Variation in Selected Wood Properties of V ochysia Guatemalensis from Four Sites in Costa Rica

Transcription

1 Variation in Selected Wood Properties of V ochysia Guatemalensis from Four Sites in Costa Rica Eugenio Gonzalez J. and Richard F. Fisher ABSTRACT. Vochysia guatemalensis D. Smith is distributed naturally from Mexico to Panama. It is a promising species for commercial reforestation on degraded lands, but little is known about the variation in im portant wood traits. Trees of this species were sampled in four natural stands to estimate the variation and radial gradients in wood specific gravity, fiber length, vessel density and vessel radial diameter. Increment cores were extracted from each tree at breast height and divided into I cm segments from pith to bark. Specific gravity was determined for each segment. The same wood cores were macerated for measuring fiber length. Vessel density and vessel radial diameter were measured for each segment from cross sections. Significant differences were found among sites (P < 0.05) for specific gravity and fiber length. Both of these variables also increased from pith to bark at all sites, and these radial gradients were significantly different among sites (P < 0.05). Vessel density was also significantly different among sites, but there was no significant change from pith to bark. No significant differences were detected among sites for mean vessel radial diameter; however, radial change in vessel diameter was significantly different among sites. For. Sci. 44(2): Additional Key Words: Geographic variation, radial gradients, wood density, fiber length, tropical hardwoods. OOD SPECIFIC GRAVITY is by far the most useful simple indicator of wood properties, as well as yield and quality of final product (Clark 1978, Zobel and Talbert 1984, Zobel and van Buijtenen 1989). Variation in wood specific gravity among sites and among geographic areas for a given species exists, and both environmental and genetic factors have been related to such variation (Rudman 1970, Zobel and Talbert 1984, Zobel and van B uijtenen 1989). Any factor that affects the growth pattern of a tree may affect its wood properties; however, the major environmental factor causing between-site variability is moisture availability (Panshin and de Zeeuw 1980, Zobel and van Buijtenen 1989). Howe (1974) found climate to have a significant effect on wood specific gravity among individual trees of three Costa Rican species. He discovered that Cordia alliodora [Ruiz & Pavon] Oken, one of the most valuable specie studied, had 60% higher wood specific gravity in tropical dry foresthan in tropical wet and moist forest. Tree to tree differences in specific gravity and other wood properties within a species or within a provenance are large (Zobel and van Buijtenen 1989) and are of a magnitude that is often greater than that between species. Among species, wood specific gravity is also highly variable. Williamson (1984), studying temperate and tropical hardwoods, found that specific gravity variation between species was greater among tropical hardwoods than among temperate hardwoods, with extremely low and high specific gravity woods found mainly in the tropics. Within trees, several patterns of variation are recognized (Zobel and van Buijtenen 1989). Ring-porous angiosperms and gymnosperms have an abrupt transition between early wood and late wood within the rings. In contrast, diffuse-porous woods have much less variability across tree stems since the cell types and their distribution tend to be more uniform (Panshin and de Zeeuw 1980). Also, wood properties are known to vary from the center Eugenio Gonzalez J. is Director, Organization for Tropical Studies, Palo Verde Biological Station, P.O. Box 676, 2050 San Pedro de Montes de Oca, San Jose, Costa Rica. Phone: (506) ; egonza@ns.ots.ac.cr; Richard F. Fisher is Professor, Department of Forest Science, Texas A&M University, College Station, Texas Phone: (409) ; r-fisher@tamu.edu. Acknowledgments: We thank the research project TRIALS and the personnel of the Organization for Tropical Studies for their support dudng the field work. We also thank Dr. R.J. Newton, Dr. M.G. Messina, Dr. G. Cobb and Dr. A.S.R. Juo for their constructive comments on earlier versions of the manuscript; Dr. H. van Buijtenen, Dr. W. Lowe, and Mr. T. Bryam for helping with the procedures and statistical analyses; and the USDA National Reseamh Initiative for financial support. Manuscript received January 27, Accepted June 10, Copyright 1998 by the Society of Amedcan Foresters Forest Sctence 44(2)

2 outward and from the base to the top of the tree. Many studies of temperate and tropical forest tree species, including both softwoods and hardwoods, have shown extreme changes from pith to bark in specific gravity, fiber length, and other wood properties (Taylor 1979, Williamson 1984, Wiemann and Williamson 1989). Most wood properties are moderately to strongly inherited, and wood specific gravity is by far the most important within-species wood characteristic to be considered in breeding. Its effect on yield and quality of fibrous and solid wood products and its strong inheritance patterns are of key importance for the breeder and the forester (Zobel and Talbert 1984, Zobel and van Buijtenen 1989). In addition, it can be changed by silvicultural practices (Williams and Hamilton 1961) as well as genetic manipulation (Zobel and Talbert 1984). Another important wood characteristic considered in breeding programs because of its great variability and strong inheritance is cell length (Smith 1967, Zobel and Talbert 1984, Zobel and van Buijtenen 1989). Understanding the source and magnitude of variation in wood properties provides a basis for improving wood utilization and selecting desirable wood traits. In this study the variation in specific gravity, fiber length, vessel density and vessel radial diameter and the radial gradient in these four wood properties was studied for Vochysia guatemalensis D. Smith [syn.: Vochysia hondurensis] (Vochysiaceae) from four natural stands in Costa Rica. Vochysia guatemalensis naturally distributed from Mexico to Panama (Whitmore and Hartshorn, 1969). Mature trees reach heights of 25 to 50 m and average from 60 to 90 cm in diameter at breast height (dbh) (Longwood 1962). It is a diffuse-porous hardwood, with an average specific gravity of 0.35, and is used for internal construction, casing, and framing for concrete construction. Anatomically its wood is reported to have an average fiber length of 1.51 mm, a vessel density of 2-5 vessel mm -2 and vessel radial diameter of 240 mm (Longwood 1962, Kukachka 1970). It grows rapidly in plantations. Trees planted in experimental trials in Costa Rica grew more than 2.0 m year -1 in height and 2.0 cm year -1 in dbh (Gonz lez and Fisher 1994, Butterfield and Espinoza 1995). Its good adaptability to degraded acid soils low in nutrients and high in aluminum has led to this species being favored for reforestation of degraded sites in the humid lowlands of Costa Rica. Material and Methods Trees of V. guatemalensis growing in natural stands at four sites in Costa Rica were core-sampled for studying the variation within trees and among sites for wood specific gravity, fiber length, vessel density, and vessel radial diameter (Table 1). At each site, 10 trees were randomly selected and sampled. Two samples at a 90 ø angle from each other were taken at breast height with a 12 mm diameter increment borer. Knots, crooks, spiral grain, and other areas of tension wood were avoided. The samples were kept in plastic tubes to avoid desiccation. Prior to the analyses the wood cores were visually checked for tension wood. Each core was divided into 1 cm segments from pith to bark for determining specific gravity according to Smith's maximum moisture content method (Smith 1954). In this method, wood cores are submersed in boiling water for about 1 hr and then placed under vacuum for several days until full saturation is reached. The samples are then blotted dry with a damp cloth, weighed to the nearest milligram, dried at 105øC, and reweighed. Specific gravity is determined from the following equation assuming a specific gravity of 1.53 for wood substance: Specific gravity = 1/((saturated weight - ovendry weight/ ovendry weight) + (1/specific gravity of wood substance)) Fiber length was determined on a subset of the specific gravity material. Macerations of every other 1 cm core section were prepared according to Franklin's modified method (Berlyn and Miksche 1976). The cores were placed in culture tubes containing 15 ml of a 1:1 solution of acetic acid and 30% hydrogen peroxide and heated in an oven at 60øC for approximately 48 hr. Following the maceration treatment, the pulp was washed thoroughly by decantation and filtration and then mounted on microscope slides without stain. Twenty-five fibers for each core section were measured using a projecting ampliscope (Echols 1959). Vessel density and vessel radial diameter were determined using the remaining subset of core segments. Crosssections of about 0.3 rnm thickness were taken at every other centimeter from pith to bark and mounted on microscope slides. Vessel density was determined by counting the number of vessels in at least 10 randomly selected ampliscope fields. Vessel radial diameter was determined by measuring 50 vessels, at the widest part, per slide. To evaluate the variation among sites and among trees within sites for specific gravity, the mean of the two core samples collected from each tree was used. An analysis of variance (ANOVA) was carried out in SAS (version 6.10) using a nested arrangement according to the following model: Specific gravity = g + sites + tree (sites) +, where g is the general mean and is the error term. Sites were treated as a fixed effect, and trees were considered a Table 1. Location and site characteristics of sampled stands of V. guatemalensis in Costa Rica. Mean dbh Latitude Longitude Elevation Precipitation a Temperature a Site (cm) North West (m) (mm yr-') (øc) La Argentina, Gu cimo ø 12' 83 ø 37' 105 3, Florencia, ø 22' 84 ø 21' 225 4, San Isidro, P6rez Zeled6n o 20' 83 ø 44' 703 3, San Miguel, ø 24' 84 ø 06' 500 4, a National Meteorological Service, San Jose, Costa Rica. 186 Forest Sctence 44(2) 1998

3 Table 2. Analysis of variance and variance components for wood specific gravity of V. guatemalensisfrom four sites in Costa Rica. Source of Variance variation df Mean square F value component (%) Sites * 26.3 Trees ** 50.5 Error NO E:* = significant at 5% level, ** = significant at 1% level. random effect. For fiber length, vessel density, and vessel radial diameter only the variation among sites was evaluated using a completely randomized ANOVA. When there were valid F test statistics, the means were compared using the Duncan's multiple range test at 5% significance level. Descriptive statistics were calculated for specific gravity, fiber length, vessel density, and vessel radial diameter for each site. To study the radial gradient from pith to bark in specific gravity, fiber length, vessel density, and vessel radial diameter among the four sites, each variable was plotted as a function of distance from pith, and subjected to an analysis of covariance. Individual measures for each segment and tree were averaged by site and tested for differences in slope and intercept. The intercept estimates each variable for the firstformed wood, and the slope measures the change in the properties along the radius. Simple correlations were also estimated among the four wood propertiestudied. Results Specific Gravity Significant differences in wood specific gravity among the four sites studied were detected (Table 2). The San Carlos site had the highest specific gravity, with a mean of and a range of (Table 3). This site had the highest minimum value for specific gravity, which was even higher than the mean of found for the Guficimo site. When estimating the between-tree variation from the two samples collected from each tree, significant differences (P < 0.01) were found. The estimated variance components obtained through the expected mean square for each source of variance showed that more than 50% of the variation in wood specific gravity was due to the tree component (Table 2). The site component represented only slightly more than 25% of total variation. The fitted regressions between specific gravity values and distance from pith indicated two patterns. First, all four sites studied showed a significant increase in specific gravity across the tree radius (Table 4, Figure 1). The coefficient of determination (R 2) for each individual site showed tha the linear regression model accounted for more than 80% of the variation for the, P6rez Zeled6n, and Sarapiquf sites. For the Guficimo site, the relationship between specific gravity and distance from pith was not strong (R2= 0.581), though it was significant (P < 0.01). Second, a 'slight decrease in specific gravity was observed for the five segments nearest the pith at three of the four sites (Figure 1). A regression analysis for the first four segments gave negative values for the slope estimator for the P6rez Zeled6n, Gu cimo, and Sarapiquf sites; however, they were not significant (P > 0.10). When the slope of the inner wood was compared individually for each site, a significant difference in slope was found. Inner wood had lower wood specific gravity than outer wood. Through a covariance analysis, the slope and intercept were compared among sites. Significant differences (P < 0.05) were found for both estimators (Table 4). The San Carlos site had the highest intercept of the four sites (0.308) with an increase of units of specific gravity for each centimeter of radial shift from the core of the tree toward the bark. Although the Sarapiquf site had a significantly lower intercept than did the site, its radial increase in specific gravity was twice that of. Table 3. Specific gravity, fiber length, vessel density, and vessel radial diameter of V. guatemalensisfrom four sites in Costa Rica. Wood property Guficimo Zeled6n Sarapiqul Specific gravity Mean c b a ab P6rez SE Minimum Maximum Fiber length (mm) Vessel density (Vessel/mm 2) Mean b a a b SE Minimum Maximum Mean ab b b a SE Minimum Maximum Vessel radial diameter (grn) Mean a a a a SE Minimum Maximum Means in the same row followed by the same letter are not significantly different at 5% level. Forest Sctence 44(2)

4 muj i , -r ,,, 1.2 (b) 1.0 ltj DISTANCE FROM PITH (cm) Figure 1. Radial shift in wood specific gravity (a) and fiber length (b) of naturally growing trees of V. guatemalensis in four sites in Costa Rica. Lines represent the average of ten trees. Fiber Length As with specific gravity, significant differences were found for fiber length across the sites (Table 3). For the site, fiber length ranged from to mm, with an average of nun, the highest mean found among the sites. The Sarapiquf site had the shortest fiber length (1.278 nun). A significant, positive linear relationship was also found for fiber length from pith to bark for all the sites. The rate of increase in fiber length (slope) was significantly higher at P6rez Zeled6n and Guficimo than at Sarapiquf and San Carlos while the intercept was significantly lower (Table 4, Figure 1). Coefficients of determination estimated for individual sites revealed that more than 75% of the varia- tion was attributable to the fitted linear relationship. The steepest increase fiber length was found at P6rez Zeled6n, with a slopestimator for the regression of nun cm-1), and the highest intercept (1.288 nun) was found at Sarapiquf (Table 4). Vessel Density and Vessel Radial Diameter Vessel density was also found to differ among the sites (Table 3). Sarapiquf had a mean of vessel cm -2(range vessel cm -2) which was significantly higher than that for P6rez Zeled6n and. This site also had the highest variation as indicated by the standard error of the mean. As might be expected, the high specific gravity San Carlos wood exhibited the lowest vessel density. wood had the highest mean vessel diameter (229 ram) followed by P6rez Zeled6n, Guficimo, and Sarapiquf, with diameters of 219 nun, 215 ram, and 211 ram, respectively. However, these values were not significantly different due to the high variation within sites (Table 3). The vessel density was found to decrease from pith to bark at the Gu cimo, P6rez Zeled6n, and Sarapiquf sites; however, the relationship was very weak and the slope estimator was not significant for any site (Table 4, Figure 2). When slopes and intercepts were compared across sites using a covariance analysis, significant differences were found (P < 0.01). Sarapiquf had the highest intercept as well as the highest rate of decrease in density (4.679 vessels mm -2 and vessels mm -2 cm -1, respectively). had a small increase in vessel density (0.006 vessels mm -2 cm-1), but it also had lowest vessel density at the pith (2.064 vessels ram-2). No clear linear pattern was found for vessel radial diameter when plotted as a function of distance from pith Table 4. Increase in specific gravity, fiber length, vessel density, and vessel radial diameter of V. guatamalensisfrom four sites in Costa Rica. Sites compared according to the intercept and slope regressed on distance from pith. Wood property Gu cimo P6rez Zeled6n Specific gravity Intercept b d a c Slope b b b a R Fiber length (mm) Intercept b b a a Slope a a b b R Vessel density (Vessel mm -2) Intercept b b c a Slope b b a c R Vessel radial diameter (urn) Intercept ns ns ns ns Slope ns ns ns ns R Values in a same row followed by the same letter are not significantly different at 5% level 2. 2 Parameter estimators not significant for any site. 3 For and sites, the parameter estimators were not significant. 188 Forest Sctence 44(2) 1998

5 (b) I I I I I I I -El Guacimo I I I I I I DISTANCE FROM PITH (cm) Figure 2. Radial shift in vessel density (a) and vessel radial diameter (b) of naturally growing trees of V. guatemalensis in four sites in Costa Rica. Unes represent the average of ten trees. I (Table 4, Figure 2). Though a slight increase with distance from pith was found for all sites (as indicated by the positive slope of fitted linear regressions), only for Gu cimo and P6rez Zeled6n was the slope estimator found to be significant. When compared through a covariance analysis, both intercept and slope were not significantly different among sites (Table 4). Linear Correlation Among Wood Properties Linear correlation coefficients among average values of the variables were calculated (Table 5). Positive and fairly strong correlations were found among specific gravity, fiber fiength, and vessel diameter as shown by the overall coefficients, calculated from pooled data. Specific gravity was not significantly correlated with vessel density across all sites. When analyzed by site, significant correlation coefficients between specific gravity and fiber length were obtained for Gu cimo,, and P6rez Zeled6n. A linear, positive, and relatively strong relationship between these two wood properties was obtained (Table 5). No significant correlation between specific gravity and other wood properties was observed at the Sarapiquf site. A significant linear relationship between fiber length and vessel diameter was observed across all sites; however, the type and degree of association varied widely among sites (Table 5). Guficimo and P6rez Zeled6n showed a strong positive linear correlation between these two properties, while the relationship was nonsignificant for and Table 5. Simple correlation coefficients (r) emong specific gravity, fiber length, vessel radial diameter, and vessel density from pith to bark for V. guatemalensis from four sites in Costa Rica. Specific gravity Gtfftcimo P6rez Zeled6n Overall Fiber Length Gu cimo P6rez Zeled6n Overall Vessel diameter Gu/tcimo P6rez Zeled6n Overall Fiber Vessel Vessel length diameter density * 0.568'* ns 0.903' 0.773' ' 0.867' 0.592' ns ns ns ns 0.720* 0.507* ns * ns * '* ns ns ns 0.747** * ns '* '* ns ' ' NOTE:* = significant at 1% level; ** = significant at 5% level; ns = not significant. Sarapiquf. The correlation between vessel diameter and vessel density was not significant across all sites. At two sites the correlation between these two variables was significant; however, it was positive at Sarapiquf and negative at P6rez Zeled6n. A significant negative linear correlation existed between vessel density and vessel radial diameter when data from all sites was considered together. On a site-by-site basis, Gu cimo, P6rez Zeled6n, and Sarapiquf had significant negative correlation coefficients while at the correlation was nonsignificant (Table 5). Discussion Specific Gravity Geographic trends in wood specific gravity have been reported for several tropical tree species (e.g., Howe 1974); however, as in the present study, the tree-to-tree variability within a provenance is usually greater than the provenance or site differences (Zobel and van Buijtenen 1989). Large variation among trees within stands is important for tree breeding program since much of the tree-to-tree variation is genetically controlled (Zobel and Talbert 1984, Zobel and van Buijtenen 1989). Consequently, genetic gains can often be obtained by selecting among trees within sites, ignoring geographic sources. Although some geographical trends in wood variation related to elevation and latitude have been reported for tropical hardwoods (Chudnoff 1976, Williamson 1984), trends in wood quality appear to follow rainfall patterns (Howe 1974). In the present study, precipitation might explain the differences in specific gravity found among the sites. Of the sites included in this study, P6rez Zeled6n has the lowest annual rainfall, but trees at this site did not have the highest specific gravity. Trees at and Sarapiquf, where much more rainfall occurs, had higher specific gravity (Tables 1 and 3). Though the general Forest Sctence 44(2)

6 pattern is to find higher specific gravity at drier sites, it is not always the case as reported previously by Howe (1974) for Cedrela rnexicana L. Other factors such as soil condi- tions and rainfall distribution, not evaluated in this study, may influence specific gravity. Differences in specific gravity among sites might be caused by differing tree age or dbh. Since a radial increase in this trait exists, smaller trees would have lower specific gravity values. Linear correlations for dbh and mean specific gravity Were calculated for all the trees sampled in this study without regard to the site. These were not significant, which can be interpreted as an indication of no age or diameter effect on mean tree specific gravity in this study. The mean specific gravity found in this study is below the average given for commercial timber. This is probably because the commercial values are for mature wood only, and the mean values in this study include many samples from low specific gravity wood near the pith. The lower values for fiber length, vessel density, and vessel diameter could also be due to the inclusion of many samples of wood from near the pith in the mean values. The maximum value for each of these properties (Table 3) lies well within the range reported for commercial wood. Radial shifts in specific gravity have been found for many tropical trees and seem to be related to ecological requirements of the species (Williamson 1984, Wiemann and Williamson 1988 and 1989). In the same way, high variation exists among sites or provenances. Butterfield et al. (1993) and Wiemann and Williamson (1989) have reported a radial increase in specific gravity from pith to bark in V. guaternalensis; however, no attempt was made by these authors to measure site-to-site variation. Our results agree with their observations and also show that specific gravity not only varies within the tree but also its rate of change is different among sites. Whether the differences found are genetically controlled or environmentally induced is not possible to explain from our results. It is important to note that a slight decline in wood specific gravity was observed at three sites for the inner wood (about 5 cm). This was also reported by Butterfield et al. (1993) for this species. Such a zone of lower specific gravity can preliminarily be interpreted as an indicator of a juvenile wood zone as found for other hardwood species (Zobel and van Buijtenen 1989); however, because many factors are associated with specific gravity variation within a tree, other wood properties, such as cell thickness, should be evaluated in order to draw final conclusions regarding juvenile wood formation in V. guaternalensis. Fiber Length In general, the fiber length found for V. guaternalensis in this study is in agreement with that reported previously for single site observations (Sotela and Carpio 1991, Butterfield et al. 1993). This species has relatively short fibers, as do many tropical hardwoods. The differences in fiber length among sites suggest the need for considering the provenance when estimating the variation of trees of this species. Tree-totree variation in fiber length was not estimated in this study, but high variability is reported for this wood property (van Buijtenen 1965, Zobel and van Buijtenen 1989). Since both the stand-to-stand variation and tree-to-tree variation in fiber length are moderately to strongly inherited (van Buijtenen 1965, Wheeler et al. 1966), relatively high genetic gains can be expected when selecting among natural stands and among trees within stands. The radial pattern of fiber length is much the same as for specific gravity; an increase fiber length occurs from pith to bark. Such radial gradients have been also observed for other tropical tree species (Briscoe et al. 1963, Wiemann and Williamson 1988 and 1989, Butterfield etal. 1993); however, not many studies have been conducted that estimate the among-sites variation. For the present case, differences in fiber length among geographic areas existed; however, how much is due to genetic factors and how much is due to environmental factors remains unknown. Vessels Density and Vessel Radial Diameter The size and distribution of vessels have a major effect on wood quality and utility (Zobel and van Buijtenen 1989) Since significant differences were found among sites for vessel density, differences in wood quality can also be expected among them. Indeed, if specific gravity is taken as one of the major wood properties indicating wood quality, t can be seen from Table 3 that the site with the lowest vessel density had the highest specific gravity. The inverse relationship was true at the Guficimo site. Th s relationship also held true within a given tree. As specific gravity increased from pith to bark, vessel density decreased (Table 4). For this species, a radial increase in the number of vessel seems to affect the solid portion of wood, which s expressed as a decrease in specific gravity. Both the mean vessel radial diameter and the radial gradient found in this study are in agreement with previous reports on this species (Sotela and Carpio 1991, Butterfield et al 1993). This was the only property found not to be different among sites, and not to show any radial gradient. Although we can offer no explanation for this, it should be noted that the large variation observed within sites for this property could be masking site variability. Relationship Among Wood Properties Wood propertie such as specific gravity, fiber length, vessel density, and diameter are often only slightly correlated: with one another (Zobel and van Buijtenen 1989). This does not appear to be case for V. guatemalensis (Table 5), especially for correlations among wood specific gravity and fiber length. These properties were found to correlate fairly well, indicating that as specific gravity increases from pith to bark, an increase fiber length can also be expected. The same can be said for specific gravity and vessel diameter. The significance of the correlations found among wood properties in this study is twofold. First, if two traits are genetically related to each other, then breeding for one could cause a favorable change in the other. Second, the degree of correlation found varies among sites, which indicates that genetic gains in one trait could depend on provenance if the differences among sites are found to be under geneti control. 190 Forest Sctence 44(2) 1998

7 Conclusions The large tree-to-tree variability in specific gravity and the potential site effect on wood specific gravity suggest that both parent and site conditions must be evaluated before meaningful conclusions can be drawn about wood quality when this species is planted for commercial purposes. The variation patterns found in naturally occurring trees of this species have several implications. First, the evaluation of specific gravity, fiber length, and vessel radial diameter for young trees can produce misleading conclusions about V. guaternalensis wood. All three properties were found to increase as tree diameter increased; therefore, wood parameters should be expressed as a function of diameter or age. Also, significant variation exists among sites and must be taken into account. Second, if final products are required to have a particular specific gravity or fiber length, the radial gradients observed for these properties can have a large impact. For example, if fibers longer than 1.5 mm are needed, trees should be more than 20 cm in dbh before harvesting. Literature Cited BERLYN, G.P., ANY J.P. MIKSCHE Botanical microtechnique and cytochemistry. The Iowa State University Press, Ames, Iowa. 326 p. BR!SCOE, C.B., J.B. HARRIS, ANt) D. WYCKOFF Variation of specific gravity in plantation-grown trees of bigleaf mahogany. Carib. For. 24: BVrmRFIELt), R.P., R.P. CROOK, R. At)AMS, ANt) R. MORRIS Radial variation in wood specific gravity, fiber length and vessel area for two Central American hardwoods: Hyeronima alchorneoides and Vochysia guatemalensis; natural and plantation-grown trees. IAWA Bull. 14: Bu -rerfield, R.P., ANt) C.M. ESPINOZA Screening trials for 14 tropical hardwoods with an emphasis on species native to Costa Rica. Fourth year results. New For. 9: CHUt)NOEE, M Density o 'tropical timbers as influenced by climatic life zones. Commonw. For. Rev. 55: CLARK, J.t)'A Pulp technology and treatment for paper. Miller Freeman Publications, San Francisco, California. 751 p. ECHOLS, R.M The ampliscope--an instrument for wood fiber measurements. J. For. 57: GONZALEZ, J.E., AND R.F. FISHER Growth of native forest species planted on abandoned pasture land in Costa Rica. For. Ecol. Manage. 70: HOWE, J.P Relationship of climate to the specific gravity of four Costa Rican hardwoods. An exploratory study. Wood Fiber 5: KUKACHKA, B.F Properties of imported tropical woods. USDA For. Serv. For. Prod. Lab. Res. Pap LONGWOOD, F.R Commercial timbers of the Caribbean. USDA For. Serv. Agric. Hahrib. No PANSHIN, A.J., AND C. DE ZEEUW Textbook of wood technology. Ed. 4. McGraw-Hill, New York. 722 p. R UDMAN, P The influence of genotype and environment on wood properties of juvenile Eucalyptus camaldulensis Dehn. Silvae Genet. 19: SMITH, D.M Maximum moisture method for determining specific gravity for small wood samples. USDA For. Serv. For. Prod. Lab. Rep. No SMITH, W.J The befitability of fibre characteristics and its application to wood quality improvement of forest trees. Siiv. Genet. 16: SOTELA, M., J. AND I. CARPIO M Vochysia hondurensis en Centroamerica. Infotree parcial INI Laboratorio de Productos Forestales, Instituto de Investigaciones en Ingenierfa, Universidad de Costa Rica. San Jos6, Costa Rica. 22 p. TAYLOR, F.W Property variation within stems of selected hardwoods growing in the Mid-South. Wood Sci. 11 (3): VAN BUIJTENEN, J.P Inheritance of fiber properties in North American conifers. Pap. presented at meeting of IUFRO Section 41. Melbourne, Australia. 12 p. WIEMANN, M.C., AND G.B. WILLIAMSON Extreme radial changes in wood specific gravity in some tropical pioneers. Wood Fiber Sci. 20: WIEMANN, M.C., AND G.B. WILLIAMSON Radial gradients in the specific gravity of wood in some tropical and temperate trees. For. Sci. 35: WHEELER, E.Y., B.J. ZOBEL, AND D.L. WEEKS Tracheid length and diameter variation in the bole of lobloily pine. Tappi 49: WILLIAMS, R.F., AND J.R. HAMILTON The effect of fertilization on four wood properties of slash pine. J. For. 59: WILLIAMSON, G.B Gradients in wood specific gravity of trees. Bull. Tort. Bot. Club. 111: WHITMORE, J.L., AND G.S. HARTSHORN Literature review of common tropical trees. University of Washington, Seattle. Inst. of For. Prod., Contrib. No p. ZOEEL, B.J., AND J. TALEERT Applied forest tree improvement. John Wiley & Sons, New York. 511 p. ZOEEL, B.J., ANt) J.P. VAN BUIJTENEN Wood variation, its causes and control. Springer-Verlag. Berlin. 363 p. Forest Science 44(2)