A research team led by Prof. CHEN Hongsong from the Institute of Subtropical Agriculture of the Chinese Academy of Sciences has revealed that lithology-driven soil properties play a critical role in shaping latitudinal variation in forest species composition in southwestern China.

This study was published in CATENA on December 3.

Beyond species richness, differences in species composition among communities provide critical insights into the mechanisms that maintain plant diversity. Traditionally, latitudinal variation in species composition has been largely attributed to climate gradients, particularly changes in temperature and precipitation that shape environmental conditions. However, growing evidence suggests that lithology, through its influence on soil environments, may also play an important role at broad spatial scales.

Along a subtropical forest transect spanning approximately 10 degrees of latitude in southwestern China, a series of coordinated studies have been conducted to disentangle the roles of climate and geology in shaping plant diversity patterns. Previous work along this transect revealed pronounced differences in the latitudinal patterns of α diversity between karst and non-karst forests, with species richness in karst forests responding to latitude in a markedly different way from that in non-karst forests. Complementary studies of soil environments further demonstrated systematic contrasts in equivalent soil depth between karst and non-karst forests, with karst forests characterized by shallower and more spatially heterogeneous soils. In addition, investigations at the hillslope scale showed that shallow epikarst water plays a critical role in supporting the growth and persistence of plants inhabiting rocky karst habitats. Together, these cross-scale findings indicate that lithology, by reshaping soil environments, may exert a strong influence on regional biodiversity patterns beyond the effects of climate alone.

In the present study, the researchers focused on latitudinal variation in species composition between forest communities. They established 60 forest plots across six latitudinal sites along the transect, encompassing paired karst and non-karst forests. Species compositional dissimilarity was quantified using β diversity metrics, including total β diversity (β_total), species turnover component reflecting species replacement (β_turnover), and species nestedness component associated with species loss (β_nestedness). Analyses were conducted separately for karst forests, non-karst forests, and the entire transect without distinguishing lithology, allowing a comparative assessment of the compositional variation under the two contrasting lithologies.

The results showed pronounced differences in species composition between karst and non-karst forests. When karst and non-karst forests were considered together, β_total and β_turnover across the entire transect were significantly higher than those observed within either karst or non-karst forest alone, while β_nestedness was significantly lower. These patterns indicate that lithological heterogeneity substantially amplifies inter community compositional differences at the regional scale. 

Further analyses revealed that latitudinal patterns of β diversity in southwestern China arise from the combined dynamics of karst and non-karst forest subsystems rather than from a single dominant process. Both β_total and β_turnover declined significantly with increasing latitude along the transect, a pattern largely driven by karst forests and closely associated with latitudinal changes in soil physicochemical heterogeneity linked to karstification processes. In contrast, β_nestedness increased with latitude, a trend primarily attributable to non-karst forests and consistent with stronger climate-driven environmental filtering at higher latitudes. These contrasting responses highlight fundamentally different mechanisms governing species compositional change in forests developed on distinct lithologies.

"This study demonstrates that lithology plays a pivotal role in shaping large scale patterns of forest species composition by mediating soil properties and habitat heterogeneity," said Prof. CHEN Hongsong, corresponding author of the study. "By integrating perspectives from geology, soil science, and community ecology, the findings advance our understanding of β diversity maintenance and underscore the importance of considering lithological context in biodiversity assessments and conservation strategies, particularly in geologically complex regions such as southwestern China."