Effects of Typhoon Disturbances on Understory Light and Seedling Growth of Six Tree Species in a Forest at Kenting, Southern Taiwan

Starting 1 mo after typhoon Dujuan swept through Taiwan in September 2003, this research
manipulated the understory light availability using artificial shading treatments in the Uplifted
Coral-reef Forest at Kenting, southern Taiwan. In this study, we examined the survivorship, height
growth, and net photosynthetic rates of seedlings of 6 tree species with advanced regeneration for
their responses to the elevated light resources after the typhoon disturbance. These species were
Diospyros discolor, D. maritima, Machilus kusanoi, Melanolepis multiglandulosa, Palaquium
formosanum, and Pisonia umbellifera. Understory light availability in this forest was about 1~2%
before the typhoon disturbance. Three weeks after typhoon Dujuan, the understory light was 6~ 9%.
This had dropped to 2~6% after 3 mo, when leaves had partially recovered. After 15 mo, it was
again 1~2%, indicating that the canopy had again closed. Seedlings of D. maritima still maintained
over 80% survival when grown under less than 1% light availability for 2 yr. However, under the
same situation, Mel. multiglandulosa and Pal. formosanum could only maintain a 12% survival
rate. The survival rate and relative height growth rate of each species, except for Pis. umbellifera,
increased with light availability but at different magnitudes. The relative height growth rates of
seedlings of Mel. multiglandulosa and D. maritima showed the most significant responses to the
elevated light, while those of D. discolor and Pis. umbellifera showed the least. There were also
significantly positive linear relations between the net photosynthetic rates and available light intercepted
by the seedlings. The highest slope of the linear regression was 0.15 for Mel. multiglandulosa,
and it ranged 0.04~0.07 for the other 5 species. Compared to the shaded plots, the increased
light availability after typhoon Dujuan increased the net photosynthetic rates of these seedlings.
Carbon yields likewise increased. As a result, the height growth of seedlings was promoted and
mortality was reduced. This was beneficial to the long-term survival of these advanced seedlings.
However, these beneficial effects diminished within 1 yr as the forest canopy gradually closed
again

2003年9月杜鹃台风（Typhoon Dujuan）侵袭中国台湾地区后1个月，本研究于台湾南部垦丁的抬升珊瑚礁林（Uplifted Coral-reef Forest）中，通过人工遮阴处理调控林下光照可利用量。本研究针对6种具有高级更新状态的乔木幼苗，探究其对台风扰动后光照资源升高的响应，观测指标包括幼苗存活率、株高生长速率及净光合速率。涉及的6个物种分别为：异色柿（Diospyros discolor）、滨海柿（D. maritima）、香楠（Machilus kusanoi）、多腺黑面神（Melanolepis multiglandulosa）、台湾胶木（Palaquium formosanum）及伞花木（Pisonia umbellifera）。
该林分在台风扰动前的林下光照可利用量约为1%~2%。杜鹃台风过境后3周，林下光照升至6%~9%；3个月后，随着植被叶片部分恢复，林下光照水平回落至2%~6%；15个月后，林下光照再次降至1%~2%，表明林冠已重新闭合。
滨海柿幼苗在光照可利用量低于1%的环境中培育2年后，存活率仍维持在80%以上。但在相同低光照条件下，多腺黑面神与台湾胶木的存活率仅为12%。除伞花木外，其余各物种的存活率及相对株高生长率均随光照可利用量升高而提升，但提升幅度存在显著差异。其中，多腺黑面神与滨海柿幼苗的相对株高生长率对光照升高的响应最为显著，而异色柿与伞花木的响应最弱。
幼苗的净光合速率与截获的可利用光照之间存在显著正线性相关关系。多腺黑面神的线性回归斜率最高，达0.15，其余5个物种的斜率介于0.04~0.07之间。与遮阴样地相比，杜鹃台风后升高的林下光照提升了各幼苗的净光合速率，碳收获量亦随之增加。由此，幼苗的株高生长得到促进，死亡率降低，这对这些高级更新幼苗的长期存活极为有利。不过，随着林冠在1年内逐渐重新闭合，这类有益效应会逐步减弱。