Ecological Characteristics Of Shrub Stage In Natural Vegetation Recovery Process In The Upper Reaches Of Minjiang River, Southwest China

Restoration of degraded plant community has been a highlight of restoring and rehabilitating degraded mountain ecosystem in the Upper Reaches of Minjiang River. Shrub stage of natural recovery in the region is not only a stage with high species richness, but also a critical stage in the process of restoration of the degraded ecosystems. From the shrub stage, degraded process of the ecosystem such as soil erosion began to decrease. Functions of the ecosystem functions such as energy assimilation, soil and water conversation and water holding capacity strengthened gradually, and more and more similar to climax community in ecosystem functioning. So natural recovery shrub stage should be an indispensable and important stage in biodiversity conservation and ecological function researches for vegetations in the Upper Reaches of Minjiang River. Based on the theories and methodologies of vegetation ecology, restoration ecology and biodiversity conservation, 1hm2 plot with 400 (5m×5m) contiguous grid quadrats was designed in the shrub stage community of the natural vegetation recovery process at Dagou valley in the Upper Reaches of Minjiang River, Southwest China. After detailed field investigation in the sample plot, community ecological characteristics(such as floristic composition, community structure, species diversity, population structure and dynamics, population niche, population spatial pattern, interspecific association , interspecies covariation) are described in this paper. Finally community microclimate characteristics in different vegetation and restoration stages were measured and analyzed..1. Although the physiognomy of the community was shrubby, the component was rich. Differed from normal shrubs, it was composed of plant species in different restoration or succession stages. The tree species in climax community of the region( such as Quercus liaotungensis )was present and somewhat dominant in the community. The species diversity of the community was high. There was a floristic composition with 43 families, 88 genera, and 135 species in the plot, in which, there were 49 species of 44 genera of 21 families of herbs and 86 species of 47 genera of 34 families of woody plants(trees and shrubs combined). The areal-types of families and genera are abundant, but the areal-types of families dominated by North Temperate and Cosmpolitan component, which accounted for 34.9%, respectively. The areal-types of genera were dominated by temperate genera, accounting for 72.8%, in which north temperate genera was 39.8%.2. The species composition, height class structure, diameter class structure and species diversity were analyzed in the 1hm2 plot. The woody plant abundance of thecommunity was 76532. Both the species richness and stem density decreased with increasing of height and basal diameter class. There was a significant positive correlation between species richness and stem density for different height and different basal diameter class in the community. The species richness and species diversity indexes were relatively higher compared with other vegetation types of Minjiang River region and some typical temperate vegetation types. Species-abundance relationship fit normal distribution. Frequency-species sequence relationship fit lognormal distribution. Closed the mountain for vegetation restoration is an effective method to restore the degraded community caused by human disturbance in the region.Stem abundance of different populations peaked at different height classes. It showed that different populations dominated at different layers. Based on the distribution of stem abundance of different populations at different height classes, the main populations in the community could be grouped into three population structural types. The joint entropy was used to measure the complexity of the community. By using improved M. Godron method, the stability of the shrub community was studied as well. The result showed that the community was not in a stable state.3. The niche breadth, niche overlap and niche percentage similarity were examined based on the investigated data in the 1hm2 plot in the community at natural recovery shrub stage in the Upper Reaches of Minjiang River. The niche breadth of most populations was narrow, only three populations' horizontal niche breadth was larger than 0.5. Quercus liaotungensis has the biggest horizontal niche breadth, its value was 0.596. Populus davidiana has the biggest height niche breadth, its value was 0.381. Height niche breadth of most of the populations(87.2%) was 0.1-0.3. Based on the niche breadth, the populations can be classified into three groups. There existed horizontal niche overlaps among 79.23% of the pairs of the populations although the niche overlap value was low. The height niche overlap existed among the all the pairs of the populations. The horizontal niche percentage similarity between the pairs of populations was low, which implied that partition of the horizontal resources for the population in the community existed. While the height niche percentage similarity between the pairs of populations was high, which indicated that those populations shared the height ecological resources dominated by light.4. Spatial pattern of main populations of the natural recovery shrub stage community were measured with 6 indexes, such as clump index, index of meancrowing, index of patching, Green's index, the clumping intensity index. Cassie' index and variance/mean ratio method as well as. All the spatial pattern of the main populations of the shrub community was clumpy. The results revealed that the distribution of the populations was influenced mainly by their biological characteristics and microtopography. In addition, competition among the species would affect the distribution pattern of the populations. With the progress of restoration, the clumping intensity would decrease, so it could be used as an indicator for plant community restoration in some degree. The relationship between these indexes was compared. The variance/mean ratio method is better than the other methods with aggregative index for its mathematical inference is rigorous and calculations process is simple with specific ecological meaning. The results of the 6 measurements were the same, while index of patching, Green's index, the clumping intensity index were more preferable than the others.5. Based on 2×2 contingency table, Variance analysis, X2 test, Dice index, Jaccard index, Ochiai index, association coefficient, the Pearson product-moment correlation coefficient and Spearman rank correlation coefficient were used to analyze the interspecific association and interspecies covariation of dominant plant populations in the natural recovery shrub stage community. The results showed that the analysis of variance can be used to explain the overall association of all species; the X2-test can be used to describe the statistical significance and the property of the interspecific association. The Dice, Jaccard and Ochiai indices can be employed to show to what degree a pair of species is associated. The variance test reveals that all tree species in the community showed a negative correlation. Among 3655 pairs of species, there were 935 pairs showed significant association, of which 553 pairs were significant positive association and 382 pairs were significant negative association. There were 2720 pairs of species showed non-significant association, accounting for 24.42%. So it indicated that most of the species distributed independently. Among 190 pairs of species, 144 pairs showed a significant interspecies covariation. Among them, 80 pairs were significant positive interspecies covariation, and 64 pairs were significant negative interspecies covariation, accounting for 42.1% and 33.7% of the total pairs of species, respectively. Based on analysis on the interspecific associations, the dominant 20 populations in the community can be classified into three ecological groups of species.6. Light intensive, relative air humidity, air temperature, soil temperature and soil moisture in six different communities in the process of community restorationwere measured in the Upper Reaches of Minjiang River, the changes of community microclimate are as follows: (1) With the development of the community, the light intensity, soil surface temperature, air temperature and their fluctuation were declined; (2)At the same stages of community restoration, the natural communities had higher air temperature and lower humidity than that of artificial communities ;(3) With the development of the community regeneration, the soil moisture of the old-field increased , the upper layer soil moisture (0-15 cm) and middle layer soil moisture (15-30cm) of artificial regenerated communities were enhanced, and lower layer soil moisture (30-45cm) was reduced, the soil moisture of natural communities was higher than others. With the development of the community restoration, the community microclimate habitats became better and steadier.