Opposing Growth Trends Created by External Disturbances in Larch Forests of the Mongolian Altai

If tree-ring series from different trees are merged to a chronology, in order to for example make inferences on climate, these series must be checked for consistency. Statistical parameters, including the Gleichläufigkeit (GL = coefficient of agreement) and standard t value have been applied for this purpose and are usually combined to boost the informative value. We tested the hypothesis that low GL and t values can, in turn, be used as indicators of external disturbance in a case study in forests of Siberian larch (Larix sibirica) in the Mongolian Altai. Since these forests were known to have been subjected to considerable selective logging, we were interested as to whether opposing trends in the radial stem increment between different tree individuals of the same forest stand can be correlated with logging intensity. We found low GL and t values at high tree stump densities and basal areas. Furthermore, other factors which are interrelated with logging were also correlated with GL and t. These factors included tree age, the dominance type (dominant versus suppressed position), the competitive pressure by neighboring trees and stand density. In conclusion, low GL and t values can be used as indicators of external disturbance. However, the prevailing type of disturbance can only be identified in combination with a study of structural stand traits. In the present case study, the tree stump density combined with GL and t values are assumed to be good proxies of selective logging intensity

Opposing Growth Trends Created by External Disturbances in Larch Forests of the Mongolian Altai

If tree-ring series from different trees are merged to a chronology, in order to for example make inferences on climate, these series must be checked for consistency. Statistical parameters, including the Gleichläufigkeit (GL = coefficient of agreement) and standard t value have been applied for this purpose and are usually combined to boost the informative value. We tested the hypothesis that low GL and t values can, in turn, be used as indicators of external disturbance in a case study in forests of Siberian larch (Larix sibirica) in the Mongolian Altai. Since these forests were known to have been subjected to considerable selective logging, we were interested as to whether opposing trends in the radial stem increment between different tree individuals of the same forest stand can be correlated with logging intensity. We found low GL and t values at high tree stump densities and basal areas. Furthermore, other factors which are interrelated with logging were also correlated with GL and t. These factors included tree age, the dominance type (dominant versus suppressed position), the competitive pressure by neighboring trees and stand density. In conclusion, low GL and t values can be used as indicators of external disturbance. However, the prevailing type of disturbance can only be identified in combination with a study of structural stand traits. In the present case study, the tree stump density combined with GL and t values are assumed to be good proxies of selective logging intensity

Diverging climate trends in Mongolian taiga forests influence growth and regeneration of Larix sibirica

Central and semiarid north-eastern Asia was subject to twentieth century warming far above the global average. Since forests of this region occur at their drought limit, they are particularly vulnerable to climate change. We studied the regional variations of temperature and precipitation trends and their effects on tree growth and forest regeneration in Mongolia. Tree-ring series from more than 2,300 trees of Siberian larch (Larix sibirica) collected in four regions of Mongolia’s forest zone were analyzed and related to available weather data. Climate trends underlie a remarkable regional variation leading to contrasting responses of tree growth in taiga forests even within the same mountain system. Within a distance of a few hundred kilometers (140–490 km), areas with recently reduced growth and regeneration of larch alternated with regions where these parameters remained constant or even increased. Reduced productivity could be correlated with increasing summer temperatures and decreasing precipitation; improved growth conditions were found at increasing precipitation, but constant summer temperatures. An effect of increasing winter temperatures on tree-ring width or forest regeneration was not detectable. Since declines of productivity and regeneration are more widespread in the Mongolian taiga than the opposite trend, a net loss of forests is likely to occur in the future, as strong increases in temperature and regionally differing changes in precipitation are predicted for the twenty-first century

Identifying drivers of non-stationary climate-growth relationships of European beech.

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species' ecological amplitude. Our results show that drought sensitivity is highly variable and non-stationary, though consistently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees' rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech's drought sensitivity, which needs to be taken into consideration when assessing this species' response to climate change

Effects of insect herbivory on the performance of Larix sibirica in a forest-steppe ecotone

The potential of insects to cause temporary spatial shifts of the forest-steppe borderline was investigated in a case study in the northern Mongolian mountain taiga, where Larix sibirica forests border on montane meadow steppe. Insect herbivores of L. sibirica in northern Mongolia include gypsy moth (Lymantria dispar) and grasshoppers, which defoliate trees. Grasshoppers have (like mice) an additional detrimental effect by decorticating stems of tree seedlings. The hypothesis was tested that insect herbivores cause spatial shifts of the forest-steppe borderline by, first, increasing the mortality of mature trees and, secondly, inhibiting rejuvenation. The first hypothesis was tested by investigating a L. sibirica-meadow steppe ecotone, which was heavily defoliated by gypsy moth in early summer 2005. Defoliation was more severe at the forest edge than in the forest interior. Though only 10% of the larch needles at the forest edge endured the gypsy moth invasion without feeding damage, trees were not sustainably affected, as trees were fully foliated in the subsequent year. This suggests that single gypsy moth invasions, which are frequent in Mongolia's forest-steppe ecotone, do not necessarily result in permanent damage of L. sibirica and, with it, not necessarily lead to local shifts of the treeline, though entire forest edges are often completely defoliated. The second hypothesis was tested by planting 2-year-old seedlings of L. sibirica along the treeline towards the meadow steppe and in the interior of the adjacent light taiga forest. Seedling mortality within 3 months was significantly higher at the forest edge (87%) than in the forest interior (40%). Seedlings at the forest edge died either due to insect and small mammal herbivory (65%) or due to drought (25%). Herbivore damage in the seedlings included defoliation by gypsy moth and grasshoppers as well as decortication by grasshoppers and mice. The high feeding pressure for seedlings at the forest edge suggests that insects and mice inhibit or at least retard forest regeneration at the treeline and can thereby lead to temporary spatial shifts of the treeline towards the steppe, after trees have died, e.g., due to fire or logging. (C) 2007 Elsevier B.V. All rights reserved

Lichen diversity on steppe scopes in the northern Mongolian mountain taiga and its dependence on microclimate

Isolated steppes on sunlit southern slopes are a characteristic feature of the northern Mongolian mountain taiga. Lichen diversity of such steppe slopes was studied in the western Khentey Mountains, located between the Mongolian capital Ulan Bator and Lake Baikal. Functional types of lichen species were analyzed and measurements of microclimate were carried out as a first approach to figure out relevant site factors that control lichen distribution in the different habitat types of the steppe slopes. Such habitats include meadow steppe, siliceous rock and savanna-like Ulmus pumila open woodlands. Dominance of chlorolichens with either yellow or orange lichen substances (usnic acid, parietin, pulvinic acid derivatives) or melanin in the cortex, which absorb both ultraviolet radiation (UVR) and visible light usable for photosynthesis (PAR) suggests that effective protection against high solar irradiation is a crucial selective factor for lichens on the steppe slopes. The slopes are subject to high irradiation because of their steepness and because of the ultracontinental climate. In Betula platyphylla-Larix sibirica light taiga forest bordering the steppe on northern slopes, lichens with and without effective photoprotection co-occur. A second important factor, apparently influencing lichen distribution of the study area is water availability. Low precipitation, which falls in numerous small rain showers, combined with frequent dewfall in summer favors chlorolichens and explains the limitation of cyanolichens to particularly moist microsites, such as water runoffs on rocks or moss cushions on inclined or horizontal tree trunks. Ecophysiological measurements to prove these hypotheses on the significance of sunlight and water availability for lichen performance in the study area have not yet been carried out. (c) 2007 Elsevier GmbH. All rights reserved

Climatic control of high-resolution stem radius changes in a drought-limited southern boreal forest

Abstract Key message Stem radius changes measured at high temporal resolution in a drought-limited southern boreal forest were primarily related to soil temperature and partly soil moisture variation, but only weakly to VPD. Abstract Forest productivity at the southern fringe of the boreal forest biome in Inner Asia is strongly drought-limited, as is evident from dendrochronological analyses. Using electronic point dendrometers, we studied the climate response of stem radius changes at high temporal resolution (10-min intervals) in a mixed larch–birch forest in northern Mongolia in a drought year and two subsequent moist years. Larch trees showed stronger stem radius fluctuations than birches, and this difference was more pronounced in dry than in moist years. Stem radius changes were most tightly related to soil temperature variation, while soil moisture was the dominant controlling factor only in birch in the dry year. Correlations with the atmospheric vapor pressure deficit (VPD), and even more so with air temperature, were much weaker. While the linkage between radius change and VPD might primarily reflect diurnal transpiration-driven stem shrinkage and expansion, soil temperature is thought to directly affect cambial cell division and elongation during wood formation. We conclude that the phenology of stemwood increment is strongly controlled by soil temperature even in drought-limited southern boreal forests under continental climate due to the combination of cold and dry climate.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Albert-Ludwigs-Universität Freiburg im Breisga

Modelling the productivity of Siberian larch forests from Landsat NDVI time series in fragmented forest stands of the Mongolian forest-steppe

The monitoring of the spatial and temporal dynamics of vegetation productivity is important in the context of carbon sequestration by terrestrial ecosystems from the atmosphere. The accessibility of the full archive of medium-resolution earth observation data for multiple decades dramatically improved the potential of remote sensing to support global climate change and terrestrial carbon cycle studies. We investigated a dense time series of multi-sensor Landsat Normalized Difference Vegetation Index (NDVI) data at the southern fringe of the boreal forests in the Mongolian forest-steppe with regard to the ability to capture the annual variability in radial stemwood increment and thus forest productivity. Forest productivity was assessed from dendrochronological series of Siberian larch (Larix sibirica) from 15 plots in forest patches of different ages and stand sizes. The results revealed a strong correlation between the maximum growing season NDVI of forest sites and tree ring width over an observation period of 20 years. This relationship was independent of the forest stand size and of the landscape’s forest-to-grassland ratio. We conclude from the consistent findings of our case study that the maximum growing season NDVI can be used for retrospective modelling of forest productivity over larger areas. The usefulness of grassland NDVI as a proxy for forest NDVI to monitor forest productivity in semi-arid areas could only partially be confirmed. Spatial and temporal inconsistencies between forest and grassland NDVI are a consequence of different physiological and ecological vegetation properties. Due to coarse spatial resolution of available satellite data, previous studies were not able to account for small-scaled land-cover patches like fragmented forest in the forest-steppe. Landsat satellite-time series were able to separate those effects and thus may contribute to a better understanding of the impact of global climate change on natural ecosystems

Late Holocene vegetation history suggests natural origin of steppes in the northern Mongolian mountain taiga

Insular occurrences of steppe vegetation are a common feature of the northern Mongolian mountain taiga. Steppe vegetation is limited here to southern slopes, whereas northern slopes and valley bottoms are principally wooded with light and dark taiga forests. In a case study in the valley of the river Eroo at Khonin Nuga Research Station in the western Kbentey Mountains, we searched for evidence of an anthropogenic versus natural origin of steppe vegetation on the southern slopes. Pollen data of three profiles covering the last 2500 years showed continuous presence of steppe throughout the late Holocene with human influence restricted to the recent past. Virtual absence of charcoal in the soil on and beneath three steppe slopes suggested that the present steppe grasslands are not replacing former forests burnt by humans or lightning. The floodplains in the center of the Eroo valley were recently deforested. This is suggested by the pollen analysis and by interviews with local people on landuse history. Steppe grasslands of the study area have probably never been used as pastures. Pastoral nomads traditionally avoided the Eroo valley near Khonin Nuga because of difficult access and high densities of wolves and bears. All our data suggest that the present vegetation pattern of the western Khentey with steppes (and single small Ulmus pumila trees) on south-facing slopes occurring as islands in the mountain taiga is driven by climate and relief and is not the result of human activities as suggested for other regions of Asia. (c) 2008 Elsevier B.V. All rights reserved