Warm springs alter timing but not total growth of temperate deciduous trees

As the climate changes, warmer spring temperatures are causing earlier leaf-out and commencement of CO₂ uptake in temperate deciduous forests, resulting in a tendency towards increased growing season length and annual CO₂ uptake. However, less is known about how spring temperatures affect tree stem growth, which sequesters carbon in wood that has a long residence time in the ecosystem. Here we show that warmer spring temperatures shifted stem diameter growth of deciduous trees earlier but had no consistent effect on peak growing season length, maximum growth rates, or annual growth, using dendrometer band measurements from 440 trees across two forests. The latter finding was confirmed on the centennial scale by 207 tree-ring chronologies from 108 forests across eastern North America, where annual ring width was far more sensitive to temperatures during the peak growing season than in the spring. These findings imply that any extra CO₂ uptake in years with warmer spring temperatures does not significantly contribute to increased sequestration in long-lived woody stem biomass. Rather, contradicting projections from global carbon cycle models, our empirical results imply that warming spring temperatures are unlikely to increase woody productivity enough to strengthen the long-term CO₂ sink of temperate deciduous forests.