A comparison of species composition and community assemblage of secondary forests between the birch and pine-oak belts in the mid-altitude zone of the Qinling Mountains, China

The mid-altitude zone of the Qinling Mountains in China was once dominated by birch and pine-oak belts but are now mainly covered by secondary growth following large-scale deforestation. Assessing the recovery and sustainability of these forests is essential for their management and restoration. We investigated and compared the tree species composition and community assemblages of secondary forests of the birch and pine-oak belts in the Huoditang forest region of the Qinling Mountains after identical natural recoveries. Both types of belts had rich species compositions and similar floristic components but clearly different community structures. Tree diversity was significantly higher for the birch than the pine-oak belt. Niche and neutral processes simultaneously influenced the species distribution and community dynamics of the belts, and these forests were able to maintain stable development during natural recoveries. The conservation and management of these forests should receive more attention to protect biodiversity and the forest resources in the Qinling Mountains.

. Six main species abundance distribution models.

Broken-stick model:
This model was first proposed by MacArthur (1957). Its analogy of placing s-points randomly on a line of unit length and simultaneously breaking it at those points into s lengths can be rephrased as a group of s series. The lengths of the segments represent the "niche sizes" of the species. According to the model, the expected size of the rth species, and â r , the expected abundance of species of species at rank r, are shown in equation (1) in Table 1. The mathematical proof of this model can be found in Pielou (1975). (Motomura, 1932) and assumes that the percentage of the total niche occupied by the first species is α, the second species occupied a percentage α of the reminder, (1 − ), and so on. The expected abundance for the rth species is equation (2) in Table 1.

Niche-preemption model This model was proposed by
Log-normal model A log-normal distribution is defined as a distribution whose variate conforms to the normal laws of probability. For SADs, the log-normal distribution characterizes a sample with relatively low abundance or very rare species (Matthews and Whittaker, 2014). Preston (1948) introduced the log-normal SAD by demonstrating a good fit to a large number of data sets covering a number of different communities. See equation (3) in Table 1.

Zipf and Zipf-Mandelbrot model The Mandelbrot model was originally developed for
information systems, assessing the cost of information (Frontier, 1985). In plant communities, the presence of a species can be seen as dependent on previous physical conditions and previous species presencesthese are the costs. Pioneer species have a low cost, requiring few prior conditions. Late successional species have a high cost, viz. the energy, time, and organization of the ecosystem required before they can invade. On this basis they will be rare (Frontier, 1987).
These differences between species give a Zipf or Zipf-Mandelbrot distribution, equations (4) and (5) in Table 1, respectively. The assumption is that a species is very likely to invade once its necessary conditions are met (Wilson, 1991). Hubbell (2001) noted that the relative abundance of species withinand the species diversity ofa community can be explained through neutral drift of individual species abundances. The model contends that the number of individuals in a metacommunity is constant, that is, all available resources in the community are saturated. This is the zero-sum assumption: if an individual dies and a portion of the resource becomes available, it will be immediately taken up by a new individual, and the community size remains constant. See equation (6) in Table 1.