Demography of the epiphytic orchid Sarcochilus japonicus, as assessed on the basis of the fallen trees disturbed by a windstorm

The demography of the epiphytic orchid Sarcochilus japonicus was investigated in a Cryptomeria japonica artificial forest disturbed by a windstorm in Asago City, which is in the northern region of Hyogo Prefecture. The number of leaf sheaths and number of fruits that each individual was currently bearing or had borne in the past were recorded. The demography of S. japonicus was assessed on the basis of the survival and reproduction rates by using a transition matrix. We studied 1335 individuals; the maximum number of leaf sheaths per individual was 53; 70 individuals were currently bearing fruits; and the minimum number of leaf sheaths for individuals that were currently bearing or had borne fruits in the past was 10. The number of fruits per individual did not exceed 4, and the mean number of fruits per individual was 1.56. We estimated the survival curve and reproduction rate assuming four conditions: (1) an individual grows 2.5 new leaves per year, (2) the number of fruits and mean establishment rate remain constant, (3) the seed input from and output to the outside of the population are balanced, and (4) the seeds are not dormant for more than one year. The survival rates were estimated by assuming that the individuals in each sheath class were exponentially distributed. These rates were calculated separately for small and large individuals; individuals were classified as small or large on the basis of a cutoff point for the number of leaf sheaths so that the sum of squared residuals was minimized. This cutoff number of leaf sheaths was eight, and the survival rates thus estimated were 0.8161 (L≤8) and 0.9131 (L>8), where L represents the number of leaf sheaths. The reproduction rate was calculated by multiplying the number of fruits per individual by the mean establishment rate per fruit and dividing this product by 2.5 assumed number of leaves grown per year. The number of fruits borne by each individual with 10 or more sheaths was regressed against the number of leaf sheaths by using the generalized linear model with Poisson errors and log link. Thus, the equation for the regression model of the number of fruits per individual was F_L=exp(0.0484L-2.2043) (L≥10), where F_L is the number of fruits per individual with L number of leaf sheaths. The mean establishment rate per fruit was calculated by dividing the number of established individuals by the number of fruits. The total number of fruits was 109, and the number of established individuals was 490; thus, the number of established individuals per fruit was 4.5. The reproduction rate was expressed as R_L=1.7994exp(0.0484L-2.2043) (L≥10), where R_L is the number of fruits per individual with L number of leaf sheaths. The finite rate of natural increase was thus determined to be 1.0235. We concluded that there may be an increase in the population of S. japonicus under the study conditions.