The Spatial Pattern and Interactions of Woody Plants on the Temperate Savanna of Inner Mongolia, China: The Effects of Alternating Seasonal Grazing-Mowing Regimes

<div><p><i>Ulmus pumila</i> tree-dominated temperate savanna, which is distributed widely throughout the forest-steppe ecotone on the Mongolian Plateau, is a relatively stable woody-herbaceous complex ecosystem in northern China. Relatively more attention has been paid to the degradation of typical steppe areas, whereas less focus has been placed on the succession of this typical temperate savanna under the present management regime. In this study, we established 3 sample plots 100 m×100 m in size along a gradient of fixed distances from one herder’s stationary site and then surveyed all the woody plants in these plots. A spatial point pattern analysis was employed to clarify the spatial distribution and interaction of these woody plants. The results indicated that old <i>U</i>. <i>pumila</i> trees (DBH ≥ 20 cm) showed a random distribution and that medium <i>U</i>. <i>pumila</i> trees (5 cm ≤ DBH < 20 cm) showed an aggregated distribution at a smaller scale and a random distribution at a larger scale; few or no juvenile trees (DBH < 5 cm) were present, and seedlings (without DBH) formed aggregations in all 3 plots. These findings can be explained by an alternate seasonal grazing-mowing regime (exclosure in summer, mowing in autumn and grazing in winter and spring); the shrubs in all 3 plots exist along a grazing gradient that harbors xerophytic and mesophytic shrubs. Of these shrubs, xerophytic shrubs show significant aggregation at a smaller scale (0-5.5 m), whereas mesophytic shrubs show significant aggregation at a larger scale (0-25 m), which may be the result of the dual effects of grazing pressure and climate change. Medium trees and seedlings significantly facilitate the distributions of xerophytic shrubs and compete significantly with mesophytic shrubs due to differences in water use strategies. We conclude that the implementation of an alternative grazing-mowing regime results in xerophytic shrub encroachment or existence, breaking the chain of normal succession in a <i>U</i>. <i>pumila</i> tree community in this typical temperate savanna ecosystem. This might eventually result in the degradation of the original tree-dominated savanna to a xerophytic shrub-dominated savanna.</p></div

Map of the Otindag Sandy Land and the location site of the study region.

<p>Map of the Otindag Sandy Land and the location site of the study region.</p

Bivariate spatial association between <i>U</i>. <i>pumila</i> trees and mesophytic shrubs in plot 2 and plot 3 with the independent null model.

<p>Bivariate spatial association between <i>U</i>. <i>pumila</i> trees and mesophytic shrubs in plot 2 and plot 3 with the independent null model.</p

Bivariate spatial association between <i>U</i>. <i>pumila</i> trees and xerophytic shrubs in plot 1 and plot 2 with the independent null model.

<p>Bivariate spatial association between <i>U</i>. <i>pumila</i> trees and xerophytic shrubs in plot 1 and plot 2 with the independent null model.</p

Univariate spatial patterns of two shrub categories with the null model of CSR.

<p>Univariate spatial patterns of two shrub categories with the null model of CSR.</p

The proportion of different age-class categories of a <i>U</i>. <i>pumila</i> tree.

<p>The proportion of different age-class categories of a <i>U</i>. <i>pumila</i> tree.</p

Distribution map of the woody plants in three surveyed plots.

<p>● U. pumila-old trees,○U. pumila-medium trees,▼ U. pumila-juvenile trees, △ U. pumila-seedlings, ■ xerophytic shrubs, and □ mesophytic shrubs.</p

Marking standard of the indices for assessing rocky desertification severity.

<p>Marking standard of the indices for assessing rocky desertification severity.</p

Parameters (PAE and N) of the revised Bagrov modeling of ETa values in rocky desertification areas.

<p>Parameters (PAE and N) of the revised Bagrov modeling of ETa values in rocky desertification areas.</p

Annual variation of (a) NDVI in July, (b) actual evapotranspiration (ETa), and (c) average precipitation (P) in different severity categories of rocky desertification areas from 2000–2013.

<p>Annual variation of (a) NDVI in July, (b) actual evapotranspiration (ETa), and (c) average precipitation (P) in different severity categories of rocky desertification areas from 2000–2013.</p