WHEN LOCAL MAY NOT BE BEST
The risks associated with maintaining the default local provenancing
strategy may be greater from an eco-evolutionary perspective than using
alternative strategies that include both local and non-local provenances
(summarised in Table 1). While this is not an exhaustive list, we
present four case studies, with the aim of highlighting the transition
from a state of known risks for non-local provenances to a state of
known risks for both non-local and local provenances.
Climate
change shifting local advantage
As climates change, there is a risk of local adaptations becoming
decoupled from environmental selective pressures, resulting in the
maladaptation of local genotypes. Evidence of such decoupling was
demonstrated in Eucalyptus gunnii , a subalpine foundation tree
species endemic to Tasmania, Australia. Consistent with local
adaptation, local provenances initially showed higher fitness in
reciprocal common garden trials (Potts 1985). However, following
two-decades of declining precipitation and increasing temperatures
(Sanger et al. 2011), local provenance fitness declined
(<30% survival), with the lower elevation, non-local
provenance showing higher fitness than the local provenance
(>70% survival; Prober et al. 2016). Similar
impacts of changing environments have been observed in other forest
species (Jump et al. 2009) and natural systems more broadly
(Parmesan 2006; Scheffers et al. 2016).
While future change may decouple local adaptations from home
environments, climate change to date may already be outpacing adaptation
– an outcome known as adaptation lag. Evidence for adaptation lag was
shown in Quercus lobata (valley oak), where two, early-age
provenance trials showed greater relative growth of provenances when
planted into cooler climates than their current homesite climate
(~10% greater growth; Browne et al. 2019). Using
a genomics-informed selection model, Browne et al. (2019) predicted
gains of up to 25% in relative growth of individuals pre-adapted to
current environments of the test site (Browne et al. 2019).
Optimal growth associated with historic rather than current climate is
consistent with adaptation lag. While growth is a plastic trait and
species may persist in suboptimal environments, similar evidence of
adaptation lag has been shown in several tree species (Carter 1996;
Aitken et al. 2008; Fréjaville et al. 2020).
Demographic
and evolutionary history
Demographic history plays an important role in structuring the genetic
and adaptive variation within and among populations (Hewitt 2000). The
historical expansion from refugia or genetic bottlenecks can, for
example, often leave evolutionary fingerprints that have fitness effects
on modern-day populations. Such demographic processes have been argued
to explain the differential survival and fecundity of two natural
populations of Arabidopsis thaliana (Thale cress) from opposing
latitudinal ends of its native European range (Sweden and Italy). While
local provenances showed higher fitness on average in reciprocal
transplant trials (Ågren & Schemske 2012), inter-provenance crosses
between the two populations tended to have higher fecundity than the
local provenance when planted in Sweden (Ågren et al. 2013).
Furthermore, several Italian alleles provided a fitness advantage over
the Swedish alleles when plants were grown in Sweden (Ågren et
al. 2013). Demographic history, in particular postglacial expansion in
northern Europe reducing genetic variation most likely led to increased
fixation of these maladaptive alleles.
Novel
environments from land use change
In some situation, environmental change, such as mining, agriculture,
and deforestation, has resulted in novel environments. This presents a
risk that the local provenances may no longer be fit for these novel,
local conditions. Evidence of this is shown in two metallophyte legumes,Mimosa acutistipula var. ferrea and Dioclea
apurensis , which are promising candidates for the restoration of
degraded areas of mineral rich soils in the Amazon, Brazil. Adaptive
genomic variation showed that local provenances would perform best in
moderately disturbed sites (e.g., sites altered by fire) (Carvalhoet al. 2020). However, no provenance in the sampled area matched
predictions for highly disturbed sites with substantially altered
conditions (i.e., ex-mining sites; (Carvalho et al. 2020). With
no suitable local provenance, regional mixing across multiple adaptive
units was recommended to capture genetic variation and enhance
adaptability, thereby de-risking revegetation plantings. Together with
previous findings of rapid, microgeographic adaptation post-mining (e.g.
Antonovics and Bradshaw 1970; Antonovics 2006), these results highlight
that provenance choice may vary dependant on site history, with
environmental changes having the potential to significantly influence
provenance fitness in novel environments.
Together, these examples highlight that current and future environmental
changes may have reduced local provenance fitness, and thus increased
the risk associated with using local provenances in particular
revegetation contexts. Further, they highlight the need to consider
current and future environmental change, climate and anthropogenic, as
well as effects of evolutionary history (e.g. demographic history) when
selecting provenances, to ensure both the short- and long-term fitness
and productivity of revegetation plantings. One approach could be mixing
local seed with seed from non-local provenances already occupying
current or future climates, which may de-risk the long-term resilience
of revegetation plantings.