Implications for restoration
Restoration practitioners frequently use D. californica to reseed
oak savanna and grassland ecosystems along the West Coast of the United
States, although chasmogamous seeds have until now been used almost
exclusively for this purpose (Maslovat 2002, Hayes & Holl, 2011, Lindh
2018). Our results from the HD common garden suggest that cleistogamous
seeds are more vigorous than chasmogamous seeds, and that
northerly-sourced seeds could germinate at a higher rate than both local
and southerly-sourced seeds. We cannot, however, claim that higher
germination probability will necessarily translate to greater fitness
advantages over the plants’ life history, or that this pattern is likely
to be replicated across the Willamette Valley. Despite finding a
significant seed type effect at both planting locations, more common
garden site replication would be needed to suggest a regional phenomenon
(but see Bischoff et al. 2006, Miller et al. 2011, Gallagher & Wagenius
2016). Future research at multiple planting sites and involving multiple
life stages across several years is required to better address questions
regarding seed translocation for restoration planting. Such a study
should include multiple species of restoration importance in the
Willamette Valley to investigate whether trends are consistent across
species. This information would support ongoing efforts to create seed
transfer zones of species used in the restoration of Willamette Valley
ecosystems (Miller et al. 2011, Ramalho et al. 2017).
On a practical note, the methods we used to facilitate cleistogamous
seed preparation substantially reduce the processing time for
outplanting cleistogamous seeds. Our sheathed and soaked manipulations
did not affect germination success for either cleistogamous or
chasmogamous seeds, demonstrating that these methods can be used to make
cleistogamous seed planting an accessible complement to chasmogamous
planting. Although post-soaked individual cleistogamous seed extraction
is a laborious process when done by hand, planting groups of
cleistogamous seeds within their intact stems is much less
labor-intensive. Our sheathed treatment demonstrates that planting
cleistogamous seeds that are still enclosed within their stem can be an
easy method to successfully germinate cleistogamous D.
californica seeds at a scale necessary for ecosystem restoration.
Restoration practitioners may benefit from incorporating cleistogamous
seed planting as an insurance policy in the event of reduced
chasmogamous germination in much the same way that the plants themselves
do (Zeide 1978, Schoen & Loyd). This practice may be especially
beneficial when local seed sourcing ability is limited.
Although the detailed
pathogen census performed by Mackin et al. (2021) demonstrates the
differences in cleistogamous and chasmogamous pathogen communities, we
were unable to assess the impact pathogens had on in-situ germination
and thus cannot be certain that pathogen escape explains the
intermediate distance advantage in cleistogamous seeds at the HD common
garden. Still, because pathogen pressure is likely a factor influencing
in situ D. californica germination, and because pathogen
communities vary between nearby sites, plug planting of larger
individuals could also present an effective method of restoring D.
californica populations, provided sterilized soil is used. Although
this method is more time consuming and expensive than direct re-seeding
(Gallagher & Wagenius 2016), accurately predicting the efficacy of
re-seeding approaches may require an intimate understanding of
epiphytic, endophytic and soil pathogen communities on both seed types,
which was beyond the scope of this study. When seeding D.
californica for restoration, practitioners should consider the pathogen
communities of both source and planting sites, use cleistogamous seeds
in addition to chasmogamous seeds, and consider sourcing cleistogamous
seeds from more distant northerly populations than their chasmogamous
counterparts.