Choimaa Dulamsuren

Water relations and photosynthetic performance in Larix sibirica growing in the forest-steppe ecotone of northern Mongolia

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 Mongolias 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 Mongolias 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 Mongolias 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 Mongolias forest-steppe ecotone is not likely to increase under the present climate, but may decrease with increasing aridity due to global warming.

Climate Warming-Related Growth Decline Affects Fagus sylvatica, But Not Other Broad-Leaved Tree Species in Central European Mixed Forests

Climate warming is predicted to extend the duration and enhance the severity of summer droughts in Central Europe, which may pose a serious risk to forest productivity and forest health. Fagus sylvatica (European beech), the most abundant tree species of Central Europe’s natural forest vegetation and one of the key species in forestry, is thought to be particularly vulnerable to drought. Here, we present a dendrochronological analysis in three mixed temperate broad-leaved forests along a precipitation gradient with the aim of comparing the climatic response of radial growth of F. sylvatica with the performance of four co-existing species (Acer pseudoplatanus, A. platanoides, Quercus petraea, Fraxinus excelsior). We hypothesized that Fagus is the most drought sensitive of the five species, which implies that it could lose its competitive advantage at drier sites in the course of climate warming. In support of this hypothesis, we found that F. sylvatica in all stands exhibited an increase in the number of negative pointer years and a decrease in radial increment in the driest stand since about 1980, in parallel to increasing summer temperatures and drought intensity. Such a response was missing in the other four species and may point to shifts in the competitive hierarchy in these mixed forests under a future warmer climate. We conclude that Central Europe’s forestry sector should consider carefully the risk of failure of beech in regions with relatively low and decreasing summer precipitation.

Increased Summer Temperatures Reduce the Growth and Regeneration of Larix sibirica in Southern Boreal Forests of Eastern Kazakhstan

The larch forests at the southern limit of the Siberian boreal forest in Central Asia have repeatedly experienced strong recent growth declines attributed to decreasing summer precipitation in the course of climate warming. Here, we present evidence from the southernmost Larix sibirica forests in eastern Kazakhstan that these declines are primarily caused by a decrease in effective moisture due to increasing summer temperatures, despite constant annual, and summer precipitation. Tree-ring chronologies (>800 trees) showed a reduction by 50–80% in mean ring width and an increase in the frequency of missing rings since the 1970s. Climate-response analysis revealed a stronger (negative) effect of summer temperature (in particular of the previous year’s June and July temperature) on radial growth than summer precipitation (positive effect). It is assumed that a rise in the atmospheric vapor pressure deficit, which typically increases with temperature, is negatively affecting tree water status and radial growth, either directly or indirectly through reduced soil moisture. Larch rejuvenation ceased in the 1950s, which is partly explained by increasing topsoil desiccation in a warmer climate and a high drought susceptibility of larch germination, as was demonstrated by a germination experiment with variable soil moisture levels. The lack of regeneration and the reduced annual stem increment suggest that sustainable forest management aiming at timber harvesting is no longer feasible in these southern boreal forests. Progressive climate warming is likely to cause a future northward shift of the southern limit of the boreal forest.

Climate response of tree-ring width in Larix sibirica growing in the drought-stressed forest-steppe ecotone of northern Mongolia

Abstract• IntroductionSiberian larch (Larix sibirica) has its southern distribution limit in Mongolia in an area of rapidly rising temperatures. Direct effects of climate on tree-ring formation due to drought stress or indirect effects via the control of insect herbivore populations are little studied.• MethodsThe hypotheses were tested that stem increment of Siberian larch is reduced by (1) drought during the growing season and (2) high snow cover, as the latter is thought to protect hibernating herbivores, including gypsy moth (Lymantria dispar).• ResultsTree-ring width increases with decreasing summer temperature, increasing precipitation during the growing season and decreasing winter precipitation.• ConclusionsThe susceptibility of stem wood formation to drought during the growing season suggests that a future climate warming will decrease productivity of Siberian larch, thus affecting its existence within the forest-steppe ecotone of Mongolia. Narrow tree rings in years following winters with low snowfall support the hypothesis that winter precipitation exerts an indirect effect on the growth of Siberian larch by controlling the survival rates of gypsy moth eggs.

Rapid recovery of stem increment in Norway spruce at reduced SO2 levels in the Harz Mountains, Germany

Tree-ring width of Picea abies was studied along an altitudinal gradient in the Harz Mountains, Germany, in an area heavily affected by SO(2)-related forest decline in the second half of the 20th century. Spruce trees of exposed high-elevation forests had earlier been shown to have reduced radial growth at high atmospheric SO(2) levels. After the recent reduction of the SO(2) load due to clean air acts, we tested the hypothesis that stem growth recovered rapidly from the SO(2) impact. Our results from two formerly damaged high-elevation spruce stands support this hypothesis suggesting that the former SO(2)-related spruce decline was primarily due to foliar damage and not to soil acidification, as the deacidification of the (still acidic) soil would cause a slow growth response. Increasing temperatures and deposited N accumulated in the topsoil are likely additional growth-promoting factors of spruce at high elevations after the shortfall of SO(2) pollution.

Relationships between the diversity patterns of vascular plants, lichens and invertebrates in the Central Asian forest-steppe ecotone

The Central Asian forest-steppe ecotone has been exposed to large alterations in grazing pressure in the last two decades, but the consequences for biodiversity have not been studied so far. We analyzed the biodiversity of the edges and the interior of Siberian larch forests in the forest-steppes of eastern Kazakhstan (Saur, Kazakh Altai) and western Mongolia (Mongolian Altai, Khangai) across different groups of organisms (vascular plants, epiphytic lichens, soil macroarthropods, oribatid mites, moths). The species richness of these groups was related to each other only at the forest edge, but not in the interior. Species richness of vascular plants, soil macroarthropods and oribatid mites at the forest edges was positively correlated. This indicates that these ground-inhabiting groups of organisms responded similarly to the variation in the grazing pressure of livestock, which is kept at spatially varying densities by mostly nomadic or transhumant herders. The species richness of epiphytic lichens was only positively correlated with that of vascular plants, and the richness of the (volant) moths was not correlated with that of any other group. The complete lack of correlation between the diversity of groups of organisms in the forest interior suggests that the diversity of the five studied groups is controlled by specific environmental factors, including light and moisture. Except for the Mongolian Altai, which was subjected to the highest grazing pressure, vascular plants, lichens, soil macroarthropods, and moths had a higher diversity at the edges than in the interior; the opposite was true for the oribatid mites. The latter probably benefit from the higher soil moisture inside the forest, whereas the other four groups are favored by increased availability of light, the proximity to the steppe with a partial mixing of species pools, and the soil macroarthropods also by increased dung abundance.

Equations for estimating the above-ground biomass of Larix sibirica in the forest-steppe of Mongolia

Biomass functions were established to estimate above-ground biomass of Siberian larch (Larix sibirica) in the Altai Mountains of Mongolia. The functions are based on biomass sampling of trees from 18 different sites, which represent the driest locations within the natural range of L. sibirica. The best performing regression model was found for the equations y = (D2H)/(a+bD) for stem biomass, y = aDb for branch biomass, and y=aDbHc for needle biomass, where D is the stem diameter at breast height and H is the tree height. The robustness of the biomass functions is assessed by comparison with equations which had been previously published from a plantation in Iceland. There, y=aDbHc was found to be the most significant model for stem and total above-ground biomasses. Applying the equations from Iceland for estimating the above-ground biomass of trees from Mongolia resulted in the underestimation of the biomass in large-diameter trees and the overestimation of the biomass in thin trees. The underestimation of thick-stemmed trees is probably attributable to the higher wood density, which has to be expected under the ultracontinental climate of Mongolia compared to the euoceanic climate of Iceland. The overestimation of the biomass in trees with low stem diameter is probably due to the high density of young growth in the not systematically managed forests of the Mongolian Altai Mountains, which inhibits branching, whereas the plantations in Iceland are likely to have been planted in lower densities.

Additions to the flora of the Khentej, Mongolia, 2

Abstract Dulamsuren, C., Kamelin, R. V., Cvelev, N. N., Hauck, M. & Mühlenberg, M.: Additions to the flora of the Khentej, Mongolia, 2. — Willdenowia 34: 505–510. — ISSN 0511-9618;

Higher climate warming sensitivity of Siberian larch in small than large forest islands in the fragmented Mongolian forest steppe

Abstract Forest fragmentation has been found to affect biodiversity and ecosystem functioning in multiple ways. We asked whether forest size and isolation in fragmented woodlands influences the climate warming sensitivity of tree growth in the southern boreal forest of the Mongolian Larix sibirica forest steppe, a naturally fragmented woodland embedded in grassland, which is highly affected by warming, drought, and increasing anthropogenic forest destruction in recent time. We examined the influence of stand size and stand isolation on the growth performance of larch in forests of four different size classes located in a woodland‐dominated forest‐steppe area and small forest patches in a grassland‐dominated area. We found increasing climate sensitivity and decreasing first‐order autocorrelation of annual stemwood increment with decreasing stand size. Stemwood increment increased with previous years June and August precipitation in the three smallest forest size classes, but not in the largest forests. In the grassland‐dominated area, the tree growth dependence on summer rainfall was highest. Missing ring frequency has strongly increased since the 1970s in small, but not in large forests. In the grassland‐dominated area, the increase was much greater than in the forest‐dominated landscape. Forest regeneration decreased with decreasing stand size and was scarce or absent in the smallest forests. Our results suggest that the larch trees in small and isolated forest patches are far more susceptible to climate warming than in large continuous forests pointing to a grim future for the forests in this strongly warming region of the boreal forest that is also under high land use pressure.