了解植物群落演替的過程及其驅動力，有助於了解影響植群結構與功能之環境與生物因子，因此是生態學上備受關注的研究議題。促進型演替模式中，演替早期物種通過直接或間接的方式，改變環境使其更適合演替後期物種的生長。在初級演替早期階段土壤環境條件惡劣，常須經由先驅物種改變土壤環境，後期物種才有機會在此生存，因此在演替早期階段常由草本及先驅木本植物引發演替促進作用，而土壤中的氮經常是此時期植物生長的限制因子。台灣中部山區的畢祿溪上游旁有著如同階梯狀的河階地形，每層河階因為經歷連續不同的演替時間，因此組成一個連續性的演替序列，在其中一階的河階上生長一片臺灣赤楊（Alnus formosana）的純林，臺灣赤楊屬於榿木屬（Alnus）的植物，許多溫帶地區的研究指出榿木屬的植物可以增加土壤中的氮含量，從而促進演替後期樹種的生長。但臺灣地區大氣氮沈降量相當高，甚至超過許多溫帶生態系的臨界負荷量，因此能與固氮微生物共生的赤楊是否仍扮演著重要的促進角色並不清楚。本試驗透過檢驗 （1）不同演替時期的土壤質地、營養元含量及物種多樣性的差異以及 （2）赤楊的密度是否與土壤條件以及松樹小苗生長率有正向關係，借此探討赤楊是否在畢祿溪上游的河階地植物群落演替中具有促進作用。本試驗地點的畢祿溪河階分成四階，接近溪流的前兩階屬於演替早期階段，植被主要以一年生草本植物主；較遠離溪流的第三階，其植被主要以臺灣赤楊為優勢物種；最遠離溪流的第四階屬於演替較晚期階段，植被以臺灣二葉松（Pinus taiwanensis）為優勢物種之天然林。然研究結果顯示，四個不同演替時期的土壤皆為沙質土壤；在赤楊為優勢物種的生物群落，土壤氮濃度及物種多樣性皆高於更早的演替時期。然研究亦發現，赤楊枯落物之覆蓋度與土壤及松樹小苗葉片之氮濃度呈現顯著正相關，但葉片氮濃度與松樹小苗的相對生長率無顯著的相關，反而位於赤楊附近之松樹小苗相對生長率有受抑制的現象。小苗所接受到的光量與赤楊密度呈現顯著負相關，然松樹小苗的相對生長率卻與接受到的光量呈現顯著正相關，推測赤楊附近之松樹小苗生長受抑制的現象可能是因光遮蔽的緣故所致。因此推斷在高氮沈降地區，赤楊似乎未能促進演替後期植物的建立。

Investigating the process of plant community succession and its driving force may contribute to the understanding in the structure and function of plant community. Therefore, it receives much attention in ecology. In the facilitation model of succession, early successional species directly or indirectly change the environment and make it suitable for the growth of later successional species. In the early stages of primary succession, pioneer species change the harsh soil environment so that the later successional species have the opportunity to survive. At this stage, herbs and pioneer woody plants initiate the succession facilitation, and nitrogen is a common limiting factor for plant growth in the soil. In the river terraces near upstream of Piluchi river in Center Mountain Ranges of Taiwan. The durations of vegetation development since the last major flood were different for each terrace surfaces, forming a chronosequence for the study of succession. Taiwan alder (Alnus formosana) dominates one terrace surface in our study area. Many studies in the temperate region indicates that Alnus can increase soil nitrogen availability, thereby facilitate the growth of later successional tree species. Under the high level of atmospheric nitrogen deposition that exceeded the critical load reported for many temperate forests, it is not clear whether nitrogen fixer like alder can still facilitate succession in regions with such high nitrogen deposition. This study examined the role of alder on community succession in Piluchi river terraces by investigating (1) the difference in the soil texture, soil nitrogen concentration and species diversity among communities of different successional stages and (2) the relationship between alder density and the growth rate of pine seedlings and the soil properties. The Piluchi river terraces are divided into four surfaces. The vegetation at the first and second terrace surfaces from the river side are considered the earlier successional stages, where major vegetation is annual herb. The third terrace surface is farther from the stream and its dominant plant is alder. The vegetation at the fourth terrace surface is considered the later successional stage and its dominant plant is pine (Pinus taiwanensis). The results indicate that the soils of four successional stages all had sandy texture. Where alder was the dominant species, soil nitrogen and species diversity were higher than those in the earlier successional stages. The coverage of alder is positively related to nitrogen concentration of soil and pine seedlings, but not related to the growth of pine seedlings. The growth of pine seedlings is inhibited by alders. Alder density is negatively related to light availability of pine seedlings. The growth of pine seedlings is likely inhibited by alders because the relative height growth of pine seedlings increased with increasing light level. Our results suggest that in regions with high nitrogen deposition, alder may not facilitate the establishment of later successional species.

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