相思樹為不擇土質、生長迅速、適應力強之木本植物，其材質堅韌耐燒，為台灣早期重要之造林樹種。近三十年由於薪炭材需求下降，相思樹人工林未撫育而放任，致使相思樹冠層下原生植物有機會生長發育而成為次生林，本研究即對此類相思樹冠層下進行原生植物多樣性分析。調查時間自2008年8月至2009年4月，於天母古道旁已放任多時且未再受干擾之均質相思樹次生林內，選取大徑相思樹為中心，其冠層下3m為半徑之圓面積為一樣區，記錄其樹蔭下之伴生植物，再以樣區內主要優勢植物之密度流程平均值均達穩定來確定此研究至少需9個樣區。9個樣區登錄之木本植物共246株分屬21種16科，除相思樹為造林樹種外均為原生植物。Simpson’s多樣性指數值為0.818，顯示在相思人工林內生長之原生植物多樣性高。9 個樣區主要優勢植物之重要值指數（IVI）以相思樹為最高（122/300），故次生林之冠層以相思樹為主，其次分別為木荷（49/300）、杜英（40/300）以及九節木（32/300）。相思樹重要值指數大是因為其胸徑大，大部分都在21cm~65cm 之間，但缺少小徑木、小苗；預測在相思樹大徑木逐漸死亡而無法自然更新狀況下，相思樹樹冠層將為其樹蔭下之原生植物取代而成上層以木荷、杜英為優勢，下層以九節木與觀音座蓮為優勢之天然林。
Acacia confusa is a fast growing and very resilient hardwood species in Taiwan. Its wood is ideal firewood, and had been widely reforested for making charcoals in the earlier days. As the demand for charcoal decreased since 30 years age, many A. confusa economic forest were left unattended, and have developed into secondary forests enriched with native plants. This study was to research the diversity of native species under the overstory of A. confusa. Based on the running means of density of 4 dominant native species, this study set up 9 circular plots in the undisturbed A. confusa forests along the Tienmu Old Trial during August 2008 and April 2009. Each plot contains a circle area with 3m radius under the overstory of a large D.B.H. A. confusar. Coexistent trees surrounding the central A. confusa of each plot were recorded. Totally 246 trees belonging to 21 species in 16 families were recorded of the 9 sample plots; excepting A. confuse all were native species. The Simpson's diversity of the 9 plots was fairly high, with an Simpson's diversity index value of 0.818. The top 4 dominant species in terms of IVI were A. confusa (122/300), Schima superb (49/300), Elaeocarpus decipiensare (40/300), and Chotria rubra (32/300). The high IVI of A. confusa was mainly due to their large D.B.H. (21-65cm). However, the absence of A. confusa saplings and seedlings, suggesting that it was not able to regenerate. In the future Schima superba and Elaeocarpus decipiens are likely to take over the overstory of A. confusa, and Psychotria rubra, Angiopteris Iygodiifolia will occupy understory by natural succession.
