Results of twenty years of research on Pinus tecunumanii by the CAMCORE Cooperative[1] (W.S. Dvorak, G.R. Hodge and J.L. Romero[2])

Pinus tecunumanii[3] has been one of the most intensively studied tropical pines in the last two decades. The CAMCORE Cooperative[4], began its first provenance seed collections of P. tecunumanii in Central America in March 1981. Since then CAMCORE has sampled 1473 trees in 48 populations of P. tecunumanii throughout southern Mexico and Central America. One hundred and fifty-three provenance/progeny trials and 50 ex situ conservation banks have been established in 9 countries. There have been 357 selections made in CAMCORE progeny tests and the best genotypes have been grafted into seed orchards in Colombia and South Africa. Other seed orchards are planned for Brazil and Venezuela. Costs for the seed collections and establishment, maintenance, and assessment of field trials over the last 20 years have been approximately US 1.0 million dollars. This paper summarizes some of what has been learned about the species as the result of these intensive research efforts.

EVOLUTION AND GEOGRAPHIC RANGE

Pinus tecunumanii appears to be a close genetic descendant of P. oocarpa based on phylogeny results from RAPD molecular marker studies. It has evolved on sites that are more fertile and humid than those where P. oocarpa is normally found. The geographic range of P. tecunumanii is from the central highlands of Chiapas, Mexico to central Nicaragua, a distance of approximately 1000 km. Several taxonomists suggest that P. tecunumanii extends as far west as Oaxaca and Guerrero, Mexico. However, examination of DNA from trees in these areas using molecular markers indicates that they are related to P. patula and P. herrerae and are not P. tecunumanii.

PRODUCTIVITY AS AN EXOTIC

Unimproved P. tecunumanii grows at a rate of 14m³/ha/yr in Venezuela and Brazil, 15m³/ha/yr in South Africa and 25m³/ha/yr in Colombia when measured at 8 years of age. In central Brazil, P. tecunumanii grows at about the same rate as good sources of P. caribaea var. hondurensis. In subtropical areas of southern Brazil, good sources of P. tecunumanii produce about 15% more volume than improved P. taeda. In both Colombia and South Africa, P. tecunumanii exhibits better growth and development than P. patula on the more tropical, low altitude sites. Pinus tecunumanii is not cold hardy and suffers great damage from hard freezes.

PROVENANCE VARIATION

Provenances of P. tecunumanii can broadly be separated into two groups in their native environment. High elevation sources (HE) are those that occur from approximately 1500 to 2900 m altitude; low elevation sources (LE) are found from about 450 to 1500 m altitude. Of the 48 provenances tested by CAMCORE, 30 are HE and 18 are LE sources. Assessment of 78 of the oldest CAMCORE tests indicated that Montebello, (Chiapas) Mexico and San Jeronimo, Guatemala were the most productive HE sources. Villa Santa, Honduras, San Rafael del Norte and Yucul, Nicaragua were the best LE provenances. Camelias and Apante, Nicaragua also demonstrated good potential followed by the Honduran sources of Culmí, San Esteben and San Francisco. The poorest provenances for volume production originate in collection areas located at the high altitudes (2200 m) in central Chiapas or those intermixed with P. oocarpa at the extreme southern edge of the geographic range of P. tecunumanii in central Nicaragua.

PROVENANCE X SITE INTERACTION

Provenance performance for volume across 78 locations in 4 countries was relatively stable. Average Type B genetic correlations for provenance effects for pairs of tests within the same country were estimated as 0.60 to 0.81. (A correlation of 1.0 would suggest no rank changes). Type B genetic correlations for pairs of tests located in different countries ranged from 0.54 to 0.58 depending on the method of calculation. Provenance performance was more stable for LE sources that for HE ones, probably because HE populations were established over a wider range of sites with altitudes ranging from near sea level to 2400 m.

GENETIC PARAMETERS

The levels of additive variance in P. tecunumanii are of the same magnitude as found for other tropical and subtropical pine species. Mean individual tree heritability estimated from the 78-provenance/progeny tests at 5 and 8 years of age was 0.16 for volume, 0.13 for stem straightness, 0.09 for branch diameter and 0.06 for forking. Individual tree heritability for wood density in four tests of P. tecunumanii assessed in Colombia at 8 years of age ranged from 0.21 to 0.47. Family x site interactions for growth traits are important for P. tecunumanii.

WOOD QUALITY

Pinus tecunumanii wood has a yellowish hue with low extractive content in most environments. With respect to the other Mesoamerican pines planted as exotics, the wood density of P. tecunumanii is generally greater than P. patula, less than that of P. oocarpa, and is greater than P. caribaea var. hondurensis when planted at mid-elevations. When planted at low altitude, the wood density of P. tecunumanii is equal to or less than P. caribaea var. hondurensi.. Studies by SAFCOL researchers in South Africa showed that P. tecunumanii had tracheid cells with much larger cross sectional diameters than either P. patula or P. taeda. Pinus tecunumanii has a low latewood percentage, which explains why pith to bark density gradients in the species are not nearly as extreme as for the US southern pines when grown in South Africa. A number of processing studies in various countries have indicated that the wood is very acceptable for pulp, paper and solid wood products.

STEM BREAKAGE

The greatest limitation for using P. tecunumanii as a plantation species in the tropics and subtropics is its propensity for the main stem to break in the upper crowns. An area of weak wood forms on the stem at the intersection of heavy branch whorls. Slight winds or even the force of gravity are enough to cause branches to break at the stem interface, weakening the main stem to the point that it easily snaps in subsequent windstorms. The stem breakage seldom kills the tree but results in great loss of volume. On the worst site stem break can affect 30 to 40% of the trees.

Initially it was thought that stem breakage was under moderate genetic control. However, recent results suggest that the defect is more related to environmental effects that vary across geographic regions. Stem breakage problems are practically non-existent in some areas of Venezuela and northern and central Brazil between 0 and 13° latitude where well-defined dry seasons are prevalent. In areas of high latitude, high precipitation, or strong seasonal winds, the breakage becomes very prominent. The individual tree heritability of stem breakage is approximately 0.06.

There are significant provenance differences in stem breakage. However, there appears to be no significant correlation between growth rate and the percentage of stem breakage. Lessening the amount of stem breakage will be dependent on good silvicultural and genetic practices. This includes establishing P. tecunumanii on the proper site using a good seed source. It also means removing damaged trees at the time of thinning and selecting against stem breakage in breeding programs. Additional work is needed to explain how environmental factors like climate, soil, and photoperiod influence the degree of stem breakage.

ROOT INSTABILITY

It has been known for some years that P. tecunumanii has a shallow root system much like P. oocarpa and can be susceptible to wind throw. However, trials established across countries indicate that container size in the nursery greatly influences root system development and subsequent stability in the field. Specifically, P. tecunumanii is very susceptible to J-rooting and root spiraling. Some nursery managers have gone to using containers with interior ridges to lessen spiraling effects and have field planted seedlings at smaller heights (10 cm to 20 cm) than in the past to circumvent the potential problem.

POTENTIAL FOR HYBRIDS

Pinus tecunumanii crosses easily with P. caribaea, P. oocarpa and P. patula. Successful crosses have also been made between P. tecunumanii and P. taeda, P. elliottii, P. greggii, and P. radiata by CAMCORE members but seed set has been low or highly variable. Pinus tecunumanii vegetatively propagates easily and hopefully low seed yields can be offset by family multiplication of most hybrid progeny. Pinus tecunumanii offers to breeders a number of traits that may be valuable in hybrid combinations such as fast growth, suitable wood quality, and in some instances superior drought tolerance (LE trees) and disease resistance. For example, LE sources of P. tecunumanii appear to be very resistant to Pitch Canker (Fusarium circinatum; syn. F. subglutinans f. sp. pini) in the seedling stage, a serious problem for both P. radiata and P. patula (in the seedling stage).

CONSERVATION

Pinus tecunumanii occurs in a series of small, disjunct populations, throughout its natural range. Allozyme studies suggest that it has average to above average levels of genetic diversity compared to other pine species. Even though it is not in immediate danger of being eradicated, woodcutters and agricultural expansion will genetically down grade or completely destroy some populations in the next 10 years. Based on our assessments in Chiapas and Central America, 30% of the P. tecunumanii populations are critically endangered, 60% are vulnerable and 10% are at low risk. Of the best seed sources, Yucul, Nicaragua and Montebello, Chiapas appear well protected. San Jeronimo, Guatemala, San Rafael del Norte and Las Camelias, Nicaragua, require greater protection in situ.

Presently, P. tecunumanii is one of the most thoroughly conserved pines ex situ. The continued future of ex situ conservation of P. tecunumanii will depend on whether the species generates sufficient interest to be used commercially, which in turn will depend on the success researchers have in reducing stem breakage through breeding and silviculture.

POTENTIAL OF PINUS TECUNUMANII AS A PLANTATION SPECIES

There are approximately 10,000 hectares of P. tecunumanii in the tropics and subtropics. Annual planting amounts are probably less than 2,000 ha per year. CAMCORE research efforts have demonstrated the great potential of P. tecunumanii when planted in the right ecological niches. Its importance may be greatly expanded if interest develops in using it in hybrid crosses. Even though the species has several disadvantages, these can be overcome with improved silviculture practices combined with selection and breeding. Breeding activities for P. tecunumanii must be carried to the second generation to determine the true worth of the species. Failing this, continued reliance will be placed on species that were used in the last century, whether they are best for all sites or not.

The advantages and disadvantages of P. tecunumanii as seen by the CAMCORE membership are summarized below.

ADVANTAGES OF PINUS TECUNUMANII

Excellent information available on provenance variation for growth and form. Grows quickly in the nursery. Captures sites rapidly when weeds are controlled. Shows better drought resistance than P. patula in South Africa in the years following establishment. Better productivity than P. oocarpa and sometimes P. caribaea var. hondurensis on most sites in the tropics where rotation age is < 16 years. Higher productivity than P. patula in areas of infrequent frost.

ADVANTAGES OF PINUS TECUNUMANII, CONT.

Higher productivity than P. elliottii at lower latitude, warmer and drier sites in the highlands of southern Africa. Lower bark content per unit volume than P. taeda. Low foxtail percent (approx. 5 to 15%, except Mountain Pine Ridge, Belize source). Lower extractive percent (approx. 4%) than some United States southern pines. Higher wood density than P. patula in Colombia and South Africa. Wood density more uniform within and between annual rings than P. elliottii, P. patula, and P. taeda in South Africa. Acceptable wood properties for pulp, paper and lumber. More resistant to Sphaeropsis sapinea (Diplodia) than P. patula and P. greggii in southern Brazil. Moderate to high tolerance to Fusarium subglutinans f. sp. pini (pitch canker) in seedling screenings. Hybridizes easily with several pine species.

DISADVANTAGES OF PINUS TECUNUMANII

Nursery container size greatly influences root architecture. Shallow rooted and prone to wind throw. Performs poorly on wet sites and when weed competition is severe. Frost tender. Upper-stem breakage common after 3 years of age at most locations. Light crown fails to completely block out understory weed competition after crown closure in some locations, resulting in a build up of fuel and an increase in the risk of fire. Does not resprout well after fire. Moderately susceptible to the Pineus pini (woolly aphid) in South Africa. Susceptible to Cinara cronartii (black aphid) attacks in southern Africa. Moderately susceptible to Cylindrocladium spp. in the lowland tropics. Very susceptible to the Hylastes sp. beetle in the seedling stage in southern Africa. Very susceptible to Cronartium quercuum f. sp. fusiforme (fusiform rust) in seedling screening trials. Preferentially browsed by livestock and antelope in South Africa. Wood has lower tear and burst strength than P. patula. Trees from HE populations do not usually produce large amounts of seed.

ACKNOWLEDGEMENTS

CAMCORE would like to thank agencies in Mexico and Central America for their assistance in making seed collections of Pinus tecunumanii. We would also like to thank CAMCORE organizations around the world that are participating in the conservation and testing of the species for their support.

REFERENCES

Most of the information in this article was taken from:

Dvorak, W. S., G. R. Hodge, E. A. Gutiérrez, L. F. Osorio, F. S. Malan and T. K. Stanger. 2000. Pinus tecunumanii. In: Conservation and Testing of Tropical and Subtropical Forest Species by the CAMCORE Cooperative. College of Natural Resources, NCSU. Raleigh, NC. USA. pp: 188-209.