Synchronized flowering events, known as general flowering, are a remarkable characteristic of Southeast Asian humid forests. These irregular, multi-year mass reproductive events involve numerous plant species across many families, most notably the Dipterocarpaceae family. While proximate cues for general flowering have been proposed—including drought linked to El Niño, cloud-free conditions, high solar radiation, and nighttime temperature drops—the impact of climate change on these events remains poorly understood. This lack of understanding stems partly from the limited availability of long-term phenological data and predictive models that accurately capture the relationship between climatic factors and reproductive phenology in tropical regions. This study addresses this gap by analyzing long-term reproductive phenology data in conjunction with meteorological data to assess past and future phenological trends in Southeast Asia, focusing specifically on the implications of climate change on community-wide flowering.

Previous research has highlighted the importance of various factors in triggering general flowering. Studies suggest that drought, associated with the El Niño Southern Oscillation, plays a significant role. Other contributing factors include cloud-free conditions, increased solar radiation, and a decrease in nighttime minimum temperature. Endogenous factors such as nutrient availability, particularly phosphorus, have also been implicated. However, for general flowering events with intervals exceeding two years, external environmental factors are more likely the primary drivers due to the relatively quick recovery of nutrient levels after heavy fruiting. Recent work emphasizes the synergistic effect of cool temperatures and drought in initiating floral induction in dipterocarp trees. This existing research provides a foundation for investigating how climate change, characterized by rising temperatures and variable rainfall in Southeast Asia, may affect these crucial reproductive events and the long-term health of the rainforest ecosystems.

The researchers analyzed historical reproductive phenology records spanning 35 years (April 1976 to September 2010) from the *Bulletin Fenologi Biji Benih dan Anak Benih*. Data were collected from the Forest Research Institute Malaysia (FRIM) arboretum, encompassing 210 species from 41 families. Strict data quality criteria were applied to ensure accuracy (e.g., ≤50% missing data, stable flowering period ≤12 months, consistent flowering/fruiting frequencies). Dipterocarpaceae was the most abundant family (45%). The fraction of flowering and fruiting species fluctuated significantly yearly, with mass flowering events occurring six times over the study period. These events correlated with general flowering events observed in natural forests. Dipterocarpaceae species showed significantly higher synchrony in flowering and fruiting compared to non-dipterocarp species. A decline in the proportion of flowering and fruiting species was observed from the mid-1970s to the early 2000s, coinciding with increased temperatures and precipitation. A phenology model, previously developed and validated for Dipterocarpaceae, was used to assess the relationship between flowering and both temperature and precipitation. The model incorporates cool units (CU) and drought units (DU), representing the accumulation of low-temperature and drought cues, respectively. Time-series clustering identified 10 phenological clusters (reduced to 6 after removing those with <5 species). Logistic regression using DU and CU × DU was performed to determine which model best explained flowering phenology for each cluster. Finally, future flowering phenology was projected under two climate change scenarios (RCP2.6 and RCP8.5) using data from three general circulation models (GCMs: GFDL-ESM2M, IPSL-CM5A-LR, and MIROC5). Bias correction was applied to the GCM data to align it with observed weather data at FRIM.

The proportion of flowering and fruiting species decreased significantly from the mid-1970s to the early 2000s, as indicated by the Mann-Kendall test (P=0.0021 for flowering, P<0.0001 for fruiting). This decline occurred concurrently with a rise in temperature (0.39 ± 0.02 °C per decade) and precipitation (0.51 ± 0.26 mm/day per decade). Analysis revealed clear reproductive seasonality for most species, with two flowering peaks (April and October) and corresponding fruiting peaks. Moraceae was the only family displaying year-round flowering and fruiting. The phenology model indicated that the flowering phenology of clusters 3 and 4 (major clusters comprising Dipterocarpaceae species) was best explained by the interaction of cool temperature and drought cues (CU × DU model). Other clusters responded primarily to drought cues (DU model). The model demonstrated acceptable discrimination ability (AUC values ranging from 0.62 to 0.79). Projections under future climate scenarios showed that the predicted flowering probabilities in clusters 3 and 4 decreased significantly under both RCP2.6 (to 57% and 49%, respectively) and RCP8.5 (to 37% and 28%, respectively) compared to the 1976-1996 period. This reduction is primarily attributable to the projected decline in low-temperature cues. In contrast, species sensitive only to drought cues showed no significant change in flowering probability. These predictions were validated across four regions in Southeast Asia (FRIM, Trang Province in Thailand, Lambir Hills National Park in Borneo, and Central Kalimantan in Indonesia), confirming the robustness of the findings. While the overall flowering probability was affected, the seasonal patterns remained consistent.

The study's findings reveal that climate change significantly impacts the reproductive phenology of tropical rainforests in Southeast Asia. The decline in flowering and fruiting observed in the past and the model's projections of further reduction under future climate scenarios highlight the vulnerability of these ecosystems. The differential responses of species to temperature and drought cues indicate that climate change will likely alter the composition and regeneration of these forests. The observed sensitivity of dipterocarp species to low-temperature cues suggests that tropical species may be more susceptible to climate change than those in temperate ecosystems. The relatively unchanged seasonal patterns of flowering, despite shifts in overall probability, represent a notable difference between tropical and temperate plant responses. This study underscores the importance of considering species-specific responses and the interplay between various environmental factors when assessing climate change's impacts on tropical ecosystems. Further research with longer-term and higher temporal resolution data is needed to refine the model and improve the accuracy of predictions at the species level.

This study provides compelling evidence that climate change, particularly the projected increase in temperature, negatively impacts the reproductive phenology of tropical rainforests in Southeast Asia. The decrease in low-temperature cues, crucial for flowering in many dipterocarp species, is a key driver of this impact. Species-specific responses to climate variables emphasize the complexity of these ecological interactions and highlight the need for further research to fully understand and mitigate the effects of climate change on biodiversity in these vital ecosystems. Future research should focus on extending the temporal and spatial scales of phenological monitoring and incorporate additional climate variables to enhance model accuracy and generalizability.

While the study uses one of the longest datasets available for tropical reproductive phenology, the relatively short time series in terms of climatic change may introduce uncertainty in predicting long-term impacts. The focus on a single arboretum, albeit with a diverse set of species, might not fully capture the diversity of responses across different rainforest habitats. Furthermore, the model's accuracy relies on the availability and quality of climatic data; inaccuracies or biases in the input data may affect the predictions. Additional factors influencing flowering and fruiting, beyond temperature and precipitation (e.g., nutrient availability, herbivory, and interactions with other species), were not explicitly incorporated into the model. Ultimately, this study necessitates continuing long-term phenological monitoring across various tropical forest sites to strengthen our understanding of the impacts of climate change on these sensitive ecosystems.