Adapting to climate change: responses of fine root traits and C exudation in five tree species with different light-use strategy.

Trees that are categorised by their light requirements have similarities in their growth strategies and adaptation mechanisms. We aimed to understand the complex responses of elevated air humidity on whole tree fine root carbon (C) exudation (ExC) and respiration rate, morphology, and functional distribution in species with different light requirements. Three light-demanding (LD) species, Populus × wettsteinii, Betula pendula, and Pinus sylvestris, and two shade-tolerant species, Picea abies and Tilia cordata saplings were grown in growth chambers under moderate and elevated air relative humidity (eRH) at two different inorganic nitrogen sources with constant air temperature and light availability. The proportion of assimilated carbon released by ExC, and respiration decreased at eRH; up to about 3 and 27%, respectively. There was an indication of a trade-off between fine root released C and biomass allocation. The elevated air humidity changed the tree biomass allocation and fine root morphology, and the responses were species-specific. The specific fine root area and absorptive root proportion were positively related to canopy net photosynthesis and leaf nitrogen concentration across tree species. The variation in ExC was explained by the trees' light-use strategy (p < 0.05), showing higher exudation rates in LD species. The LD species had a higher proportion of pioneer root tips, which related to the enhanced ExC. Our findings highlight the significant role of fine root functional distribution and morphological adaptation in determining rhizosphere C fluxes in changing environmental conditions such as the predicted increase of air humidity in higher latitudes.

Responses of fine root exudation, respiration and morphology in three early successional tree species to increased air humidity and different soil nitrogen sources.

Global climate change scenarios predict an increase in air temperature, precipitation and air humidity for northern latitudes. Elevated air humidity may significantly reduce the water flux through forest canopies and affect interactions between water and nutrient uptake. However, we have limited understanding of how altered transpiration would affect root respiration and carbon (C) exudation as fine root morphology acclimates to different water flux. We investigated the effects of elevated air relative humidity (eRH) and different inorganic nitrogen sources (NO3- and NH4+) on above and belowground traits in hybrid aspen (Populus × wettsteinii Hämet-Ahti), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) grown under controlled climate chamber conditions. The eRH significantly decreased the transpiration flux in all species, decreased root mass-specific exudation in pine, and increased root respiration in aspen. eRH also affected fine root morphology, with specific root area increasing for birch but decreasing in pine. The species comparison revealed that pine had the highest C exudation, whereas birch had the highest root respiration rate. Both humidity and nitrogen treatments affected the share of absorptive and pioneer roots within fine roots; however, the response was species-specific. The proportion of absorptive roots was highest in birch and aspen, the share of pioneer roots was greatest in aspen and the share of transport roots was greatest in pine. Fine roots with lower root tissue density were associated with pioneer root tips and had a higher C exudation rate. Our findings underline the importance of considering species-specific differences in relation to air humidity and soil nitrogen availability that interactively affect the C input-output balance. We highlight the role of changes in the fine root functional distribution as an important acclimation mechanism of trees in response to environmental change.