Old-growth mixed dipterocarp forests show variable losses and gains in aboveground biomass and standing carbon over forty years

Background No studies have documented long-term trends in aboveground biomass (AGB) for mixed-dipterocarp forests (MDF), the dominant rain forest type in tropical wet equatorial Asia. In our study, we sought to document such trends over forty years across three sites representing lowland to lower montane elevations.

Methods To do this, we established fifty 100 ​m ​× ​25 ​m plots in 1978 across three sites sampled along an elevation gradient, identified as mature old-growth forest. We measured trees for diameter at breast height that we identified to species and tagged. We took wood samples to calculate species wood-specific gravity. We re-measured plots in 1998 and again in 2018.

Results We show standing AGB for all sites combined to be 517.52 ​Mg·ha⁻¹ in 1978, but this declined by 17% over 40 years to 430.11 ​Mg·ha⁻¹. No differences exist among sites in AGB primarily because of considerable within site variation; but interactions of time with site show declines across sites were not uniform, one remained about the same. Relatively few species represented a high proportion of the AGB with the top five species comprising between 34% and 65%, depending upon site and year sampled. One species, Mesua nagassarium, represented a disproportionately large amount of AGB and decline over time, particularly at the low elevation site.

Conclusions Our results are directly relevant to estimating AGB and standing carbon sequestered in MDF. Our study is the first to demonstrate varying but overall, declining trends in amounts of AGB among forests making predictions of biomass and standing carbon in MDF difficult over wide regions.