Forests have a key role in global ecosystems, hosting much of the world’s terrestrial biodiversity and acting as a net sink for atmospheric carbon1. These and other ecosystem services that are provided by forests may be sensitive to climate change as well as climate variability on shorter time scales (for example, annual to decadal)2,3,4. Previous studies have documented responses of forest ecosystems to climate change and climate variability2,3,4, including drought-induced increases in tree mortality rates5. However, relationships between forest biomass, tree species composition and climate variability have not been quantified across a large region using systematically sampled data. Here we use systematic forest inventories from the 1980s and 2000s across the eastern USA to show that forest biomass responds to decadal-scale changes in water deficit, and that this biomass response is amplified by concurrent changes in community-mean drought tolerance, a functionally important aspect of tree species composition. The amplification of the direct effects of water stress on biomass occurs because water stress tends to induce a shift in tree species composition towards species that are more tolerant to drought but are slower growing. These results demonstrate concurrent changes in forest species composition and biomass carbon storage across a large, systematically sampled region, and highlight the potential for climate-induced changes in forest ecosystems across the world, resulting from both direct effects of climate on forest biomass and indirect effects mediated by shifts in species composition.
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Funding was provided by US Department of Agriculture Forest Service agreements 11-JV-11242306-059 and 16-JV-11242306-050 to J.W.L., and by the European Regional Development Fund (Centre of Excellence EcolChange) and the Estonian Ministry of Science and Education (institutional grant IUT-8-3) to Ü.N.
Extended data figures
This file contains Supplementary Methods 1-7: (1) Description and evaluation of the DT index; (2) examples of Δ (̅DT ) in remeasured plots; (3) influence of tree harvesting on estimated Δ ("DT" ) ̅ responses; (4) stand-level analysis of remeasured inventory plots; (5) partitioning Δ ("DT" ) ̅ into component contributions; (6) species influences on Δ ("DT" ) ̅ responses; and (7) species influence on ΔAGB responses.
This file contains Supplementary Tables 1-5: Tables (one per forest age class) of species influence statistics. The statistics quantify how excluding (vs. including) each species affects the estimated slopes of Δ ("DT" ) ̅ vs. ΔPDSI, ΔAGB vs. ΔPDSI, and ΔAGB vs. Δ ("DT" ) ̅. Forest age classes are 0-20, 20-40, 40-60, 60-80, and 80-100 years.
This file contains Supplementary Tables 6-20: Tables of abundance change between the 1980s and 2000s for common species within each forest age class and each of the three sub-regions (north-central, northeastern, and southeastern USA). ‘Common species’ are those comprising at least 1% of AGB in a given sub-region and age class in either or both decades.