Topography mediates forest distribution and productivity at the sub-boreal ecotone, Mongolia

Background/Question/Methods

The forests in the Mongolian forest-steppe are part of the boreal forest’s ‘trailing edge’. They form an important buffer between the Gobi Desert and the Eurasian boreal forest. Conservation of these forests will likely require an approach distinct from that needed in other parts of the biome. This study aims to identify areas of conservation priority by characterizing the relationship between topography, forest distribution and forest productivity in this region. Forest patches that have lower productivity and are located in more fragmented parts of the landscape may be at greater risk for an ecological state change to grassland. We used satellite imagery to study forest patches within the sub-boreal ecotone in north-central Mongolia. We compiled data from a digital elevation model, Landsat TM imagery, and a 12-year timeseries of the MODIS NDVI 16-day composites (MODQ13). Across a 310,000km2 study area, we modeled regional-scale relationships between primary topographic attributes (elevation, slope and aspect, and latitude/longitude), forest distribution (percent of the landscape covered by forest) and forest productivity (NDVI). We used CART analysis and multiple linear regression to identify relationships between topography, forest presence and NDVI. We used hierarchical cluster analysis to identify zones of similar productivity patterns within the forest-steppe region.

Results/Conclusions

We found that forest cover and forest productivity interact with topography in different ways. Forest cover is mediated by latitude, slope, elevation and aspect. Higher percent forest cover was found at steeper slopes, lower elevations, and north-facing aspects, but the specific topographic threshold at which forest cover became sparse varied with latitude. Forest productivity was highest at easterly longitudes and at low elevations, aligned with the precipitation gradient in the region. Relationships between topographic attributes and NDVI were generally most pronounced during greenup and senescence, but different attributes had the strongest relationship with NDVI at different points in the season. Five forest zones of 100km to 200km each were evident across the landscape, with forests in each zone sharing certain topographic attributes and similar patterns of NDVI anomalies. Topographic attributes play a significant role in determining the pattern of forest cover and productivity in the Mongolian forest-steppe, and the relationship varies annually and seasonally. A regional latitudinal gradient in forest cover interacts with a longitudinal gradient in forest productivity, resulting in distinct forest zones within the largely single-species forest system. Forests growing in the most inhospitable topographies have consistently lower NDVI and may be vulnerable to forest dieback.