Landform and lithospheric development contribute to the assembly of mountain floras in China

Mountains globally serve as both cradles and museums of biodiversity, concentrating a significant portion of terrestrial life, especially in the tropics. China, with its diverse mountainous regions, exemplifies this phenomenon, harboring a vast array of plant and animal species. Numerous hypotheses attempt to explain this biodiversity, including climate stability, habitat heterogeneity, and energetics. However, existing explanations often fall short, particularly in accounting for pantropical diversity disparities. An integrated framework considering both ecological and geological processes is needed to predict montane biodiversity effectively. The 'mountain geobiodiversity hypothesis' (MGH) suggests a combination of mountain uplift, geodiversity evolution, and Neogene-Pleistocene climate changes as drivers. This study focuses on understanding the links between biotic processes and topographic erosion, highlighting the role of geological and lithological processes – particularly uplift and erosion – in influencing species formation, immigration, and extinction. Mountains create diverse topographic complexities and niches, facilitating the formation of endemic species specialized to specific bedrock types. The study emphasizes the underappreciated impact of geological processes on local species assembly, often overshadowed by ecological factors like local climate. The significant similarities in plant family and genus compositions across mountains sharing the same bedrock, such as those in eastern Asia and eastern North America, highlight the potential role of landform type in constraining floristic assembly.

Previous research has explored various factors influencing montane and global biodiversity. Climate stability, habitat heterogeneity, and energetics have been proposed as key drivers. Latitudinal gradients reveal the impact of environmental energetics, particularly potential evapotranspiration (PET) and average annual temperature. However, contemporary climate alone cannot explain global diversity patterns, suggesting that habitat heterogeneity, or geodiversity, plays a more significant role. The unique evolutionary history of taxa in mountainous regions also significantly influences local biodiversity. The MGH, initially proposed for the Tibeto-Himalayan region, integrates mountain uplift, geodiversity evolution, and climate changes to explain montane plant diversity. Geological processes, especially uplift and erosion, are known to impact montane biodiversity through their effects on species formation, immigration, and extinction. Mountains' formation and subsequent bedrock erosion create topographic complexities and new niches. The edaphic specialization of some species, which are dependent on specific bedrock types, further complicates the picture. While climate change drives plant migration, edaphic specialists are restricted in their ability to migrate. Studies demonstrating the relationship between edaphic conditions and plant diversity remain scarce. The unique contributions of geological and lithological processes are often overshadowed by ecological factors, necessitating a more integrated approach.

This study compiled a dataset of 17,576 angiosperm species from 140 Chinese mountain floras, categorized into five landforms based on bedrock: karst, granitic-karst, granitic, Danxia, and desert. 'Flora' refers to the collection of all angiosperm species on a specific mountain. Species richness, phylogenetic diversity (Faith's PD), phylogenetic structure indices (PDI, NRI, NTI), and mean divergence times (MDT) were calculated for each flora. Regression models were constructed, one using landform as a predictor ('landform model') and a second incorporating landform along with tectonic, climatic, and geographic variables ('full model'). This allowed comparison of landform's relative importance in explaining floristic assembly relationships. The study used the recently published, dated megaphylogenetic tree GBOTB.extended.LCVP.tre as a backbone, adding missing species using the R package V.PhyloMaker. Phylogenetic diversity and structure were analyzed using Faith's PD, PDI, NRI, and NTI. MDT, MDT of the youngest 25% of species (MDTyoungest), and MDT of the oldest 25% of species (MDToldest) were calculated. Regression models assessed the effects of landform and climate variables on divergence times. Spatial error models were used to account for spatial autocorrelation in the data. Geographic information (longitude, latitude, area, median elevation, elevational range), landform type, tectonic plate, and climatic variables from CHELSA climate data were used as predictor variables. GLMs were employed to determine the effects of landform, tectonics, and climate on various biodiversity metrics. AIC model selection and a 'leave one out' approach were used for model evaluation.

Angiosperm species richness was higher in granitic and karst-granitic floras compared to karst, Danxia, and desert floras. The landform model explained 28.95% (GLM) and 31.40% (SEM) of species richness deviance, while the full model explained 62.8% (GLM) and 63.7% (SEM). Phylogenetic diversity showed a similar pattern to species richness. PDI was highest for karst and lowest for desert, indicating greater floristic stability in karst. The landform model explained 70.89% (GLM) and 77.15% (SEM) of PDI deviance, while the full model explained 88.09% (GLM) and 88.25% (SEM). MDT, MDToldest, and MDTyoungest also followed a similar pattern, with karst having the highest median age. Landform effects had greater explanatory power for MDToldest than for MDT and MDTyoungest. The full model showed that high temperature annual range (TAR) and mean temperature of the coldest quarter (TCQ) were negatively correlated with divergence times. NRI was significantly higher for desert landforms, indicating phylogenetic clustering, while granitic landforms showed more overdispersion. Landform explained 64.91% (GLM) and 77.43% (SEM) of NRI variance in the landform model, and 1.22% in the full model. Differing landforms also exhibited differences in floristic compositions, with karst floras dominated by Malvales, Rosales, and Lamiales, desert by Poales, Asterales, and Caryophyllales, and granitic by Magnoliales, Saxifragales, and Ericales. The study revealed strong interactions between landform type and temperature and precipitation, indicating the importance of considering both factors in predicting species richness.

The findings highlight the unique effects of landforms on species richness and phylogenetic diversity, suggesting that habitat heterogeneity is a key driver of richness. While temperature and precipitation are correlated with species richness, the full model indicated a negative relationship with mean temperature of the warmest quarter (TWQ) which may be attributable to landform interactions. Igneous bedrock (granitic and karst-granitic) yielded higher species richness than sedimentary bedrock (karst, Danxia, and desert), likely due to differences in water cycle processes and rock erosion rates. Phylogenetic structure varied across landforms, with sedimentary bedrock floras showing more clustering than igneous bedrock floras, potentially due to local speciation and habitat filtering effects. The study demonstrates the role of landform constraints on species dispersal and establishment, with restricted dispersal between landforms contributing to variation in species composition. The 'floristic geo-lithology hypothesis' posits that montane species differentiation is driven by the lithospheric cycle, bedrock-constrained landform development processes, and the interaction between landform and environment. This contrasts with MGH, which focuses on alpha diversity rather than beta diversity. The study introduces the concept of 'landform flora', acknowledging the unique floras shaped by bedrock erosion and landform development processes.

This study demonstrates the significant role of bedrock and landform in shaping the assembly and differentiation of mountain floras, particularly in China. The proposed 'floristic geo-lithology hypothesis' offers a novel framework integrating geological and ecological factors. Recognizing 'landform floras' will enhance predictions of mountain biodiversity, inform conservation strategies, and advance our understanding of plant diversification worldwide. Future research should explore the specific mechanisms underlying these relationships and extend these findings to other mountain ranges globally.

While the study uses a large dataset of Chinese mountain floras, it is limited to angiosperms and may not fully represent the entire biodiversity of these regions. The reliance on existing species checklists and the use of a megaphylogenetic tree with potential uncertainties in phylogenetic relationships could influence the results. The interactions between landform and climate variables necessitate more detailed investigation.