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

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