Spatial Upscaling of Tree-Ring-Based Forest Response to Drought with Satellite Data

Warming unsynchronised tree radial growth and regional vegetation canopy growth in semi-arid areas of north-eastern China--a case of Pinus sylvestris var. mongolica in plantation

Exploring the synchronisation between radial growth from a specific tree species and the regional vegetation canopy growth covering a certain area (obtained from remote sensing data) and their climate responses contributes towards clarifying the influence of climate change on aboveground forest biomass. We assessed the variation and correlation between the radial growth of Pinus sylvestris var. mongolica (PM) and regional vegetation canopy growth along with their climate responses in the semi-arid area of northeastern China, investigating the synchronisation and temperature limitation of the two growths. We also clarified the variation in the synchronisation of radial and canopy growth in a warming climate. The radial growth of PM and canopy growth of regional vegetation increased significantly. Positive correlations between tree-ring width index (RWI) and leaf area index (LAI) during May and June were higher than those during the rest of the months and month-combinations of a year. The synchronisation of the radial growth and the canopy growth significantly increased along increasing gradients of latitude, and significantly decreased along increasing gradients of temperature. Radial and canopy growth were limited in July by minimum temperatures in the northern high-latitude sample sites (cold and arid) of the study area. Warming induced the unsynchronised radial and canopy growth in the semi-arid area. The synchronous change of the two growth types will weak in the study area in the future; the decoupling of tree growth is expected to occur earlier in the cold, dry areas than in the warm, wet areas. Weakened or broken statistical linkages, such as the synchronisation between the radial growth of a tree species and the canopy growth of the regional vegetation, indicate that the decreased effectiveness of a specific tree radial growth as an indicator of regional vegetation growth complicates the up- or down-scale assessment of forest biomass dynamics and its carbon sequestration potential.

Quantitative analysis of vegetation restoration and potential driving factors in a typical subalpine region of the Eastern Tibet Plateau

Vegetation restoration is an essential approach to re-establish the ecological balance in subalpine areas. Changes in vegetation cover represent, to some extent, vegetation growth trends and are the consequence of a complex of different natural factors and human activities. Microtopography influences vegetation growth by affecting the amount of heat and moisture reaching the ground, a role that is more pronounced in subalpine areas. However, little research is concerned with the characteristics and dynamics of vegetation restoration in different microtopography types. The respective importance of the factors driving vegetation changes in subalpine areas is also not clear yet. We used linear regression and the Hurst exponent to analyze the trends in vegetation restoration and sustainability in different microtopography types since 2000, based on Fractional Vegetation Cover (FVC) and identified potential driving factors of vegetation change and their importance by using Geographical Detector. The results show that: (1) The FVC in the region under study has shown an up-trend since 2000, and the rate of increase is 0.26/year (P = 0.028). It would be going from improvement to degradation, continuous decrease or continuous significant decrease in 47.48% of the region, in the future. (2) The mean FVC is in the following order: lower slope (cool), lower slope, lower slope (warm), valley, upper slope (warm), upper slope, valley (narrow), upper slope (cool), cliff, mountain/divide, peak/ridge (warm), peak/ridge, peak/ridge (cool). The lower slope is the microtopographic type with the best vegetation cover, and ridge peak is the most difficult to be afforested. (3) The main factors affecting vegetation restoration in subalpine areas are aspect, microtopographic type, and soil taxonomy great groups. The interaction between multiple factors has a much stronger effect on vegetation cover than single factors, with the effect of temperatures and aspects having the most significant impact on the vegetation cover changes. Natural factors have a greater impact on vegetation restoration than human factors in the study area. The results of this research can contribute a better understanding of the influence of different drivers on the change of vegetation cover, and provide appropriate references and recommendations for vegetation restoration and sustainable development in typical logging areas in subalpine areas.

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.