Typically, the first breeding cycle is based on phenotypic selection, which involves the selection of trees based on their appearance. The second breeding cycle involves genotypic selection, which distinguishes trees based on their pedigree. The results of this process are shown in chart 8 to the left.

The Benefits Of Breeding.
There is absolutely no doubt that tree breeding works. Like it's more established cousins, animal and plant breeding, traditional tree breeding techniques have proven themselves as an effective way of achieving truly impressive results.

At the North Carolina State University - Industry Cooperative Tree Improvement Program, for example, the loblolly pine breeding program has achieved volume gains of 12% from their first generation rogued seed orchard. They expect to reach gains of 18% from rogued second-generation orchards.

It's also important to note that these improvements in gain represent an averaging out over large numbers of parents. Indeed, the three top crosses within this program are expected to generate volume gains of 40%, while the single best cross is expected to generate volume gains of 60%.

Nor were these benefits limited merely to volume gains. The top crosses were also substantially more resistant to rust infection, with a 50% increase in resistance.

B. Li, S. McKeand and R. Weir. 1999. Tree improvement and sustainable forestry - impact of two cycles of loblolly pine breeding in the U.S.A. Forest Genetics 6:229-234).

The Different Kinds Of Genetic Modes.
While tree breeders select for specific, highly desirable attributes, it's important to remember that these traits are not just a product of a tree's genetics. Rather, they are a function of genetics, environment and the interaction of the two. Therefore, quantitative geneticists have distinguished two major types of genetic modes - additive (V_A) and dominance (V_D) genetic variances.

V_A expresses the effect of the genes themselves (ie. predictable from knowledge of the parents), while V_D expresses the effect of a specific combination of genes. These specific combinations are randomly formed after every cycle of sexual reproduction, which involves genetic recombination through a process called "meiosis". As a result, additive variances (V_A) are fairly easily controlled through selection programs by increasing the number of desirable genes within the population. Dominance variances (V_D), however, are more random in nature and are therefore more difficult to control.

As a result, maximizing these dominance variances (V_D) means creating a large numbers of crosses, which in turn permits the creation of random combinations through sexual reproduction. Only then does one select the individuals with the desired combinations.

Put another way, the production of seed from seed orchards generates a high level of additive variances (V_A), while the random nature of dominance variances (V_D) makes the process fairly difficult to control.

That's where Somatic embryogenesis 

A process of initiation and development of somatic embryos in vitro from somatic cells and tissues.

really comes in.

By taking the elite crosses from the top parents of a traditional breeding program, somatic embryogenesis can produce several cell Lines 

When used in the context of plant propagation, the term refers to a collection of plants produced asexually either from a single plant or part of a plant. They have the exact same genetic make-up.

within each cross. This would then be followed by a testing program to identify the seeds with the desirable and unique V_D. Once these individuals are identified, they can then be mass-produced through somatic embryogenesis. Chart #9 at the left illustrates the gain that can be achieved over time through breeding and testing of materials produced through somatic embryogenesis.

Maintaining Genetic Diversity.
A successful breeding program needn't come at the expense of genetic diversity. In fact, the inclusion of a sufficiently wide range of trees into a breeding program can actually generate a level of genetic diversity that matches that seen in a natural population.

That genetic diversity is what allows for future improvements, while simultaneously providing a good buffer, or insurance policy, for unknown contingencies.

Indeed, studies have found that the amount of genetic diversity present in selected breeding populations was actually substantial, and in most cases actually exceeded that present in the natural population.

El-Kassaby and Ritland 1995 a and b, Stoehr and El-Kassaby 1997).

A summary table of this phenomenon is shown below, comparing levels of genetic variation in seed orchards to those present in natural populations.

TABLE: Heterozygous parameters comparison between phenotypic selection (seed orchards) and natural populations.

		HETEROZYGOSITY PARAMETERS
SPECIES	POPULATION	% polymorphic loci	# of alleles/locus	mean expected heterozygosity
Douglas-fir1	Seed orchard Natural populations	62.5 52.6	2.3 2.1	0.172 0.171
"Interior" spruce2	Seed orchard Natural populations	64.7 64.7	2.4 2.7	0.194 0.189

¹El-Kassaby and Ritland 1995a,b     ²Stoehr and El-Kassaby 1997