| Plateau zokor (Eospalax baileyi syn. Myospalax baileyi) has the large negative impact on the original community structure by excavating burrow systems and feeding plant roots on subalpine meadow. Succession initiated when a new zokor mound was formed. In subalpine meadow, zokor mounds in space and time constitute a series of successional mosaic, because mound formate at the different time and the succession process is asynchronous. Most studies on the restoration succession of plant communities on zokor mounds were in an identity matrix community which was in a degraded state. However, multiple restoration states occurred in subalpine meadow. How local communities of zokor mounds to develop under different matrix communities has been an emerging task for ecologists.In present study we selected mound communities generated by zokor disturbance and corresponding contrast quadrats under three matrix communities. Along the successional gradient, we compared species richness differences on zokor mound communities, analyzed the distribution patterns of plant communities with quantitative classification and ordination methods, calculated the convergence level during the recovery time by two beta-diversity indices (Jaccard and Bray-Curtis index), estimated species negative co-occurrence patterns by calculating checkerboard scores at each successional stage, evaluated the relative effects of surrounding vegetation and three environmental factors (available light radiation, soil temperature, and moisture) on species turnover of mound communities using multiple regressions on distance matrices, and calculated the niche index of dominant species. At patch community, mound metacommunity, block metacommunity and micro-landscape scales, we compared differences and correlations of ecological factors on mound communities and matrix communities by variance analysis and Pearson correlation analysis.The main results of the study are drawn as follows:(1) Two-way indicator species analysis successfully distinguished matrix communities from zokor mound communities. Meanwhile, based on this analysis, we divided mound communities into communities as2～4years old and fresh mound communities. A total of90species in26families were recorded in our study field. The total species in block A is58, block B is71, and block C is80. Early successional species are r-strategist, and mostly belong to annual forbs, e.g. Elsholtzia densa, Chenopodium glaucum, Polygonum convolvulum, Pedicularis chinensis, Slamium amplexicaule and Lagedium sibiricum. These species were often found in fresh mound communities, but occurred only sporadically in2～4years old mound communities. Late successional species can be divided into three sub-groups:a) species belonging to intermediate type in r-K continuum, which mainly occurred on fresh mound community, and also dominated in2～4years old mound communities and matrix communities, e.g. perennial forbs Artemisia tangutica,Potentilla anserine, and Lancea tibetica; b) species belonging to intermediate type in r-K continuum, which mainly occurred on2～4years old mound communities and matrix communities, e.g. perennial forbs Potentilla bifurca, Plantago depressa, Aster tataricus, Gueldenstaedtia diversifolia, Poa pratensi, and Elymus nutans; c) species belonging to K-strategist, which only dominated in matrix communities, e.g. perennial plants Kobresia humilis, Festuca sinensis, and Koeleria glauca.During the natural recovery of zokor mound communities, species with larger niche breadth had a larger niche overlap with other species, e.g. Artemisia tangutica, Potentilla anserine, and Lancea tibetica. However, species with smaller niche breadth also had a high niche overlap when their distributions were synchronous in a given succession stage. For example, the niche overlap index between Slamium amplexicaule and Lagedium sibiricum was up to0.963in the early succession.There were three main recolonization strategies for early species in new-formed mound communities:seed germination (e.g. Elsholtzia densa, Geranium sibiricum, Pedicularis chinensis), regrowth of buried plants (e.g. Lancea tibetica), and clonal propagation from residual roots and rhizomes in soil mound (e.g. Potentilla anserina, Artemisia tangutica, and Cirsium setosum).(2) There were four levels of recovery convergence during the successional processes of zokor mound communities:a) In each matrix community, species composition on zokor mound communities became more convergent over time (In terms of Bray-Curtis dissimilarity index, the dissimilarity of mound communities was gradually reduced from0.835to0.432in block A, from0.891to0.501in block B, from0.925to0.503in block C); b) Species composition between zokor mound community and corresponding matrix community became more convergent along the successional gradient (Along the successional gradient, dissimilarity of species composition measured by Bray-Curtis dissimilarity index between zokor mound community and corresponding matrix community gradually changed from0.991to0.592in block A, from0.974to0.671in block B, from0.988to0.665in block C); c) Three types of matrix communities as a whole, mound communities became more convergent over time (Bray-Curtis dissimilarity of mound communities has been gradually reduced from0.859to0.591along the successional gradient); and d) Three types of matrix communities as a whole, species composition between zokor mound community and matrix community became more convergent along the recovery stage (Bray-Curtis dissimilarity of mound communities has been gradually reduced from0.987to0.706along the successional gradient). The visual result of DCA ordination indicated:a) along the recovery stage, community structure of all the selected mounds were gradually changed from random distribution to concentrated distribution pattern; and b) zokor mound communities become convergent to background-based over time in the DCA plot.(3) The checkerboard scores on fresh mound communities were higher than their random expectation. Community structure was in line with the checkerboard compositional pattern, which indicated that species among mound communities were segregated, and species interactions were weak. However, the checkerboard scores of mound communities at late successional stage were lower than their random expectation. Community structure didn’t display the checkerboard compositional pattern, which indicated that species among mound communities were aggregated, and species interactions were strong. Multivariate regression analysis based on distance matrix showed that stochastic factors affected community structure in the early restoration stage. Along the successional gradient, the regression coefficients between matrix communities and mound communities were-0.570,3.693,2.314,5.152,8.259,12.364, which implied that the effect of surrounding vegetation on mound communities became more stronger over time.(4) During the recovery process, species richness of zokor mound communities in our study were ranged from2to28, and increased with the recovery time. The number of species on different spatial scales was also different. Take Block B as an example. At the patch scale, species richness of mound community (14.6) was lower than matrix community (25.7). However, at the mound metacommunity scale, block metacommunity scale and micro-landscape metacommunity scale, species richness of above three metacommunities (59,69and90, respectively) was higher than matrix community (46).(5) Compared with matrix communities, mound communities had the following characteristics:soil available phosphorous, soil total phosphorous, soil nitrate nitrogen, soil total available nitrogen, soil total nitrogen, soil total carbon, plant tissue carbon and plant tissue nitrogen were higher. In addition, soil bulk densities, soil moisture, soil temperature and pH were also higher. Based on these results, we draw the following conclusions:(1) Zokor disturbance played a substantial role on the composition of meadow communities. Species with r-strategy are gradually displaced by species in intermediate type of r-K continuum during the recovery process. Artemisia tangutica, which is an intermediate species in r-K continuum, has a higher niche breadth and niche overlap with other species. Species with smaller niche breadth also has a high niche overlap within a special succession stage. In2～4years old mound communities, Potentilla anserine, Lancea tibetica, Cirsium setosum and Trigonella ruthenica were main sub-dominant species of Artemisia tangutica. The identity of dominant species is variable. The main recolonization strategies for early species were seed germination, regrowth of buried plants, clonal propagation from residual roots and rhizomes in soil mound.(2) Succession of zokor mound communities experiences a change from ’multiple alternative states’ to ’background-based deterministic community assembly’. Most newly formed mound communities were highly influenced by stochastic factors, while later co-occurring species were primarily a product of species interactions and were highly influenced by surrounding vegetation.(3) At the patch scale, species richness of early mound communities was low, with a continuous increase along restoration stage. At the metacommunity scale, mound metacommunity has higher species richness than matrix communities, while block metacommunity (including both mound communities and matrix community in a block) and micro-landscape metacommunity (including matrix communities and all mound communities of three blocks) has much higher species richness.(4) Disturbance of zokor burrow activities changed the original species composition of ecological communities, created a dynamic mosaic of soil nutrient conditions and patchy vegetation, provided a refuge for r-strategy species, and increased species diversity on the regional scale. Therefore, plateau zokor may be ecosystem engineers in sub-alpine meadow. |
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