This dataset contains data from a study considering the relationship between decay class, wood density and C content of CWD in old-growth mixed monsoon montane tropical forests in Vietnam based on the inventory of 359 CWD pieces.

The CWD inventories were conducted on forty one (3-4 per site) 50 m long and 4 m wide transects. The first dataset for contains site description, plot and transect numbers with geographical coordinates. All CWD pieces of more than 6 cm in diameter were inventoried. The dataset contains position of CWD: stumps, standing dead trees (snags), lying logs, leaning logs and branches, diameter, proportion consumed by termites, decay class, bulk density and moisture of wood and mass loss. The second dataset contains the data on the wood density of tree species present in the study area according to the global database compiled by Zanne et al. (2009).

The study was carried out in 2018-2019 in November-December in Central Highlands in the Bi Doup - Nui Ba (Vườn quốc gia Bi Doup - Núi Bà) national park (N12° 10.885' - 11.235'; E108° 40.469' - 41.406').

CWD inventories were conducted on forty one (3-4 per site) 50 m long and 4 m wide transects. All CWD pieces of more than 6 cm in diameter were inventoried. They were distributed by position as follows: stumps and standing dead trees (snags) (N=21), lying logs (N = 109), leaning logs (N = 72) and branches (N = 42). In total, 359 CWD pieces of different position were sampled.

We assigned each sampled CWD individual to a decay class using five decay classes that span a spectrum from fresh mortality to nearly complete decay with collapsed bole shape.

Density sampling for CWD wood was performed using a hand saw and a knife at 1-3 locations per CWD piece, including the sides of logs. The type of rot (white vs. brown) was identified, when possible, based on visual characteristics, mainly color and structure (cubes vs. fibers), of residual material. An adjustment for volume losses during decomposition was made; the initial shape of highly decomposed samples was reconstructed. Small samples of regular shape up to 100 cm³ in size were taken. Their length, width and thickness were measured in the field. In 2019, 248 samples were weighed in order to determine relative wood moisture by decay classes. The samples were collected during the dry season, i.e. in relatively dry conditions.

In the laboratory, the samples were oven-dried at 103°C for 48 h and weighed. The bulk density of a wood sample (ρ, g cm^-3) was calculated by dividing the dry mass by the fresh volume of a sample. The total bulk density of a CWD item was calculated taking into account the proportion of wood consumed by invertebrates (void). The mass loss was calculated based on the loss of total density, i.e. it included all decomposition processes: respiration, consumption by invertebrates, fragmentation, leaching etc.

The initial bulk density of wood was estimated for the tree species (genera) dominating in the studied forest stands based on the global wood density database (Zanne et al. 2009) for tropical South-East Asia. The tree community-level mean density was calculated as a weighted mean.

Chemical analysis of 259 wood samples was conducted in the Analytical Laboratory of Forest Research Institute KarRC RAS. In the laboratory, the samples were liophilically dried at -30 -40°С. Mechanical trituration of the samples by laboratory mill using liquid nitrogen was used to fix tissues. The C mass concentrations were measured for freeze-dried material (1.5-2.5 mg) using a Perkin Elmer 2400 Series II CHNS/O Analyzer (USA), calibrated with the organic analytical standard acetanilide (Perkin-Elmer №0204-1121).