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40The interaction between xylem phenology and climate assesses forest growth and productivity 41 and carbon storage across biomes under changing environmental conditions. We tested the annual temperature, from 83.7 days at -2 °C to 178.1 days at 12 °C, at a rate of 6.5 days °C -1 . 54April-May temperatures produced the best models predicting the dates of wood formation. 55Our findings demonstrated the uniformity of the process of wood formation and the 56 importance of the environmental conditions occurring at the time of growth resumption. 57Under warming scenarios, the period of wood formation might lengthen synchronously in the 58

Climate influences the timing, rate and dynamics of tree growth from sub-hourly to multicentennial timescales. Monitoring of stem radius variations on the temporal scales at which many physiological processes operate is necessary to obtain insight into intra-annual stem dynamics and contribute to the understanding of climate impacts on growth processes. Here, we quantify the response of radial conifer stem size to environmental fluctuations via a novel assessment of tree circadian cycles from four years of sub-hourly data collected from 56 larch and spruce trees growing along a natural temperature gradient of ~6°C located in the central Swiss Alps. During the growing season tree stem diameters were greatest at mid-morning and smallest in the late evening, reflecting the balance of transpiratory water loss and uptake from the roots. Along the gradient, amplitudes calculated from the stem radius cycle were ~50% smaller at the upper site (~2200 m a.s.l.) relative to the lower site (~800 m a.s.l.). We show that typical growing season diurnal cycles are substantially modified by changes in precipitation, temperature and cloudiness; amplitudes were nine times smaller on days with rain (>10 mm) and daily amplitudes increase by ~40% if mean daily temperature is between 15-20°C compared to 5-10°C. We find that over the growing season in the sub-alpine forests, spruce show greater daily stem water movement than larch. However, under projected future warming, larch could experience up to 50% greater stem water-use, which on already dry sites may severely affect future growth. Our data further indicate that the confounding influences of radial growth and short-term water dynamics on stem size likely lead to an overstated importance of water-linked meteorological variables on intraannual tree growth using conventional methodology. We suggest that intra-seasonal measurements of cellular development and consideration of reversible changes in stem diameter due to climatic factors will be necessary to reduce possible biases in quantifying and attributing changes in forest productivity in response to future warming.