臺灣中部烏石坑地區森林下層植群與上層的關聯及其環境影響因子

Floristic Associations Between Understory and Overstory Vegetation and Their Relationships to Environmental Factors in Broad-leaved Forests in Wushyrkeng Area, Central Taiwan

本研究以鮮為人注目的森林下層植群為研究對象，測試烏石坑地區林下植群與上層植群的關聯程度，並探討森林下層植群如何受到外在環境的影響。我們於烏石坑地區選取 3 個樣區進行森林上層及下層植物調查，並蒐集環境因子資料，利用植物社會分群分析及直接梯度分析，探討植物組成與環境因子的關係。結果顯示於相同調查範圍內，森林下層植群植物總種數為上層的 1.2 倍，雖然樣區內林下層植物較上層多樣性低，但樣區間物種組成變化卻較上層植物大。烏石坑地區森林下層植群在跨樣區的大尺度下，大致可與上層植物社會分群對應，主要可區分為 A、B 及 C 三型植物社會，其組成隨海拔梯度變化。而在單一樣區的小尺度下，烏石坑地區之森林上下層無法呈現良好對應，顯示除去強烈的海拔梯度因子後，森林上下層植群各自有其限制因子。我們也發現烏石坑地區的林下植群組成，與上層樹木密度顯著關係，此也印證過去曾經提出林下層植群受到上層林冠結構調控的觀點。 To investigate the rarely studied understory vegetation in broad-leaved forest, we tested the degree of association between understory and overstory vegetation, and explored how understory vegetation is affected by exogenous environmental factors in forests in Wushyrkeng Area, central Taiwan. Three plots were set up and the overstory and understory plants in each plot were identified and measured. Classification analysis of plant communities was conducted to compare species composition between understory and overstory strata. Data of environmental factors were collected for direct gradient analysis to explore the relationships between species composition and environmental factors. The results showed that the total number of species in understory layer was 1.2 times more than that in the overstory layer. Although plant diversity of the understory layer is lower than that of the overstory stratum within a study plot, the variations in understory species composition were more apparent than those in the overstory layer. When all plots are combined, clustering analysis generated three comparable groups (A, B, and C community types) along altitudinal gradient for each forest layer. However, within a single plot, there was no significant correlation between the two forest strata, indicating that each forest layer had its own limited factors after eliminating the effect of elevation. We also found that the patterns of understory vegetation composition in Wushyrkeng Area was significantly related to overstory trees density; this result is also consistent with previous studies in which the canopy structure contributed to the differentiation of understory plant communities.

To investigate the rarely studied understory vegetation in broad-leaved forest, we tested the degree of association between understory and overstory vegetation, and explored how understory vegetation is affected by exogenous environmental factors in forests in Wushyrkeng Area, central Taiwan. Three plots were set up and the overstory and understory plants in each plot were identified and measured. Classification analysis of plant communities was conducted to compare species composition between understory and overstory strata. Data of environmental factors were collected for direct gradient analysis to explore the relationships between species composition and environmental factors. The results showed that the total number of species in understory layer was 1.2 times more than that in the overstory layer. Although plant diversity of the understory layer is lower than that of the overstory stratum within a study plot, the variations in understory species composition were more apparent than those in the overstory layer. When all plots are combined, clustering analysis generated three comparable groups (A, B, and C community types) along altitudinal gradient for each forest layer. However, within a single plot, there was no significant correlation between the two forest strata, indicating that each forest layer had its own limited factors after eliminating the effect of elevation. We also found that the patterns of understory vegetation composition in Wushyrkeng Area was significantly related to overstory trees density; this result is also consistent with previous studies in which the canopy structure contributed to the differentiation of understory plant communities.