Abstract
Increasing quadrat area or quadrat number for the study of interspecific associations in plant communities aims at adding overall sample area. The overall sample area that was chosen by many scholars on studying interspecific association were between 3 000 m 2 and 40 000 m 2, and the quadrat areas were 25 m 2、100 m 2 and 400 m 2. The overall sample areas were limited in some cases (e.g endangered species or monodominant communities) because the distribution areas are often limited. Therefore overall sample area for studying interspecific association could not be determined arbitrarily. Under condition of fixed overall sample areas, the sampling strategy needs to be explored. The typical sampling area of Quercus liaotungensis forest in Dongling Mountain in Beijing was chosen. The transect that its overall area was 1 250 m 2 (5 m×250 m) was designed. Three quadrats that its overall area were 1 200 m 2 (3×20 m×20 m) was chosen for the study. The transect was segmented equably according to long border, the area summation of all quadrats was equal to that of the overall transect. Because three quadrats were discontinuous, every one was divided according to definite border or area, and the area summation of all divided quadrats was equal to the overall area of three quadrats. The sample area was divided into some grids. The coordinate and quantity of every individual was measured in all grids. Under conditions of given sample area, some interrelations of quadrat number and area and affects of interspecific association were discussed based on quadrat and transect design. The results showed that positive and negative interspecific association ratio, species average appearance ratio, total multi_species χ 2_test, χ 2_test and significant ratio of species pairs and variation range of interspecific association coefficient could be adopted and applied synthetically to choose optimal quadrat number and area. Under condition of 1 200 m 2 sample area, regularity of those parameters was showed obviously on transect design for sampling technique of Quercus liaotungensis forest, and the choice of 50 quadrats and 25 m 2 for each quadret were optimal quadrat number and area for analysis of interspecific association in this area. The quadrat design was revealed some shortages, and total sample area must be added for gained optimal quadrat number and area. Based on study results of interspecific association analysis according to optimal quadrat number, effects of transect design were better then that of quadrat design. Therefore, under condition of limited total sample area, transect design was better choice for analysis of interspecific association.
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