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.