| Summary: | 博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 99 === Understory layer is the most diverse component of forest communities and understory woody seedlings play an important role in forest regeneration. However, this layer is least understood, not except for in subtropical region under monsoon climate. In this study, the following questions were asked: 1) under the influence of monsoon, do forest strata interact with each other? (how overstory vegetation affects the understory composition?) 2) how the understory vegetation reflect on exogenous factors? and 3) what are the factors that control tree seedling establishment?
In order to better understand the ecological role of understory vegetation, resident species (including herbaceous species, climbers, small shrubs and tree ferns < ca.1.5 m high) were investigated on seven transects, each 5 m wide and 300 m long in a wind-stressed forest of Lanjenchi in Nanjenshan, southern Taiwan. Seedlings of Transient species (canopy, subcanopy and shrub species) with diameter at breast height < 1 cm were censused in three of the seven transects. Species of understory plants in transects were identified, and the percent coverage of residents or species abundance of transients was estimated for every 5 m × 5 m contiguous quadrat along transects. I detected possible intrinsic and extrinsic factors of understory abundance by detecting spatial autocorrelation patterns with Moran’s I. The composition patterns of understory plants and their corresponding overstories were compared with clustering analyses. The relationships between understory composition and environmental factors were examined with direct ordination and spatial models. The direct and indirect effects of monsoon on herb coverages were tested with path analysis based on a causal model. The interaction (competition or facilitation) between woody seedlings and resident cover was evaluated with correlation analysis. The contributions of parent tree distribution and environmental factors on seedling abundance were evaluated with regression models.
A total of 94 resident species, belonging to 44 families and 83 genera were recorded. There were 16,925 transient individuals from 102 tree and shrub species, representing 37 families and 76 genera in seedling census. Most (98%) of understory species had significant autocorrelation in fine scale within 50 m. Eighteen % of species showed a significant autocorrelation at a 180m-class which correspond to the local replicates of topographical features in hilltops. According to the presence of spatial scales of understory species, it is suggested that the understory distribution patterns are influenced by species intrinsic traits (e.g. individual size and prolonged leaves and stems) and topographical characteristics in the plot.
The resident compositions shifted with overstory community types along a topographical change, from windward ridges (northeast-facing slopes with strong effect of winter monsoon) to leeward creeks (southwest-facing slope with weak wind effect in winter). The spatial distribution of transient communities also presented a consistent pattern with the overstory composition. The separated direct ordination analyses showed that the understory resident and overstory compositions had very similar responses to every environmental vector, indicating similar habitat requirements of overstory and resident communities. It also showed that monsoon exposure (angle from zenith to skyline in northeast monsoon direction) was one of the most influential factors explaining spatial patterns of both resident and transient vegetations. However, canopy openness, a light availability index in understory, did not show significant correlation with the compositions/abundances of neither residents nor transients. A causal model indicated that monsoon exposure was the major determinants of understory vegetation variation. Path coefficient between monsoon exposure and fern coverage was higher than those between canopy openness/tree density and coverage of ferns which were considered as the indicators of wind effect. This suggested that monsoon might have a strong direct effect on fern coverage, but relatively weak indirect effect via altering canopy structure.
Based on the abundance models of species seedlings, many species showed significant responses to parent abundance and monsoon exposure. This revealed that monsoon was the most important environmental factor for seedling establishment, and parent abundance also play an important role in shaping spatial patterns of seeding abundance. Owing to the positive correlations between local numbers of seedlings and parents for most species, it indicated that there was a short dispersal range but no strong density dependences for species in Lanjenchi. In the term of competitive effect of understory resident, strong negative correlations between resident structure/species and seedling abundances were absent. Among seedling species which abundances were negatively correlated with resident coverage, most of them exhibited positive correlation of abundance between seedling and parents. I concluded that the competitive effect of residents was not strong enough to attenuate seedling-parent co-occurrence. In summary, high similarity of spatial patterns between understory and overstory compositions should be attributed to that they both respond similarly to exogenous factors rather than interacted with each other.
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