaO49, in collaboration with the Empress Brasileira de Pesquisa Agropecuaria (EMBRAPA), Brazil. The support of the EMBAPA staff is gratefully acknowledged. The authors would also like to thank Joao Queiroz and Mireille Kirmse for help with data collection, Keith Owens and Alan Carpenter for manuscript review, and Drs. David Turner and Fred Provenza for advice on statistical analysis. Manuscript accepted January IO, 1985. JOURNAL OF RANGE MANAGEMENT 38(5), September 1985 easily obtained plant dimensions and foliage weight. A number of studies have shown that canopy volume is highly correlated with foliage weight (Cook 1960, Ludwig et al. 1975, Uresk et al. 1977, Lyon 1968, Kelley and Walker 1976, Bryant and Kothmann 1979, Guy 1981). Study Area and Methods The study area was located in Ceara State of Northeast Brazil (3.Y south latitude, 41° west longitude), much of which is dominated by semiarid tropical woodland, called the caatinga (Pfister et al. 1983). The test plants werejurema (Mimosa acutistipula Benth.) and pau branco (Auxemma oncocalyx [Fr. Alem.] Taub.), 2 common and widely distributed species. Morphological characteristics, habitat preferences and management values of these 2 species are described by Kirmse et al. (1983). Jurema is a thorny, small evergreen leguminous tree and pau branco is a deciduous tree; both readily coppice after being cut near ground level and develop a shrubby growth form. Fifteen coppicing jurema and pau branco plants were selected over a range of sizes. They graded in height from .45 to 2.00 m for jurema and .55 to 1.70 m for pau branco and represented the range of sizes available for goat browsing. The jurema plants had been cut 2 years previously; the pau branco plants were first year resprouts. The field data were collected in May, 1980, during the latter part of the rainy season. Reference Unit Technique The technique was tested by 3 observers (judges), 1 of which had 2 years previous experience sampling sagebrush (Artemisia tridentata Nutt.) biomass with the reference unit method. Prior to the beginning of the trial the experienced judge trained the inexperienced observers. This training involved estimating and verifying foliage biomass of 3 jurema shrubs of different sizes and was completed in 2 hours. Jurema The reference unit method was easily adapted to jurema because the growth form of a resprouted plant provided clearly distinguishable branches (Fig. 1). The procedure involved 3 steps: I) Representative branches, selected from plants not being tested, provided the reference units. The leafy growth on the branch comprised the reference unit per se. Andrew et al. (1979) suggested that the preferred size of the reference unit be IO-20% of the foliage weight of the average sample plant. Three branches (units) were used for jurema in this study. The smallest weighed 36 g (dry weight of foliage) and represented 7% of the average foliage weight of the 15 test plants. Two larger branches, each representing about 19% of the average plant, were also used for estimating jurema. Of these larger units, one had compacted branching and dense foliage typical of the test plants and the other had more dispersed foliage. For convenience, these 3 reference units will be referred to as small, large-compacted, and large-dispersed. 2) The number of times the foliage of the reference unit was replicated in the test plant was counted. Only the leaves were considered in the biomass estimations. The estimations were conducted in early morning in order to avoid any bias associated with wilting and curling of the leaves on these reference

Comparison of the Reference Unit Method and Dimensional Analysis Methods for Two Large Shrubby Species in the Caatinga Woodlands