Nutrient retranslocation strategies associated with dieback of Pinus species in a semiarid sandy region of Northeast China

Abstract

In the semiarid sandy region of Northeast China, Mongolian pine (Pinus sylvestris var. mongolica) suffers dieback after the age of 35, while Japanese red pine (Pinus densiflora) and Chinese pine (Pinus tabuliformis) stay healthy. Foliar nutrient retranslocation reflects the nutrient conservation and utilization mechanism of plants in response to their habitats. However, the nutrient retranslocation strategies employed by three Pinus tree species to cope with nutrient limitations remain largely unknown. For this study, we investigated the seasonal variations in nitrogen (N) and phosphorus (P) concentrations of Mongolian pine, Japanese red pine, and Chinese pine plantations in terms of the green needles of all ages, senesced needles, and soil. Further, the N retranslocation efficiency (NRE), and P retranslocation efficiency (PRE), and correlations between the N:P ratios of needles and soil were analyzed. The results showed that, except for the spring NRE in 1-year-old needles of Mongolian pine, the spring NRE and PRE in 1- and 2-year-old needles of the three tree species were greater than zero. The autumn PRE was higher than zero for Mongolian pine, but lower than zero for Japanese red pine and Chinese pine. Among the three Pinus species, Mongolian pine showed greater spring PRE in 2-year-old needles, and PRE from 1- to 2-year-old needles, and from 2-year-old needles to litter. However, Japanese red pine had higher P concentrations and lower N:P ratios in senesced needles, while greater PRE was found in Chinese pine litter. Significant relationships between the N:P ratios were found in the current year and 1-year-old needles and soil in the Mongolian pine plantation, while there was an insignificant relationship between the N:P ratios of the needles and soil in the Chinese pine plantation. Thus, for Mongolian pine, the removal of P from needles in autumn, and higher P translocation from older needles under P-deficient soil may have contributed to the tree dieback. In contrast, Japanese red pine and Chinese pine stored P in their needles during autumn. Japanese red pine returned more P to the soil via litter, while Chinese pine maintained N:P homeostasis and increased P withdrawal prior to needle abscission.