Environmental distance and latitude did not drive germination
patterns
Environmental distance between source population and common garden sites
did not explain the observed seed-type and scale-dependent germination
patterns. Although regional climate models do show that southern sites
tend to be warmer and drier (PRISM Climate Group 2020), it is possible
that the environmental covariates we used to estimate environmental
distance were not meaningful, masking a potential environmental source
effect on germination that can be difficult to detect (Hereford 2009).
Other common garden studies have found that germination success is
influenced by parental environment (Gallagher & Wagenius 2016). More
source populations and environmental data could possibly reveal such a
trend if one exists.
We expected to see higher germination of nonlocal seeds sourced from
southern compared to northern sites due to recent warming patterns in
the region (PRISM Climate Group 2020) but were surprised to find the
reverse pattern in our study. A categorical comparison of locally
sourced seeds to those sourced to the north and south of the HD common
garden revealed that northern seeds performed significantly better than
both local and southerly sourced seeds. This pattern was not as easily
detected in a latitude-based germination model; as latitude increased,
chasmogamous germination also increased slightly, while cleistogamous
germination slightly decreased which produced a significant latitude by
seed type interaction. It is likely that the highly reduced germination
probability of cleistogamous seeds at only our most northerly site
created this negative latitudinal trend for cleistogamous seeds. This
suggests there may be a northern limit for seed sourcing success at
Willamette Valley planting locations.
These results could be linked to our observation that seed weight
slightly increased with latitude, although this trend is not
significant. It is unlikely that the opposing effects of latitude on
chasmogamous and cleistogamous seeds observed in our continuous latitude
model are biologically meaningful. Perhaps sourcing from more sites
would allow us to examine the impact of latitude on germination more
accurately. Indeed, the latitudinal range for D. californicaextends from British Columbia to southern California (Darris &
Gonzalves n.d.). Of course, our study only measured the germination
probability within one growing season, which makes the detection of
climate-driven latitudinal fitness patterns unlikely (Gaillart et al.
2018, DeMarche et al. 2019). Other local adaptation studies have found
significant latitudinal responses (van Boheemen et al. 2019, Zhang et
al. 2019), although they may not necessarily be tied to climate
(DeMarche et al. 2021).