Low temperature limits of root growth in deciduous and evergreen temperate tree species

1. To assess low temperature limits of root growth in woody plants from periodically cold climates, we exposed seedlings of broadleaved and conifer taxa to contrasting soil temperature gradients under unlimited nutrient supply. 2. Five of the six species tested (Alnus viridis, Alnus glutinosa, Picea abies, Pinus sylvestris, Pinus cembra) produced hardly any (< 3%) new roots at temperatures below 6 degrees C, while Betula pendula did produce a few roots. Across all species, 85% of all new roots in the cold profile were confined to the rooting zone above 9 degrees C. Total root production in the gradual 16 to 2 degrees C temperature profile was only 40% of that obtained at a constant temperature of 16 degrees C. 3. New shoot growth (21-23 degrees C) was unaffected by these soil temperature differences. Neither specific root length nor root width responded to treatments. Low and high elevation taxa did not differ in any of the traits or responses tested. 4. Given that shoots were experiencing optimal conditions, the root data suggest a direct growth (sink) limitation by low temperatures during spring at low elevation, and potentially year-round limitation at the high-elevation climatic treeline. The critical temperature for significant root growth is approximate to 6 degrees C, which is close to the worldwide mean soil temperature at climatic treelines.