Ecophysiological characteristics of leaves and fine roots in dominant tree species in a subalpine coniferous forest of western Sichuan during seasonal frozen soil period

Aims There is an obvious seasonal freezing and thawing process (early winter freezing-thawing, late winter freezing, and early spring freezing-thawing) in the subalpine coniferous forests, where there is also a strong effect of climate change in winter. Hence, global warming will likely affect the seasonal freezing and thawing process in this area. Our objective was to determine the ecophysiological characteristics of leaves and fine roots in dominant tree species in a subalpine coniferous forest of western Sichuan during seasonal frozen soil period, in order to improve our understanding of the ecological processes in subalpine coniferous forests. Methods We analyzed the changes in malondialdehyde (MDA) content, osmoregulation substance content, tissue water content, peroxidase (POD) activity, and nitrate reductase (NR) activity in leaves and fine roots of Picea asperata and Abies fargesii var. faxoniana seedlings, and measured specific root length, specific surface area, diameter and tissue density of fine roots over the course of the seasonal frozen soil period. Important findings The POD activity and the contents of proline and soluble proteins in leaves were significantly lower than in fine roots, despite that the daily fluctuations of soil temperature was less than that of air temperature during the seasonal frozen soil period, suggesting that fine roots were more susceptible to the seasonal frozen soil than leaves. In comparison with the soil freezing period, a greater daily fluctuations of air and soil temperature resulted in an increase in the soluble sugar content in leaves of P. asperata and the POD activity and 344 植物生态学报 Chinese Journal of Plant Ecology 2014, 38 (4): 343–354 www.plant-ecology.com proline content in leaves of the two species during the soil freezing-thawing period, whereas the tissue water content was significantly decreased and the contents of proline, soluble proteins and soluble sugars were significantly increased in fine roots, indicating that the effects of soil freezing-thawing on plants were stronger than soil freezing. In soil freezing-thawing period, the POD activity and osmolyte contents were significantly increased in both A. fargesii var. faxoniana and P. asperata, but changes in the MDA content and the NR activity in the fine roots and leaves were not consistent in the two species. In A. fargesii var. faxoniana, the MDA content was significantly increased in fine roots and the NR activity was significantly reduced in both leaves and fine roots. A change in the MDA content was only observed in leaves of P. asperata, which was significantly decreased, indicating that P. asperata had more tolerance to soil freeze-thaw cycles. Moreover, no significant changes in fine root morphology and growth were observed during the seasonal frozen soil period.