Water movement through the soil‐plant‐atmosphere continuum can be tracked through its hydrogen and oxygen isotopic composition. How the isotopic composition of water evolves as it moves through this continuum provides critical information on ecosystem hydrology and climate change. Cryogenic vacuum extraction is the most applied method to extract waters from soil and plant material for such studies. However, recent experiments where oven dried soils were spiked with a reference water suggest potential for these techniques to bias results. We present results from a greenhouse‐based plant water uptake and cryogenic vacuum distillation experiment using Salix viminalis cuttings. We tested the capacity for cryogenic vacuum extraction to recover irrigation water and the plant‐available water pool from 4 soils that were subject to continuous irrigation as would occur in nature. Results show that δ2H and δ18O values of extracted soil waters reflect those of irrigation water, but with some influence of evaporation that varied with differences in soil humic and clay content. This suggests that isotope effects observed in laboratory experiments with oven‐dried soils may not be relevant under natural conditions. Cryogenic vacuum extraction also recovered xylem water δ2H values that matched the δ2H values of extracted soil water but revealed small, consistent offsets between xylem and soil water δ18O values of 0.84 ± 1.13‰. Although these effects may not significantly bias water isotope sourcing applications, they do result in large extrapolation errors when estimating original source water isotope composition from extracted xylem waters using the back extrapolation to the global meteoric water line method.

Cryogenic vacuum extraction is the well-established method of extracting water from soil for isotopic analyses of waters moving through the soil-plant-atmosphere continuum. We investigate if soils can alter the isotopic composition of water through isotope memory effects, and determined which mechanisms are responsible for it. Soils with differing physicochemical properties were re-wetted with reference water and subsequently extracted by cryogenic water distillation. Results suggest some reference waters bind tightly to the soil and not all of this tightly bound water is removed during cryogenic vacuum extraction. Kinetic isotopic fractionation occurring when reference water binds to the soil is likely responsible for the O-depletion of re-extracted reference water, suggesting an enrichment of the tightly bound soil water pool. Further re-wetting of cryogenically extracted soils indicates an isotopic memory effect of tightly bound soil water on water added to the soil. The data suggest tightly bound soil water can influence the isotopic composition of mobile soil water. Findings show that soils influence the isotope composition of soil water by (i) kinetic fractionation when water is bound to the soil and (ii) equilibrium fractionation between different soil water pools. These findings could be relevant for plant water uptake investigations and complicate ecohydrological and paleohydrological studies.

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Summary1. The elevational limit of trees (henceforth, the 'tree line') is widely considered to be a sensitive indicator of environmental change. Here, we document the 20th century tree line advance and increase in the tree population at the tree line ecotone, along a Pinus sylvestris-dominated slope in north-eastern Finland, in conditions where growth and recruitment have generally been linked to temperature variation. 2. Using tree recruitment ages (growth to 1.3 m height) along an elevational transect, we compared recruitment to variation in environmental conditions associated with tree line dynamics, seed and cone crops, and reindeer densities. We further investigated the relationships among temperature and tree-level growth variables. 3. Results show the existence of a former tree line at approximately 400 m a.s.l. and an advance of trees that began in the 1920s and reached the top of the fell (at 470 m a.s.l.) in the 1980s. During this time, the population density of P. sylvestris increased from 4 to 468 trees within the 5.6 ha study plot. Needle and shoot lengths were positively related to air temperature but recruitment was not. 4. Average rate of P. sylvestris advance over the 20th century was consistent with earlier studies, but the temporal patterns of both upslope advance and population density increase were unexpected: both were characterized by a strongly stepwise pattern, culminating in a rapid advance and density increase in the 1970-1980s, and returning to low levels in the 1990s. 5. Despite a positive relationship between growth and temperature variables at the tree level, climatic variables, or seed and cone crops were inconsistent with recruitment patterns. For most part of the 20th century, the increase corresponded to the gradual atmospheric CO 2 increase, but of the variables screened only the changes in reindeer densities coincided with the stepwise pattern. 6. Synthesis. Our results confirmed the connection between tree-level processes and temperature variability as expected from earlier studies. However, recruitment was not correlated with any of the environmental variables. Our findings point towards complex tree line dynamics, in which biotic agents may play a major role in mediating tree line response to environmental change.