Characterization of ecotones on north- and south-facing slopes in northern Mongolia

Background/Question/Methods

Ecotones, the transitional zones between ecosystems, are sensitive to climate and land-use changes, both occurring rapidly in northern Mongolia. However, the pattern of species’ distributions and abundances that define ecotones in this region are not well understood, impeding their protection and management. The region’s landscape is a patchwork of steppe, occuring on the south-facing slopes and base of the valleys, and taiga forest (almost entirely Siberian larch, Larix sibirica), covering most of the north-facing slopes. We set out to measure changes in larch and understory plants through ecotones between forest and steppe in the Dalbay valley of Mongolia’s Hövsgöl National Park. For understory plants, we recorded presence/absence in 1m by 1m plots every 5m along ten, 75m transects in 2012. For larch, we sampled trees on 100m long transects, 24 through the south-facing ecotone in 2009 and 25 through the north-facing ecotone in 2011. All transects started in the steppe and well outside the forest. For each tree along the transect, we recorded its position on the transect, diameter at breast height (dbh), distance to nearest neighbor, and the nearest neighbor’s dbh. The “breakpoint” between forest and ecotone on each transect was determined using a linear regression model.

Results/Conclusions

Distance to nearest neighbor (tree density)—not dbh or nearest neighbor’s dbh—provided the most consistent measure of a structural breakpoint between forest and ecotone on both the north- and south-facing slopes. Both slopes had large spatial variability across their ecotones, but the north-facing slope tended to have a larger ecotone containing an average of 8.4 trees between the steppe and the forest-ecotone “breakpoint”, while the south-facing ecotone contained an average of 3.9 trees before transitioning to forest. Additionally, while tree density increased through the ecotone from steppe to forest, only on the north-facing slope was this increase statistically different from any change in density within the forest. Understory and steppe vegetation changes in plant community align well with the change in larch density. This suggests Larix sibirica may be a good indicator for defining changes in overall plant community. Our study demonstrates that aspect plays a prominent role in structuring ecotone size and species’ distributions in northern Mongolia. However, the variability across a single ecotone and between ecotones on north- vs. south-facing slopes suggests caution in attempts to generalize about the nature and sharpness of the steppe-taiga ecotone.