The forest ecology literature is rife with debate about how to: (i) define a ‘forest’ and distinguish it from similar systems, such as woodlands, savannas, parklands or plantations; (ii) identify transitions from ‘forested’ to ‘non-forested’ states and, most challengingly; (iii) quantify intensities of degradation. Here we argue that past attempts to define forests and forest degradation, focusing on attributes of living trees (e.g., height, canopy cover), combined with regenerating processes such as recruitment and succession, whilst useful, are ecologically incomplete. These approaches do not adequately represent processes that, operating over long time scales, determine whether a forest system is structurally healthy (as opposed to degraded), functional and persistent. We support our case using a conceptual model to illustrate how deeper-time processes, as well as instantaneous or chronic disturbances that cause degradation, might be revealed through analysis of the patterns of size structure and density of the fallen wood, in relation to the living trees and standing dead. We propose practical ways in which researchers can incorporate dynamic, long-term processes into definitions of forests and forest degradation, using measurements of dead and fallen trees. Doing so will improve our ability to manage and monitor forest health under global change.