Notes on Dipterocarps. {No. 3} The seedling of Shorea robusta, Roxb., and the conditions under which it grows into pure forests
I.H. Burkill, writing in 1918 from his position as a dipterocarp specialist at the Singapore Botanic Gardens, examines the seedling biology and forest ecology of Shorea robusta (Sal) in India to explain how this species produces pure forests—a phenomenon largely absent among its Malayan congeners. The article argues that the success of Sal is rooted in a distinctive seedling morphology that aligns it more closely with the genus Dipterocarpus than with Malayan Shorea species, combined with a physiological adaptation to deep, well-drained soils of the Himalayan foothills.
Summary
Burkill’s central concern is taxonomic and ecological: understanding why certain dipterocarps form monodominant stands while others do not. He traces the seedling of S. robusta from germination through establishment, noting that the radicle is thrust into the soil by elongation of the cotyledon stalks while the cotyledons themselves remain loosely apposed and non-photosynthetic. This mode of germination diverges sharply from the Malayan Shorea species Burkill had previously described (notably S. leprosula) and instead parallels the behaviour of Dipterocarpus, in which the cotyledons remain imprisoned within the fruit wall and are depleted through their stalks as the hypocotyl elongates. He cites Heim’s 1890 anatomical study, which placed S. robusta in the section Eu-Shorea and noted its vascular bundle distribution and resin canal count as linking it to Dipterocarpus.
The ecological discussion turns to the role of forest fires, a subject of considerable debate among Indian foresters at the time. Burkill reviews the arguments of Brandis, Troup, Haines, and others that fires clear leaf litter, destroy fungal pathogens, and damage competing vegetation, thereby facilitating Sal establishment. He ultimately rejects the notion that fire is the agent maintaining pure Sal forests, noting that over ninety-nine per cent of falling seeds fail for want of room even in the most favourable conditions, and that every experienced Indian forester advocates fire protection as a principle. He instead locates the explanation in the nature of the soil itself—deep, open, rapidly developed by Himalayan rivers—where Sal possesses an as-yet unanalysed physiological advantage.
A further distinguishing feature is the remarkable capacity of Sal seedlings to regenerate a lost primary stem repeatedly from the axils of the cotyledons, a resilience Burkill had not observed in Malayan species. Hole’s work is cited for evidence that this die-back is most intense during the rains and may be caused by a toxic body produced in the decomposition of leaf litter, rather than by water contact itself. Burkill notes a critical difference: Malayan Shorea seedlings die out entirely in wet periods, whereas Sal seedlings merely die back and recover—a distinction he flags as potentially connected to Sal’s greater success and as demanding further investigation.
Key Findings
- S. robusta seeds contain approximately 60 per cent starch on dry weight and 8 per cent tannins, the latter rendering them toxic to humans (causing indigestion, constipation, and ultimately death) while serving as a partial protection against animal predation (p. 41).
- A good seed crop is yielded only about once in three years, despite annual flowering in March or April; McIntyre (1909) attributed annual failures to unfavourable weather, a point Burkill flags as requiring investigation (p. 40).
- Hole demonstrated that Sal seedlings grow healthily under artificial shade reducing light to 0.015, confirming the species’ capacity to tolerate the deep shade of its own pure forests (p. 42).
- Pure dipterocarp forests are produced by four species in four distinct geographic regions: S. robusta (rim of Bengal plains), S. assamica (Upper Assam), Dipterocarpus (Burma, Siam, Indo-China), and Dryobalanops (Sumatra, Borneo, Malay Peninsula)—each in its own region, not in competition (p. 42).
- Hole found that mixing Sal leaves with soil increases seedling die-back, suggesting a toxic body produced during decomposition; death can be reproduced in pots without neighbouring plants, ruling out allelopathic excretion (p. 43).
- Over ninety-nine per cent of Sal seeds that fall to the ground fail to establish, even under the most favourable conditions of deep, well-drained soil (p. 43).
Conclusion
Burkill’s definitive takeaway is that the formation of pure dipterocarp forests is not a product of human fire management but of intrinsic physiological and morphological adaptations to specific soil conditions. The seedling of S. robusta, with its Dipterocarpus-like germination strategy and its extraordinary capacity for stem regeneration, represents a distinct evolutionary solution that Malayan Shorea species have not replicated. This has direct implications for the classification of the order, as the Indian species S. robusta and S. obtusa stand apart from their Malayan relatives in both ecological performance and seedling anatomy.
Context
- Burkill was the leading dipterocarp taxonomist of his era, based at the Singapore Botanic Gardens, and this article is the third in a series of “Notes on Dipterocarps” published in the Journal of the Straits Branch of the Royal Asiatic Society, reflecting the colonial botanical infrastructure of the Straits Settlements.
- The article draws heavily on the Indian forestry literature of the period (Brandis, Troup, Haines, Hole, McIntyre), situating Burkill’s work within the broader Anglo-Indian forest administration’s interest in the ecology and management of Sal, the most commercially important timber species of northern India.