Abstract:

Aims Pinus sylvestris var. mongolica (Mongolian pine) is one of the main tree species used for forestation in the northern China and grows well in the early growth stage on sandy lands in the semi-arid area. In recent years, some of trees of Mongolian pine plantations in the sandy lands have been found to be degraded. Study of foliar traits is key to understanding the adaptability and survival strategy of this tree species on sandy land. Our major objective is to examine the impact of needle age on the main eco-physiological foliar traits of Mongolian pine in Horqin sandy land and thus to highlight the importance of needle age for coping with water and nutrients in the semi-arid area.
Methods We chose trees of Mongolian pine planted in early 1980s on a sand dune in the southern margin of Horqin sandy land, located in the permanent experimental plot of the Naiman Desertification Research Station, Cold and Arid Regions Environment and Engineering Research Institute, Chinese Academy of Sciences (42°55′ N,120°43′ E). We measured foliar photosynthetic rate, transpiration rate, carbon content, nitrogen content and morphological parameters (leaf length, width and thickness) for different-age needles. Based on these measurements we calculated transpiration coefficient (K_c) and photosynthetic nitrogen use efficiency (PNUE).
Important findings Net photosynthetic rate (P_n), K_c and PNUE varied among different needle ages and were significantly lower for 4-year old needles than 1-, 2- and 3-year old needles. This can be accounted for by foliar nitrogen content, with decrease in nitrogen leading to the decline in photosynthesis. Strong correlations were found between foliar form (e.g., leaf length, width, thickness and specific leaf area) and precipitation of the year prior to leaf formation and between foliar carbon content and precipitation of the year prior to leaf formation. In terms of transpiration coefficient and PNUE, juvenile needles (mainly 1- and 2-year old needles) were more efficient in using water and nutrients than old needles (4-year old needles), and hence a tradeoff between juvenile and old needles in water and nutrient use is a mechanism for growth and survival of Mongolian pine on sandy land. In addition, shedding old leaves under environmental stresses, as we observed in the field, has significant implication for this species to adapt to harsh habitat in the semi-arid area.

Key words: needle age, photosynthetic nitrogen use efficiency, photosynthetic rate, Pinus sylvestrisvar. mongolica, transpiration coefficient