Discussion
Collectively, our findings highlight that the simultaneous study of
multiple sources of genetic diversity in trees is important for
understanding how trees will respond to environmental change in coming
years. We found little evidence to suggest that the relatively uncommon
occurrence of triploidy in most of aspen’s range has overarching
implications for the ecology of trembling aspen. In particular, across
16 trembling aspen genotypes native to Wisconsin, we found that cytotype
typically explained only a small portion of overall variation in a wide
array of individual traits and their responses to environmental stress
(defoliation and/or drought). Trembling aspen is known for its
extraordinarily high intraspecific genotypic variation in both
constitutive traits and trait plasticity (Barker et al., 2019; Callahan
et al., 2013; Mitton & Grant, 1996; Rubert-Nason et al., 2015). Our
study underscores the importance of this genetic diversity, which fuels
environmental adaptation and evolution (R. J. Petit & Hampe, 2006;
Westerband, Funk, & Barton, 2021) as a potential buffer against climate
change impacts on aspen.
Notable exceptions to our general conclusion above include the
observation that foliar tremulacin levels were higher in triploids than
in diploids (Appendix 2), which accords with findings from other studies
that polyploidy can lead to altered secondary chemistry (Gaynor,
Lim-Hing, & Mason, 2020; Park et al., 2021; Te Beest et al., 2012).
Although the ploidy effect on tremulacin level was moderate
(~1.5% across defoliation and drought treatments),
tremulacin is a key defensive compound exhibiting biological activity
against an array of herbivores (Boeckler, Gershenzon, & Unsicker, 2011;
R. J DeRose, Gardner, Lindroth, & Mock, 2021; Lindroth & St. Clair,
2013). Thus, even small contrasts in tremulacin concentration might
affect herbivore activity. Especially for juvenile trees with limited
biomass, small differences in the extent of herbivory can have
long-lasting performance impacts (Massad, 2013).
The influence of ploidy level on light-saturated photosynthesis in our
Wisconsin genotypes (Fig. 3a) is also generally consistent with previous
findings. Greer et al. (2018) reported that mature triploid aspen clones
in the Rocky Mountains exhibited higher net carbon assimilation rates
than did diploid clones. In contrast to our results, however, the key
driver of enhanced photosynthetic performance in mature triploids was
increased stomatal conductance rather than heightened rubisco activity
(V cmax, Appendix 3). Regardless, given the
central role that leaf photosynthetic performance plays in plant carbon
balance, differences in photosynthesis between diploid and triploid
trees might ultimately drive other trait-level divergences between the
two cytotypes. For example, increased photosynthetic rates in triploids
possibly contributed, via greater availability of assimilated carbon, to
higher tremulacin levels in triploid foliage. Enhanced photosynthetic
activity would also likely increase biomass growth in triploids, but
corresponding growth differences were not observed in our study. We
acknowledge, however, that our ability to thoroughly assess links
between growth and its determinants is precluded by the absence of data
on belowground biomass allocation. Other studies on mature aspen have
found that triploid genotypes exceed diploids in stem growth rates and
stem diameter (R Justin DeRose et al., 2015).
The absence of marked cytotype differences in trait response to
defoliation and/or recurring drought in our study agrees with results
from other research that, for the most part, diploid and polyploid
plants share a similar degree of trait plasticity (C. Petit & Thompson,
1997; Te Beest et al., 2012; Vilas & Pannell, 2017; Wei, Cronn, Liston,
& Ashman, 2019). Nevertheless, we observed a key difference from those
general patterns: triploid genotypes displayed greater drought
resilience than did diploid genotypes, as measured via increased
production of new lateral shoot tissue growing on the main stem in
post-drought recovery (Fig. 4). During recovery, triploid trees also
invested more in leaf rather than height growth (Fig. 4, Appendix 5). As
a consequence, young triploid trees remained shorter than diploid trees
(Fig. 4), potentially placing the former at a competitive disadvantage
for light. Yet, when growing in regions frequented by drought, investing
resources in additional leaf tissue and higher overall photosynthetic
capacity (see above) could allow triploids to generate more carbohydrate
reserves upon return to favorable growth conditions. If this explanation
is valid, triploid trees would possess more resources and improved
capacity to recover from re-occurring drought events.
A cytotype difference in drought resilience could help explain the
comparative abundance of triploid clones in the arid regions of North
America (Mock et al. 2012; but see Latutrie, Tóth, Bergeron, and
Tremblay (2019)). Mock et al. (2012) suggested that triploid aspen,
which can reproduce only clonally, may possess an advantage over
diploids with respect to clonal expansion rates or ramet growth. Asexual
(vegetative) reproduction may be the most reliable means of persisting
in dry environments, where seedling establishment is less common
(Landhäusser, Pinno, & Mock, 2019). In contrast, in more mesic
environments such as the Great Lakes region of North America, polyploidy
may not provide an overall benefit. While juvenile triploid trees may
possess an advantage due to slightly increased levels of phytochemical
defenses (Appendix 2) they also suffer from a competitive disadvantage
by growing more slowly than their diploid counterparts early in stand
development (Fig.1). Therefore, the establishment of individual
polyploid aspen in the Great Lakes region likely depends on genotype-
rather than ploidy-associated trait properties.
We note that defoliation rivaled or exceeded polyploidy with respect to
the magnitude of its positive impact on our measure of drought
resilience (Fig. 4, Appendix 5). This outcome can be explained in part
by the fact that leaves of defoliated trees sustained stomatal opening
and photosynthesis at soil moisture deficits well beyond the threshold
causing pronounced stomatal closure (and decreased photosynthesis) in
foliage of nondefoliated trees (Fig. 3b, Appendix 4). A similar
influence of defoliation on leaf photosynthetic response to water stress
has been observed in other studies (McGraw, Gottschalk, Vavrek, &
Chester, 1990; Pinkard, Eyles, & O’Grady, 2011). It may stem from the
effects of defoliation on the fundamental balance between
transpirational demand and moisture supply, which is thought to exert
considerable control over stomatal behavior (Sperry, Adler, Campbell, &
Comstock, 1998). Consistent with the observed growth responses of
droughted trees in this study (Fig. 1) and others (Jacquet, Bosc,
O’Grady, & Jactel, 2014; Pinkard et al., 2011), a comparative increase
in leaf carbon gain would help mitigate the negative effects of
defoliation on biomass growth. Correspondingly, the increase would allow
defoliated trees to accumulate more carbohydrate reserves and, during
subsequent drought recovery, invest more resources into biomass
production relative to nondefoliated trees. These phenomena and their
underlying mechanisms warrant further investigation, as we have not
found published corroboration of the defoliation impact on drought
recovery as observed in our study. With the advance of climate change,
forests are increasingly exposed to multiple co-occurring environmental
stressors (Aber et al., 2001; Seidl et al., 2017). Accordingly, the
possibility that herbivore-mediated defoliation in addition to ploidy
and genotypic determinants can buffer negative drought effects is an
important consideration when assessing the consequences of future
stressors on forest ecosystems.