Plant Survival Strategies Under The Background Of The Asian-Indian Plate Collision

The Asian-Indian plate collision and the accompanying climate change have been hot fields in geological researches.However,due to the complexity of tectonic evolution,the timing and the impacts of it are still highly debated.Plant fossils,as the important carriers in geological history,witnessed the whole process of this collision and climate change between India and Asia.Therefore,the study of phytogeography,paleoclimate and plant paleoecology in related regions during the particular geological ages could provide a key window for understanding the scenarios of this geological event.Nanning and Baise of Guangxi,low-latitude Asia,where affected directly by this collision event,were selected as the ideal study areas.Based on the Oligocene plant materials in this study areas,with the multiple perspectives analysis,the discussions focus on the following questions:1)whether there was a Paleogene plant exchange between mainland Asia and India and when did this biogeographic link arise?2)what was the paleoclimate of the study area during this collision event?3)which kind of plant ecological strategy has been adopted to adapt the corresponding environment condition in the context of the Asian-Indian plate collision.The study of plant systematic classification provides a foundation for solving the above problems.In the present paper,macro-and microstructures of fossils from the late Oligocene Nanning,Guangxi were observed by some treatments i.e.general cuticle treatment,paraffin section treatment,and Micro-CT analysis.Combined the traditional morphological comparison,principal component analysis with the previous molecular phylogenetic studies,a total of 3 species of Annonaceae,including 2 new species,were identified.The present fruit and seed fossils are the first report of annonaceous reproductive organs in the low-latitude East Asia.Meanwhile,the descriptions of leaf epidermis and vascular bundles fill the gap of the previous study about the fossil leaf microstructure of Fissistigma.Furthermore,a total of 18 species from the early Oligocene Baise,low-latitude Asia were also identified preliminarily.Based on fossil occurrences and previous molecular clock study,the origin and migration of annonaceous were reconstructed.The result suggests the most parsimonious dispersal scenario for the annonaceous taxon from the late Oligocene of low-latitude Asia is the Out-of-India route after the suturing between these two plates.Floral exchanges between India and mainland Asia have also been inferred in many other different taxa during this period,suggesting that unimpeded floristic migrations commenced no later than the late Oligocene.Above phenomena are also consistent with the fact that the early Oligocene Baise flora lacks typical plants from India.Meanwhile,from the perspective of paleophtogeography,such frequent plant exchange reflects the adaptive strategies under the background of Asian-Indian plate collision event and might also suggest the suturing between these two plates no later than the late Oligocene.The palaeoclimate of Baise,Guangxi during the early Oligocene were quantitatively reconstructed using leaf physiognomic analyses.The results suggest a slightly warmer and more humid climate without obvious seasonality in Baise during the early Oligocene than today.Besides,the paleovegetation were reconstructed based on the Integrated Plant Record(IPR)analysis.Using this method,3 floras from the early Oligocene Baise,the early Oligocene Nanning and the late Oligocene Nanning are reconstructed as broad-leaved evergreen forests.Meanwhile,compared with the vegetation components from the early Oligocene Nanning,the proportion of broad-leaved evergreen component from the late Oligocene Nanning increased obviously.In contrast,the proportion of broad-leaved deciduous,sclerophyllous,legume-type and herb components from the late Oligocene Nanning is lower than that from the early Oligocene Nanning.Such direction of vegetation succession is consistent with the global climate change trend in the contemporaneous period,which reflects the response and adaptation of vegetation to the environment under the background of Asia-India collision.Using the relationship between petiole width-leaf area and leaf mass per area(M_A),the paleo-M_As of 23 species from the early Oligocene Baise were estimated.Furthermore,based on these reconstructed paleo-M_As and the leaf economics spectrum theory,the paleoecological trade-off strategy of early Oligocene Baise flora have been reconstructed.The results show the Oligocene Baise flora was dominated by?slow-return‘species but mixed with some important components of?fast-return‘species.By comparing the M_A distribution of the Baise flora with selected extant sites,the similar ecological strategy has been found in tropical riparian broad-leaved evergreen forest from Damingshan,Guangxi.Such kind of ecological strategy might promote the community to maximize the use of limited living resources,and at the same time to better withstand the potential extreme situations and relive the pressure of competition.This might imply the Baise flora has established a relatively stable ecological strategy to adapt to the living environment in the period of Asian-Indian plate collision.