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.

Topography alters tree growth–climate relationships in a semi‐arid forested catchment

“…We hypothesize that measured negative growth relationships with high spring (P. ponderosa) and fall (P. contorta) temperatures may be related to an extended growing season. These high temperature patterns may lengthen the growing season and increase water stress (in the absence of significant summer rainfall) ultimately decreasing total carbon assimilated and/or increasing use of stored carbon (Oberhuber et al 2011, Brzostek et al 2014. Hu et al (2010) found that longer growing seasons decreased forest productivity in Colorado Front Range subalpine forests due to increased water stress.…”

Root-induced changes in nutrient cycling in forests depend on exudation rates

“…We then multiplied daily exudation rates by the number of days in the growing season in 2013 (200 days) derived from measurements of canopy phenology in a nearby forest with similar tree species (the Morgan Monroe State Forest; Brzostek et al, 2014). To account for the relative abundance of trees at GW, we used a weighted average based on the average exudation rate for AM and ECM trees, and the relative abundance of AM (57% of the basal area) and ECM trees (43% of the basal area) in 30 randomly located 15 m  15 m plots at GW.…”

Drought Influence over Radial Growth of Mexican Conifers Inhabiting Mesic and Xeric Sites

“…Water deficit has been reported as one of the major triggering factors of forest growth decline, dieback, and tree mortality [3][4][5]. Droughts reduce radial growth and alter the hydraulic conductivity of trees [6][7][8], with a negative impact on carbon sequestration [9,10] and forest productivity [11,12]. Moreover, given that aridity is expected to intensify in the future in many forested areas, this may have negative and long-term implications on forest productivity [7,13].…”