Aggregated Distribution of Herbaceous Plants in Restored Vegetation Community in a Semi-arid Area: Evidence from the Loess Plateau of China

The spatial pattern of herbaceous plants is closely associated with the reestablishment of species diversity as well as other ecological benefits in restored vegetation communities. However, it is still not clear how the distribution of herbaceous understory plants occur after the vegetation restoration. Additionally, the relationship between spatial heterogeneity index and number of herbaceous plants in semi-arid areas are still not well-understood. We selected two types of 35-year-old afforested lands and one type of grassland with areas of 50 x 50 m in Ansai County, Northern Shaanxi, Loess Plateau of China (36 degrees 46 ' 42 ''-36 degrees 46 ' 28 '' N, 109 degrees 13 ' 46 ''-109 degrees 16 ' 03 '' E). Each plot was evenly divided into a total of 100 (5 x 5 m in size) cells. The herbaceous plants in each cell were surveyed in three randomly selected 1 mx1 m S-quadrats. The power law model was used to define the relationship among the variance in the number of plants for species i (v(i)) and the mean total number of plants (m(i)) to reflect the spatial heterogeneity index. Results indicated that the relationship between v(i) and m(i) in afforested land and grassland demonstrated good fit with the power law model (R-2 > 0.85). The spatial heterogeneity indexes of all herbaceous species in the three plots were >0, and the regression lines of the power law model were significantly above the y = x. The species number, Shannon index, and overall spatial heterogeneity indexes of the afforested land were greater than those of the restored grassland. There was a positive correlation between spatial heterogeneity index and mean number of plant individuals. Additionally, there was a negative correlation between overall spatial heterogeneity index and total number of herbaceous plant individuals. These results indicated that afforestation has positive effects on the reestablishment of diversity for herbaceous species. The herbaceous species in afforested land and restored grassland communities exhibited an aggregated distribution in the Loess Plateau.