Effects of soil warming on fine root growth and morphology of Chinese fir ( Cunninghamia lanceolata ) seedlings

Currently, there is large quantity of research on the responses of fine roots to warming, but there are still some controversies. Fine root growth has a direct effect on fine root biomass, and thus affects the plants’ abilities to acquire soil resources and resist environmental stresses such as drought. Fine root morphology is an important feature of root function, and directly affects the ability of fine roots to absorb and transport water and nutrients, yet how root growth and morphology respond to soil warming is not clearly understood. In the present study, a simulated soil warming experiment was carried out in the Chenda State-owned Forest Farm in Sanming City, Fujian Province, China, to investigate the effect on fine root growth and morphology of Chinese fir (Cunninghamia lanceolata) seedlings. Both the soil warming treatment and the control (non-warming) had five replicated 2 × 2 plots, and four 1-year-old Chinese fir seedlings were planted in each plot before beginning of the experiment. Soil warming was accomplished with buried heating cable technology, and soil temperature was elevated to 5℃ above ambient. Other plants that naturally sprouted in the plots were removed at regular intervals. Fine roots were sampled at the second (in January and July 2015) and third (in January and July 2016) years of soil warming using the in-growth donut method. Root samples were scanned with an Epson scanner at 300 dpi, and root morphology, including specific root length (SRL), specific root area (SRA), and root tissue density (RTD), was analyzed with WinRHIZO Pro2009 b software. Fine root growth was measured by the fine root dry mass obtained from the in-growth donut every half-year. The results showed that: (1) with the growth of Chinese fir seedlings, the effect of soil warming on fine root growth was initially inhibited, then no significant effect, and finally promoted. (2) The soil warming effect on root morphological characteristics varied at different sampling times: soil warming significantly increased fine root SRL and SRA in July (summer), but there was no significant effect on either SRL or SRA in January (winter). (3) RTD of 1-2 mm diameter roots was significantly increased by soil warming at the second and third samplings. It is concluded that the effect of soil warming on fine root growth of Chinese fir seedlings was related to the seedling growth stages. At the same time, Chinese fir seedlings can adjust their fine root morphology (by increasing SR