樟子松固沙林林水关系研究进展及对营林实践的指导

中国有着世界上最大面积的人工林, 如何维持人工林的可持续性已成为气候变化背景下需要面对的重大挑战。樟子松(Pinus sylvestris var. mongolica)以其抗旱、抗寒、耐贫瘠等优良特性成为中国北方生态治理中最主要的常绿针叶树种之一, 近70年来发挥了巨大的防风固沙与生态固碳功能。然而, 随着林分的生长与气候变化, 樟子松人工固沙林正经历着越来越严峻的环境胁迫, 部分地区出现了林分“早衰”或死亡的现象, 引起了人们对樟子松固沙林适应与应对气候变化能力的担忧。该文在回溯樟子松基本生物学特征与引种推广历史, 系统总结近年来樟子松林林水关系研究新成果的基础上, 全面分析了樟子松固沙林林水关系存在的主要矛盾, 并提出了基于林水关系相协调的林分经营措施的调整: 由倡导防护功能为主的单一目标向包含林分稳定性、生态固碳功能、可持续发展等多目标平衡方向调整; 由以沙地森林景观培育为主向以良好土壤生境培育为主的方向调整; 由倡导天然更新为主向以人工造林与天然更新相结合的世代更新方向调整。在立足于北方沙地脆弱生境与气候变化客观现实的基础背景下, 应坚持樟子松在固沙林生态系统演化过程中的先锋种与建群种地位, 基于“以水定绿”原则, 采取“隔行带伐+再造林”等方式开展林分密度动态调控, 促进林分向异龄林结构演化, 促进樟子松固沙林生态服务的优质化和生态固碳功能的最大化。

Research advances on forest-water relationships in Pinus sylvestris var. mongolica plantations for sand dune immobilization and guidance to forest management practices

China has the largest area of tree plantations in the world, and how to maintain the long-term stability of plantations has become one of the major challenges in the context of climate change. Pinus sylvestris var. mongolica is one of the most important coniferous evergreen tree species in the well-known “Three-North” Shelterbelt Construction Program in China, due to its properties of tolerance to drought, low temperature, and open land condition. Afforestation with P. sylvestris var. mongolica in northern China provides great benefits to environmental protection and ecosystem carbon fixation. However, with the growth of the stands and the ongoing climate change, P. sylvestris var. mongolica plantations in a fragile habitat of sandy land have been subjected to increasingly severe water stresses. Typically, the phenomenon of premature dieback has occurred in some of the P. sylvestris var. mongolica plantations, such as at the sites of Zhanggutai in the southern edge of Horqin Sandy Land, which causes great concerns on the stability of the local ecosystems. Based on recollections of studies on the basic biological characteristics and introduction history of P. sylvestris var. mongolica, we summarized the recent research achievements in understanding the forest-water relationships in P. sylvestris var. mongolica plantations, and examined the main contradictions in the forest-water relationships in P. sylvestris var. mongolica plantations under the climate change scenario. We recommend adjustment of several stand management measures based on the compromised forest-water relationships: (1) turning the single goal for environmental protection into multi-objectives with balanced consideration for stand structural stability, carbon sequestration, and sustainable development; (2) adopting more measures for improving soil conditions rather than for developing forest landscapes; and (3) establishing the sustainable shelterbelts by combining afforestation with natural regeneration rather than by merely depending on natural regeneration processes. Based on the condition of fragile sandy land and climate change, we should still regard P. sylvestris var. mongolica as a pioneer and construction tree species. In consideration of the principle of “determining forest cover by water supply”, the uneven-aged forest stands can be established by conducting alternate-row harvesting followed by replanting. As a result, the environmental protection function and carbon sequestration capacity can be continuously improved, the ecological services and productivity can be maximized, and the biodiversity and stability of the ecosystem can be protected and guaranteed. The findings from this study can be of important values for guiding the systematic management of northern ecological shelterbelts.