Dulamsuren Choimaa, Hauck Markus, Bader Martin, Osokhjargal Dalaikhuu, Oyungerel Shagjjav, Nyambayar Suran, Runge Michael, Leuschner Christoph
Department of Plant Ecology, Albrecht von Haller Institute of Plant Sciences, University of Göttingen, D-37073 Göttingen, Germany.
Tree Physiol. 2009 Jan;29(1):99-110. doi: 10.1093/treephys/tpn008. Epub 2008 Dec 3.
Shoot water relations were studied in Siberian larch (Larix sibirica Ledeb.) trees growing at the borderline between taiga and steppe in northern Mongolia. Larix sibirica is the main tree species in these forests covering 80% of Mongolia's forested area. Minimum shoot water potentials (Psi(m)) close to the point of zero turgor (Psi(0)) repeatedly recorded throughout the growing season suggest that the water relations in L. sibirica were often critical. The Psi(m) varied in close relation to the atmospheric vapor pressure deficit, whereas Psi(0) was correlated with monthly precipitation. Young larch trees growing at the forest line to the steppe were more susceptible to drought than mature trees at the same sites. Furthermore, isolated trees growing on the steppe exhibited lower Psi(m) and recovered to a lower degree from drought overnight than the trees at the forest line. Indications of drought stress in L. sibirica were obtained in two study areas in Mongolia's forest-steppe ecotone: one in the mountain taiga of the western Khentey in northernmost Mongolia, the other in the forest-steppe at the southern distribution limit of L. sibirica on Mt. Bogd Uul, southern Khentey. Larix sibirica growing in riverine taiga with contact to the groundwater table was better water-supplied than the larch trees growing at the forest line to the steppe. Larch trees from the interior of light taiga forests on north-facing slopes, however, exhibited more critical water relations than the trees at the forest line. Frequent drought stress in mature trees and even more in young larch trees at the forest-steppe borderline suggests that L. sibirica does not have the potential to encroach on the steppe under the present climate, except in a sequence of exceptionally moist and cool years. A regression of the present borderline between forest and steppe is likely to occur, as average temperatures are increasing everywhere and precipitation is decreasing regionally in Mongolia's taiga forest region. Higher stomatal conductance concomitant to lower Psi(m) in trees of northern-slope forests compared to trees from the forest line to the steppe may be the result of a recent increase in drought intensity that affects better drought-adapted trees at the forest edge less than the trees in the forest interior. We conclude that drought is a key factor explaining the forest-steppe borderline in northern Mongolia. The proportion of forests within the present vegetation pattern of forests on north-facing slopes and the grasslands on south-facing slopes in Mongolia's forest-steppe ecotone is not likely to increase under the present climate, but may decrease with increasing aridity due to global warming.
在蒙古北部泰加林和草原交界处生长的西伯利亚落叶松（Larix sibirica Ledeb.）树木的嫩枝水分关系进行了研究。西伯利亚落叶松是这些森林中的主要树种，覆盖了蒙古80%的森林面积。在整个生长季节反复记录到的接近零膨压点（Ψ(0)）的最低嫩枝水势（Ψ(m)）表明，西伯利亚落叶松的水分关系常常处于临界状态。Ψ(m)与大气水汽压差密切相关，而Ψ(0)与月降水量相关。生长在森林与草原交界处的幼龄落叶松比同一地点的成熟树木更容易受到干旱影响。此外，生长在草原上的孤立树木的Ψ(m)较低，且一夜之间从干旱中恢复的程度比森林交界处的树木低。在蒙古森林草原交错带的两个研究区域获得了西伯利亚落叶松干旱胁迫的迹象：一个在最北部肯特省西部的山地泰加林，另一个在肯特省南部博格达乌勒山西伯利亚落叶松分布南界的森林草原。生长在与地下水位接触的河边泰加林的西伯利亚落叶松比生长在森林与草原交界处的落叶松水分供应更好。然而，生长在北向山坡上轻度泰加林内部的落叶松，其水分关系比森林交界处的树木更为严峻。森林草原交界处成熟树木频繁遭受干旱胁迫，幼龄落叶松更是如此，这表明在当前气候条件下，除非出现一系列异常湿润凉爽的年份，西伯利亚落叶松没有向草原扩张的潜力。随着蒙古泰加林地区各地平均气温升高且区域降水量减少，森林和草原之间目前的边界可能会发生退缩。与从森林交界处到草原的树木相比，北向山坡森林中的树木气孔导度较高且Ψ(m)较低，这可能是近期干旱强度增加的结果，这种干旱对森林边缘适应干旱能力较强的树木的影响小于森林内部的树木。我们得出结论，干旱是解释蒙古北部森林草原边界的关键因素。在当前气候条件下，蒙古森林草原交错带目前北向山坡森林和南向山坡草原植被格局中森林的比例不太可能增加，但可能会随着全球变暖导致干旱加剧而减少。
