Some of these trees are not like the others: Individual tree contributions to population growth over five centuries in colonizing ponderosa pine populations

Background/Question/Methods

How individuals contribute to population growth is important for understanding ecological and evolutionary change in populations. Determination of which, and how many, individuals contribute to population growth is fundamental to understanding allele frequencies, phenotypes and population structure through time. Furthermore, how reproductive performance changes throughout the lifetime of individuals is important for understanding population development and structure.  

We studied a total of 1128 trees across four disjunct populations of ponderosa pine, in the Bighorn Basin of north-central Wyoming, to examine how individual trees contribute to population growth. These populations are separated from continuous ponderosa pine forest by distances ranging from 15 to over 100 km. There is strong evidence that the initial colonizers are still present, and that there has been no erasure of past trees at the sites, giving us a complete record of population history. All trees in each of the populations were aged using tree-ring based techniques and needle tissue was collected and used for population genetic analysis at nine microsatellite loci. For each individual, parentage analysis was used to determine the number of direct offspring and the total number of descendents over multiple generations.

Results/Conclusions

Results showed that the majority of individuals (>40 percent) in each population produced no offspring. Seventy-five percent of individuals in each population had two or fewer offspring. In all four populations, a small number of individuals (1-3) had a much higher than average number of offspring (7-19 offspring/parent). Determination of descendents over multiple generations showed that a small number of trees were related to a large proportion of the population (>30 percent). Furthermore, results showed that, on average, trees younger than 150 years old produced more successful offspring than older trees, even though older trees were still producing viable seed. Our results empirically demonstrate that very few individuals are responsible for population growth and that beyond 150 years reproductive performance diminishes.