Performance of Pinus elliottii, Pinus caribaea, their F1, F2 and backcross hybrids and Pinus taeda to 10 years in the Mesopotamia region, Argentina

Performances of Pinus taxa were studied to 10 years of age in two trials in each of Misiones and Entre Ríos provinces across the Mesopotamia region of Argentina. Taxa comprised 22 populations from sources in Argentina, Australia, Brazil and Zimbabwe including Pinus elliottii var. elliottii (Pee), Pinus caribaea var. hondurensis (Pch), their four, inter-specific hybrids (F₁, F₂ and backcrosses from F₁ to Pch and to Pee—all as broadly based bulks); other Pee and Pinus taeda (Pt) comprised narrower or unspecified bulks. Variable numbers of taxa were missing at each site. Mean survival across sites at age 10 years ranged 53.2–91.3% averaging 74.2%. Analysis of variance of plot means indicated population effect was statistically significant (p < 0.05) for all or most growth and quality traits at all sites. However, significant differences from the nominated check seedlot at the Entre Ríos sites (Pee, Australia) were extremely rare, while quite common at the northern, Misiones sites (check seedlot a Pt population). In the Misiones trials, F₁, F₂ and both backcross hybrids showed better stem straightness than Pee and Pt from Argentina, generally with statistically significant differences (p < 0.05). Pt showed lowest forking scores (desirable). Taxon × environment interaction was statistically significant (p < 0.01) for growth traits only (p > 0.05). However, this interaction contributed an average of only 34.1% of the taxon variance suggesting a lack of practical importance. Taxa most suitable for deployment in the Mesopotamia region, Argentina are suggested.     

我国南方杂交松生长和材性的研究

对广东湛江 15年生和广西东门林场 7年生的杂交松试验林的生长和材性进行研究 ,结果表明 :广东湛江 15年生最优良杂交松组合 (正种加勒比松河 6×湿地松混合花粉 )的材积较对照正种加勒比松材积大 1.66倍 ;广西东门林场 7年生杂交松 (湿地松×洪都拉斯加勒比松 ) 3个家系的平均材积较湿地松 60个家系的大 2倍 ,最优良杂交松 (家系 62号 )的材积较湿地松大 2 .88倍。以上优良杂交松可分别在我国热带及南亚热带推广。杂交松的年轮平均密度和晚材率大致处于正种加勒比松和湿地松之间 ,年轮平均密度的差异主要是由于晚材率差异所致。 3种松木的变异模式都是从髓心起逐渐增高 ,到第 8～ 9年变化趋缓 ;杂交松在近树皮处的纤维长度介于两个亲本之间 ,近髓心处的纤维长度略大于两个亲本的纤维长度     

Tolerance of Pinus caribaea var. hondurensis seedlings to five post-emergence herbicides

Five herbicides were evaluated for their potential to reduce reliance on directed white (mineral) spirits applications and hand weeding for grass control in P. caribaea var. hondurensis nurseries. Post-emergent applications of haloxyfop-methyl, fluazifop-P, fenoxaprop-ethyl and sethoxydim were identified as being selective to P. caribaea var. hondurensis seedlings whilst providing effective grass control. Ametryn was toxic to P. caribaea var. hondurensis seedlings at 1 kg ha^–1.     

First-year freeze hardiness of pure species and hybrid taxa of Pinus elliottii (Engelman) and Pinus caribaea (Morelet)

Six pine taxa were compared for freeze hardiness following 2 whole plant freeze tests carried out in early and late winter 1993–94. Pure species included slash pine (Pinus elliottii var. elliottii Engelman) (PEE), Caribbean pine (Pinus caribaea Morelet) var. bahamensis (PCB), and var. hondurensis (PCH). The same six slash pine parents served as the female parents of the pure slash pine taxon and were control pollinated to create the F₁ hybrids of slash pine × Caribbean pine var. bahamensis (F1B), the F₁ of slash pine × Caribbean pine var. hondurensis (F1H), and the backcross of slash pine × (slash pine × var. hondurensis (B1H). Four traits related to adaptation (bud set, foliage/stem damage, survival) were compared. Significant differences existing among the three species at all temperatures for both freeze tests. The 3 hybrid taxa were intermediate to parental taxa for all measured traits and only at extreme temperatures was there an indication of hybrid vigor. However, inspection of foliage damage curves to estimate temperatures at which 50 percent damage occurs indicated that the hybrids deviated from the mid parent value toward the more freeze susceptible parent. Discriminant functions which included foliage damage and survival traits provided the best segregation of taxa groupings and the combining of canonical variable 1 for these discriminant functions placed hybrids intermediate to parental taxa. Parent × taxa interaction was significant, suggesting that within these taxa, family to family variation occurred, but different maternal parents were associated with the freeze hardly families in different taxa.This is Florida Agricultural Experiment Station Journal Series No. R-04852 of the Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL, 32611     

The flexural properties of young Pinus elliottii x Pinus caribaea var. hondurensis timber from the Southern Cape and their prediction from acoustic measurements

The objectives of this study were to determine the bending strength and stiffness properties of young Pinus elliottii x P. caribaea var. hondurensis timber from the Southern Cape, South Africa, and to evaluate the predictability of these properties from acoustic measurements on standing trees, logs and their sawn boards. The timber has good bending strength (MOR) properties but the mean stiffness (MOE_stat) was significantly lower than that required for structural-grade timber. The mean MOE_stat was also about 30% lower than a P. radiata control sample. The MOE_stat of sawn boards could be predicted from acoustic stress wave measurements on standing trees with a correlation (r) of 0.48 and from acoustic resonance frequency measurements on sawn boards with a correlation of 0.85. The MOR of sawn boards was not significantly correlated with acoustic stress wave measurements on standing trees and only moderately correlated with acoustic resonance frequency measurements on sawn boards (r = 0.45).     

Long-term impacts of harvest residue management on nutrition, growth and productivity of an exotic pine plantation of sub-tropical Australia

Residue retention is an important issue in evaluating the sustainability of production forestry. However, its long-term impacts have not been studied extensively, especially in sub-tropical environments. This study investigated the long-term impact of harvest residue retention on tree nutrition, growth and productivity of a F1 hybrid (Pinus elliottii var. elliottii × Pinus caribaea var. hondurensis) exotic pine plantation in sub-tropical Australia, under three harvest residue management regimes: (1) residue removal, RR₀; (2) single residue retention, RR₁; and (3) double residue retention, RR₂. The experiment, established in 1996, is a randomised complete block design with 4 replicates. Tree growth measurements in this study were carried out at ages 2, 4, 6, 8 and 10 years, while foliar nutrient analyses were carried out at ages 2, 4, 6 and 10 years. Litter production and litter nitrogen (N) and phosphorus (P) measurements were carried out quarterly over a 15-month period between ages 9 and 10 years. Results showed that total tree growth was still greater in residue-retained treatments compared to the RR₀ treatment. However, mean annual increments of diameter at breast height (MAID) and basal area (MAIB) declined significantly after age 4 years to about 68–78% at age 10 years. Declining foliar N and P concentrations accounted for 62% (p < 0.05) of the variation of growth rates after age 4 years, and foliar N and P concentrations were either marginal or below critical concentrations. In addition, litter production, and litter N and P contents were not significantly different among the treatments. This study suggests that the impact of residue retention on tree nutrition and growth rates might be limited over a longer period, and that the integration of alternative forest management practices is necessary to sustain the benefits of harvest residues until the end of the rotation.     

QTL detection for growth traits in Pinus elliottii var. elliottii and P. caribaea var. hondurensis

Quantitative trait locus （QTL） detection was carried out for growth traits in 122 F1 progenies of Pinus elliottiivar. elliottii （PEE） x tR caribaea var. hondurensis （PCH） hybrid. The PCH male parent linkage map contained 108 mark- ers in 16 linkage groups, while the PEE female parent contained 93 markers in 19 linkage groups. Sequence-related amplified polymorphism （SRAP）, microsatellite （SSR）, expressed sequence tag polymorphism （ESTP） and inter-simple sequence repeat （ISSR） were selected from an existing linkage map. Growth traits investigated were height at age five （HT05） and six years （HT06）, diameter at breast height at age five （DBH05） and six years （DBH06） and annual growth from age five to six years of height （AGHT） and diameter at breast height （AGDBH）. KruskaI-Wallis and interval mapping approaches were used to estimate levels of significance, the number of QTLs, the percentage of the phenotypic varia- tion explained by each of QTLs and their positions on the genetic linkage maps. Twenty six QTLs with significance levels p 〈 0.05 were detected on the parental maps for the six growth traits, which explained more than 15% of the phenotypic variation, suggesting an existence of major-effect genes. Several QTLs had the pleiotropy controlling more than one sin- gle growth trait. Overall, the proportion of phenotypic variation explained by QTLs ranged from 5.9% to 40.6% for HT05 and HT06, from 6.6% to 42.0% for DBH05 and DBH06 and from 5.8% to 22.3% for AGHT and AGDBH. The results from this study provide a basis for marker-aided selection （MAS） in PEE x PCH hybrids.     

Indole-3-butyric acid accelerates adventitious root formation and impedes shoot growth of Pinus elliottii var. elliottii?×?P. caribaea var. hondurensis cuttings

Many plantation tree species are cloned to achieve the growth, disease resistance and wood quality characteristics required for a successful economic venture. However, clonal propagation is limited by declines in adventitious root formation with increasing stock plant age. We examined the effects of immediate or delayed IBA application on adventitious root formation and subsequent root and shoot development of cuttings harvested from 8-year-old clonal hedge plants of Pinus elliottii var. elliottii × P. caribaea var. hondurensis. IBA applied at the time of setting accelerated root formation, elevating the percentage of cuttings with roots at 13 weeks post-setting from 45 to 78% and from 83 to 93% for a low- and a high-rooting clone, respectively. Final rooting percentages for the same treatments and clones (78 and 85%, and 88 and 100%, respectively, at 20 weeks post-setting) were not significantly affected by IBA application. IBA increased the root:shoot ratio of rooted cuttings by decreasing shoot weight compared with untreated cuttings, without affecting root weight, root length, root surface area or root volume. IBA was only effective when applied at the time of setting. A simple IBA treatment for cuttings from 8-year-old clonal hedges, by accelerating root production, has potential for reducing nursery costs and increasing the root system quality of containerised pine cuttings.     

Vegetational indicators as independent variables in forest growth prediction in West-Central Alberta,Canada

Vegetational variables were added to conventional edaphic and topographic properties as independent variables in tree growth prediction equations. Cover estimates of individual understory species, litter cover, deadfall cover, and advance regeneration density accounted for significant amounts of variability in lodgepole pinr (Pinus contorta Dougl. var. latifolia Engelm.) and white spruce (Picea glauca (Moench) Voss) mean annual increment in total volume (MAI) and site index (SI). R² values for lodgepole pine MAI and SI increased from 0.24 to 0.66 and 0.49 to 0.71, respectively, and for white spruce MAI and SI increased from 0.53 to 0.86 and 0.58 to 0.91, respectively, with addition of vegetational variables. The inclusion of certain forest understory species as independent variables accounted for more MAI and SI variability than did some of the more frequently used soil and site properties. Inclusion of vegetational variables in a multiple regression may facilitate refinement of a productivity-oriented ecological classification of forest stands.     

Sawn-timber and kraft pulp properties of Pinus elliottii × Pinus caribaea var. hondurensis and Pinus patula × Pinus tecunumanii hybrids and their parental species

A study was undertaken to evaluate the quality of the timber produced by Pinus elliottii × Pinus caribaea var. hondurensis (PECH) and the Pinus patula × Pinus tecunumanii low elevation (PPTL) and high elevation (PPTH) hybrids and their parent species for both kraft pulp and sawn-timber production. Trees were taken from unthinned tree improvement trials managed for pulpwood, ranging in age between 15 and 19 years. All sawn boards produced by study trees met the minimum wood density requirement for S5-grade structural timber (360 kg m^?3), but approximately 17% of the boards failed to meet the other requirements for the grade, largely due to knot-related defects. Results of dynamic modulus of elasticity assessments performed on all of the boards suggested that a large percentage of boards would not meet the specified average stiffness (7 800 MPa). This was confirmed by the results of static bending tests performed on a subsample of boards. All boards tested for all species and hybrids met the required fifth percentile bending strength value for grade S7 (15.8 MPa) according to SANS 6122 (2008) specifications. The kraft pulping results indicated that the samples taken from the upper part of the stem yielded slightly better results on average than the samples representing the whole tree with respect to uniformity in the kappa vs charge and temperature, and yield vs kappa traits, with slight improvement of pulp yield (52% vs 50%) and some strength properties compared with whole tree pulping. Samples from the upper part of the stem had a close delignification rate in the 60–80 kappa range. The delignification rate for PPTL in the 60–80 kappa range was slower and the yield was slightly lower than P. patula (53.17% vs 52.72%) despite a higher kappa number. The pulp strengths short-span compressive test, breaking length and tearing strength of PECH were similar to those of P. elliottii, which were in turn generally lower than those of P. patula. The pulp strengths of PPTL and PPTH were similar to those of P. patula, whereas P. caribaea strengths were intermediate between those of P. patula and P. elliottii. With the exception of a slightly lower pulp yield, PPTL emerged as the best all-round hybrid for both pulp and sawn-timber properties.     

Genetic variation and demographic contraction of the remnant populations of Mexican Caribbean pine (<Emphasis Type="Italic">Pinus caribaea</Emphasis> var. <Emphasis Type="Italic">hondurensis</Emphasis>: Pinaceae)

• Introduction   

The recognition of endangered species takes into account geographic isolation and small size of populations, as is the case of two populations of Pinus caribaea var. hondurensis distributed in México.     

Growth and dynamic modulus of elasticity of Pinus patula × Pinus tecunumanii hybrids in Mpumalanga,South Africa

Field establishment of South Africa’s most important commercial pine species, Pinus patula, is severely hampered by the pitch canker fungus, Fusarium circinatum. Importantly, hybrids between P. patula and other pine species tolerant to the pitch canker fungus, such as P. tecunumanii and P. oocarpa, have been identified as an alternative planting stock. In this study, variation in tree volume and dynamic modulus of elasticity (MOE_dym) of the P. patula × P. tecunumanii (low- and high-elevation [LE and HE] ecotypes) hybrid was compared with the P. elliottii × P. caribaea hybrid, and the pure species P. tecunumanii (LE) and P. patula. The MOE_dym was assessed using the Fakkop TreeSonic microsecond instrument across three sites. The results of the study showed that P. patula × P. tecunumanii LE performed significantly better than P. patula × P. tecunumanii HE for volume and MOE_dym, which in turn was significantly better than P. patula. The MOE_dym and tree growth decreased with an increase in elevation. There was significant taxon × site interaction for volume and MOE_dym. The results of these trials suggested that P. patula × P. tecunumanii LE is a suitable alternative to P. patula in the Sabie region of Mpumalanga in South Africa on frost-free sites, in terms of the traits that were assessed.     

Genetic variation of isolated Picea balfouriana populations from the southeast of the Qinghai-Tibet Plateau

? The objective of this work is to estimate the level of genetic variation and pattern of genetic structure of isolated Picea balfouriana populations.

? Nine SSR markers and six STS markers were assayed in ten natural populations of P. balfouriana, which is a regionally distributed conifer species in the southeast of the Qinghai-Tibet Plateau.

? Expected heterozygosity ranged from 0.592 to 0.710 based on SSRs, and from 0.489 to 0.635 based on STS markers. The SSR and STS markers revealed that 11% and 12% of variation, respectively, was present among populations. However, the SSRs showed no deviation from the Hardy-Weinberg equilibrium (F _IS = ?0.030), unlike the STS markers (F _IS = 0.249). In addition, assignment methods showed that individuals from the same sampling site usually cluster together.

? Our results indicated that the distribution of genetic variation and population genetic structure of P. balfouriana may be attributed to habitat fragmentation and heterogeneous environments caused by the complex topographic environment in the Qinghai-Tibet Plateau. The population genetic information obtained in our study will benefit the development and utilization of appropriate conservation and breeding strategies for P. balfouriana.

     

Growth of Eucalyptus tereticornis and Pinus caribaea at different sites in Northern Nigeria

Trial plantings of exotic tree species commenced in the northern areas of Nigeria over 20 years ago. Based on data from permanent sample plots of Eucalyptus tereticornis and Pinus caribaea, two of the promising exotic tree species tried, growth studies involving development of top height and total volume production (tvp) trends have led to the derivation of growth figures for each of the species.The results showed that, on an average site for E. tereticornis with a top height of 24.8m at a reference age of 15 years, a tvp of about 286 m³/ha at the peak of mai (age 19) could be obtained, while for an average site for P. caribaea, with a top height of 23.4m at age 20, a tvp of about 726 m³/ha at the peak of mai (age 30) could be obtained. The growth figures, which were compared with those obtained in other countries, are discussed in relation to the management of the species.     

有无枯落物覆盖对加勒比松林地表径流及其K流失的影响

为了解有无枯落物对K随地表径流损失的影响,研究比较了有无枯落物加勒比松林地表径流中的K流失的特点,结果表明:有、无枯落物加勒比松林的年地表径流量分别为12.6、51.8 mm,主要发生在夏季,对应的地表径流系数分别为0.60%、2.48%,表明前者能有效削弱地表径流。2种林分的降水量和地表径流量的关系可用二项式方程表示。有、无枯落物加勒比松林各月的地表径流K浓度分别为3.2~36.3、1.8~6.0 mg.kg-1,通过地表径流输出的K量分别为701、982 g.hm-2。2008年5、6月为有枯落物加勒比松林的K流失高峰,6月为无枯落物加勒比松林的K流失高峰。地表径流的K浓度和径流量之间存在对数关系,K的流失量主要由径流量决定。     

Nitrogenase activity (C2H2 reduction) in the forest floor of a Pinus elliottii plantation following superphosphate addition and prescribed burning

The C₂H₂ reduction method was used on eight occasions between January 1983 and January 1984 to measure the effects of adding superphosphate (60 kg P ha⁻¹ applied in September 1979) and of prescribed burning (May 1981) on nitrogenase activity in the forest floor of an 18 year old Pinus elliottii plantation in southeast Queensland.Averaged over sampling days, superphosphate increased nitrogenase activity in the forest floor 4.0-fold in the unburnt plots and 2.0-fold in burnt plots. In the absence of superphosphate, burning increased activity 3-fold but in plots receiving superphosphate a response to burning was seldom evident. The responses were attributed to increased nutrient availability. Nitrogenase activity was highest in the F1 layer (weathered needles in the centre of the forest floor) and lowest in the L layer (recently fallen needles). Moisture content has a strong effect on forest floor nitrogenase activity and, in laboratory dried material, activity increased most rapidly for moisture contents between 80 and 170%.Mean nitrogenase activity in the forest floor on individual sampling days ranged from 3 nmol C₂H₄ g⁻¹ (L layer, control treatment) to 233 nmol C₂H₄ day⁻¹ (F1 layer, combined refertilized and burnt treatment). Activity was also detected in dead wood, female cones, thinning debris, soil (0–5 cm) and roots, but was negligible in bark and green needles.     

Developing a taper model for the Pinus elliottii × P. caribaea var. hondurensis hybrid in South Africa

A relatively new hybrid, Pinus elliottii × P. caribaea var. hondurensis (Pexc), for which a taper model does not exist, is being planted commercially in South Africa. This study primarily focused on developing a taper model for Pexc in South Africa. Taper data were collected from a total of 363 trees, in the Mpumalanga and Limpopo provinces, using a random sampling method. A subsample was selected to determine if altitude, rainfall, temperature or soil have a significant influence on the taper of Pexc. Only rainfall significantly influenced the overall taper. The Max and Burkhart segmented polynomial taper model, as well as the Kozak88, Kozak01 and Kozak02 variable exponent taper models, were fitted, compared and tested using the statistical analysis system (SAS). The predictive ability of the models was evaluated based on the results from the mean bias, standard deviation, the standard error of prediction and the average percentage deviation. The Kozak02 model had the best fit overall followed by the Max and Burkhart model (MB76). The MB76 model, however, predicted the volumes more accurately than the Kozak02 model.     

Evaluation of tropical forage legumes under Pinus caribaea var hondurensis in Turrialba,Costa Rica

A number of tropical forage legume species were sown in Turrialba., Costa Rica, in plots in the open and under an even cover of Pinus caribaea var hondurensis of basal area 30m²/ha. Mean light level under the trees was 18% full sunlight. After 19 months, species showing good shade tolerance and adaptation to local conditions were: Centrosema spp., Desmodium spp., Flemingia congesta, Galactia striata. Species of poor performance in the shade were: Stylosanthes spp., Indigofera hirsuta, Cajanus cajan, Stilozobium sp.. Canavalia ensiformis, an annual, grew well in the shade, as did Lablab purpureus and Vigna unguiculata. It is suggested that further work be done on the shade tolerant species for eventual use as components of silvopastoral systems. Recommendations are made concerning germplasm collections, evaluation methodology and research for silvopasture development.

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Resumen Se sembraron especies diversas de leguminosas forrajeras en Turrialba, Costa Rica, en parcelas al descubierto y bajo una cobertura uniforme de Pinus caribaea var hondurensis con area basal de 30 m²/ha. El nivel medio de iluminación bajo los pinos era 18% del nivel a pleno sol. Despues de 19 meses las especies que mostraron buena adaptación a la sombra eran: Centrosema spp., Desmodium spp., Flemingia congesta, Galactia striata. Especies de bajo rendimiento en la sombra eran: Stylosanthes spp., Indigofera hirsuta, Cajanus cajan, Stilozobium sp., La planta anuál Canavalia ensiformis creció bien bajo sombra, como tambien lo hicieron Lablab purpureus y Vigna unguiculata. Se sugiere que se investiguen las especies mencionadas como bien adaptadas al sombreamiento para uso en sistemas silvopastoriles. Se presentan recomendaciones sobre colecciones de germoplasma, metodologia de evaluación y investigación para el desarollo de sistemas silvopastoriles.

     

Investigation on the hydrogen abstraction from methyl glucoside by active oxygen species under oxygen delignification conditions IV: appearance of kinetic isotope effect in the reaction between methyl glucoside and deuterated methyl glucoside

In our previous studies, a clear kinetic isotope effect was observed when a pair of carbohydrate model compounds, methyl ??-d-glucopyranoside (MGP??) and a deuterated MGP?? labeled at the anomeric (methyl ??-d-(1-²H)glucopyranoside) or C-2 position (methyl ??-d-(2-²H)glucopyranoside), was reacted with active oxygen species (AOS) generated by reactions between O₂ and a phenolic compound, 2,4,6-trimethylphenol (TMPh), under oxygen delignification conditions. These results indicate that the AOS abstract the anomeric and C-2 hydrogens of MGP??. Contrarily, no clear kinetic isotope effects were observed when AOS were generated by reactions between O₂ and another phenolic compound, 4-hydroxy-3-methoxybenzyl alcohol (vanillyl alcohol, Valc), and hence, the abstraction of the anomeric and C-2 hydrogens by the AOS was not confirmed. In this study, a pair of MGP?? and the deuterated MGP?? labeled at all the positions, (²H₃)methyl ??-d-(1,2,3,4,5,6,6-²H₇)glucopyranoside, was reacted with AOS generated from TMPh or Valc under oxygen delignification conditions to further examine the appearance of a kinetic isotope effect. A clear kinetic isotope effect was observed when either of TMPh or Valc was the origin of AOS, indicating that some AOS abstract at least a hydrogen of MGP?? in either case. The results are further discussed focusing on the type of AOS generated from TMPh and Valc.     

Family variation in diameter growth and acoustic velocity in three 5‑year‑old South African‑grown Pinus elliottii×Pinus caribaea progeny trials established on three diverse sites

This paper discusses family differences in the stiffness of young Pinus elliottii×P. caribaea wood and how these differences relate to differences in growth rate and altitude. Trees for measurement were obtained from three 5?year?old progeny trials established at three diverse sites on the estate of Komatiland Forests (Pty) Ltd. The trials included the following controls: P. elliottii, P. patula, P. taeda, P. patula×P. tecunumanii and P. tecunumanii. Diameter at breast height and sound velocity (which has a direct relationship with wood stiffness) were measured on all healthy trees. Virtually all of the species and hybrids exhibited marked decreases in growth rate and velocity (stiffness) with increasing altitude; the effect on velocity was most likely partly due to the inverse relationship that normally exists between wood density and altitude in most pines. Differences in growth rate explained only a small proportion of the total variation in velocity. The average sound velocities of P. patula, P. patula×P. tecunumanii and P. tecunumanii surpassed the velocity values of most of the P. elliottii×P. caribaea crosses at all three sites. Results of Spearman rank correlations of average treatment velocity between sites suggested only some similarity in the ranking orders among sites. The inverse effect of altitude on wood stiffness suggests that trees grown at increased altitudes are likely to yield wood with higher proportions of wood not meeting the minimum stiffness requirement for structural lumber at final harvest. The wood of some P. elliottii×P. caribaea crosses would be particularly vulnerable. Considering the large variation found, presumably largely genetic, there can be little doubt that a good opportunity exists for the development of genotypes more suitable for planting at higher altitudes.