A new study, conducted by researchers of the Institute of Applied Ecology (IAE), the Chinese Academy of Science (CAS), shows that Mongolian pine (Pinus sylvestris var. mongolica) trees grown on sandy lands respond to severe drought stress by adjusting hydraulic architecture with increasing tree size. Since this adjustment is unsustainable in terms of long-term drought stress and carbon imbalance, it eventually leads to growth decline and dieback of trees.
In 1953, researchers of the Institute of Forestry and Soil (now known as IAE, CAS) successfully established the first sandy land Mongolian pine (SLMP) plantation in Zhangwu Country, a water-limited region in western Liaoning province. Since then, Mongolian pines have been widely used for establishing windbreak and sand-fixation forests in the ‘Three-North’ (northwest, north, and northeast) region of China.
Unexpectedly, the SLMP plantations in Zhangwu Country began to exhibit severe decline and massive mortality in the early 1990s. Compared to these plantations, natural distributed SLMP forests in the Great Xing’an Mountains grow well even at ages older than 100 years.
To reveal the mechanisms causing the decline of SLMP plantations, Dr. LIU Yanyan, Prof. HAO Guangyou, Prof. ZHU Jiaojun and their colleagues compared hydraulic architecture and photosynthetic physiology of different sized Mongolian pine trees from plantations in Zhangwu Country and from the Honghuaerji Natural Reserve where the pure SLMP forests are naturally distributed.
The researchers found that trees in SLMP plantations showed lower stem hydraulic conductivity, lower leaf to sapwood area ratio (LA/SA) and more severe syndromes of drought stress with increasing tree size, whereas no such trends were observed at the natural forest site.
Lower LA/SA may mitigate the effect of increased water stress in larger trees, but may also result in greater risk of carbon imbalance, eventually leading to growth decline or even death of trees at the plantation site. The findings of this study thus provide a mechanistic explanation for the decline of SLMP plantations.
“Reducing the planting density and tree thinning would contribute to the mitigation of plantation decline,” the researchers said. In the long run, however, trees in SLMP plantations may eventual decline and “mortality may become unavoidable due to water limitation.” They also noted that “savanna-like grassland with sparse trees” might be a stable vegetation state in those water-limited regions.
The study entitled “Hydraulics play an important role in causing low growth rate and dieback of aging Pinus sylvestris var. mongolica trees in plantations of Northeast China” has been published in Plant, Cell and Environment. Dr. LIU is the first author, while Prof. HAO is the corresponding author of this paper.
This study is financially supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, Key Research Project from the Bureau of Frontier Science and Education of CAS.