Interannual variation in leaf expansion and outbreak of a teak defoliator at a teak stand in northern Thailand

摘要

The leaf area index (LAI) is a key factor affecting tree growth in forests. Following the outbreak of a defoliator, the LAI declines, serving as a useful indicator in forest management. In this study, daily radiative transmittance from above the canopy, which decreases exponentially with increasing LAI, was measured in a teak plantation (Tectona grandis L. f.) in northern Thailand from March through July in 2001-2008. Volumetric soil moisture was also measured at depths of 0.1, 0.2, 0.4, and 0.6 m. The negative logarithmic value of the ratio of daily downward solar radiation on the forest floor to that above the canopy (NLR;-ln[S_b↓/S↓.]), was calculated as an indicator of leaf flush and subsequent leaf expansion. The NLR data indicated that leaf expansion began in late March and continued to the beginning of May during all eight years (with the day the leaves began to expand defined as DB). In addition, the peak in NLR values (NLR_P), corresponding to the lowest value of a 99% confidence interval, occurred in July. The day when NLR first reached NLRP was defined as DP, which always occurred in June, 31-85 days after D_B. The NLR indicated an increase in the population of Hyblaea puera (a teak defoliator) that was associated with greatly decreased leaf areas during two growth periods (D_B-D_P): the earliest D_B-D _P in 2001 and the second-earliest D_B-D_P in 2008. In almost all cases, soil moisture data indicated that leaf expansion occurred after increases in soil moisture at depths of 0.1-0.4 m even without increases at 0.6 m; in contrast, increases in shallow soil moisture (0.1-0.2 m) were insufficient to trigger leaf expansion at the stand level. Periods of soil drought at 0.1-0.4-m soil depths inhibited leaf expansion, resulting in prolongation of the interval between D_B and D_P during those years in which the D_B's occurred chronologically close to one another. Moreover, when drought did not limit leaf expansion, the D _B-D_P growth periods characterized by earlier D _B's tended to be longer than those with later D_B's.