Growing‐season warming and winter soil freeze/thaw cycles increase transpiration in a northern hardwood forest

Abstract

Climate models project higher growing‐season temperatures and a decline in the depth and duration of winter snowpack throughout many north temperate ecosystems over the next century. A smaller snowpack is projected to induce more frequent soil freeze/thaw cycles in winter in northern hardwood forests of the northeastern United States. We measured the combined effects of warmer growing‐season soil temperatures and increased winter freeze/thaw cycles on rates of leaf‐level photosynthesis and transpiration (sap flow) of red maple (Acer rubrum) trees in a northern hardwood forest at the Climate Change Across Seasons Experiment at Hubbard Brook Experimental Forest in New Hampshire. Soil temperatures were warmed 5°C above ambient temperatures during the growing season and soil freeze/thaw cycles were induced in winter to mimic the projected changes in soil temperature over the next century. Relative to reference plots, growing‐season soil warming increased rates of leaf‐level photosynthesis by up to 85.32 ± 4.33%, but these gains were completely offset by soil freeze/thaw cycles in winter, suggesting that increased freeze/thaw cycles in winter over the next 100 yr will reduce the effect of warming on leaf‐level carbon gains. Soil warming in the growing season increased rates of transpiration per kilopascal of vapor pressure deficit (VPD) by up to 727.39 ± 0.28%, even when trees were exposed to increased frequency of soil freeze/thaw cycles in the previous winter, which could influence regional hydrology in the future. Using climate projections downscaled from the Coupled Model Intercomparison Project, we project increased rates of whole‐season transpiration in these forests over the next century by 42–61%. We also project 52–77 additional days when daily air temperatures will be above the long‐term average daily maximum during the growing season at Hubbard Brook. Together, these results show that projected changes in climate across both the growing season and winter are likely to cause greater rates of water uptake and have no effect on rates of leaf‐level carbon uptake by trees, with potential ecosystem consequences for hydrology and carbon cycling in northern hardwood forests.

Soil frost affects stem diameter growth of Norway spruce with delay

“…However, our results suggest that climate warming, which increases soil frost severity, may initiate physiological responses and damage that result in an overall decline in tree growth rates (cf. Reinmann et al 2019;Harrison et al 2020). Thus, the positive effects on forest growth via a longer growing season as a consequence of increasing temperatures may not be fully realised due to the negative effects of winter warming.…”

“…Evapotranspiration of trees, particularly conifers, is high in spring due to high solar irradiance, temperature fluctuation and wind (Sakai 1968;Tranquillini 1982). Frozen soil inhibits water uptake, potentially causing severe water stress and injuries to trees (Bonan and Shugart 1989;Harrison et al 2020). Although tree roots may tolerate short-term low temperatures in their frost-hardy state (Bigras et al 2001), mild but long-lasting soil freezing may damage fine roots (Tierney et al 2001(Tierney et al , 2003Comerford et al 2013;Sutinen et al 2014).…”

Climate change influences foliar nutrition and metabolism of red maple (Acer rubrum) trees in a northern hardwood forest

“…Therefore, to minimize potential artifacts associated with measuring photosynthesis on excised branches in this study, we submerged the ends of excised branch shoots in water, cut 2-3 cm above the break to reduce the impact of embolisms on gas exchange rates, and measured rates of photosynthesis within 5-7 min of excision from the trees. We also allowed leaves to stabilize for approximately 1-2 min and then made five measurements of photosynthesis and conductance at 15-s intervals for 1 min and utilized the mean of these five measurements (Harrison et al 2020a).…”

“…2013, Harrison et al. 2020 a , b ). Changes in forest N cycling have the potential to alter primary production in temperate, northeastern U.S. forests facing a changing climate, as N availability and uptake by trees are closely linked with foliar N allocation and rates of photosynthesis in predominantly N‐limited forests.…”

“…Additionally, increased soil freezing has been shown to decrease soil microbial biomass and rates of soil net nitrogen (N) mineralization in the following growing season (Dur an et al 2014, which may limit microbial decomposition and soil N availability in the future. However, while winter soil FTCs damage roots and reduce plant N uptake (Sanders-DeMott et al 2018), warmer growing season temperatures increase rates of soil microbial N cycling and tree N uptake (Bai et al 2013, Harrison et al 2020a. Changes in forest N cycling have the potential to alter primary production in temperate, northeastern U.S. forests facing a changing climate, as N availability and uptake by trees are closely linked with foliar N allocation and rates of photosynthesis in predominantly N-limited forests.…”