Introduction
Seed provenance is an important consideration for restoration
practitioners seeking to re-seed grassland ecosystems (Bischoff et al.
2006, Vander Mijnsbrugge et al. 2010, Bischoff et al. 2010, Breed et al.
2018). Seemingly minor differences in the fitness of seeds sourced from
different populations can have profound effects on the establishment of
focal plant populations at an ecosystem scale (Middleton et al. 2010,
Seifert & Fischer 2010). In 2020, the United Nations declared 2021-2030
the “Decade on Ecosystem Restoration” (UNEP and FAO, 2020). Grasslands
have high potential for restoration under this declaration, but careful
planning is needed to ensure long term success (Dudley et al. 2020). To
achieve ambitious global restoration targets for grassland ecosystems,
research on the relationship between seed provenance and plant fitness
is urgently needed (Breed et al. 2018).
Restoration practitioners must consider the degree of local adaptation -
the superior fitness of local genotypes - for plant species used in
their projects. Populations under intense selective pressure are more
likely to show local adaptation, providing them with a distinct “home”
advantage over nonlocal populations at a given location (Joshi et al.
2001, Breed et al. 2018). Although intuitive from an evolutionary
perspective, local adaptation is certainly not universal (Bischoff et
al. 2006, Leimu & Fischer 2008, DeMarche et al. 2019). Ecologists
frequently use reciprocal transplant and common garden experiments to
measure the degree to which local adaptation exists in plant populations
(Hereford 2010). Approximately 70% of reciprocal transplant studies
show local adaptation (Leimu & Fischer 2008, Hereford 2009), which
likely depends upon three variables (Hereford 2009): the difference in
selection pressure between local and nonlocal genotypes (Schluter &
Grant 1984), the amount of gene flow between populations (Garcia-Ramos
& Kirkpatrick 1997, Lenormand 2002, Kawecki & Ebert 2004), and the
genetic structure of each population (Linhart & Grant 1996).
Differences in selection pressure between populations are likely linked
to differences in site environmental characteristics, which is often
closely related to geographic distance (Leimu & Fischer 2008, Hereford
2009). Theory predicts that as environmental and geographic distance
increase between populations, so too should the magnitude of local
adaptation (Garcia-Ramos & Kirkpatrick 1997, Joshi et al. 2001). The
underlying logic is simple; genotypes proven to perform well in a site
should continue to do so in the future, while genotypes sourced from
elsewhere may not, especially as the differentiation between sites
increases. The degree of local adaptation thus varies among populations
and can be difficult to predict (Leimu & Fischer 2008, Gaillart et al.
2019). With germination being the critical first step in plant
establishment, understanding how seeds germinate near and far from their
maternal plants can help elucidate the degree of local adaptation in
plant populations.
Another consideration for choosing seed sources for restoration is how
to best maintain genetic diversity (McKay et al. 2005). Although most
plant species produce a single type of seed, many exhibit seed
heteromorphism – the production of multiple seed types. Nearly 700
angiosperm species exhibit cleistogamy, a breeding system that includes
permanently closed, obligately self-pollinated flowers (Culley &
Klooster 2007). The majority of these species are classified as
dimorphically cleistogamous, producing seeds from both cleistogamous and
chasmogamous (more typical, externally pollinated) flowers (Culley &
Klooster 2007, Baskin & Baskin 2017). As such, cleistogamous seeds are
likely to have less genetic diversity than their potentially outcrossed
chasmogamous counterparts and could be more prone to inbreeding
depression (Culley & Klooster 2007). Cleistogamous seeds also typically
disperse much shorter distances than chasmogamous seeds (Schoen & Loyd
1984, Culley & Klooster 2007, Auld & de Casas 2013, Baskin & Baskin
2017), and average seed weight can differ substantially between the two
types (Waller 1982). There are, however, several evolutionary advantages
to cleistogamy, including insurance in the absence of external
pollination, the reduced energy cost of production, and the retention of
locally adapted gene complexes (Schoen & Loyd 1984, Culley & Klooster
2007, Baskin & Baskin 2017). Indeed, a review of field and lab studies
comparing the germination of cleistogamous and chasmogamous seeds found
that a higher proportion of cleistogamous seeds germinated in two-thirds
of cases (Baskin & Baskin 2017). The inherent differences in genetic
diversity and dispersal between these two seed types suggest that
chasmogamous seeds might be better suited for success in novel
environments, while cleistogamous seeds may perform better in the
immediate vicinity of their maternal plant (Schoen & Lloyd 1984, Culley
& Klooster 2007).
To date, researchers have mostly recommended the use of locally-sourced
seeds for restoration (Bischoff et al. 2010, Vander Mijnsbrugge et al.
2010, Bucharova et al. 2017), despite approximately 30% of plant
populations surveyed not showing local adaptation (Hereford 2009). In
these cases, stringent seed sourcing restrictions likely inhibit the
genetic diversity of the restored population, which may have negative
effects on the population’s ability to respond to changing environmental
conditions (Broadhurst et al. 2008, Miller et al. 2011). There is
growing support for the use of nonlocal seeds sourced from populations
that may be better adapted to future climatic conditions, as the use of
climate-adapted genotypes could facilitate the maintenance of ecosystem
services and critical habitat structure (Broadhurst et al. 2008,
Bischoff et al. 2010, Kreyling et al. 2011, Ramalho et al. 2017).
Climate-motivated translocation of seeds is controversial, however, as
it relies on a series of assumptions that are difficult to test. These
assumptions include that the seeds are sufficiently adapted to their
local climate, that this climate adequately matches the future climate
of the restoration site, that the nonlocal seeds will germinate and
establish in a restored site under current conditions (Kreyling et al.
2011), and that climate is the most important driver of performance
(DeMarche et al. 2019). While considering the long-term effects of
introducing novel genotypes, the germination of nonlocal seeds in a
novel environment needs further study to ensure such an approach is
feasible in the first place (Bucharova et al. 2017, Breed et al. 2018).
Danthonia californica , a perennial bunchgrass native to western
North America, is commonly used in the restoration of prairie ecosystems
in the Pacific Northwest, USA (Buisson et al. 2006, Hayes & Holl 2011,
Stanley et al. 2011, Pfeifer-Meister et al. 2012). Individuals produce
both chasmogamous and cleistogamous seeds (Appendix A; A,B), the latter
of which are enclosed within the stem. Cleistogamous seeds are difficult
to remove manually, potentially contributing to their infrequent use inD. californica restoration (Hayes & Holl 2011). Although mating
system generally does not influence the degree of local adaptation
across species (Hereford 2010), many studies have shown different
fitness and local adaptation patterns for conspecific seeds produced via
different mating systems (Schmitt & Gamble 1990, Lovell et al. 2014,
Rushworth et al. 2020). However, the applied aspects of mating-system
dependent seed selection for restoration are relatively rare in the
literature (Coulter 1914, Charlesworth 2007, Rushworth et al. 2020).Danthonia californica thus provides an excellent opportunity to
study the impacts of sourcing distance and mating system on local
adaptation in an ecosystem restoration context.
Here, we devised a common garden experiment using both chasmogamous and
cleistogamous seeds collected from eight natural populations of D.
californica across a latitudinal gradient in western Oregon and
Washington, USA (Figure 1A). Our design allowed us to ask whether the
effects of seed source origin (local vs. nonlocal) on germination are
dependent on seed type, and whether there are other factors about seed
source origin, such as the distance or direction (north or south) from
the common garden, latitude, or average seed weight, that help explain
germination patterns across source populations. Additionally, we
experimented with seed processing techniques to facilitate cleistogamous
seed preparation and decrease processing time for heteromorphic seed
planting. This is necessary because cleistogamous seeds remain in the
stem and are difficult to remove and separate.
We hypothesized the following: H1a: At each common garden, we
predicted that both cleistogamous and chasmogamous seeds originating
from that site (local seeds) would outperform seeds originating from
other source populations (nonlocal seeds), regardless of whether the
nonlocal seeds originated to the south or north of the common garden.H1b: However, we expected that the degree of local adaptation
would depend on seed type. If inbreeding depression compromises local
adaptation, then we would expect local chasmogamous seeds to outperform
local cleistogamous seeds. Alternatively, if gene flow limits local
adaptation, then we would expect local cleistogamous seeds to outperform
local chasmogamous seeds. H2: Furthermore, we expected
germination to decrease with increasing distance between source
population and common garden (both geographic and environmental distance
- the similarity in environmental conditions such as temperature,
precipitation, etc.), considering that the magnitude of local adaptation
between common garden and source sites should increase as distance does.H3: Finally, we predicted that nonlocal seed germination would
decrease with increasing latitude, as seeds sourced from southern
populations would outperform seeds sourced from northern populations due
to recent climate warming. Demographic studies of natural D.
californica populations, including most of the populations studied
here, revealed that population growth rate decreases with increasing
latitude and that locally, the population growth rate decreases under
warmer and drier conditions (DeMarche et al. 2021). Thus, it follows
that the higher-performing nonlocal seeds at the two common gardens
should be those adapted to warmer and drier conditions (i.e., more
southern populations).