Why? Because the maximum level of genetic gain can occur only if the seed orchard is a closed, controlled and perfect population.

Moreover, three key prerequisites must be met to ensure this maximum level of gain. First, pollination of all trees within an orchard must take place in synchrony. Second, all seed orchard trees must produce seed and pollen of the same quantity. Third, there must be minimal pollen contamination from outside sources.

Obviously, this is never the case. Here's why.

1. There's No Reproductive Synchrony.
   Trees within the same seed orchards pollinate at different times, as shown in Chart #5. This randomness is explained by the fact that trees within a seed orchard are selected from a range of geographic locations, and therefore have evolved in such a way as to pollinate at different times. In Chart #5, we see the timing of pollen release from different loblolly pine 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.

   in a seed orchard.

   Source: Friedman, S.T. and W.T. Adams. 1985. Estimation of gene flow into two seed orchards of loblolly pine (Pinus taeda L.). Theor. Appl. Genet. 69:609-615 and El-Kassaby, Y.A., D. Rudin and R. Yazdani. 1989. Levels of outcrossing and contamination in two Pinus sylvestris L. seed orchards in northern Sweden. Scand. J. For. Res. 4:41-49).

   The result, in both cases, is a reduction in the possible genetic mix among the seed orchard's parents, which results in reduced genetic gain.
   
   
2. Trees do not produce the same amount of gametes.
   Trees within the same orchard produce pollen and seeds in vastly different amounts. In fact, in many species, 80% of the seed yield is generated by only 20% of the orchard. This discrepancy in seed production is illustrated in Chart #6. This situation is so common that scientists at the North Carolina Tree Improvement Cooperative have dubbed it "The 80-20 phenomenon".

   Source: Anonymous. 1976. Twentieth Annual Report on cooperative Tree Improvement and Hardwood Research Program. North Carolina State University, Raleigh, NC, USA).

   This inequality in reproductive output among trees in the same orchard hinders the optimal gain levels that could be achieved.

3. Pollen contamination is a constant.
   All seed orchards are located in environments where other trees of the same species exist. Given this, pollen contamination from outside sources is very high, ranging from 30% to 70%

   Source: Friedman, S.T. and W.T. Adams. 1985. Estimation of gene flow into two seed orchards of loblolly pine (Pinus taeda L.). Theor. Appl. Genet. 69:609-615 and El-Kassaby, Y.A., D. Rudin and R. Yazdani. 1989. Levels of outcrossing and contamination in two Pinus sylvestris L. seed orchards in northern Sweden. Scand. J. For. Res. 4:41-49).

   contamination. In fact, some studies have shown that without this cross contamination, many orchards would fail to achieve their seed production goals.

   El-Kassaby, Y.A. and S. Reynolds. 1990. Reproductive phenology, parental balance and supplemental mass pollination in a Sitka spruce seed orchard. For. Ecol. Manage. 31:45-54.

   Chart #7 shows the impact of pollen contamination on the genetic gain of a seed crop.

   Pollen contamination creates another problem for foresters. Specifically, seed orchards planted outside their target planting zone often under-perform due to a phenomenon known as "maladaptation". This failure is a direct result of pollen contamination. The end result is a seed that is adapted to the wrong location.

   Source: Johnsen, Ø. 1989a. Phenotypic changes in progenies of northern lines of Picea abies (L.) Karst. grown in southern seed orchard. I. frost hardiness in a phytotron experiment. Scand. J. For. Res. 4:317-330, Johnsen, Ø. 1989b. Phenotypic changes in progenies of northern lines of Picea abies (L.) Karst. grown in southern seed orchard. II. seasonal growth rhythm and height in field trails. Scand. J. For. Res. 4:331-341, and Johnsen, Ø. 1989c. Phenotypic changes in progenies of northern lines of Picea abies (L.) Karst. grown in southern seed orchard. III. climatic damage and growth rhythm in a progeny trial. Scand. J. For. Res. 4:343-350).

   For foresters, the impact is a loss in yield and productivity.

   Some operators have reacted to these limitations by using controlled crosses for producing genetically improved seeds. Controlled crosses offer the option of pedigree selection and can overcome the risk of pollen contamination and maladaptation. But controlled pollination, or CP orchards, requires a large investment in specialized orchards to produce large quantities of seed. This is a labor intensive process, which results in increased seed cost.

   A viable CP delivery system using vegetative propagation through rooted cuttings has been developed for radiata.