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Eastern deciduous forests are changing in species composition and diversity outside of classical successional trajectories. Three disturbance mechanisms appear central to this phenomenon: fire frequency is reduced, canopy gaps are smaller, and browsers are more abundant. Which factor is most responsible is a matter of great debate and remains unclear, at least partly because few studies have simultaneously investigated more than one process. We conducted a large‐scale experiment in mesophytic forests of West Virginia, USA, to test three key hypotheses: (1) the fire hypothesis (fire suppression limits diversity to few shade‐tolerant, fire‐intolerant species that replace and suppress many fire‐tolerant species); (2) the gap hypothesis (small gaps typical of today's forests promote dominance of a few shade‐tolerant species); and (3) the browsing hypothesis (overbrowsing by deer limits diversity to a few …