Physiological drought-like water limitation at low root temperature in temperate tree species with different elevational distribution limits

Low temperature is the main driver behind the upper elevational and higher latitudinal distribution range of trees. As the limit for tree growth in general, the alpine treeline is traditionally in the focus of most studies on the effect of low temperature limits of trees, but all other tree species that reach their distribution limits below the treeline have species-specific low-temperature limits, as well (Körner, 2021). Restricted water uptake and deteriorated hydraulic relations at low root zone temperatures might be one of the drivers for the species-specific cold distribution limits of trees. Negative cold soil effects on the hydraulic conductivity of trees can thus potentially amplify the direct effects of cold temperatures on growth and contribute to the cold limit of temperate tree species. Thus, we put forward two hypotheses: (1) the natural cold distribution limit of temperate tree species is related to their capacity to take up water at cold root temperatures; (2) cold root temperatures lead to drought-like water limitations that are more severe in low- compared to high-elevation species. In this study, we investigated the low temperature sensitivity of root water uptake and transport in seedlings of 16 European broadleaved and conifer temeprate tree species, which reach their natural upper elavation distribution limits at different distances to the alpine treeline acsross a ca. 1500 m elevational range. We tested the temperature sensitivy of root water uptake and whole tree water transport by exposing the seedlings to three different constant root temperatures (15, 7 and 2°C) while all seedlings received the same warm abovground temperatures. To quantify the negativ low temperature effects on water transport, we used 2H-H2O pulse labelling of the source water in hydroponic systems. The result showed a correlation of the thermal distance to treeline of the natural upper distribution limits of each species with its relative water uptake at 7°C root temperature revealed a moderate but significant linear relationship, whereby water uptake and transport tends to be more limited the larger the species’ thermal distance to treeline (i.e. the lower the high elevation distribution limit). In contrast, 2°C root temperatures strongly reduced water uptake, with consequently no significant correlation between relative water uptake and the species-specific distribution limits. We concluded low root temperatures lead to species-specific restrictions of water uptake and drought-like stress with reduced water potentials and stomatal conductance. Furthermore, species that reach their upper distribution at higher elevations are less sensitive to low root temperatures and vice versa. Low temperature-caused hydraulic restrictions might thus contribute to the cold distribution limits of temperate tree species.