Phosphorus and rock outcrops shape hidden diversity in China’s steep karst rainforests

On the razor-edged limestone hills of the Nonggang National Nature Reserve in Guangxi, southwestern China, a single walk of a few hundred meters crosses a gradient that many ecosystems would need kilometers to express. Closed, funnel-shaped depressions sit at the feet of clustered peaks, and from the flat depression floor to the upper slope the terrain transitions from deep, fertile soil to bare rock, thin soils, and intense solar radiation over remarkably short horizontal distances. A new study published in Nature Conservation has now mapped how plant diversity is distributed across this steep topographic mosaic, and the results carry direct implications for where conservation resources should be concentrated in one of the world’s recognized biodiversity hotspots.

The research focused on the tropical karst seasonal rainforests, or TKSR, of the Nonggang reserve, a 10,080-hectare protected area characterized by typical karst peak-cluster depression landforms. Karst landscapes, sculpted by rainfall and groundwater dissolving carbonate bedrock, cover roughly 15 to 20 percent of Earth’s ice-free land surface, with the largest continuous expanse of about 0.51 million square kilometers spanning southwestern China. These regions are exceptionally vulnerable to disturbance, yet they harbor extraordinary numbers of endemic species. The Nonggang reserve alone contains 1,752 recorded vascular plant species across 810 genera and 184 families, including 33 rare and endangered species and 278 species endemic to karst habitats.

To capture the full vertical structure of the forest, the team established 40 plots of 20 by 20 meters at four slope positions: depression, lower slope, middle slope, and upper slope. Within each plot, nested subplots of 5 by 5 meters and 1 by 1 meters were used to survey the shrub and herb layers respectively, while all trees were measured for diameter at breast height and height. Species identifications were verified by two plant taxonomy experts. Soil samples collected at depths of 0 to 20 centimeters were analyzed for pH, exchangeable calcium and magnesium, available and total phosphorus, available and total potassium, ammonium and nitrate nitrogen, organic matter, total nitrogen, and surface water content. Rock outcrop rate, slope aspect, and slope degree were also recorded for each plot.

The analysis combined classical species diversity indices, including species richness, the Simpson index, the Shannon-Wiener index, and the Pielou index, with phylogenetic metrics that account for the evolutionary history shared among coexisting species. A phylogenetic tree of all recorded vascular plants was constructed using the V.PhyloMaker package, integrating seed plant branches from GBOTB and fern branches from an expanded phylogenetic system. The Faith Diversity Index quantified phylogenetic diversity, while the net relatedness index and nearest taxon index, computed against null models with 999 randomizations, revealed whether species within communities were more closely or more distantly related than expected by chance.

The most striking finding was an inverted U-shaped pattern in species richness across the herb, shrub, and tree layers. Diversity was lowest on the middle slope and highest at both the depression and the upper slope. In the herb layer, species richness on the middle slope was significantly lower than on the lower and upper slopes, and the Pielou index of evenness increased steadily with slope position. Phylogenetic diversity followed the same trajectory, initially decreasing and then increasing with slope position, with significantly higher values on the lower and upper slopes than on the middle slope in the herb layer. Across all three layers, phylogenetic diversity was highest in the tree layer, followed by the herb and shrub layers.

Phylogenetic structure told a different story depending on the forest stratum. The herb and tree layers exhibited phylogenetic clustering, meaning species within these communities were more closely related than expected under a random null model, a pattern consistent with habitat filtering as the dominant assembly process. In contrast, the shrub layer showed phylogenetic divergence, indicating that competitive exclusion, rather than environmental filtering, shaped which shrub species coexisted. This divergence may be linked to the relatively small sampling area of the shrub layer, where restricted space and resources intensify competition among species occupying different niches. The authors note that as spatial scale increases, phylogenetic structures in tropical forests typically shift from divergent to aggregated, a trend documented in other tropical rainforest systems.

Stepwise multiple regression, guided by the Akaike Information Criterion, identified the environmental factors behind these patterns. Soil total phosphorus and soil available phosphorus emerged as significant determinants of species diversity across all three layers, while rock outcrop rate significantly affected phylogenetic structure. In the shrub and tree layers, phosphorus was also a key driver of phylogenetic diversity. Notably, the study found that potassium, alongside phosphorus, may replace nitrogen as a primary limiting nutrient in this karst environment, aligning with previous work in subtropical Guangxi and highlighting the biochemical uniqueness of karst ecosystems. Basal area of woody species negatively affected species diversity in the shrub and tree layers, suggesting that large-diameter trees suppress smaller plants through negative interactions at fine scales.

The harsh conditions of the middle slope, with water scarcity, thin soil, and strong solar radiation, appear to prevent any single species from becoming dominant, promoting higher species diversity on the upper slope. Meanwhile, the depression, with deep and fertile soils, supports communities with longer evolutionary histories and greater evolutionary diversity. Sunny slopes with favorable light conditions promoted greater species richness and higher phylogenetic diversity than shaded slopes in the herb and shrub layers, likely because plants on sunny aspects can reproduce more rapidly and diversify to occupy suitable niches.

The conservation implications are significant. The study identifies depressions and upper slopes as priority areas for protection, as these positions harbor both the highest species richness and the greatest phylogenetic diversity. Karst depressions, with their deep, fertile soils, are especially susceptible to conversion into agricultural and plantation lands, which has already caused substantial natural forest loss in southwestern China. Because communities with higher phylogenetic diversity possess greater evolutionary potential and a stronger capacity to cope with risks associated with global change, protecting these areas safeguards not only current species richness but also the evolutionary legacy embedded in the forest’s lineages. The authors argue that conservation strategies should incorporate phylogenetic diversity into assessment frameworks alongside traditional species metrics, and that areas with high and unique phylogenetic diversity should be prioritized in reserve planning.

This research provides a template for understanding how environmental heterogeneity shapes biodiversity in steep, topographically complex landscapes. By demonstrating that deterministic processes, specifically habitat filtering and competitive exclusion, dominate community assembly in the TKSR, while some communities also show influence from neutral processes, the study refines the ecological theory underlying karst forest management. As karst regions across Asia face mounting pressure from land-use change and climate shifts, the fine-grained mapping of diversity across slope gradients offers a practical tool for targeting conservation where it matters most, ensuring that the evolutionary treasures hidden within these limestone forests are not lost before they are fully understood.

Subject of Research: Ecology and Conservation

Article Title: Changes in species and phylogenetic diversity in tropical seasonal rainforests on steep karst hillslopes in southwestern China: implications for conservation

Article References: Pang, Q., Hu, C., Xu, C., Zhong, C., Zhang, Z., & Hu, G. (2025). Changes in species and phylogenetic diversity in tropical seasonal rainforests on steep karst hillslopes in southwestern China: implications for conservation. Nature Conservation, 60, 73-97. https://doi.org/10.3897/natureconservation.60.162595

Image Credits: AI Generated

DOI: 10.3897/natureconservation.60.162595

Keywords: karst, phylogenetic diversity, tropical seasonal rainforest, biodiversity hotspot, habitat filtering, soil phosphorus, Nonggang National Nature Reserve, conservation priority, community assembly, southwestern China, Changes, species