366
T.J .B. Boyle
(British Columbia Ministry of Forests, 1993). Linkages among reserves
will be maintained or established and managed specifically for that purpose. Although reserves are permanent, linkages are temporary features,
with individual linkages being replaced by others over time. The major
objective of forest management outside reserves is to maintain a variety
of several stages, with not less than 30% of the total forest area less than
20 years old, and a variable cut block size, with an average of 40 ha.
Maintenance of structural diversity through retention of dead wood,
snags-at least ten large diameter snags per hectare, green trees, and
coarse woody debris are also features of the stand-level component of this
management strategy.
Harvesting is not the only forest management operation affecting diversity: several others are reviewed by Hunter (1990). Of particular interest
to those involved in tree improvement is regeneration, particularly the
use of genetically improved and clonal planting stock . Despite dramatic
increases in annual planting programs in recent years, only 20% of land
that is harvested or lost to fires and pests each year is currently regenerated
by planting (Forestry Canada, 1990). Most of the planting stock still
comes from "wild" collections, although seed orchard seed is now approaching or meeting demands is some provinces. The proportion of
planting stock that is clonal remains tiny, but more than a hundred
hectares of experimental black spruce (Picea mariana) stecklings have
been panted in northern Ontario, and Quebec is undertaking a similar
program.
A typical seed orchard in Canada may consist of several hundred openpollinated families, resulting in tens of thousands of possible interfamily
pollinations. The permutations in clonal orchards may be fewer, but
greater fecundity is likely to result in a more even representation of
possible crosses. Temporal and spatial variation in fecundity of natural
forests reduces the potentially infinite number of crosses to a much
smaller effective number, which may be similar to those resulting from
seed orchards. (See Boyle , Liengsiri, & Piewluang, 1990, for an example
of low heterozygosity in open-pollinated seed from a natural forest.)
Plant ing with seed from seed orchards managed to promote outcrossing
is unlikely to reduce genetic diversity of the planted species, but see
EI-Kassaby (1993) for a discussion of other factors . Except in very simple
ecosystems , however , tree species and age structure diversity are likely to
be reduced as a result of site preparation and planting.
As discussed previously, however, frequency of abundnce are not the
only important aspects of diversity, spatial heterogeneity or pattern may
also have important ecological consequences. The spatial arrangement of
genetic diversity on planted sites will be completely artificial. This is of
particular concern when dealing with clonal planting stock, when choices
must be made as to the size and arrangement of monoclonal blocks, or
clonal representation in an intimate mixture. Again, although arbitrary
T.J .B. Boyle
(British Columbia Ministry of Forests, 1993). Linkages among reserves
will be maintained or established and managed specifically for that purpose. Although reserves are permanent, linkages are temporary features,
with individual linkages being replaced by others over time. The major
objective of forest management outside reserves is to maintain a variety
of several stages, with not less than 30% of the total forest area less than
20 years old, and a variable cut block size, with an average of 40 ha.
Maintenance of structural diversity through retention of dead wood,
snags-at least ten large diameter snags per hectare, green trees, and
coarse woody debris are also features of the stand-level component of this
management strategy.
Harvesting is not the only forest management operation affecting diversity: several others are reviewed by Hunter (1990). Of particular interest
to those involved in tree improvement is regeneration, particularly the
use of genetically improved and clonal planting stock . Despite dramatic
increases in annual planting programs in recent years, only 20% of land
that is harvested or lost to fires and pests each year is currently regenerated
by planting (Forestry Canada, 1990). Most of the planting stock still
comes from "wild" collections, although seed orchard seed is now approaching or meeting demands is some provinces. The proportion of
planting stock that is clonal remains tiny, but more than a hundred
hectares of experimental black spruce (Picea mariana) stecklings have
been panted in northern Ontario, and Quebec is undertaking a similar
program.
A typical seed orchard in Canada may consist of several hundred openpollinated families, resulting in tens of thousands of possible interfamily
pollinations. The permutations in clonal orchards may be fewer, but
greater fecundity is likely to result in a more even representation of
possible crosses. Temporal and spatial variation in fecundity of natural
forests reduces the potentially infinite number of crosses to a much
smaller effective number, which may be similar to those resulting from
seed orchards. (See Boyle , Liengsiri, & Piewluang, 1990, for an example
of low heterozygosity in open-pollinated seed from a natural forest.)
Plant ing with seed from seed orchards managed to promote outcrossing
is unlikely to reduce genetic diversity of the planted species, but see
EI-Kassaby (1993) for a discussion of other factors . Except in very simple
ecosystems , however , tree species and age structure diversity are likely to
be reduced as a result of site preparation and planting.
As discussed previously, however, frequency of abundnce are not the
only important aspects of diversity, spatial heterogeneity or pattern may
also have important ecological consequences. The spatial arrangement of
genetic diversity on planted sites will be completely artificial. This is of
particular concern when dealing with clonal planting stock, when choices
must be made as to the size and arrangement of monoclonal blocks, or
clonal representation in an intimate mixture. Again, although arbitrary