Evidence for habitat fragmentation induced genetic degradation in remnant Syzygium maire (Myrtaceae) populations

Context. Wetland restoration is critical for the recovery of biodiversity, ecosystem functions and services. In highly modified landscapes, conservation must navigate complex trade-offs among these goals. Local-scale information on the genetic structure and trajectory of species can support the design of restoration strategies that maximise diversity. Objectives. We investigated population genetics of Syzygium maire (maire tawake), a critically threatened canopy tree of swamp forests in the Greater Wellington Region of Aotearoa New Zealand. Methods. We sampled leaves of adult trees and seedlings in 12 sites across the region applying low-coverage whole genome resequencing to each individual. We characterised genetic structure in the adult and seedling cohorts, examining patterns of kinship and diversity using hierarchical Analysis of Molecular Variance. Further, we compared the decay in spatial autocorrelation among trees in this region to that of a relatively large and intact reference population. Results . Our data suggest historical homogeneity and admixture across extensive pre-fragmentation swamp forests, but fine-scale genetic structure and decay of spatial autocorrelation within a few kilometres indicates recent fragmentation now constrains gene flow. Small yet significant reductions in genetic diversity and increased inbreeding in seedlings point to early genetic erosion in remnant habitat patches. Although genetic loss is not yet severe, continued isolation risks accelerating degradation in future generations. Conclusions. Restoration should prioritise reconnecting fragmented populations, maximizing genetic diversity by replanting with broadly sourced seed and reestablishing habitat for avian seed-dispersers. These findings highlight how genetic information can inform restoration planning and improve the long-term resilience of habitat specialist.