Authors: Allison Perkins, Aubrey Hawks, Talia Karasov
Mentors: Talia Karasov
Insitution: University of Utah
Over the past two decades, studies have documented a 20% decline in Quaking Aspen (Populus tremuloides) populations in western North America (Worrall et al., 2015; Stanke et al., 2021). This phenomenon has been fittingly characterized Sudden Aspen Decline (SAD), and is an increasingly pressing issue as the role of aspen as an ecologically irreplaceable keystone species impacts the health of the surrounding forests (Singer et al., 2019). SAD has been attributed to the interplay of climate change-driven drought and other biotic and abiotic factors that are less well characterized (Anderegg et al., 2013a). One potential contributor to SAD is biotic pests and pathogens (Marchetti et al., 2011; Anderegg et al., 2013a; Worrall et al., 2015).
My study system includes both natural populations of aspen representing a precipitation gradient and a controlled garden experiment. The field experiments span five distinct sites across Utah & Colorado, selected and montintered by the Anderegg lab of the University of Utah. The experimental garden contains approximately 300 tree saplings subjected to various levels of drought stress, managed by the Anderegg lab on the University of Utah campus. Through the integration of both controlled and natural experiments, my research aims to comprehensively evaluate the impact of drought exposure on pathogen abundance and chemical defenses in aspen trees.
Over the past two decades, studies have documented a 20% decline in Quaking Aspen (Populus tremuloides) populations in western North America (Worrall et al., 2015; Stanke et al., 2021). This phenomenon has been fittingly characterized Sudden Aspen Decline (SAD), and is an increasingly pressing issue as the role of aspen as an ecologically irreplaceable keystone species impacts the health of the surrounding forests (Singer et al., 2019). SAD has been attributed to the interplay of climate change-driven drought and other biotic and abiotic factors that are less well characterized (Anderegg et al., 2013a). One potential contributor to SAD is biotic pests and pathogens (Marchetti et al., 2011; Anderegg et al., 2013a; Worrall et al., 2015).
Recent investigations have indicated a link between SAD and specific microbial diseases, suggesting that the increasing frequency and severity of droughts due to climate change might make aspen more vulnerable to certain pathogens, even though many of the most common pathogens of aspen in general require more abundant water (Aung et al., 2018). For example, the foliar Melamspora fungal pathogens require abundant water and are not frequently observed in drought stressed trees.
On the other hand, Cytospora, which causes a devastating canker disease in aspen trunks, occurs at higher frequency in plots of trees suffering damage from drought (Guyon, 1996). Lin et al. (2023) shows changes to phyllospheric microbiome in aspen during drought, but far less is known about the leaves specifically. Could drought lead to an altered microbiome in aspen leaves? If different or possibly opportunistic pathogens are better able to colonize the leaf tissue under drought stress, this may be the case.
It’s known that plants with a reduced diversity of microbiomes are more susceptible to pathogens (Zheng et al., 2020), but the there is little understanding how drought may reduce microbial diversity in aspen. Aspen have two main groups of chemical defenses (SPGs and CTs) that occur in relatively high levels in the leaf (Lindroth et al., 2023). It is well established that these defend against insect herbivores and that they come with a trade-off for plant growth (Marchetti et al, 2011). There is some observational evidence that these secondary compounds have an effect on pathogens (Jacoby et al., 2021), but there are few controlled studies on this idea. Additionally, Metlen et al. 2009 describes how trees in North America produce higher rates of these metabolites under wetter conditions, attributed to possible UV protection. However, the impact of additional environmental changes have of on the abundance of these compounds is understudied.
Aspen are an ideal system to study forest disease and drought because the genus’ (Populus) genomes are easily sequenced and relatively tractable, aspen generally exhibit rapid vegetative growth, their defensive chemistry is relatively well known, and most importantly, they grow in cloned groves that reduce the genetic variation in experiments (Lindroth et al., 2023, Luquez et al., 2007).