4.2 Impacts of provenance but mixed evidence for local
adaptation
In comparing the growth and survival of upland and lowland provenances
between the DRO and Thiaki we had hoped to isolate the impacts of
temperature - and given the proximity of maternal populations to
planting sites (Figure S1) the prevalence of local adaptation in our
study species. Although we found that the lowland provenance often
outperformed the upland provenance at the lowland site, we rarely found
the reciprocal to be true i.e. upland provenances did not
outperform lowland provenances in the upland site, indicating a partial
lack of local adaptation sensu Kawecki and Ebert (2004). Despite this
study only including three sites and thus having relatively low power to
detect local adaptation (Blanquart et al . 2013), many examples
supporting our results challenge the notion that this is widespread
(Leimu & Fischer 2008, Bucharova et al. 2016, Mushagalusaet al. 2020). Our findings resemble those observed in the
reciprocal transplant of Quercus oleoides populations conducted
by Deacon & Cavender-Bares (2015). In their study, the lowland
provenance also exhibited higher mean fitness at all sites compared to
the upland provenance. Given that faster growth rates are typically
observed in lowland environments (Bradford et al. 2014, Baumanet al. 2022), it is possible that lowland populations of
rainforest trees generally have evolved higher growth rates than upland
populations, potentially at the cost of stress tolerance or other
fitness metrics (Willi & Van Buskirk 2022). Another possibility is that
the lowland provenance seedlings in our study were sourced from more
intact populations with higher reproductive population size and
therefore genetic diversity, which has been linked to higher fitness
(Reed & Frankham 2003). Confirming this requires further work assessing
within-species genetic diversity and differentiation across populations.
However, it is possible that upland provenances may possess greater
stress tolerance, and a full life-cycle assessment is needed to
determine any true differences in population fitness. Furthermore, in
Deacon & Cavender-Bares (2015) study, the advantage of the lowland
provenance was only temporary with provenance differences no longer
evident by the third year, highlighting the importance of continued
monitoring of such studies when assessing patterns of local adaptation
(Preece et al. 2023).