A comparative analysis of biomass production in five tropical tree species

A comparative analysis of biomass production in five tropical tree species

Forest Ecology and Management, 31 (1990) 153-166 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 153 A Comparative Analys...

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Forest Ecology and Management, 31 (1990) 153-166 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands


A Comparative Analysis of Biomass Production in Five Tropical Tree Species ARIEL E. LUGO 1, DEANE WANG2 and F. HERBERT BORMANN 3 1Institute of Tropical Forestry, Southern Forest Experiment Station, USDA Forest Service, Call Box 25000, Rfo Piedras, Puerto Rico 00928-2500 2Center for Urban Horticulture, GF-15, University of Washington, Seattle, Washington 98195

(U.S.A.) 3School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut (U.S.A.) (Accepted 27 January 1989 )

ABSTRACT Lugo, A.E., Wang, D. and Bormann, F.H., 1990. Comparative analysis of biomass production in five tropical tree species. For. Ecol. Manage., 31: 153-166. Trees of Casuarina equisetifolia, Albizia procera, Eucalyptus robusta, and two varieties of Leucaena leucocephala (an exotic and a native) were grown for maximum biomass production under the same climatic (1102 mm mean annual rainfall and 25°C mean annual air temperature) and edaphic conditions in the Lajas Valley of Puerto Rico (lat. 18 ° N., long. 67 ° W.). Management was intensive during early growth and establishment phases. Rates of large-branch/stem production ( diameter >/2.5 cm ) at age 5.5 years were 27.8, 20.4, 10.1, 7.7, and 5.5 t h a - 1year - 1for Casuarina, Albizia, Eucalyptus, exotic Leucaena, and native Leucaena, respectively. Stemwood biomass production of 40 tropical tree plantations established elsewhere for biomass production averaged 7.9 t h a - 1 year- 1. Species ranking in terms of total above-ground biomass and litter accumulation followed the same order. At age 5.5 years, the litter accumulation in plantations was (in the same order): 16.2, 10.2, 11.8, 7.0, and 6.5 t ha -1. Thirty-five natural tropical forest stands averaged 6 t h a - 1. Intensive management on fertile soils increases biomass yield of energy plantations, but not all species respond equally well to such treatments.


The rate of tree plantation establishment in the tropics has increased considerably since 1970. Sixty percent of the 11 million ha of tropical tree plantations are less than 10 years old (Brown et al., 1986). Many of these new plantations were established for energy production. These energy plantations use plantings at close spacing and short rotations to maximize biomass production per unit area. However, the effects of intensive management practices on the sustainability of site productivity are little understood. Also, energy



plantations have potential for being integrated with conventional herbaceous crops (permanently as wind breaks or short-term for soil improvement) in agricultural lowland valleys. To evaluate such uses and effects, data on biomass production and nutrient demands on the site are needed. Here we present results of an experiment in which five tree species were intensively managed under identical environmental conditions to compare their efficiency of biomass production and nutrient utilization. A forthcoming companion article will contain the nutrition aspects of the study. Studies such as these are needed to assess species suitability for sustained-yield energy plantations in the tropics. STUDYSITE The study was conducted in the Lajas Valley, Puerto Rico (lat. 18°N., long. 67 °W.), at less than 100-m elevation in the subtropical dry-forest life-zone (sensu Holdridge, 1967). The region is described in detail by Carter (1965). Between 1952 and 1980, rainfall at the site averaged 1102 m m year-1, and the mean annual temperature was 25 ° C. Throughout the 5.5-year study (198085), annual rainfall was 856, 1203, 1146, 1067, 1447, and 1150 mm, and air temperatures averaged 25.7, 25.9, 25.3, 25.8, 25.0, and 25.1°C, respectively. Soils at the site were classified by Roberts (1942) as intrazonal soils in the group planosol. Carter (1965) further described the soil as being in the Fraternidad series, specifically as Fraternidad clay. This soil (very-fine, montmorillonitic, isohyperthermic, Udic Chromustert) is high in natural fertility, but it cracks when dry and has poor drainage. The surface soil is very dark grayish-brown, friable, granular clay. Depth to calcareous material ranges from 15 cm to 90 cm, but the soil above this material is slightly acid to mildly alkaline. The subsoil is likely to contain soluble salt. High-salt control practices are in effect in the Lajas Valley. The study area was once planted with sugarcane, but before 1980 the site was abandoned. A comparison study was established simultaneously in the humid north coast of Puerto Rico on 15°-30 ° slopes. METHODS Originally, six taxa representing five genera were selected for the project: Albizia procera (Roxb.) Benth; Leucaena leucocephala (L.) Benth (two varieties, a native variety identified as P.R. and an exotic variety identified as K8); Eucalyptus robusta J.E. Smith; Cassia siamea Lam.; and Casuarina equisetifolia L. Cassia siamea was direct-seeded. However, after 6 months this species suffered high mortality and poor development and was excluded from the study. Leucaena and Albizia seeds were collected from the north coast of Puerto Rico. Seedlings of Eucalyptus and Casuarina were obtained from a Common-



wealth Government nursery in Dorado, Puerto Rico. All material was grown in the nursery at the Institute of Tropical Forestry. Five-month-old seedlings of five taxa (excluding Cassia) were transplanted from the Institute of Tropical Forestry nursery to the study site in February 1980. A randomized block design was used with six plots, one for each taxon, arranged in six parallel replicated blocks. Albizia and Leucaena were planted bare-rooted, while bagged Eucalyptus and Casuarina were used. Planting density was 10 000 seedlings/ha in each 10-m × 10-m plot (1-m × 1-m spacing), with 3 m of unplanted buffer zone between each plot in a block. Blocks were separated by a 5-m buffer zone. Trees were managed as agricultural commodities. The site was plowed for planting with a weighted disk-harrow drawn by a D-5 Caterpillar tractor, followed by light disking and land plane for border irrigation. The site was penetrated twice with Roundup to destroy wild grasses. Soils were treated with 112 kg N ha-1 and herbicide. Weeding and fertilization (112 kg N ha-1) were continued at 6-month intervals through the first 2 years. Irrigation was applied on an 'as-needed' basis to assure maximum survival of seedlings. Systematic thinnings were conducted at 6 months (spacing changed from 1-m X 1-m to 1m × 2-m) and 12 months (spacing changed to 2-m × 2-m). Measurements included annual height and diameter increment of seedlings and trees, as well as collection of material for nutrient-content and biomass estimates. These measurements were done on trees thinned from the plantation. In the 5.5 years of growth, trees in some plots suffered from outside intervention that disrupted the planting scheme. Of the six plots available for study of Leucaena K-8, only three could be used for the 5.5-years biomass study. To maintain a balanced design, three randomly chosen plots (out of all available for each species) were studied. Thus, a total of 15 plots were included in the harvest at age 5.5 years. We harvested plots for biomass determinations at age 5.5 years. The aboveground portion of trees in the central 4-m X 4-m subplot within each 10-m × 10m plot was harvested and weighed green in the field with spring scales (one accurate to 0.1 kg and one accurate to 10 mg). These subplots usually contained 4 trees but, due to sprouting or mortality, the number varied from 2 to 7. Harvested trees were divided into three categories: (a) large-branch/stems (arbitrarily defined as part of the large-branch/stem if greater than 2.5 cm diameter ); (2) small live branches (including leaves ); and (3) dead branches. This separation of biomass components permitted convenient measurement of biomass in the field but obscured tree form. Casuarina and Eucalyptus were essentially excurrent in form; most large-branch and stem wood was found in the excurrent single stem. Albizia and Leucaena K-8 were deliquescent with an ill-defined central stem, thus much large-branch and stem wood was located in large branches. Leucaena P.R. is a small tree with shrub-like characteristics. Although our data do not reflect these differences, these growth characteristics



are important in plantation management because they strongly affect the ease of harvesting. One tree in each plot was systematically subsampled by tissue type (leaves, bark, and wood) for fresh- to oven-dry-weight conversions, leaf:branch weight ratios, percentage of bark, and for tissue chemistry, which will be reported in the companion paper. Bark samples were taken from the main stem. Four randomly selected 0.5-m × 0.5-m samples of litter were collected from the center 4-m × 4-m subsection of each plot. Litter dry-weights were corrected for loss on ignition to avoid contamination from mineral soil. Within each litter plot, four samples were randomly collected from the top 3 cm of the mineral soil. Soils were combined to achieve one composite sample per tree plot. The soils were dried to constant weight at 105 ° C. Oven-dried samples (to constant weight at 60 °C) of litter and plant material were subsequently ground and passed through a 20-mesh Wiley mill before chemical analysis. Soil organic matter was determined with the Walkley-Black Method. All tests of significance were made at 95%, using Duncan's multiple-range procedure. RESULTS No significant soil differences existed among species plots for cation exchange capacity (48 c m o l ( + ) kg-1), Ca (4.9 mg g-1 soil), Mg (1.5 mg g - l ) , K (0.32 mg g - l ) , or P (0.09 mg g - l ) . Nitrogen averaged 0.19% and exhibited differences among species that can be attributed to species effects (e.g., Nfixers vs. non-fixers) rather than to intrinsic differences in the soil. Sodium, averaging 0.085 mg g-1 soil, exhibited similar species differences, to be discussed in the companion paper. Average percent organic matter for soils, with standard error and number of plots in parentheses, were 3.13 {0.06, 6), 3.81 (0.22, 6), 3.63 (0.21, 4), 4.33 (0.04, 3), and 4.17 (0.19, 5) for the Eucalyptus, Casuarina, Albizia, Leucaena K-8, and Leucaena P.R. plots, respectively. Height growth was nearly linear through the first 2 years in all species except the leucaenas (Fig. 1 ). The native Leucaena had the slowest growth rate, followed by Leucaena K-8. Casuarina had the tallest trees, followed by Albizia and Eucalyptus. Average height growth for the fastest-growing species exceeded 4 m year- 1. The height growth of Leucaena K-8 slowed to less than 2 m year-1 during the 2nd year of growth, and the native Leucaena P.R. grew only 1 m more after growing 2.4 m the first year. At age 5.5 years, the species maintained the same relative rankings, but rates of height growth changed. Casuarina averaged 3 m year -1, and Albizia averaged 2.7 m year-1. Declines in the height growth rates of Eucalyptus were faster than those in Leucaena K8, and both species had similar tree height at age 5.5 years. Height growth in Leucaena P.R. was the slowest, averaging at 1.3 m year- 1. Diameter growth-rate during year 1 was highest in Albizia and Eucalyptus




• []

Albizia procero E u c a l y p t u s robusta




vor K-8



v o r P.R.



I0 E

Cn .m 4U









Fig. 1. Height growth to age 5.5 years of five tree species grown u n d e r similar conditions at Lajas, Puerto Rico. S t a n d a r d error of the m e a n is shown with vertical bars. E a c h value is based on 30 trees.

and lowest in Leucaena P.R. (Table 1 ). During the 2nd year, diameter growthrate decreased in all species except Casuarina. The reduction in diameter growth of Eucalyptus and Leucaena K-8 was dramatic. At age 5.5 years, the ranking of species in terms of diameter growth was the same as at age 2 years, with Albizia being the fastest-growing species, followed closely by Casuarina. Leucaena P.R. was the slowest grower. The biomass accumulation by Leucaena K-8 trees at age 1 year was similar to that by Casuarina, much higher than that by the P.R. Leucaena, and lower than the accumulations observed in Albizia and Eucalyptus (Table 1 ). Above-ground biomass expressed on a unit-area basis (Table 2) resulted in

4.4 7.5 7.2 5.3 2.0

(30) (30) (29) (30) (30)

Year I

5.4 6.3 4.9 2.7 1.2

(145) (108) {129) (74) (144)

Year 2

2.0 2.3 1.6 1.2 0.6

(12) (10) {13) (13) (38)

Year 5.5

Diameter growth (cm year -~ )b

9733 6400 8633 5167 7367

Tree density at 4 months (trees ha- 1)

1.40 1.18 1.37 0.60 0.29


0.58 0.45 0.95 0.48 0.10

Small branches ¢

0.77 2.90 1.82 1.22 0.37

Large branchstems d

2.76 4.54 4.14 2.30 0.76

( 147 ) {112) (133) (93) {158)


Average tree biomass accumulation, year 1 (kg tree- 1)

aNumbers in parentheses represent the number of trees measured. Diameters were measured at breast height. bBased on diameter divided by age. < 2.5 cm diameter. d> 2.5 cm diameter.

Casuarinaequisetifolia Albiziaprocera Eucalyptusrobusta Leucaenaleucocephala (K-8) Leucaenaleucocephala (P.R.)


Early growth rates of five tree species a at Lajas, Puerto Rico







Total above-ground oven-dried biomass ( m e a n + 1 SE) at ages 1 year a n d 5.5 years a of five tree species growing in Lajas, P u e r t o Rico


Above-ground biomass (t ha- ' )

Litter (t ha -I)

Casuarina equisetifolia Albizia procera

Eucalyptus robusta Leucaena leucocephala (K-8) Leucaena leucocephala (P.R.)

Year 1b

Year 5.5 c

13.6 17.0 18.9 8.0 4.0

199.0 (71.6) 124.0 (25.1) 67.0 (14.2) 47.4 (6.2) 33.2 (5.2)

(1.0) (3.5) (1.9) {1.3) (0.6)

16.2 10.2 11.8 7.0 6.5

(0.9) b

(1.2) d (0.8) b (0.9) c (0.9) ~

aLitter accumulation corresponds to age 5.5 years. Size of area sampled was different for each of

the three rows of replicated blocks (see M e t h o d s ) . bn~_6. On=3. tin__.4.

en___5. TABLE 3 Rates of net above-ground biomass accumulation (t h a -1 year -1) in five tree species growing in Lajas, Puerto Rico a t age 5.5 years

Species Casuarina equisetifolia Albizia procera

Eucalyptus robusta Leucaena leucocephala (K-8) Leucaena leucocephala (P.R.)

Large branch-


stems a


27.8 20.4 10.1 7.7 5.0

36.2 22.5 12.2 8.6 6.0

Total b

39.1 24.4 14.3 10.0 7.2

"Including bark. bIncluding litter.

different rankings of species after 1 and 5.5 years of growth. After 5.5 years, Casuarina had the highest biomass, while Eucalyptus dropped from highest biomass at 1 year to the 3rd rank at year 5.5. Albizia was second to Casuarina, and the two leucaenas accumulated low amounts of biomass. Eucalyptus ranked above Albizia in litter accumulation on the forest floor, while Casuarina accumulated much more litter than any other species (Table 2). Above-ground biomass distribution (Fig. 2) shows Albizia and the leucaenas with the lowest proportion in leaves and Eucalyptus and Casuarina with the highest. Albizia had the largest proportion of its biomass in stem and largebranch wood, followed by Leucaena K-8 and Casuarina. Eucalyptus had a large




] Small Branches




r 7.6°/



~,I b i z i o




.6% ..6 Yo LI%











-- u c a l y p t u s


~ig. 2. Distribution of above-ground biomass at age 5.5 years for five tree species grown under similar conditions in Lajas, Puerto Rico. Table 2 :ontains the total biomass accumulation for each species.



.orcJe Branch and Stem




1.9 %



proportion of biomass in bark, which apparently reduced the proportion in large-branch/stemwood. Casuarina had a high proportion of small branches. Casuarina had the fastest rate of stemwood and total above-ground biomass accumulation, followed by Albizia, then Eucalyptus, with the leucaenas having the lowest rates of stemwood biomass accumulation (Table 3). DISCUSSION

The growth-rates observed in this study were high in comparison with results from other tree plantations ~n Puerto Rico and elsewhere (c.f. Lugo et al., 1988). For example, 1 m year -1 is considered adequate height growth in tree plantations. Yet, at age 2 years height growth of all species except native Leucaena was in excess of 3 m year -1, and after 5.5 years of growth the slowest grower (Leucaena P.R.) averaged 1.3 m year-1 (Fig. 1). Diameter growth of 2.5 cm year-1 is also considered adequate for tree plantations, but after 2 years all species except the native Leucaena P.R. exceeded this norm (Table 1 ). At age 5.5 years, diameter growth rates were much lower, but trees of the fastestgrowing species had already reached commercial size (10 cm Dbh). The high growth rates measured in this experiment are a result of intensive management during the site-preparation and tree-establishment phases. Seedbed preparation, irrigation, and weed control through establishment and early juvenile growth, coupled to excellent site conditions, combined to stimulate the fast tree growth. Clearly, trees responded well to intensive agronomic practices. Table 4 summarizes biomass production by tropical tree plantations managed for biomass production. The 30 stands with annual aboveground biomass production data averaged 11.1 (SE = 1.2 ) t ha-1, and the 40 stands with data on mean annual stemwood biomass production (/~a) averaged 7.9 (SE = 0.6) t ha -1. Three of the species in this study were highly productive (Table 3), if these mean values are used as a basis for comparison (c.f. Lugo et al., 1988 for a detailed discussion of biomass production in tropical tree plantations). The native Leucaena P.R. was clearly below average in biomass production, while the K-8 variety was close to average in terms of large-branch/stemwood production. However, in Costa Rica (Table 4) an unknown variety of the same species of Leucaena was planted in wetter environments where the biomass yields were higher than those in this study. In general, tree plantations increase biomass production with increasing water availability, provided the climate is not excessively wet (Lugo et al., 1988). The large reduction of biomass production in Eucalyptus between ages 1 (Table 2) and 5.5 years (Table 3) was not expected, and it may have been caused by salinity in the lower soil profile. In spite of that reduction in biomass production, Eucalyptus averaged rapid rates of biomass accumulation. Our

Guazuma ulrnifolia (Lain.)



(139) (n.a.)d (78.1) (64) (48.4) (8.3)



197c 245

(114.5) (71)


9 9 8 5 8 2

E. tereticornisSm.


( 82 ) (161)

(95) (46.3)

(61) (61.8) (75.9) (88) ( 140.0 ) (64)


8 9

E. grandis Hill ex Maid. E. saligna Sm.

123 195






7 16

E. globulus Labillardi~re.

Gmelina arborea Roxb.

8 8

10 8 8 15 8 9

Total" ( t ha - l)


21.9 27.3


17.6 12.2




12.1 b

Net production (t ha -1 year -1)

Age Above-ground biomass (years)

E. degluptaBlume.

E. citriodora Hook

Casuarina equisetifolia L. Eucalyptus camaldulensis Dehnh.

Cassia siamea Lain.





15.5 n.a. d 9.8 11.8 6.1 4.2

14.3 7.9

11.6 10.1

11.9 5.8

6.1 7.7 9.5 5.8 17.5 7.2





India India Colombia Costa Rica Colombia Panama

Colombia Brazil

India India

Costa Rica Colombia

Nigeria Colombia Colombia India Colombia Brazil


Tropical dry

Tropical dry

Tropical dry

Tropical dry Tropical dry Tropical dry Subtropical dry Tropical dry Subtropical moist Tropical premontane wet Tropical dry Tropical lower montane moist Tropical lower montane moist Tropical dry Subtropical moist Subtropical dry Subtropical dry Tropical dry Tropical moist Tropical dry Tropical dry


Escobar and Sutherland, 1986

Ladrach, 1987 Rodrlgues Pereira et al., 1984 Singh and Sharma, 1976 Singh, 1982 Ladrach, 1987 Rose and Salazar, 1983 Ladrach, 1987 Escobar and Sutherland, 1986 Escobar and Sutherland, 1986 Escobar and Sutherland, 1986

Negi et al., 1984 Negi et al., 1984

Oh-Adams, 1976 Ladrach, 1987 Ladrach, 1987 Prasad et al., 1984 Ladrach, 1987 Rodrlgues Pereira et al., 1984 Navarro, 1985 Ladrach, 1987


Age, total and net above-ground biomass production, mean annual stemwood biomass increment (]~ai), country grown in, life zone, and literature source for species grown for maximum biomass in the tropics


c~ o



9.2 8.8 5.0 9.7 9.0 29.8

(27.8) (43.1) (18.7)

31.2 46.7 22.7 52.5 72 119

3.4 5.3 4.5 5.4

2.9 5.2 4.6 8.3 13.5

(32) (90) (104) (191) (482)

46 105 118 217 541

16 20 26 26 4O

"Stemwood biomass in parentheses. bSecond-rotation coppice. ¢Canal-sideplantation. dNot available. ~Irrigated.





Shorea robusta Gaertn. f.

(51) (38) (49.0)

8 4 8

Populus deltoides Marsh Prosopisjuliflora (Sw.) DC. Samanea saman (Jacq.)


1.6 14.0 10.8 16.9 5.0

(n.a.) (69.7) (22.3) (84.1) (18.6)

5• 75.4 26.0 89.8 20.7

14.3 7.9 4.4

3 5.4 2.4 5.3 4.1

(23.4) (5.2) (3.9)

L. leucocephala (Lain.) de Wit.

37.3 10.3 6.1

2.6 1.3 1.4

Leucaena diversi[olia (Schlecht.) Benth.

Costa Rica Costa Rica Costa Rica Costa Rica India India Colombia India

8.2 8.1 4.2 9.0 6.4 9.4 6.1 3.2


4.5 4.0 7.3


India India India


India Costa Rica Costa Rica Costa Rica Costa Rica

n.a. 12.9 9.3 15.9 4.5


Costa Rica Costa Rica Costa Rica

9.0 4.0 2.8

Subtropical dry

Subtropical dry Subtropical dry Subtropical dry

Subtropical dry

Premontane moist Premontane wet Premontane rain Subtropical dry Tropical dry Tropical moist Premontane moist Premontane wet Tropical moist Tropical moist Tropical moist Premontane wet Subtropical moist Subtropical dry Tropical dry Subtropical dry Raman, cited in DeAngelis et al., 1981 Raman, cited in DeAngelis et al., 1981 Raman, 1976 Raman, 1976 Raman, cited in DeAngelis et al., 1981 Raman, cited in DeAngelis et al., 1981

Salazar, 1985 Salazar, 1985 Salazar, 1985 Salazar, 1985 Kaul et al., 1983 Gurumurti et al., 1984 Ladrach, 1987

Nerkar, 1984 Salazar, 1985 Salazar, 1985 Salazar, 1985 Salazar, 1985

Salazar, 1985 Salazar, 1985 Salazar, 1985




C~ *q

o~ O


Cb O



results are comparable to those of other species of Eucalyptus managed for biomass production elsewhere in the tropics (Table 4). The rate of biomass accumulation by Casuarina in this study was higher than any reported in Table 4. This high rate of biomass accumulation can be explained by the intensity of management and perhaps approaches the maxim u m production that can be expected from tree plantations. In the global review by Lugo et al. (1988), only two plantation stands were found to be as productive as this Casuarina stand. The highest value reported by t h e m was a 5-year-old Eucalyptus grandis stand in the tropical montane moist-forest lifezone that accumulated stemwood biomass at an annual rate of 38 t h a - 1. The large accumulation of litter in the study plantation is notable (Table 2 ). Casuarina and Albizia accumulated litter biomass in 5.5 years that was the equivalent of almost 10% of their above-ground biomass. For the other species, litter accumulation was equivalent to over 15% of the above-ground biomass, reaching a high of 20% for the native Leucaena. Litter biomass ranged from about 1.5 times to 4.0 times the leaf biomass of the trees, suggesting accumulation in excess of 1-year leaf-litter deposition. In contrast, the amount of litter in 35 naturally occurring tropical forest stands averaged 6 t h a - 1 ( SE ----0.5 ), or the equivalent of 2% of the above-ground biomass in those forests (Brown and Lugo, 1982). Apparently, the amount and proportion of litter relative to aboveground biomass is a fundamental difference between natural forests and plantations in the tropics. This accumulation of mass and nutrients (unpublished data) in plantations may be important in the moderation of erosion and in support of forest growth of the next rotation if the reservoir is conserved during harvest and stand-regeneration operations. The proportion of the above-ground biomass production that was largebranch/stemwood ranged from 76% in Casuarina to 91% in Albizia (Table 3 ). These high proportions reflect the favorable characteristics of these species as fuelwood producers. Because Albizia is deliquescent, it may not be a good charcoal species, although it may be excellent for firewood. In contrast, the average for the stands in Table 4 was 70%. High rate of biomass production, high proportion of biomass in stem and branches, and observed vigorous coppicing after harvesting support the value of Albizia and Casuarina as excellent fuelwood species under the conditions of our experiment. ACKNOWLEDGEMENTS This study was initially funded by a grant from the Center for Energy and Environment Research (University of Puerto Rico) to A. Alexander and J. Whitmore. Grants from the A.W. Mellon Foundation to F.H. Bormann, and from the Tropical Research Institute, Yale School of Forestry and Environmental Studies to D. Wang, supported measurements at age 5.5 years. All work was conducted in cooperation with the University of Puerto Rico. Sandra



Brown, Alex Alexander and three anonymous reviewers reviewed the manuscript.


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