Northern seeds outperform southern seeds at HD common garden
It is possible that seeds sourced from the south were of a generally inferior quality than northern seeds, which may be linked to greater climate-related environmental stresses at lower latitudes. Other studies have found that climate-based environmental stresses lead to decreased germination (Moreira de Oliveira et al. 2019, Yi et al. 2019, Ribeiro et al. 2021), which may help explain our finding that northern seeds outperform southern seeds at both common gardens.
Our prediction that southern seeds would outperform northern seeds also assumed that southern populations had adequately adapted to historically warmer conditions and that these conditions match recent climate warming. Given the rapid rate of climate change in the region and the high degree of habitat fragmentation throughout the Klamath Mountain and Willamette Valley ecoregions (Wilson 1998, Floberg et al. 2004), it is possible that southern populations may not yet be adequately adapted to their current climates. Because our study only lasted one growing season, we may have assessed germination during an abnormal winter that favored northern-sourced seed. Although a multi-year analysis of germination was outside the scope of this study, multi-year studies allow for a more thorough understanding of the mechanisms driving differential germination and establishment patterns (Rice & Knapp 2008, Pfeifer-Meister et al. 2012, Merges et al. 2020)