Invasions of non-native insects are a major driver of change for forest composition, structure, and carbon storage capacity, especially in invasion hotspots such as the Northeastern United States. The goal of this dissertation is to understand causes and quantify consequences of tree mortality from invasive insects. The two insect-host systems featured – spongy moth (Lymantria dispar) defoliation of oaks (Quercus), and hemlock woolly adelgid (Adelges tsugae) infestation of eastern hemlock (Tsuga canadensis) – exhibit contrasting virulence and magnitude of ecosystem change. The three chapters of the dissertation each addressed a different question and approach. Chapter 1 examined the carbon starvation hypothesis by evaluating sugar and starch stores in oak trees defoliated by spongy moth. Defoliated trees drew down their sugar and starch stores, sometimes to zero. All the trees that died had extremely low energy reserves (<1.5% dry weight of sugars and starches in their roots and stems), providing empirical evidence that these trees starved to death. Chapter 2 evaluated the longer-term (15 years) results from an experimental manipulation that contrasted hemlock death from simulated insect damage with salvage logging. Many differences between the treatments either converged within 15 years or are projected to converge within the next decade, with notable exceptions of understory species richness, dead wood quantity, and aboveground carbon stocks. The differences that persist provide the girdled plots with greater biodiversity, aboveground carbon stocks, and dead wood microhabitats, and thus a potentially greater capacity for resilience to ongoing and future stressors and disturbances. Finally, Chapter 3 contrasted spongy moth outbreaks in the 1980s and the 2010s across a forested landscape and asked how defoliation and forest stand development interact to determine oak mortality. Greater loss of trees and biomass occurred during the 2010s outbreak than the 1980s outbreak, partially because the average tree that died between 2010-2020 was larger and contained much more biomass than the average tree that died between 1980-1990. However, other stressors likely contributed as well, highlighting an increased risk of tree mortality in an era of accelerating global change. Taken together, this work highlights how invasive insects alter forested ecosystems and reduce their capacity to respond to human needs and climate change.