What is the likelihood that two adjacent trees of the same species share a parent? Addressing this question is an essential first step in my work to evaluate the adaptive capacity of California trees. Many tree populations exhibit “spatial genetic structure,” in which trees that are located near one another are generally more closely related than trees located farther apart. This pattern is largely a product of the fact that many seeds from each mother tree fall within a relatively restricted radius. When there are few potential parent trees scattered across a landscape (for example, following a severe wildfire), the seedlings that establish in a given area are most likely offspring of the nearest adult tree. However, when adult trees are abundant, a given site often contains offspring from many different parents, and adjacent seedlings may be only distantly related.
I am conducting a study of the relatedness among the Douglas-fir trees in my study sites in order to refine the sampling design for my main study of adaptive potential. For that study, I need to sample trees that are as closely spaced as possible without sharing any parents; the trees must be spaced closely to minimize differences in environmental variables such as topographic slope and soil depth among trees, but they cannot share parents because my sample must represent the range of genetic variation present within the site. If I sample many closely-related trees, I will not know whether certain genotypes are common on a site because (a) they are highly adapted to the site or (b) they all came from the nearest parent tree (or both).
Last week I received an initial, draft set of set of molecular genetic data from a microsatellite assay performed for me by the National Forest Genetics Lab (NFGEL). The lab analyzed DNA from 127 Douglas-fir trees from 9 of my study sites in the central Sierra. Within each site, I sampled multiple clusters of adjacent trees so that I could analyze relatedness across a range of distances from very near (adjacent trees) to very far (different study sites). The sampling design is illustrated in the map below.
By comparing the genotypes of any two trees, it is possible to estimate how closely related they are; trees that have more DNA in common are more closely related, and the exact amount they have in common roughly translates into a familial relationship (e.g., full-siblings, half-sibling, cousins, etc.). I used a software package called SPAGeDi (Spatial Pattern Analysis of Genetic Diversity) to estimate relatedness among every pair of sampled individuals and determine how the degree of relatedness varies with spatial distance among the trees.
The preliminary analysis across all study sites reveals that trees that are closer together are generally more closely related, which in genetics jargon means that there is “spatial genetic structure” within populations. In the figure above, a relatedness coefficient of 0.125 represents the relatedness of half-siblings (trees that share a mother but have different fathers). Relatedness near the half-sibling level appears to be common at a distance of about 0 to 50 meters (~150 feet). Above 50 meters, relatedness is much lower–below the relatedness of cousins. Based on this analysis, an appropriate sampling distance for my climate adaptation project is between 50 and 100 meters. This analysis is based on draft data; NFGEL is performing a second run to fill in data gaps, and once I have the final data I will repeat this analysis and evaluate each study site separately to determine if the patterns of relatedness vs. distance vary among sites.