Tree water source variation in semi-arid ecosystems is important to understand because climatic shifts towards lower snowpack and increased drought affect water availability in subalpine forests of the western US. Here, we use daily in situ measurements of stable isotopes (2H) in soil and tree water, soil matric potential and sap flow to study tree water uptake dynamics. We instrumented three soil profiles down to 90 cm, as well as three Aspen and Engelmann Spruce trees near Gothic, Colorado, in the East River watershed. We observed the fate of natural 2H variations in rainfall, soil, and plants from June to October 2022, and in August 2023 we conducted a 2H labeled water irrigation experiment. Our observations showed that transpiration was reduced by all trees, but partially compensated by shifting the dominant water source from 60 to 90 cm within days of a dry period. Intense rainfall quickly shifted the plant water uptake partially to top soil layers. Changes in water uptake depths were similar between aspen and spruce, but rainfall infiltration was low in the spruce stand due to high canopy interception. Therefore spruce transpiration was lower and relied more on snowmelt. However, both species relied on snowmelt to sustain transpiration and groundwater recharge from monsoonal rains was not observed. These findings highlight the important role of snowmelt stored in the deep soil layers for subalpine forest drought response and the dominant fate of monsoonal rainfall to become transpiration rather than recharging groundwater and streams in the Upper Colorado River.

The rapidly warming Arctic and its effects on sea ice extent, hydrology, and nutrient availability influence terrestrial and marine carbon cycles in a number of interrelated ways. While these changes likely have shared effect on adjacent land and ocean systems, we often study them in isolation, making it difficult to understand response patterns and trajectories in these carbon cycle hotspots. Using almost two decades of remotely-sensed Gross Primary Productivity (GPP) in Arctic coastal margins, we test how the magnitude and direction of change in productivity covary. We observed that coastal marine productivity is four times that of coastal tundra productivity in the pan-Arctic. From 2003-2020, GPP in both the coastal land and ocean increased by approximately 12%. This common trajectory seems to be a product of increasing open water conditions, increased terrestrial water balance, and nutrient availability as driven by the regional warming. On a sectoral scale, we proposed a Coastal Synchrony Index (CSI) to compare the rate of change of ocean productivity relative to land productivity and show that ocean productivity is increasing faster than land in inflow margins of Barents, Bering, and Okhotsk, outflow margins of Canadian Arctic Archipelago (CAA) and Greenland/Iceland, and in interior margin of Eurasia. Additionally, we see strong coherence between land and ocean GPP on 4–5-year cycles illustrating that coastal synchrony observed over decadal timescales is mirrored over interannual timescales. These cycles align with variations in open water duration, emphasizing the pivotal role of reducing shorefast ice on terrestrial and marine productivity trajectories.

The isotopic effects of sublimating ice is poorly understood and disagreement from diverging results from studies spans decades. The core question is whether sublimation occurs layer-by-layer with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. Here, small ice spheres were suspended in an unsaturated atmosphere and a Rayleigh distillation model was used to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, fractionation (103lna18O of ~ -0.6‰ (ɑ = 0.999) and 103lna2H -3 to -6‰ (ɑ = 0.994 to 0.997 ) was observed, smaller than predicted for equilibrium fractionation at this temperature and humidity. Assuming a modest porosity of 0.0005%, porosity could sufficiently increase diffusivity to explain the observed fractionation. The results help reconcile how sublimation varies between experimental and observational studies where uncontrolled porosity varies substantially across a continuum from porous firn layers to low porosity ice deep in glaciers.

Rising summer temperatures are expected to increase tree mortality rates across the western United States. Here, we analyze subfossil wood samples from Colorado dating to the last interglacial to assess the response of two common conifers to a previous warm period. The trees experienced comparable growth rates and water use efficiency during the interglacial relative to modern despite evidence from model simulations of a ~30% increase in evaporative demand during the peak of the growing season. High-resolution isotopic analysis of the wood samples show an enrichment in the late season cellulosic δ18O relative to modern samples, which we find was associated with increased reliance on summer rain. The data are consistent with other proxy data and climate model simulations showing the interglacial was associated with wetter summers across the western US. We propose enhanced summer rain during this warm period compensated for drought stress imposed by higher evaporative demand.