Although lowland tree species in the ever-wet regions of Southeast Asia are characterised by the supra-annual cycle of reproduction, the reproductive phenology of montane tree species remains poorly understood. In this study, we investigated the reproductive phenology of montane tree species using litter samples that were collected every two weeks from six rainforest sites, consisting of three elevations (1700, 2700, and 3100 m), on Mount Kinabalu, Borneo. At each elevation, one site was on infertile ultrabasic soil and one was on relatively fertile non-ultrabasic soil. We used a composite sample from 10 or 20 litter traps per site and sorted it by species. Therefore, the obtained data captured reproductive phenology in the population of each species rather than in an individual tree. Ten-year time series of flower and fruit litterfall were obtained for 30 and 39 tree species, respectively. Fourier analysis was used to identify the dominant cycle of each time series. The most abundant cycle across species was supra-annual, followed by sub-annual, and annual cycles. Many species at higher elevations showed supra-annual cycles of flower litterfall, whereas species in the 1700 m sites often showed annual or sub-annual cycles regardless of soil type. No systematic differences were found among sites for fruit litterfall. Mechanisms underlying these elevational patterns in the reproductive cycle remain unclear but may include more severe El Niño droughts, lower primary productivity, lower soil fertility, and the absence of some sub-annually or annually reproducing families at higher elevations.

Litter samples were collected every two weeks between February 1996 and March 2006 by earlier studies (Kitayama et al., 2015, 2021). Twenty, ten, and ten litter traps (0.5 m² area) were placed at 10 m intervals at 1700, 2700, and 3100 m sites, respectively. After drying at 70°C, these samples were combined per sampling date. Therefore, only a single composite sample was obtained per site at each sampling date. Reproductive litter sample was sorted by species and weighed after being grouped into two categories: flower and fruit. Flowers included inflorescences with flower buds, petals, stigmas, and stamens. The remaining reproductive organs, with immature and mature fruit, were categorised as fruit. Regarding conifers, male cones always had pollen scales, whereas the developmental stages of seed cones were not easily identifiable. Male-cone and seed-cone litter were grouped with flower and fruit litter, respectively. In the dataset, these groups will be referred to as ‘flower/male-cone’ and ‘fruit/seed-cone’ litters, respectively. Exceptionally, the flowers of Lithocarpus and Quercus at 1700 m non-ultrabasic site and the flowers and fruits of Syzygium at 1700 m sites were sorted by genus. In the dataset, the unit of litterfall data is dry mass (g) per trap. The area of each trap was 0.5 m². Dates in the first row correspond to sampling dates.