Climate Change at Hövsgöl ILTER
One of the most important—if not the most important—environmental issues we face today is climate change through global warming. Some of the most dramatic examples we see come from retreating glaciers in high mountains or melting polar ice sheets. Many scientists have focused on studying the effects of global warming in these frigid regions in part because they appear to be more vulnerable to the early stages of climate change, and because ice melts are readily documented.
Climate change is also the subject of intense study at Hövsgöl ILTER. Indeed, the Lake Hövsgöl watershed is arguably the world's most comprehensively studied mid-continental region in regards to this issue. One of the primary research goals of the Institute is to develop sustainable landuse policies for East and Central Asia in the face of climatic change.
A Transitional Zone Ideal for Study
Ecological and environmental changes, such as those brought about by changes in the climate, are often best observed in ecotones. These are the transitional zones between ecological communities that can range in scale from small patches to vast biomes. The Lake Hövsgöl watershed is ideally situated at the transition between two Eurasian biomes: the Siberian Taiga (a boreal coniferous forest) and the Central Asian Steppe (a temperate grassland).
The transitional nature of the Hövsgöl landscape is most evident in the patchwork of vegetation that's found along the eastern shore of the lake. Here, streams originating in low mountains to the east create long and narrow valleys as they flow to the lake. The northern slopes of these valleys, which receive less direct exposure to the sun, are underlain with permafrost (permanently frozen soil). Coniferous forests dominated by Siberian Larch (Larix sibirica) typically cover the upper parts of these north-facing slopes while moist steppes cover the lower areas. The south-facing slopes are covered by dryer steppe vegetation and usually lack permafrost. The valley bottoms contain a mixture of steppe, wetlands and riparian vegetation.
Observed Changes and Impacts
The climate in Mongolia—and in other mid-continental parts of Eurasia—is notoriously harsh and variable. Dramatic differences from one year to the next can obscure any longer-term changes. However, two major changes are evident. One is the that mean annual temperatures have increased almost 2°C (3°F) over the last 40 years. (Warming is greatest during the winter and spring, but has occurred in all seasons.) The other change is the length of the growing season. It's increased by about a month during the last 10 years.
The first impression is that warmer temperatures and longer growing season might improve a landscape that's subjected to long and frigid winters. However, Mongolia is also a land of little precipitation. The region around Lake Hövsgöl is among the wettest in the country, but it still averages only about 30 cm (13 in) of precipitation per year. (This is about one third of the annual precipitation for Philadelphia.)
Consequently, the impacts of warming in Mongolia may be greatest in terms of the availability of water. Studies at Hövsgöl have recorded increases in evaporation and plant evapotranspiration, as well as decreases in soil moisture. These changes contribute to a reduction in plant cover, which serves as a natural insulator. The resulting increases in surface soil temperatures through loss of this insulation further increases evaporation and soil moisture loss.
Changes are also evident in watershed's permafrost. (The Hövsgöl region is at the southern boundary of the boreal permafrost zone.) Permafrost may limit plant growth, but it can also store water in landscapes where the climate is both cold and dry. The upper layers of these frozen soil melt during the summer and support plant growth during the long summer days. The soil refreezes with the coming of winter. Plants adapted to this cycle of melting and freezing can thrive.
However, climate warming has altered this cycle by permanently melting some of the permafrost. Normally, the layer of soil that thaws during the summer, known as the active layer, is located near the surface where it's accessible to plant roots. Prolonged thawing drives this active layer deeper and the upper soil can dry out.
The effects of increased soil evaporation and prolonged permafrost thaw can be exacerbated by overgrazing. People have been herding livestock in Mongolia for thousands of years, but grazing pressures have accelerated in recent decades as a result of greater human and livestock populations as well as changes in grazing practices.
Some of the changes in grazing practices result from the increasing market demand for the wool of cashmere goats. The fading of traditional pastoralism during the Soviet era has also contributed to the problem. Faced with the economic dislocations following the decline of the Soviet Union, many Mongolians returned to subsistence herding, but they often lacked experience to properly manage their herds and rangelands. Researchers at the Hövsgöl Long Term Ecological Research site are working on developing sustainable herding practices and communicating these practices to pastoralists in Mongolia and elsewhere in Central Asia.
These impacts on soil moisture and permafrost degradation are best understood in the steppe habitats used by livestock. However, there are signs that the taiga forests may also be vulnerable. The dynamics of forest regeneration are largely unknown, but drier soils may inhibit seed germination and seedling establishment for the dominant tree, Siberian larch.
Climate change may also be a factor in the unexpected and severe outbreak of the Asian gypsy moth (Lymantria dispar dispar) during 2002-2004. Larch saplings were particularly hard hit during the outbreak. Some lost more than half their leaf area and growth rates were reduced by an estimated 40%.
This forest pest is native to Eurasia and is a serious introduced pest in eastern North America forests. There are historical and governmental records of gypsy moth outbreaks in central and eastern Mongolia during 1928, 1970-1974, and 1989, but it wasn't even listed as a forest pest at Hövsgöl in government reports published as recently as 2000.
It's known that the feeding rate and development of gypsy moth larvae are dependent on air temperature, so it's reasonable to infer that warming is a contributing factor. The research is not yet conclusive, but mean summer temperatures at Hövsgöl didn't exceed the suspected temperature threshold for successful larval development until 2000.