Subcontinental impacts of an invasive tree disease on forest structure and dynamics

Summary 1. Introduced pests and pathogens are a major source of disturbance to ecosystems world‐wide. The famous examples have produced dramatic reductions in host abundance, including virtual extirpation, but most introductions have more subtle impacts that are hard to quantify but are potentially at least as important due to the pathogens’ effects on host reproduction, competitive ability and stress tolerance. A general outcome could be reduced host abundance with concomitant increases in the abundance of competitors. 2. Beech bark disease (BBD) is a widespread, fatal affliction of American beech ( Fagus grandifolia ), currently present in c. 50% of beech’s distribution in eastern North America. Despite high adult mortality, beech remains a dominant component of the forest community. 3. Employing spatially extensive data from the national Forest Inventory and Analysis program of the United States Forest Service, we show that forests have changed dramatically in the presence of BBD. Within the 2.3 million km 2 range of beech, size‐specific mortality was 65% higher in the longest‐infected regions, and large beech (>90 cm diameter at breast height) have declined from c. 79 individuals km −2 to being virtually absent. Small stem beech density was dramatically higher (>350%) such that infested forests contain a roughly equivalent cross‐sectional (basal) area of beech as before BBD. 4. There was no evidence for compensation by sugar maple or other co‐occurring tree species via increased recruitment or adult survivorship at the landscape scale. Overall, community composition remained roughly unchanged as a result of BBD. 5. Surprisingly, trajectory of stand dynamics (shifts in stem density and mean tree size reflecting normal stand maturation (self‐thinning) or retrogression (more abundant, smaller trees over time)) did not differ between affected and unaffected regions. Variance in stand dynamics was greater in afflicted forests, however, indicating that predictability of forest structure has been diminished by BBD. 6. Synthesis. Forests of eastern North America have shifted to increased density and dramatically smaller stature – without notable change in tree species composition – following the invasion of a novel forest disease. Our results reinforce the conclusion that introduced diseases alter fundamental properties of ecosystems, but indicate that the spectrum of potential effects is broader than generally appreciated.