Dominant Yet Vulnerable: Dipterocarps in Southeast Asia’s Changing Forests

Trees from the Dipterocarpaceae family, commonly known as dipterocarps, dominate the forest canopies of Southeast Asia, from Borneo to Sri Lanka and India, up through southern China, and down through Indonesia to Papua.

With more than 680 species identified globally, dipterocarps represent one of the most diverse and ecologically important tree families in the region. Yet despite the huge number of species within this family, it is unusual for a single family to be so dominant, with dipterocarps accounting for up to 80 percent of the canopy in many of these forests. In tropical ecosystems, where diversity is often spread across many plant families, this kind of canopy dominance by one family stands out.

This dichotomy of diversity (forest canopies dominated by a single highly diverse family) is of great ecological interest and raises important questions for conservation and regeneration. Dipterocarps are highly valued for their timber and have been preferentially logged throughout the region. As a result, many forests that still support substantial biodiversity across other trophic levels and ecological groups have been selectively stripped of their dominant canopy species. While dipterocarp dominance may be greatly reduced in heavily logged forests, many ecological processes evolved in close association with these trees and may be disrupted by their absence.

 

There are key aspects of dipterocarp reproductive ecology that are critical for understanding how these forests regenerate and how they should be managed. Unlike many temperate species, dipterocarps do not establish a seed bank in the soil. Instead, their seeds germinate shortly after falling and form a persistent seedling bank in the understory. These seedlings can survive for years, in some cases decades, under low light conditions, waiting for a gap in the canopy that provides enough sunlight to trigger growth. If, however, logging removes the mother trees and disturbs the seedlings on the forest floor, the seedling bank may be depleted, slowing the natural regeneration cycle.

Further complicating matters, dipterocarps do not flower and fruit annually, unlike many temperate species that rely on predictable seasonal cues. Instead, dipterocarps reproductive cycles are irregular, with mass flowering and fruiting events typically occurring only every 3 to 10 years, or even longer. These events are thought to be correlated with climatic signals, including dry periods and brief temperature drops associated with El Niño Southern Oscillation (ENSO) events, though the exact mechanisms remain uncertain. With our changing climate, there is growing concern that the environmental cues triggering dipterocarp reproduction may become more erratic, reducing flowering opportunities and further disrupting regeneration across the region, especially if more severe droughts begin to hinder seedling growth and survival.

Taken together, these characteristics make the recovery of dipterocarp forests a slow, complex, and sometimes uncertain process. In some cases, more isolated or heavily disturbed sites may require active restoration. In many others, logged-over areas may still be able to regenerate naturally, especially where seedling banks and surrounding forest structure remain intact. It is crucial that these areas of intact logged-over forest are protected now, while natural regeneration is still possible.

Borneo is the center of dipterocarp diversity, with around 267 species recorded on the island, of which 162 are endemic. This exceptional concentration underscores the island’s critical role in conserving these ecologically significant trees. In the Malaysian state of Sabah, the Kuamut Rainforest Conservation Project (KRCP) is helping protect and monitor this diversity, with over 50 dipterocarp species already identified in field plots alone. Efforts like this are vital not only for protecting local biodiversity, but also for sustaining dipterocarp populations that are essential to maintaining the ecological integrity of these forests.

 

Sources

https://doi.org/10.1034/j.1600-0706.2002.970317.x [flowering 3-10 years or longer..] 

https://doi.org/10.1034/j.1600-0706.2002.970317.x [general reproductive evolution ideas] 

https://www.ias.ac.in/article/fulltext/jbsc/018/04/0457-0474 [temp drop idea, and no seedling bank] 

https://www.nature.com/articles/s42003-022-03245-8 [climate change decreased flowering] 

https://www.bgci.org/wp/wp-content/uploads/2024/01/GCC-for-Dipterocarps.pdf [680 species] 

https://www.annualreviews.org/content/journals/10.1146/annurev.es.19.110188.002023 

https://en.wikipedia.org/wiki/Borneo_lowland_rain_forests  [80% canopy] 

https://www.bgci.org/resources/bgci-tools-and-resources/the-red-list-of-bornean-endemic-dipterocarps/ [267 in borneo, 162 endemic] 

 

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