Exploring the role of genetic diversity and relatedness in tree seedling growth and mortality: a multi‐species study in a Bornean rain forest

Where conspecific seedlings occur at high densities, density dependent
processes tend to depress their performance and survival relative to
co‐occurring heterospecifics. We extend this observation to within‐species
genetic diversity and relatedness. We posit that seedling growth and
survival increase where there is higher genetic diversity, and lower
relatedness, among seedling populations, under the expectation that
increased genetic dissimilarity among conspecific seedlings affords
greater resistance to pathogens. We used estimates of individual seedling
genetic diversity (multilocus heterozygosity (sMLH)) and genetic
relatedness among conspecific seedlings (pairwise kinship coefficients
(LRI)), under high and low conspecific seedling density, coupled with
censuses of seedling growth and mortality over 30 months to
explore the role of genetic diversity and relatedness on growth and
mortality of 1,485 seedlings of four dipterocarp species in an undisturbed
Bornean tropical rain forest. We hypothesized that more genetically
diverse and less related co‐occurring seedlings would display higher
survival and growth rates. In three of our four species lower genetic
diversity increased the probability of mortality over 30 months.
We observed no effect of genetic diversity on relative growth rates.
Contrary to our expectations only one species showed a negative effect of
increased relatedness on mortality. In two of the four species studied,
seedlings that were more genetically related to neighbouring conspecific
seedlings in the sample plot were less likely to die, but grew more
slowly. Synthesis: Our results confirm that genetic diversity and
relatedness among seedlings shapes survival probabilities differentially
across species. In contrast we found no differences in mortality and
growth rates between plots with low and high seedling density. Our results
suggest that a greater abundance of pollen donors could contribute to
cohort seedling survival in some species, though not all, and the specific
mechanisms by which genetic relatedness determines seedling demography
remain uncertain. These results have conservation and management
implications in view of changing patterns of gene flow in fragmented,
exploited, and degraded tropical rain forests, which might differentially
influence natural regeneration among species.