T.D. Byram1, W.J. Lowe2 and G.D. Gooding1
The Western Gulf Forest Tree Improvement Program (WGFTIP) is a cooperative tree breeding project founded in 1969 with the objective of providing the best genetic quality seed for use in forest regeneration programs in the Western Gulf Region of the United States. Current members include five states and 11 industrial members. Genetic variation is the natural resource on which all breeding and genetic improvement programs are based and therefore conservation of genetic diversity is considered good natural resource stewardship, a prerequisite for evolutionary change and an obligation to future human generations. The cooperative is conserving and improving populations of five southern pine species and several hardwood species. This paper focuses on WGFTIP's work with conservation of loblolly pine (Pinus taeda L.).
Loblolly pine (Pinus taeda L.) can by no means be considered an endangered or threatened species. In fact, it is one of the most abundant and widely adapted and distributed of the southern pines (Dorman 1976). Its range extends along the Atlantic Coast from Maryland to Florida in the east and to Texas and Oklahoma in the west (Figure 1). Its natural range is limited by cold weather in the north and by decreasing rainfall in the west. The loblolly pine population is more or less continuous across its entire range with only one major discontinuity at the Mississippi River. Small populations also exist in central Texas at the western edge of its range in isolated pockets known as the "Lost Pines".
Natural variation in traits such as growth rate (Stonecypher, Zobel and Blair 1973, Wells and Wakeley 1966), nitrogen use efficiency (Li, McKeand and Allen 1991), wood specific gravity (Byram and Lowe 1988, Jett, McKeand and Weir 1991), drought resistance (van Buijtenen 1966), stress tolerance (Hodge and Weir 1993) and disease resistance (Arabatzis, Gregoire and Lenhart 1991, Sluder 1989) have been extensively studied for this species. Genetic variation in adaptive traits has been documented among regional populations, among stands within regions, and among individual trees within stands (Wells and Wakeley 1966, Yeiser, van Buijeten and Lowe 1981). There are important regional differences for most traits; however, loblolly pine is an extremely heterozygous species with a large amount of tree to tree variation (Roberds and Conkle 1984, Williams, Hamrick and Lewis 1995). While growth and adaptability traits have been most frequently studied (e.g. McKeand, Weir and Hatcher 1989), it is also likely that there is substantial genetic variation in numerous traits that have not been examined.
Fig. 1 The natural range of Pinus taeda (from Dorman 1976)
Table 1: Area (in thousands of acres) of loblolly pine forest type in the U.S. by ownership class
|Ownership Class||All||Stand Origin|
|County and municipal||177.3||117.1||60.1|
|Other private corporate||4,121.7||2,616.4||1,505.3|
|Other private individual||17,443.3||12,799.5||4,643.8|
Information retrieved 15 October 1997 from the U.S. Department of Agriculture-Forest Service Southern Research Station Forestry Inventory and Analysis Database Retrieval System. ( http://www.srsfia.usfs.msstate.edu/scripts/ew.htm).
Because loblolly pine is so abundant and widespread, there is no reason to fear an immediate loss of genetic variation. Loblolly pine covers approximately 50.5 million acres (20.4 million ha.) in the southern United States (Table 1). Natural regeneration is the predominant regeneration method on public and non-industrial private land and accounts for approximately 29.7 million acres (12.0 million ha.) of the total loblolly pine ecosystem (Table 1). These naturally regenerating populations act as a repository of genetic diversity and allow for the continued emergence of adaptive genetic variation within the natural population.
In situ conservation through natural regeneration is a major contributor to the overall conservation of the genetic variation of the species. For the purpose of advanced tree improvement, it may be desirable to complement in situ with ex situ conservation. Gene frequencies in natural stands vary over time with the reproductive success of the parents. This can be affected by changes in the environment, silvicultural practices, and random climatic fluctuations between years. Genetic variability at any given time can be determined, but without a base line population, it will not be possible to assess long-term trends. New techniques in the field of molecular genetics are more useful when baseline data are available for an individual's relatives and their physical characteristics. Furthermore, genetic variation on a regional scale is very difficult to assess as stands are distributed over large areas and many ownerships.
Tree improvement practices affect genetic variation in both breeding and deployment populations. Breeding stock is initially selected from the wild population. The level of genetic variation that is maintained in this subpopulation is a function of the sampling method, the number of selected individuals and how subsequent breeding is structured. The production population is likely to be under an even more rigorous selection regime. Maintaining genetic variation in commercial plantations must be accomplished through the continued infusion of new material from the breeding population. These issues have been reviewed for loblolly pine by Namkoong (1997) and examined in the context of an applied tree improvement program by McKeand and Svensson (1997).
An organization's resources and objectives, as well as other ongoing efforts, influence the choice of an appropriate genetic conservation strategy. As noted above, a great deal of in situ gene conservation for loblolly pine occurs as a consequence of natural regeneration. The number of diverse populations maintained by the cooperative tree improvement programs will also contribute to the genetic conservation program, allowing continued evolution (Namkoong 1997). In assessing the role of tree improvement programs in genetic conservation, the Western Gulf Forest Tree Improvement program (WGFTIP) has recognized that its first-generation population represents a unique resource that can meet several objectives complementary to other ongoing genetic conservation efforts.
Applied forest genetics, the deliberate selection and conservation of vigorous, well-adapted trees, was first initiated in the Western Gulf Region of the United States by the Texas Forest Service in the early 1950's. Other states and several private concerns soon started independent programs which matured into regional tree improvement cooperatives. The WGFTIP, the primary tree improvement program for loblolly pine in the Western Gulf Region, is one such cooperative. Current members include five states and 11 industrial members.
One of the primary conditions for membership in the cooperative was that each organization select 100 individuals from undomesticated stands of loblolly pine for each of their breeding programs. The members of the WGFTIP screened thousands of acres to identify vigorous, well-adapted individuals for inclusion in the tree improvement program. This process, known as mass selection, depends solely on an individual's phenotypic appearance as compared to that of neighboring trees and is therefore not very efficient in changing gene frequencies. It is likely that a wide variety of genes for all traits was included in the selected trees. Neutral genes were sampled at random and should in principle be represented at the same frequencies in the WGFTIP selected population as in the wild population. Therefore, the individuals selected by the WGFTIP represent one of the best available samples of the loblolly pine population in the Western Gulf Region of the United States prior to domestication.
Once trees were selected, the genotypes were conserved by grafting the individuals into scion banks (Figure 2). The 2,745 loblolly pine selections grafted by the members of the WGFTIP cooperative originated from 126 counties and parishes across the states of Arkansas, Louisiana, Mississippi, Oklahoma and Texas. Records are available for most selections and these include the county or parish in which the tree was selected and subsequent progeny evaluations for survival, growth rate, form characteristics and wood specific gravity.
The members of the WGFTIP are committed to conserving loblolly pine selections from the Western Gulf Region of the United States to meet the following objectives:
Genetic conservation is an integral part of the cooperative's long-range breeding plan. A strategy of ex situ conservation through grafting into scion banks is the preferred method for conserving this material. Long-term seed storage, an alternative used in several agricultural crop species, does not appear to be practical for loblolly pine. Long-term seed storage has proven unreliable for loblolly pine. A genetic conservation strategy using seed would require that stands of trees be grown to replenish seed stocks when the seed in storage loses viability. As inbred lines do not exist, low frequency genes would be at risk as they might be lost due to chance in sexual reproduction. Scion banks already exist and grafted material is more readily available and easier to use than seed or pollen in cryopreservation.
Grafting offers several benefits for genetic conservation ex situ complementary to in situ conservation efforts. Genetic "copies" can be produced by making many grafts that can then be distributed across multiple locations. In this way, grafting can be used to centralize or decentralize selections to locations where they can be easily protected and accessed for testing or use. It can also be used to distribute the same genetic material to different scion banks to prevent loss of genetic material due to natural disasters (Figure 3). The cooperative's policy is to establish each selection at two separate locations, and the members of the WGFTIP are committed to ensuring that these selections are perpetuated indefinitely by serial grafting. Most southern pines require five to six years from seed to reach sexual maturity. Because most of the WGFTIP selections were established in scion banks several years ago, seed as well as vegetative tissue is available from most of these sources.
Records on all the selections are maintained by the WGFTIP staff and are available through the USDA-ARS Germplasm Resources Information Network (GRIN).
Access to this information can be obtained by submitting a request to:
Gene Conservation Program
Forest Science Laboratory
Texas A&M University
College Station, TX 77845-2585
or through the World Wide Web at http://www.ars-grin.gov/wgftip/index.html
Requests for plant material can also be submitted to the above address.
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