Abstract: Taking 8-year old subtropical coastal shelter forest tree species Ligustrum lucidum, Fraxinus pennsylvanica, Casuarinn equiestifolia, and Taxodium mucronatum as study objects, this paper investigated their root growth characteristics with relation to soil salt content, and compared the root biomass pattern, functional root distribution percentage, specific root length, and root length density among the four tree species. The results indicated that soil salt content was the most important factor limiting the vertical root growth, and the latter had a negative relationship with horizontal root growth. The test tree species had different salt-tolerance characters, resulting in the difference of their functional root biomass pattern. The smaller the lateral root diameter, the larger the lateral root water- and nutrient absorbing capacity; the larger the lateral root diameter, the stronger the capacity of the lateral roots in anchoring and supporting plant. For T. mucronatum, its salt threshold was > 0.45%, and its vertical root distribution was at the soil depth of 0-95 cm, which resulted in a good capability to resist typhoon and soil salt stress. For C. equiestifolia and F. pennsylvanica, their tap root length was 25 cm and 29 cm, and their anchor root occupied 87.2% and 56.3%, respectively. These two tree species had definite capability to resist typhoon, and could be planted on the coastal tidal flat with salt content < 0.36%. For L. lucidum, its salt threshold was < 0.21 %, tap root length was 20 cm, and anchor root occupied 51.0%, suguesting that it was not adaptable to plant in the coastal areas with high salt content and typhoon.

Key words: Poplar clone seedlings, Osmotic stress, Photo inhibition of photosynthesis, Reactive oxygen species, Protecting enzymes