Conclusions
Our results show that populations are significantly differentiated with
respect to growth, morphology, and phenology traits, supporting a
multi-trait hypothesis of divergent selection (QST> FST). Phenology traits were the most
differentiated among populations with the largest QSTvalues, while SLA and height supported the divergent selection
hypothesis in some gardens, but not others. In contrast, basal trunk
diameter was the only trait to show evidence of stabilizing selection
and only in the coldest common garden (Fig. 4). In addition, we found
source climate is significantly correlated with trait differences across
the gardens, suggesting the large climatic gradient experienced by these
Arizona populations is an agent of selection. Interestingly, the
magnitude of trait variation detected among populations depended, in
part, on their growing environment. We found most traits had the
greatest population differences with highest QST values
in the warmest garden and declined as the trees were planted in cooler
environments. Specific leaf area was the only trait measured with the
opposite response of higher population differentiation in the cold
garden (Fig. 4). Populations exhibited local adaptation in growth and
phenology traits, with many populations growing largest in the gardens
that most closely matched their home climates. This study demonstrates
that experimental common gardens simulating climate change, across even
a portion of a species range, can have a substantial impact on how
important functional traits are differentially expressed among
populations. The gradient of climate-driven selection may lead to the
identification of a geographic mosaic of local adaptation that may also
cascade to affect associated species and communities (e.g., Thompsonet al . 2005; Smith et al. 2011; Wooley et al.2020). Importantly, we found that the detection of past selection on
population-level trait differences, as measured by
QST-FST analysis, is modified by growing
environment. This finding suggests past climate can interact with the
current and future climates to affect population responses. Strategies
for management of widespread species like Fremont cottonwood would
benefit from considering the climatic selection pressures of source
locations to anticipate their performance under changing environmental
conditions. Acknowledgements
This research was supported by NSF-IGERT and NSF GK-12 Fellowships (HF
Cooper), NSF Bridging Ecology and Evolution grant DEB‐1914433 (RJ Best,
GJ Allan, R Lindroth, TG Whitham), NSF MacroSystems grant DEB-1340852
(GJ Allan, TG Whitham, CG Gehring, & KC Grady), NSF Macrosystems grant
DEB-134056 (KR Hultine), NSF DBI-1126840 (TG Whitham), which established
the Southwest Experimental Garden Array. We thank our agency partners
for helping to facilitate use of the common gardens: Dana Warnecke and
Kelly Wolf at Arizona Game and Fish (Agua Fria), Erica Stewart at the
Bureau of Land Management (Yuma), and Barry Bakker, Phil Adams, and the
Redd family at The Nature Conservancy’s Canyonlands Research Center at
Dugout Ranch. We acknowledge Christopher Updike, Zachary Ventrella,
Davis Blasini, Dan Koepke, and Matthew McEttrick, along with many
volunteers for help establishing and maintaining the common gardens. We
thank Helen Bothwell for her help developing and troubleshooting the SNP
library. Lastly, thanks to Jacob Cowan, Michelle Hockenbury, Teresa
Reyes, Michelle Bem, and Jackie Parker for assistance with data
collection in the field, and the Cottonwood Ecology and Community
Genetics Lab for their constructive comments and reviews.