Mating system and early viability resistance to habitat fragmentation in a bird-pollinated eucalypt

Abstract

Habitat fragmentation has been shown to disrupt ecosystem processes such as plant-pollinator mutualisms. Consequently, mating patterns in remnant tree populations are expected to shift towards increased inbreeding and reduced pollen diversity, with fitness consequences for future generations. However, mating patterns and phenotypic assessments of open-pollinated progeny have rarely been combined in a single study. Here, we collected seeds from 37 Eucalyptus incrassata trees from contrasting stand densities following recent clearance in a single South Australian population (intact woodland=12.6 trees ha(-1); isolated pasture=1.7 trees ha(-1); population area=10 km(2)). 649 progeny from these trees were genotyped at eight microsatellite loci. We estimated genetic diversity, spatial genetic structure, indirect contemporary pollen flow and mating patterns for adults older than the clearance events and open-pollinated progeny sired post-clearance. A proxy of early stage progeny viability was assessed in a common garden experiment. Density had no impact on mating patterns, adult and progeny genetic diversity or progeny growth, but was associated with increased mean pollen dispersal. Weak spatial genetic structure among adults suggests high historical gene flow. We observed preliminary evidence for inbreeding depression related to stress caused by fungal infection, but which was not associated with density. Higher observed heterozygosities in adults compared with progeny may relate to weak selection on progeny and lifetime-accumulated mortality of inbred adults. E. incrassata appears to be resistant to the negative mating pattern and fitness changes expected within fragmented landscapes. This pattern is likely explained by strong outcrossing and regular long-distance pollen flow.

Figure 1

Map showing location of E. incrassata mother trees (filled circles) from both landscape contexts. Shaded areas represent native vegetation. Inset maps A, B, C give a closer view of sampled adult mother trees within the intact woodland context. All other sampled mother trees were in isolated pasture contexts. The cross ( × ) indicates the location of the common garden experiment.

Figure 2

Spatial autocorrelation of genetic relatedness of adult E. incrassata mother trees. Significant autocorrelation between pairwise kinship within a distance class is indicated if mean observed pairwise kinship (filled circles±s.e.) falls outside confidence interval (CI) limits (open squares). If observed autocorrelation is above the upper 95% CI line, then individuals within that distance class were more genetically similar than expected at random, and a significant positive spatial structure is observed.