INTRODUCTION
Climate change-associated drought events are impacting forest ecosystem structure, function and distribution worldwide at a magnitude and speed unparalleled in human history (Allen et al. 2015; Anderegg et al. 2013a; Batllori et al. 2020). In coming years, droughts are predicted to further increase in intensity and frequency in many parts of the world, including temperate regions (Gazol et al. 2017; Lecina‐Diaz et al. 2021; Samaniego et al. 2018). The capacity of many forest trees to persist in future environments will be determined by mechanisms that enable them to cope with recurrent and intense drought stress on a regular basis.
Drought impacts on forests ecosystems are frequently coupled to other climate change-related stressors (Millar and Stephenson 2015). Drought and warming, for example, can facilitate outbreaks of insect herbivores (Anderegg et al. 2015; Kolb et al. 2016; Seidl et al. 2017). Consequently, trees must cope with multiple, potentially interacting abiotic and biotic stressors (Aber et al. 2001; Niinemets 2010). To advance our understanding of how climate change affects tree performance, controlled experiments are needed to disentangle the individual and interactive effects of multiple co-occurring stressors and to elucidate the underlying mechanisms that enable trees to cope with environmental stress.
Genetic diversity is a key determinant of a plant species’ ability to evolve in response to environmental stress (Estravis-Barcala et al. 2020; Schueler et al. 2013). Conspecific genotypes can vary considerably in trait expression and phenotypic plasticity - i.e., the capacity to express different phenotypes under different environmental conditions (Bradshaw 1965; Valladares et al. 2007). Different genotypes from the same plant species can show marked differences in their responses to abiotic (Albert et al. 2010; Cooper et al. 2019; Huang et al. 2015; Kreyling et al. 2019) and biotic (Aartsma et al. 2019; Barton et al. 2015; Rubert-Nason et al. 2015; Silfver et al. 2009; Wurst et al. 2008) stress. Intraspecific genotypic variation is thus an important element facilitating ecological and evolutionary responses of plant populations to environmental change (Westerband et al. 2021).
Another less-understood aspect of genetic diversity in plants is polyploidy - i.e., the possession of more than two paired sets of chromosomes in somatic cells. Naturally occurring polyploid cytotypes can differ from diploid cytotypes in many traits, including growth, chemistry and physiology (Diallo et al. 2016; Greer et al. 2018; Li et al. 1996; Meng et al. 2014; Niwa and Sasaki 2003). Polyploidy has long been considered a mechanism that could increase plant tolerance to stressful environments (Levin 1983; Madlung 2013; Van de Peer et al. 2021; Van de Peer et al. 2017). However, the importance of polyploidy as a stress-tolerance enhancer is still debated and the mechanisms underlying ploidy-driven stress responses remain largely unknown (Fox et al. 2020; Van de Peer et al. 2021; Van de Peer et al. 2017).
For plant species or populations with a high intraspecific genotypic and cytotype diversity, both genotype and cytotype may be important for plant stress adaptation. Yet, the relevance of trait variation due to genotype relative to variation due to cytotype has rarely been documented (but see Blonder et al. (2021), Wei et al. (2020)). Few if any results have been published from controlled experiments that simultaneously evaluated the significance of trait variation due to genotype and cytotype in a climate change context. Trembling aspen (Populus tremuloides; hereafter “aspen”), the most broadly distributed tree in North America (Elias and Little 1980), is among the species predicted to be most severely affected by climate change (Anderegg et al. 2013b; Ashraf et al. 2015; Zolkos et al. 2015). In recent decades, aspen stands have experienced large-scale declines throughout the Intermountain West of North America (Rehfeldt et al. 2009; Stanke et al. 2021). These declines have been attributed in part to extended drought events and insect outbreaks (Chen et al. 2018; Worrall et al. 2013). Aspen is characterized by high levels of intraspecific genetic diversity and phenotypic plasticity (Barker et al. 2019; Mitton and Grant 1996). Aspen is generally diploid (individuals with two sets of chromosomes). Autopolyploid, triploid genotypes (i.e., triploidy wherein all three sets of chromosomes derive from the same species rather than from the hybridization of two species) appear to be common across the western USA but are rare in most other parts of the USA, including the Great Lakes Region (Every and Wiens 1971; Mock et al. 2012).
Preliminary research suggests that larger aspen clones are frequently triploid (Bishop et al. 2019; Mock et al. 2008), and that triploids grow faster and differ in photosynthetic capacity when compared with their diploid counterparts (DeRose et al. 2015; Greer et al. 2018; Mock et al. 2008). Additionally, Blonder et al. (2021) reported that triploid aspen have reduced recruitment on drought-prone plots relative to diploids. Finally, Greer et al. (2018) found that triploid aspen may have a lower resilience than diploids to drought stress, due to larger stomatal size and lower stomal sensitivity to increasing vapor pressure deficit.
The goal of this study was to explore the effects of autopolyploidy levels (cytotype), in the context of conventional genotypic variation among sexually generated individuals, on both trait expression and its plasticity, in aspe
n, in the first of three field studies we assessed variation in growth, biomass allocation, phytochemistry and leaf physiology across diploid and triploid aspen genotypes from the Intermountain West (Utah, USA) and the Great Lakes region (Wisconsin, USA). In two follow-up experiments involving the same diploid and triploid genotypes from Wisconsin, we evaluated plasticity in response to two environmental stressors that heavily influence aspen performance: drought and defoliation (simulated herbivory). In Experiment 2, we assessed how ploidy levels were associated with growth, chemistry, and physiology of trees subjected to moderate drought stress, defoliation, and their combination. In Experiment 3, we explored how ploidy levels influenced aspen recovery from prolonged drought stress and pre-drought defoliation. We aimed to disentangle the effects of ploidy level per se from the effects of different genotypes on trait variation. Hence, for all three experiments, we quantified ploidy effects by using individual genotypes as statistical units, allowing us to incorporate individual genotypic variation.