Quaking aspen (Populus tremuloides) is the most widely distributed tree in North America (Lindroth and St Clair 2013), and a keystone species in our western montane forests (Worrall et al. 2015). Aspen has become a model organism for studies of genetics and physiology in woody plants (Bradshaw et al. 2000, Taylor 2002). Aspen is also economicallyimportant (Worrall et al. 2015) – wood is harvested for various uses, its scenic beauty helps sustain the tourism economy in many areas, and it has recently been studied as a possible source of biofuel (Sannigrahi et al. 2010). Aspen is also a species of conservation concern, due to recent large-scale deterioration and decline of many aspen forests in the last two decades (Worrall et al. 2013). Several causal factors have been identified: fire suppression (Calder et al. 2011, Smith et al. 2011), increased ungulate herbivory (Kay and Bartos 2000), disease (Marchetti et al. 2011), and climate change (Worrall et al. 2013). My thesis focuses on two different biotic stressors of aspen: a fungal pathogen and ungulate herbivory. Understanding the relationship between aspen and their biotic stressors adds to our knowledge of aspen ecology and helps manage the increasing risk of decline in our aspen forests. Chapter 1 is a study of the relationship between aspen and a necrotrophic fungal pathogen (Drepanopeziza sp.) during a major disease outbreak in 2015. I quantified the relationship between Drepanopeziza infection severity and aspen leaf functional traits, including morphological, chemical and phenological traits. I found that severe Drepanopeziza infection was associated with low concentrations of a key class of herbivore defense compounds (phenolic glycosides), and strongly associated with early budbreak and leaf-out in aspen stands. The association between infection and early budbreak was likely caused by unusually rainy conditions in May of 2015, which may have exposed leaf tissue to wet conditions that favor thedispersal of Drepanopeziza spores. Chapter 2 is an experiment designed to determine whether the mode and timing of herbivory can influence aspen's defensive response. I specifically asked whether removing leaves, twigs and meristems together and removing leaves alone had unique effects on aspen sucker growth, survival, and phytochemistry. Additionally, I applied these simulated herbivory treatments to suckers on different dates to see whether early- or late-summer herbivory had greater effects on suckers. I found strong mode and timing effects on growth and survival, but not foliar chemistry.