Abstract:

Ever since Darwin, competition has been considered to be one of the major forces that shape the morphology and life history of plants and affect the structure and dynamics of plant communities. Competition among plants occurs when resource availability falls below the sum requirement of the community needed for optimal growth. Competition includes intraspecific and interspecific competition for both aboveground and belowground resources and takes place over different spatial and temporal scales. Although many researchers have determined the zone of competition according to their past experiences, it is important to define the zone of influence by using a competition index (CI) that includes all competitors and sources of competition for scarce resources. Phenotypic plasticity is thought to be an adaptive trait evolved in plants growing in spatially or temporally heterogeneous environments. Plasticity of growth and morphology in response to different levels of light, nutrient and water availability in herb plant species has been extensively documented. However, very few studies have addressed phenotypic responses to neighborhood competition in hardwood plants, especially concerning morphology. Larix chinensis is an endemic species in China distributed only in the alpine and sub-alpine belt of the Qinling Mountains. It is also an endangered species listed in the Chinese Plant Red Data Book. It is a dominant species at altitudes of 2 560-3 500 m with special functions that maintains the ecological balance at timberline. In order to define the zone of competition, intraspecific and interspecific competition in L. chinensis communities were investigated using a method that gradually increases the zone of interaction and Zhang's competition index model for individual trees. Data for this study were collected in the Taibai Mountain Natural Reserve of Shaanxi Province in China. Ten 40 m 2-plots were investigated, and every plot was stem-mapped, and species, DBH, total height, and crown length were recorded for each tree that had a diameter of at least 5 cm at breast height. In order to study neighborhood competition effects, morphological characteristics, such as the branching angle, total branch-number, current-year branch length, average branch length, and living branch number of the main stem in different strata were investigated for L. chinensis under different levels of neighborhood competition intensity. The results showed that as a tree increased in size and the distance between plants increased that intraspecific competition intensity decreased due to self-thinning. L. chinensis is the dominant tree in the community, and the few other species tree species in the forest community are all much smaller than L. chinensis. As a result, intraspecific competition was more intense than interspecific competition. The order of competition intensity was: L. chinensis-L. chinensis >Abies fargesii-L. chinensis > Betula platyphylla-L. chinensis > other species-L. chinensis. The relationship between competition intensity and growth of the tree of interest closely followed the following equation: CI×AD -B. The change in competition intensity is very small when the diameter of the objective tree reaches 35 cm. The model can simulate and predict intraspecific and interspecific competition efficiently. The morphology of L. chinensis, such as the branching angle, total branch-number, current-year branch length, average branch length, and living branch number of main stem, was modified in response to different intensities of neighborhood competition. The study provides a new approach to ascertain the zone of competition among trees for studying plant competition. L. chinensis can improve its capacity to intercept light and adapt to different levels of neighborhood competition through morphological changes.

Key words: Larix chinensis, Competition intensity, Intraspecific competition, Interspecific competition