臺灣北部福山亞熱帶雨林種子雨與小苗之長期動態

Abstract

在植物生活史當中，種子和小苗是死亡率最高的一個階段，對於森林的動態與物種共存有著很重要的影響。為了解台灣北部福山亞熱帶雨林(24°45′ N, 121°35′ E)樹種的繁殖生態，本研究長期監測福山森林種子雨與小苗的動態(2002至2011年)。福山森林中大多數的樹種每年都會開花結果，整個植物群落的開花、結果高峰也呈現一個以一年為週期的律動，此一季節性與福山地區的氣候、資源變化有關，而在譜系上親緣較近的植物有較為相似的開花、結果季節。在族群和群落的層級上，開花和結果量都有很大的年間變化，並與福山地區的氣象因子、聖嬰-南方振盪現象(El Nino Southern Oscillation)有關，此外，本研究也觀察到2005年的霜害對於植物開花、結果量有著長遠影響。這些結果顯示，在群落層級上，氣候的年間變化與極端氣候事件皆會影響植物的開花、結果量。 由於植物結果量有著很大的年間變化，福山森林底層的小苗數量在時間軸上亦有著很大的波動。先前研究認為福山地區頻繁的颱風干擾導致其森林冠層破碎、林下光線充足，應該有利於樹種的更新，然而本研究並不支持此一論述。大多數的新增小苗會在六個月內死亡，且平均每年僅生長1.55 ± 0.20 cm，這些結果顯示福山大部分樹種都有更新上的限制。種子數量與生育地的狀況都會影響各樹種小苗的數量，而影響小苗數量多寡的因子通常與影響小苗存活的因子不同，顯示植物對於生育地的偏好在生活史不同階段可能會有所改變。雖然各樹種小苗的更新狀況與種子數量或生育地有關，但其相對重要性在各樹種間的差異並沒有辦法由植物生活史特徵所解釋。儘管如此，本研究觀察到植物的更新在時間上與個體間都很大的差異，而這些變異可能對於福山森林樹種的共存有著很重要的影響，這些現象再再突顯長期研究對於了解森林動態的重要性。

Seed and seedling are critical bottlenecks during the life cycles of plants with implications for forest dynamics and species coexistence. To understand reproductive ecology of tree species in a subtropical rain forest, Fushan, northern Taiwan (24°45′ N, 121°35′ E), I monitored the seed rain and seedling dynamics during a 9-year period (2002–2011). Most species in the Fushan forest reproduced annually. The community-wide reproductive patterns exhibited distinct annual rhythms, associating with the climatic seasonality of resource availability. I also detected the phylogenetic signal in community reproductive patterns, suggesting the existence of phylogenetically structured reproductive niches. There were great interannual variations in the flower and fruit production at the population and community level. Both the El Nino Southern Oscillation (ENSO) indices and local climatic variables could explain these interannual variations. I also observed that an extreme event, frost in 2005, has a long-lasting impact on the fruit production. These results demonstrated that the interannual climatic variation and extreme weather event can significantly influence the flower and seed production at the community level. The interannual variation in fruit production led to the great temporal fluctuations in seedling density. Previous studies showed that frequent typhoon disturbances contributed to the high canopy openness and high understory light availability at Fushan, which might benefit tree regeneration. However, the results do not support this idea. Most of the newly recruited seedlings died within six months and only grew 1.55 ± 0.20 cm per year, suggesting that the majority of tree species in Fushan were recruitment limited. The seedling abundances depended on a species-specific manner on seed arrival and habitat conditions. The factors that influenced recruit abundance generally were unrelated to the factors that influenced seedling survival, suggesting habitat association patterns might change over time. However, the interspecific variation of seedling response to seed limitation and habitat conditions was not related the life-history traits. Nevertheless, the large temporal and individual variations observed in seedling recruitment might have great consequences for species coexistence in Fushan, emphasizing the importance of long-term studies to the understandings of forest dynamics.