Abstract
Tropical forest is being cleared and fragmented at a rapid pace.
Unsustainable forestry practices are eroding natural resources where
they are badly needed and contributing to the decline of global
biodiversity. Increasingly, efforts are being made to address the
problem, through conservation of remaining forest and restoration of
areas of degraded land. However, for policies to be successful in the
long term, it will be necessary to understand the level and distribution
of natural variation within forest species and the effect that human
impacts may have on its maintenance. This thesis aims to use
molecular methods to investigate the genetic diversity and population
structure of two commercially important neotropical tree species in a
human-influenced Central American landscape.
Red Yemeri, Vochysia ferruginea Mart., is recommended as a
potentially highly useful species for forestry reclamation of degraded
land. Populations from throughout its range in Costa Rica were
analysed for both organellar (chloroplast specific PCR-RFLP) and
total genomic (AFLP) variation. All populations were fixed for one of
two chloroplast DNA (cpDNA) haplotypes identified. One haplotype
was distributed exclusively throughout the Atlantic watershed, whilst
the second was found only on the Pacific slope. AFLP diversity was
partitioned primarily within populations (80.5%) and small but
significant between-population differentiation was detected (®st =
0.195). The relationships between populations fitted an isolation by
distance model when an ecological distance metric linking populations
through suitable habitat was used, and indicated gene flow around a
central mountain range. In combination with the cpDNA data, these
data suggest that pollen flow around the mountain range is maintained
whilst seed flow appears to be more restricted and cpDNA structure is
more probably a consequence of historical population processes.
A Central American rangewide study of cpDNA variation in C.
odorata identified a total of five haplotypes. These formed three
groups reflecting geographic distribution, representing Northern-,
Central- and Southern-Central American populations. However, the
genetic relationships between groups did not follow the geographic
pattern. The Northern group was most strongly differentiated from the
other two, although it is morphologically similar, geographically
proximate and occurs in similar habitat to the Central type. This
divergence most likely reflects a very ancient colonisation event. In
the south, the Central (dry) type and the Southern (wet) group were
differentiated along environmental boundaries. Given the distributions
of the latter two cpDNA types and taking AFLP diversity into
account, it seems most probable that this pattern reflects colonisation
and extinction processes that would have accompanied the vegetation
and climate changes in the region during the glacial cycles of the last
million years.
The data obtained on these two important species are discussed in
terms of application to their conservation and sustainable use. In
particular, the identification of Evolutionarily Significant Units in C.
odorata will help to shape an effective policy for its long term
survival.
