Critically low soil temperatures for root growth and root morphology in three alpine plant species

Limited rates of tissue formation most likely constrain plant growth at low temperatures. Yet, temperature thresholds and growth dynamics under critically cold conditions have still to be identified. We utilized microhabitat contrasts in soil temperature along glacial and melt water streams to study temperature effects on root extension growth and root morphology of Ranunculus glacialis, Homogyne alpina and Poa alpina at ca. 2500 m elevation in the Swiss Alps. Plants were grown in containers filled with glacial silt, embedded in stream water, with root windows allowing the monitoring of root development for 41 days in naturally 'cold' substrate temperatures (hardly ever exceeding 5 °C), and 5-8 K warmer substrates. Plants grown in the cold substrate had significantly less roots and roots were pale and unbranched. Under cold conditions, total root length was 83 % shorter and total root dry mass was 67 % lower than in warm substrate. In contrast, aboveground biomass was hardly affected. Mean root elongation rates were 47 % lower under cold substrate conditions. The frequency distribution of root temperatures and root elongation rates showed a substantial reduction of apical root growth below 3-5 °C, with growth approaching zero at ca. 2 °C. A thermal sum of ca. 20°h > 2 °C per four-day interval is at least required for detectable root elongation, At a 5 °C threshold, thermal sums of ca. 40°h > 5 °C within 4 days allowed the formation of a fully developed root system during the short alpine summer. Substrate temperatures above 10 °C exerted no further stimulation in root growth.