Mating system and reproductive fitness traits of eastern white pine (Pinus strobus) in large, central versus small, isolated, marginal populations

Multilocus (t(m)) and single-locus (t(s)) outcrossing and actual inbreeding rates and seed traits were determined for eastern white pine (Pinus strobus L.) in six small, remnant, and marginal populations from two regions (East and West) in Newfoundland and in three large populations from the center of the species' geographic range in Ontario to examine the effects of small population size and fragmentation on mating system parameters and reproductive fitness. The population t(m) ranged from 0.867 to 0.991, with a mean of 0.924 over all nine populations. The mean t(s) ranged from 0.672 to 0.908, with a mean of 0.797 over the nine populations. The Ontario populations, on average, showed higher but statistically similar outcrossing rates (t(m) = 0.947, t(s) = 0.848) to the Newfoundland populations (t(m) = 0.912, t(s) = 0.772). The Newfoundland West populations, on average, showed the lowest outcrossing rates (t(m) = 0.889, t(s) = 0.716). Individual family outcrossing rates, although slightly higher, were similar to their respective population outcrossing rates, and no significant differences were observed among families within populations. The mean ts were significantly lower than their corresponding t(m), and the differences were significantly and positively correlated with the number of loci showing significant regression of pollen allele frequency on ovule genotype, suggesting possible occurrence of consanguineous mating. The Ontario populations showed the highest and the Newfoundland West populations the lowest reproductive fitness, with Newfoundland East populations ranking higher than Newfoundland West but significantly lower than Ontario populations. Actual inbreeding rates, determined by combining allozyme-based estimates of selfing in the filled seed component with estimates of inbreeding from the proportions of empty seeds, ranged from 7.4 to 31.6%, with an average of 22% for all populations and 11.1% for the Ontario, 24.7% for the Newfoundland East, and 30.1% for the Newfoundland West populations. Multilocus outcrossing rates were significantly correlated (i) negatively with the average distance to the five nearest neighboring trees (a surrogate measure for within-stand densities of reproductively mature trees) and (ii) positively with the proportion of filled seeds per cone. The filial seed progeny fixation index was positively correlated with both (i) average nearest-neighbor distances and (ii) proportion of empty seeds per cone. Thus, we detected strong interrelationships between the within-stand density of reproductively mature trees and both outcrossing rates and filled seed production. Interestingly, there was no relationship between the fixation index of the mature parent stands and their density. The genetic status or integrity of the extant parental populations may have been largely unaffected by the large-scale population decline experienced by eastern white pine early in the 20th century, a decline that showed an adverse effect on reproductive fitness of these populations.