Cupressus torulosa plantations - dark and inhospitable to incoming native tree seedlings. Thinning and enrichment planting could help convert them into native forest.
In 1999, Forest Restoration Research Unit (FORRU) assisted Doi Suthep-Pui National Park Authority with enrichment planting of an aging plantation of Cupressus torulosa D.Don (Cupressaceae) (an exotic species), on the summit of Doi Pui, with indigenous framework tree species. The plantation had a very dense canopy and a dense root mat, visible at the soil surface. Consequently, seedlings of native forest trees had not established on the site.
In May 1999, 1,440 trees of 24 species from FORRU-CMU's Doi Suthep nursery were planted over 4 rai of the site beneath the C. torulosa trees. Totally 240 trees of 23 species, were monitored 5 times for growth and survival until August 2002.
The overall percent mortality of the planted trees was 41%, whilst the growth of surviving trees was exceedingly slow. The pioneer species suffered 100% mortality, whereas survival rates of climax species were around 70 %. Most trees failed to grow taller than 1 m in 3 years (from 30-50 cm tall at planting time). The most likely explanation was that the planted trees were deprived of light and suffered from root competition, due to the dense of the C. torulosa plantation, which contributed to the poor performance. Therefore, FORRU-CMU recommended, the C. torulosa trees should be thinned before enrichment planting is made at this site.
This recommendation was applied in 2005, 200 saplings of framework tree species were planted in a thinned-out subplot of the C. torulosa plantation, whilst another 200 were planted in a control (non-thinned) sub-plot. The planted trees were monitored 4 times for growth and survival over three years after planting from 2005 until January 2008.
Survival was higher in the thinned plot (35.61%) than in the non-thinned plot (17.27%) - even though both numbers were unacceptably low. However, almost all of the planted trees in the thinned plot grew better than in the non-thinned plot. Root collar diameter and height growth of almost all trees were higher with thinning than in the non-thinned plot. Therefore, thinning of C. torulosa trees at this site (at least 30% or higher) is highly recommended, before any further enrichment planting is implemented. However, although thinning would very likely improve the performance of planted framework tree species at this site, their performance would most likely remain low compared with other FORRU-CMU field trials in more open areas.
Detailed results are available in the project reports via the download panel.
Cupressus torulosa D.Don (Cupressaceae), an exotic conifer, was planted over Doi Pui summit about 50 years ago - restoring native forest to such a dense, dark forest plantation is challenging - but thinning can help.