Evaluation of the Impact of an El Niño Drought on the Dynamics of Tropical Forests at Different Altitudes and of Different Forest Types

Forest Research Centre, Sabah Forestry Department, Malaysia

FY 2017-2019 A Grant for Environmental Research Projects from The Sumitomo Foundation

MIYAMOTO, Kazuki (Department of Forest Vegetation)

Previous studies have predicted that tree mortality in tropical forests is increasing due to the increased frequency of extreme droughts. Supporting this, increased mortality and decreased growth of trees as a result of drought have been reported in the lowland forests of Borneo. However, studies in tropical montane forests and heath forests (a forest type that occurs on nutrient-poor and acidic sandy soil known as podzol) have shown that tree mortality and growth depend on the elevation and substrata or exhibit no clear differences pre- and post-drought. Thus, the effect of drought on tropical forests is considered to vary with elevation and forest type.

The aim of this research was to assess the impact of the drought that was caused by the 2015–2016 El Niño on the growth and mortality of tropical forest trees growing at different elevations and in different forest types that are associated with particular soil conditions. To achieve this, we obtained tree community dynamics data from tropical forests in Borneo and compared these between the pre- and post-drought periods.

We conducted field surveys in tropical heath forests and mixed dipterocarp forests in Sabah, Borneo. We established three 50 m × 50 m plots at a low-elevation site [ca. 500 m above sea level (a.s.l.)] and four 50 m × 50 m plots at a high-elevation site (ca. 1000 m a.s.l.). We then carried out tree size measurement and species identification for all trees in the plots that were ≥15 cm in girth at breast height (ca. ≥5 cm diameter at breast height) in 2013, and conducted re-censuses and tree mortality surveys in 2015 and 2018.

Tree mortality during the El Niño drought period was higher than during the predrought period, indicating the effects of drought on trees. Specifically, mortality rate tended to be higher in forests at lower elevations than those at higher elevations.

Most forest reserves in Borneo are located at low altitudes (below several hundred meters) and are susceptible to the effects of climate change, such as rising temperatures and dryness; therefore, it has been predicted that they will become less suitable for wildlife in the future. Forest reserves in high-altitude areas are expected to function as refuges for wildlife because they offer cooler environments and are less susceptible to the effects of climate change. By sharing this study with the relevant organizations in Sabah, we believe that the findings can be used to formulate forest management plans, such as the establishment of local forest reserves and corridors linking forest reserves.

Kazuki Miyamoto, Shin-ichiro Aiba, Reuben Nilus (2019) Effects of drought on growth and survival of trees under different elevations and soil environments in Borneo. Abstract of the 29th Annual Meeting of the Japan Society of Tropical Ecology (JASTE29), 28.

Kazuki Miyamoto, Shin-ichiro Aiba, Reuben Nilus (2020) The 2015–2016 El Niño-induced drought effects on aboveground biomass under different elevations and soil environments in Borneo. Abstract of the 30th Annual Meeting of the Japan Society of Tropical Ecology (JASTE30) (Online International Conference), P05.

Miyamoto, K., Aiba, S., Aoyagi, R. Nilus, R. (2021) Effects of El Niño drought on tree mortality and growth across forest types at different elevations in Borneo. Forest Ecology and Management 490: 119096, DOI：10.1016/j.foreco.2021.119096