There are over 100,000 species of forest tree species. Despite this, while the genomes of many animal species have been sequenced rapidly, genomics research on trees has been taking rather longer and has been restricted to a relatively small number of species. In addition to the more restricted available research funding and the limited number of researchers (<1000), a further difficulty is the large size of tree genomes. Conifers have particularly huge genomes. For instance, Pinus (pine) species have a genome size that is 21-37 Gb (7-12 x the human genome size of approximately 3 Gb); that of the Picea (spruce) is 31-40 Gb and ;that of Pseudotsuga menziesli (Douglas fir) is 37Gb. Quercus (oaks) are not quite as large, at 1.14-2Gb and Castanea (chestnut) is in the same range, at 1.53-1.63 Gb.
On a more technical note, there are, however, some interesting features of forest tree genomes that will assist future genetic studies. There is a particularly high abundance of sequence variants called single nucleotide polymorphisms (SNPs), with as many as 1 per 100 bp. Also the blocks of “linkage disequilibrium” (i.e. the regions of tightly linked genetic markers) are relatively small, with the result that genetic marker SNPs are more likely to be closely associated with the genes of interest.
For a more detailed source of reference, please see the informative review published by Neale and Kremer (2011) in Nature Reviews Genetics.