Beech leaf disease reduces nitrogen retranslocation at leaf fall, altering litter chemistry

Beech leaf disease (BLD) poses a serious threat to the health of beech forests throughout the northeastern USA and Canada. Caused by invasive nematodes, BLD first appeared in 2012 in Ohio and has rapidly spread eastward. We investigated the effects of BLD on leaf and litter chemistry and leaf litter decomposition rate from four infected beech stands in Falmouth, Massachusetts. Attached leaves exhibiting differential infection intensity were collected from trees in October 2023 and analyzed for metrics of carbon (C) and nitrogen (N) content including %C, δ¹³C, %N, and δ¹⁵N. Severely infected and asymptomatic newly fallen leaf litter was gathered in November 2023 and analyzed for %C and %N only. We tested the effect of BLD-altered litter chemistry (separate from BLD-altered leaf structure) on litter decomposition rates by incubating ground litter in soil. Despite notably lower C:N in severely infected (C:N ~ 25) vs. asymptomatic (C:N ~ 43) litter, the difference in CO₂ evolution was negligible in short-term incubations. Percent N was significantly higher in severely infected (cupped) vs. asymptomatic litter (P = 0.00076), but significantly lower in severely infected leaves compared to asymptomatic green leaves (P = 0.021). δ¹³C tended to increase with infection intensity, possibly reflecting increased water use efficiency with infection stress; δ¹⁵N showed no clear pattern. The severely symptomatic cupped leaves (gathered in October) and litter (gathered in November) had indistinguishable %N, suggesting trees retranslocated far less N out of infected leaves than out of asymptomatic leaves prior to leaf fall.