title: Chronic water stress reduces tree growth and the carbon sink of deciduous hardwood forests.
abstract: Predicted decreases in water availability across the temperate forest biome have the potential to offset gains in carbon (C) uptake from phenology trends, rising atmospheric CO2 , and nitrogen deposition. While it is well established that severe droughts reduce the C sink of forests by inducing tree mortality, the impacts of mild but chronic water stress on forest phenology and physiology are largely unknown. We quantified the C consequences of chronic water stress using a 13-year record of tree growth (n = 200 trees), soil moisture, and ecosystem C balance at the Morgan-Monroe State Forest (MMSF) in Indiana, and a regional 11-year record of tree growth (n > 300 000 trees) and water availability for the 20 most dominant deciduous broadleaf tree species across the eastern and midwestern USA. We show that despite ~26 more days of C assimilation by trees at the MMSF, increasing water stress decreased the number of days of wood production by ~42 days over the same period, reducing the annual accrual of C in woody biomass by 41%. Across the deciduous forest region, water stress induced similar declines in tree growth, particularly for water-demanding 'mesophytic' tree species. Given the current replacement of water-stress adapted 'xerophytic' tree species by mesophytic tree species, we estimate that chronic water stress has the potential to decrease the C sink of deciduous forests by up to 17% (0.04 Pg C yr(-1) ) in the coming decades. This reduction in the C sink due to mesophication and chronic water stress is equivalent to an additional 1-3 days of global C emissions from fossil fuel burning each year. Collectively, our results indicate that regional declines in water availability may offset the growth-enhancing effects of other global changes and reduce the extent to which forests ameliorate climate warming.
authors.affiliation: Department of Biology, Indiana University, Bloomington, IN, 47405, USA.
authors.forename: Edward R
authors.initials: ER
authors.lastname: Brzostek
authors.forename: Danilo
authors.initials: D
authors.lastname: Dragoni
authors.forename: Hans Peter
authors.initials: HP
authors.lastname: Schmid
authors.forename: Abdullah F
authors.initials: AF
authors.lastname: Rahman
authors.forename: Daniel
authors.initials: D
authors.lastname: Sims
authors.forename: Craig A
authors.initials: CA
authors.lastname: Wayson
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Johnson
authors.forename: Richard P
authors.initials: RP
authors.lastname: Phillips
mesh_terms: D057965:Carbon Sequestration; D065928:Forests; D007196:Indiana; D019684:Magnoliopsida / Q000254:growth & development*; D012987:Soil / Q000737:chemistry; D013312:Stress, Physiological; D014197:Trees / Q000254:growth & development; D014867:Water / Q000032:analysis*
chemical_list: D012987:Soil; D014867:Water
keywords: carbon storage; deciduous hardwood forests; phenology; tree growth; water stress