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The Upper Mountain Forest and Tree Response to Climate Change in South Siberian Mountains

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The Upper Mountain Forest and Tree Response to Climate Change in South Siberian Mountains The upper mountain forest and tree response to climate change in south Siberian Mountains Vyy,acheslav I. Kharuk, and K. Jon Ranson Forest Monitoring Lab, Sukachev Institute of Forest, Krasnoyarsk, Russia NASA’s Goddard Space Flight Center Greenbelt, MD 20771 Siberia is the zone of most pronounced observed and predicted climate change Tree response to climate change is primarily expected in the areas where temperature and precipitation limits growth, e.g., in alpine forest- tundra ecotone. The area of investigation Is the upper elevation forests and forest-tundra ecotone of Altai - Sayan Mountain country. This is the southern border of Siberian taiga and Mongolian steppes and deserts The purpose of this report is to address the following questions: 1. How has tree line changed during the last millennium? 2. How does forest response to climate variables depend on topography? 3. How do the main tree species (Larix sibirica and Pinus sibirica) respond to climate change? Study was based on in situ measurements within elevation transects, time series of satellite data and forest maps, dendrochronology data, and DEM. The study parameters were apical and radial increments, tree natality/mortality, tree physiognomy, regeneration abundance and age structure. The study area is a zone of Siberian larch and Siberian pine contact. These species differ in precipitation sensitivity. Larch can survive at semi desert level of precipitation (~ 250 mm/yr), whereas Siberian pine is precipitation sensitive, and has optimal growth with precipitation level >1000 mm/yr. Life span of Larix sibirica and Siberian pine within study area is 600 -700 yr and 400 - 500 yr, respectively. Results I. The tree line evolution Several tree lines were defined and georeferenced. 1. The historical tree line (i.e., highest tree line during last millennium) was marked by subfossil trees. 2. Refugee tree line The maximal tree retreat during Little Ice Age was marked by old-growth trees (A>300yr). This “refugee line” is ~70 m lower in comparison with historical line. 3. A current tree line 4. Regeneration line Notably that majority of regeneration appeared mainly during the last 2-3 decades. Siberian pine regeneration age structure and heights distribution. 1 – number of viable seedlings, 2 – dead and dying samples; 3 – regeneration height. Tree line evolution during last millennium Tree mortality was observed during Little Ice Age. The new tree establishment wave Appeared at the end of the 19th century. Since 19th century trees were “diffusing” into tundra area A mean rate of tree line migration into the alpine tundra during the last century was about 1.0 m/yr; past three decades of warming increased migration rate to 2.5 m/yr. In the beginning of 21st century tree line surpassed its historical record by 20 m; regeneration line surpassed it by about 80-100 m Siberian pine and larch now survive at elevations up to 150 m higher in comparison with the maximum tree line retreat during Little Ice Period. Warming (+1.2ºC) was observed mainly during the winter . Winter temperatures increase promotes regeneration recruitment by reducing stem and needles desiccation and snow abrasion. II. Changes in tree physiognomy (prostrate vs. arboreal ) A wide-spread transformation of krummholz into arboreal forms has been observed. The beginning of this process occurred in the mid 1980s. These changes in tree morphology coincide with similar events on the western end of Eurasia, in Swedish Scandia (Kullman, 2007). This indicates the Eurasian scale of the phenomena, caused some temperature threshold “trigger” effect. III. Climate-driven forest response and topography 3500 100 Year 90 3000 2002 80 1983 2500 1960 70 2000 60 50 1500 increment (ha) 40 increment (%) a a Area 1000 30 Area 20 500 10 0 0 1800- 1900- 2000- 2100- 2200- 2300- 1900 2000 2100 2200 2300 2400 Elevation (m) The forest area increment dependence on elevation. 1 – total area increment (ha) (2002 vs 1960 yr); 2, 3 – area increment in percents (2002 vs 1960 yr and 2002 vs 1983 yr, respectively). 25 20 15 Slope (º) Slope 10 1960 1983 2002 5 Terrain 0 1800- 1900- 2000- 2100- 2200- 2300- 2400- 1900 2000 2100 2200 2300 2400 2500 Elevation (m) The dependence of “slope steepness medians” on elevation. Plotted values are median steepness values for forested slopes over a range of elevations for 1960, 1983, and 2002. For comparison the median slope value for all terrain in the study area (i.e. forested and non-forested) is shown. 350 Elevation ((m)m) 1900-2000 300 2100-2200 2300-2400 250 200 150 a increment (%) e Ar 100 50 0 0 5 10 15 20 25 30 35 Slope (º) Forest area increment dependence on slope steepness (2002 vs. 1960 yrs.). Winds impact at least as important as temperature for tree survival at upper limit due to desiccation and snow abrasion Distribution of forests with respect to azimuth. The radius-vector shows the dominant direction of forest distribution. Areas of forest for a given year are proportional to the areas within the boundaries. Winds forms a “flag- like” crown shape, and caused minimal radial increment on the wind- ward side Winter winds in concert with temperature are forming tree line boundary. On the wind-protected areas trees grow 100- 200 m higher. In the “cross-fire” zone of upper tree limit regeneration is located in the wind-protected sites IV. Larch vs Siberian pine reaction to climate change Larch surpasses Siberian pine in arid zone, whereas with precipitation increase Siberian pine gets competitive advantages Larch surpasses Siberian pine in its cold and wind resistance, and is growing arboreal where pine is still prostrate …whereas Siberian pine response to warming is stronger, than larch (as tree increment dynamics showed). Dynamics of apical and radial increments of Siberian pine and larch. Siberian pine: 1 –- apical increment, 2 -- radial increment. Larch: 4 –- radial increment. 3, 5 –- pre-warming period. With temperature and precipitation increase Siberian pine is increasing its proportion within upper elevation belt. It is notable that similar changes are predicted in the European Alps: increasing proportion of Pinus cembra (which is, like P. sibirica, is a five-needle pine) vs. L. decidua at higher altitudes (Bugmann et al, 2005). Conclusions 1. A temperature increase of 1Сº allows regeneration to occupy areas 10-- 40 m higher in elevation. Winter temperatures increase is essential for regeneration recruitment. 2. A mean rate of tree line migration into the alpine tundra during the last century was about 1.0 m/yr. Past three decades of warming increased migration rate to 2.5 m/yr. 3. Siberian pine and larch trees now survive at elevations up to 150 m higher in comparison with the maximum observed tree line retreat during the last millennium, and surpass the upper historical line up to 80-100 m. 4. Forest response to warming strongly depends on elevation, azimuth, and slope steepness. Warming promotes tree migration to areas less protected from winter desiccation and snow abrasion. 5. A warming climate provides competitive advantages to Siberian pine in comparison with larch in areas with sufficient precipitation. Increased Siberian pine proportion, as well as tree growth and migration into alpine will also decrease albedo, which may increase warming at the regional level. 6. Climate-induced forest response significantly modified the spatial patterns of high elevation forests in southern Siberia during the last four decades. Thank you!.
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