Small‐scale genetic structure and mating patterns in an extensive sessile oak forest (Quercus petraea (Matt.) Liebl.)

Oaks (Quercus) are major components of temperate forest ecosystems in the
Northern Hemisphere where they form intermediate or climax communities.
Sessile oak (Quercus petraea) forests represent the climax vegetation in
eastern Germany and western Poland. Here, sessile oak forms pure stands or
occurs intermixed with Scots Pine (Pinus sylvestris). A large body of
research is available on gene flow, reproduction dynamics, and genetic
structure in fragmented landscapes and mixed populations. At the same
time, our knowledge regarding large, contiguous and monospecific
populations is considerably less well developed. Our study is an attempt
to further develop our understanding of the reproduction ecology of
sessile oak as an ecologically and economically important forest tree by
analyzing mating patterns and genetic structure within adult trees and
seedlings originating from one or two reproduction events in an extensive,
naturally regenerating sessile oak forest. We detected positive spatial
genetic structure up to 30 meters between adult trees and up to 40 meters
between seedlings. Seed dispersal distances averaged 8.4 meters. Pollen
dispersal distances averaged 22.6 meters. In both cases, the largest
proportion of the dispersal occurred over short distances. Dispersal over
longer distances was more common for pollen but also appeared regularly
for seeds. The reproductive success of individual trees was highly skewed.
Only 41 percent of all adult trees produced any offspring while the
majority did not participate in reproduction. Among those trees that
contributed to the analyzed seedling sample, 80 percent contributed 1-3
gametes. Only 20 percent of all parent trees contributed four or more
gametes. However, these relatively few most fertile trees contributed 51
percent of all gametes within the seedling sample. Our study demonstrates
that extensive, apparently homogenous oak forests are far from uniform on
the genetic level. On the contrary, they form highly complex mosaics of
remarkably small local neighborhoods. This counterbalances the levelling
effect of long-distance dispersal and may increase the species’ adaptive
potential. Incorporating these dynamics in the management, conservation
and restoration of oak forests can support the conservation of forest
genetic diversity and assist those forests in coping with environmental
change.