Abstract:Plant populations comprise individuals of the same plant species existing in a specific area and time. Population structure and spatial patterns are important topics studied by population ecologists. In particular, monitoring plant population structure can help us understand the processes within and between different populations and identify drivers that control plant population dynamics. Analysis of the spatial distributions of populations is important because spatial patterns can reveal many biological characteristics of populations, their intra- and interspecific relationships with co-existing populations, as well as indicate factors underlying environmental factors and related concerns. Thus, population structure and spatial patterns are important indicators of population processes.
We studied Betula platyphylla populations which dominate a mountainous area of forest vegetation on Wula Mountain of Inner Mongolia. Pure stands of this tree species are found on shaded or semi-shaded slopes at altitudes above 1800m. We analyzed the demographic structure and spatial patterns of B. platyphylla stands in the Wulashan Nature Reserve by establishing three 30m×30m sample plots, and classified older plants into three age groups: middle age, near maturity, and mature plants. Spatial clustering and population dynamics were examined over a range of distances between trees while using tree diameter classes serving as a proxy of tree age, and point pattern analysis (Ripley's K-function). Point pattern analysis is widely used for the research of population spatial distribution pattern and inter-species relationships at different scales. In this paper Ripley's K-function method, which is a type of point pattern analysis, is also used in this analysis. To derive age classes based on diameter, we selected one representative tree in each plot, measured the diameter at breast height (DBH) and later classified the trees into three classes (9.4, 15.7, and 19.0 cm DBH), did trunk analysis, measured their ages and grouped them into tree ages classes of 30, 54, and 90 years, respectively, and used these data for the basis of dividing the trees into diameter and age classes. Based on our analyses and other existing reports, we can classify the B. platyphylla population on Wula Mountain to eight diameter classes: seedling (class I), young trees (class II), middle aged-trees (classes III-IV), and mature trees (classes V-VIII).
Our results suggest: (1) the demographic structure of the B. platyphylla population on the Wulashan Nature Reserve follows a typical "pyramid" shape suggesting a high regeneration rate and growth of many young plants in this population; (2) self-thinning is common in this population, which is explained by high intraspecific competition for space and resources and a higher death rate in age classes III and IV; (3) the survivorship curve of the B. platyphylla population is close to a Deevey I curve; (4) the B. platyphylla population exhibits an aggregated distribution when it is dominated by young and middle aged plants. Distributions are random when it is dominated by mature and old trees. Therefore, we predict that as the population matures, its spatial distribution will change from aggregated to random. We found spatial clustering occurs most strongly at distances of less than 1.5 m. This means two or more individual plants can form a cluster. Based on our findings we conclude the spatial patterns of the B. platyphylla population in this area of uniform environment conditions are controlled by biotic interactions and local ecological characteristics.
In summary, we conducted a detailed analysis of the population age structure and spatial patterns of a B. platyphylla population in Wulashan Nature Reserve to elucidate peculiarities of population processes and their drivers. This paper attempts to provide a theoretical basis and help develop strategies for balanced natural resource protection in and around the Wula Mountain area. Our data can also serve as a starting point for more in-depth research.