Spatial structure and distribution pattern of natural Toona ciliata populations in the Enshi Region

Key words: 
• forest ecology /
• Toona ciliata /
• population /
• spatial structure /
• distribution pattern /
• natural population

Abstract: Toona ciliata is a key protected tree species at the second national level, its natural population acts as a dominant species in its existing community, is rarely seen in subtropical regions of China. This study was to research the structure of age class, the quantity dynamic conditions, the trend of development and the correlation between the distribution pattern and the structure of age class, the quantity dynamic condition of different Toona ciliata populations. Four sample plots were selected in the Enshi Region of Hubei. The sample plots, entirely covering 4 populations, were set up, each in 5 m 5 m grid, by using contiguous grid quadrate method, with T1 10 m 30 m, T2 20 m 20 m, T3 20 m 20 m and T4 10 m 40 m in size respectively. Based on field survey data, a specific life table was established, survival curve was determined, a dynamic estimation of the survival number for different age classes was made, and spatial distribution patterns of different Toona ciliata populations were analyzed to determine the causes of spatial structures and distribution patterns in different sampling plots. Analysis included Deevey curves, regression, ratio method of variance mean value with t test, Morisita indices （I） with F test and negative binomial parameter （K）, patchiness index （m*/m）, and mean crowding index （m*）. Results of the human disturbance intensity for the four populations was T4 ＞ T1 ＞ T2 ＞ T3 with dynamic indices （Vpi） of T1 = 23.8%, T2 = 34.1%, T3 = 27.8%, and T4 = 32.3% showing growth form. Also, the qx values of different populations lagged in age-class with external disturbance intensities, and the life expectancy peak values （ex） varied as follows: T3 ＞ T4 ＞ T1 ＞ T2. Survival curves did not agree with any Deevey curves, but 4 cubic functions represented a regression of the four survival curves: y=－0.121S3 +2.469S2－14.814S+31.071, y=－0.136S3 +2.162S2 －11.297S + 23.286, y=－0.287S3 +4.410S2 －19.992S +31.143, y=－0.058S3 +1.492S2－10.64S + 24.357; With 25 m2, 50 m2, and 100 m2 as sample sizes, a ratio of variance and mean value to t test, I with F test, and K all demonstrated that T1 and T4 populations had clumped distributions, but T2 and T3 had Poisson distributions. The m*/m varied with a sequence of T4 ＞ T1 ＞ T2 ＞ T3; whereas, m* was T1 ＞ T4 ＞ T3 ＞ T2. This research demonstrated that in a natural state, the Toona ciliata population was in growth form with a stable population susceptible to potential external disturbances and having a Poisson distribution, but with human disturbance a clumped distribution emerged that with proper human objective regulations, population regeneration, species protection, and maximum ecological effect could be enhanced.［Ch, 2 fig. 5 tab. 25 ref.］