Mean shade tolerance of adult trees did not increase on 51,433 remeasured eastern US forest plots, 2002–2026, although saplings and stand age predicted an increase

Question: Do eastern US forest canopies become more shade tolerant (mesophication)? We tested this on the same plots measured twice. Approach: We used 51,433 USDA Forest Inventory and Analysis plots in 37 eastern states, measured in 2002–2010 (T1) and again 12–18 years later (T2). We followed each tree and gave each species its Niinemets–Valladares (2006) shade-tolerance score (0–5). We calculated the basal-area mean of adult trees (diameter at least 12.7 cm) and divided its change exactly into natural death, harvest, other loss, survivor growth and ingrowth. We used a 1-degree cell bootstrap for 95% intervals. We also tested two indicators: the T1 gap between saplings and adults, and the T1 relation between stand age and mean tolerance. Results: The pooled mean decreased by 0.015 per decade (−0.020 to −0.010). It increased in the North (+0.015), did not change clearly in the Central states (−0.006; −0.012 to +0.001) and decreased in the South (−0.025). Loblolly pine (basal-area share 13.5% to 16.5%) caused most of the decrease. Without it, the change was +0.002 (−0.002 to +0.007), and for hardwoods only it was −0.004 (−0.007 to −0.000). Harvest increased the mean (+0.043), but faster growth of intolerant survivors (−0.025) and intolerant ingrowth (−0.038) cancelled this. Saplings were 0.31 units more tolerant than adults at T1, but adults moved −7% of that gap, and ingrowth was 0.51 units less tolerant than T1 saplings. In undisturbed natural stands, the stand-age relation predicted +0.085 per decade; the observed change was −0.003 (−0.009 to +0.002). In oak–hickory forests the change was +0.023 in the North, +0.004 in the Central states and −0.046 in the South. Basal-area drought tolerance increased (+0.027). Limitations: the test covers about 15 years; there is one trait source; with published score errors the pooled interval includes zero (−0.037 to +0.006); with stem weights the pooled change is −0.004 (−0.013 to +0.007); regions are state groups; fire tolerance was not tested.

mesophication · shade tolerance · Forest Inventory and Analysis · community-weighted mean · demographic decomposition · stage-for-time · space-for-time · stand age · loblolly pine · oak-hickory · eastern United States · forest dynamics · remeasured plots