Tall Trees, Rough Bark: What a Critically Endangered Indonesian Climber Demands of Its Hosts

Deep in the montane forests of East Java, Indonesia, a critically endangered climbing plant is quietly revealing the rules it follows when choosing where to live. Smilax nageliana, a spiny climber in the family Smilacaceae, is found nowhere else on Earth, and only two natural populations are known: one within Bromo Tengger Semeru National Park at Ranu Darungan, and another on Mount Kawi. Despite its precarious status, the ecological relationships that tie this species to the trees around it have remained almost entirely unexplored. A new study published in Plant Biosystems by researchers from Indonesia’s National Research and Innovation Agency (BRIN) has now provided the first systematic look at which trees S. nageliana climbs, which traits of those trees matter, and what those preferences mean for saving the species from extinction.

The research team surveyed 148 purposively established plots covering a combined area of 2.9104 hectares across the two known populations. Within each plot, they recorded every climbing individual of S. nageliana along with its host tree, measuring host attributes such as species identity, height, diameter, and basal area, and noting where on the host stem the climber anchored itself. They also documented the surrounding vegetation at two strata, sapling-to-tree and shrub-to-seedling, to understand the broader community context in which the climber persists. The survey revealed that S. nageliana is far from indiscriminate in its climbing habits: seventeen host species belonging to seven families were recorded, with the Myrtaceae, at 17.65 percent of host species, and the Lauraceae, at 11.76 percent, emerging as the most species-rich host families.

To move beyond simple observation, the researchers applied a suite of statistical tools. Logistic regression and correlation analyses were used to test which host attributes best predicted the presence and growth of the climber, while Principal Coordinates Analysis (PCoA) and PERMANOVA based on Bray–Curtis dissimilarity quantified differences in vegetation composition between the two sites. Indicator species analysis then pinpointed which plant species significantly distinguished one locality from the other. This combination of approaches allowed the team to separate genuine host preference from the mere availability of particular trees in the landscape, a distinction that is critical for interpreting climbing plant ecology.

The standout result was unambiguous: host tree height was the most influential attribute governing S. nageliana growth. The correlation coefficient between host height and climber growth was 0.387, and the logistic regression yielded a coefficient of 0.051 with an odds ratio of 1.053 (95 percent confidence interval: 1.002 to 1.124; P = 0.074). Host height accounted for 7.4 percent of the variation in climber growth (R² = 0.074; P = 0.001). In practical terms, every additional increment of host height modestly but consistently increased the odds that the climber would thrive. By contrast, host diameter and basal area showed no significant effects, suggesting that the sheer bulk of a tree matters less to this climber than how far its canopy reaches toward the light.

Field observations added two further layers of preference. The vast majority of climbing individuals, 68.4 percent, selected rough-barked hosts rather than smooth-barked ones, a pattern consistent with the mechanical demands of attachment. Climbing plants that grip their supports rely on friction and anchorage, and bark texture can make the difference between a secure hold and a fatal slip. Additionally, 69.2 percent of the climbers occupied the upper stem zone of their hosts, positioning themselves high on the trunk where they can access brighter light and potentially reach the canopy more quickly. Together, these patterns paint a picture of a climber that seeks tall, structurally reliable supports with surfaces it can grip effectively.

The community-level analyses revealed that the two populations inhabit strikingly different vegetation contexts. PCoA and PERMANOVA detected significant differences in community composition between Ranu Darungan and Mount Kawi at both strata: for the sapling-tree layer, F = 7.337 with p < 0.001, and for the shrubs-seedlings layer, F = 6.387 with p < 0.001. Indicator species analysis identified eight sapling-tree species and eighteen shrubs-seedlings species as significant discriminators between the sites. This means that the two last strongholds of S. nageliana are not merely two patches of the same forest; they host distinct plant communities, and the climber persists within both despite these ecological differences. That flexibility at the community level, combined with clear preferences at the level of individual host traits, is an encouraging sign for conservation planning.

The findings carry direct implications for how this species should be protected. Because host height is the key driver of climber growth, conservation strategies that preserve tall, mature trees are likely to be far more effective than those focused on tree species identity alone. The authors argue that conservation efforts should prioritise native forest restoration and the protection of key host trees to secure the long-term survival of the climber. In fragmented Indonesian landscapes, where old-growth structure is often lost to logging and land conversion, the loss of tall host trees could quietly undermine even legally protected populations of the species. Restoration plantings that aim to support S. nageliana would need to include fast-growing native species capable of reaching substantial heights, and ideally species with rough bark that facilitates attachment.

The study also situates S. nageliana within the broader science of liana and climbing plant ecology. Climbers are increasingly recognised as influential players in tropical forests: they can suppress tree regeneration, compete aggressively for light, and alter forest carbon dynamics. Yet most research on host preference has focused on widespread lianas in the Neotropics and Southeast Asia, leaving narrow endemics like S. nageliana almost entirely unstudied. Understanding what a rare climber needs from its hosts adds a conservation dimension to this literature, because a species restricted to two sites cannot afford to be picky about resources that are disappearing. The genus Smilax itself is globally significant, valued in traditional medicine and increasingly studied at the genomic level, which makes the fate of this particular endemic species of interest well beyond Java.

For now, S. nageliana remains on the edge, clinging, quite literally, to the tallest and roughest-barked trees its shrinking forests can offer. The new research transforms that image from a poetic one into a practical checklist: protect tall native trees, maintain rough-barked host species, restore forest structure at both sites, and monitor the distinct plant communities at Ranu Darungan and Mount Kawi separately. As the authors conclude, vertical forest structure is not just a backdrop to this climber’s life but a key structural attribute determining its suitability as habitat. In a world where forest canopies are being lowered and simplified at pace, the humble preference of a rare Javan climber for tall trees may prove to be one of the clearest warnings yet about what is lost when forests lose their height.

Subject of Research: Host tree preference and conservation ecology of the critically endangered endemic climber Smilax nageliana in East Java, Indonesia

Article Title: Host preference in the endemic and threatened climber Smilax nageliana

Article References: Rinandio, D. S., Sofiah, S., Robiansyah, I., Sulistyaningsih, L. D., Metusala, D., Trimanto, Yulia, N. D., & Setyorini, D. (2026). Host preference in the endemic and threatened climber Smilax nageliana. Plant Biosystems, 160(4), Article 232. https://doi.org/10.1007/s44473-026-00227-0

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00227-0

Keywords: Smilax nageliana, host preference, lianas, climbing plants, critically endangered, endemic species, East Java, forest ecology, conservation, host tree height, bark texture, Plant Biosystems