Management implications
Given that local adaptation and phenotypic differentiation in forest
trees (Savoleinen et al . 2007) has been closely tied to variation
in climate (Alberto et al. 2013), populations may become
increasingly maladapted as climate change continues (Shaw & Etterson
2012; Franks et al. 2014, Aitken & Bemmels 2015). Maladaptation
due to climate change is expected to be greatest in populations from the
warmest extent of their range, while populations at the cold edge may
benefit from slightly warmer temperatures (Aitken & Bemmels 2015).
Provenance trials have shown this response experimentally with tree
productivity declining as the climate distance transferred between home
site and garden site increases (O’Neill et al. 2008; Evanset al. 2016; Grady et al. 2015). Transfer functions can
help determine how far a population can be moved before growth declines
below a specified level; this tool, combined with climate change
forecasts, is one of the best ways to implement assisted migration in
order to manage for future forest health and productivity. This method
has been used to recommend seed transfer zones and distances for
economically important conifer species in British Columbia (O’Neillet al. 2008, 2017), and as a caution to move trees at a
reasonable, step-wise pace to track climate change (Grady et al.2015). Based on our results of declining tree performance as climate
transfer distance increases, we may expect decreases in tree
productivity and increasing maladaptation as local conditions become
increasingly arid, especially for populations in southern Arizona that
are close to the thermal and low elevation edge of their distribution
(Ault et al. 2014; see Fig. 1).
In addition to tracking declines in growth metrics with climate transfer
distance, it is important to consider how shifts in phenology in forest
trees will affect dependent communities (Whitham et al. 2020).
Numerous studies have shown trophic-level asynchronies across different
ecosystems due to mismatches in phenological changes (Thackeray et
al. 2010; Renner & Zohner, 2018). For example, warmer, earlier springs
can facilitate earlier phenology of many tree species, which may or may
not be synchronized with the emergence and reproductive cycles of
important community members (Visser et al. 2006; Kudo & Ida
2013; van Asch et al. 2013). This can cause a phenological
mismatch between plants and the species that rely on them for food or
habitat (Renner & Zohner 2018), disrupting species interactions and
trophic cascades (Bailey et al . 2006; Smith et al. 2011).
Since cottonwood trees provide food and habitat for thousands of
dependent species (Whitham et al . 2006; Lamit et al.2015), changes in growth, morphology, and phenology with altered climate
will likely affect species interactions, community composition, and
functionality (Whitham et al . 2020).