A Review of the Interaction Mechanism and Law between Vegetation and Rock Geochemical Background in Karst Areas

Karst landforms are mainly special landscapes formed by carbonate dissolution.They are characterized by calcium abundance, lack of soil resources, and insufficient water resources. The growth and development of vegetation in the karst area is restricted by the bedrock. It is very important to understand the synergistic interaction between vegetation metabolism and geochemistry of carbonate rocks in karst areas to maintain the stability of structure and function of the karst ecosystem.

The mechanism and law of interaction between vegetation and rock geochemical background in karst areas from two aspects is expounded in this paper: vegetation community promotes weathering of carbonate rocks and geochemical background restricts vegetation.

Through summarizing: (1) Plants promote the weathering and dissolution of carbonate rocks through physicochemical and biological actions, such as secreting carbonic anhydrase organic matter, improving the water retention performance of rock surface through boring by organisms and accelerating the disintegration of broken rocks through root splitting, thus forming a unique karst geochemical background of drought, high calcium, shallow soil layer and lack of nutrients in the soil layer.

(2) With the long-term interaction between plants and the karst environment, plants adapt to environmental stress by adjusting their own structures and physiological functions, and even their unique plant succession rules. The plants that survived eventually evolved into unique karst plants that were drought-resistant, adaptable to high-calcium environments, and able to cope with nutrient deficiencies.

Due to the solubility of carbonate rocks, the hydrologic system forms a two-layer spatial structure of surface and underground, which makes it difficult to utilize groundwater resources. As a result, the available water resources of local plants are limited, and are prone to drought stress. The drought resistance of plants adapts to the drought or water shortage environment mainly through physiological and biochemical processes, morphological structure and water use. In the morphological structure of the plant, through the stomatal regulation and the xeric structural characteristics of the leaves, the transpiration water loss of the plant is minimized. Some karst plants can cope with drought stress through physiological and biochemical processes, which can reduce the damage caused by drought stress by increasing the activities of antioxidant enzymes and accumulating osmoregulatory substances through phytochemicals. Karst plants improve water use efficiency and reduce transpiration through different water use methods in the dry season. For example, some plants absorb deep soil, deep bedrock water or groundwater water through developed deep roots, and some plants even use fog water.

The adaptability of plants to high calcium is realized through physiological structure and process. In a high calcium environment, karst plants can limit the excess calcium transfer upward by forming calcified roots and keeping the calcium content in plants in a relatively stable state through the regulation of calcium oxalate crystal cells and leaves. Plants can also control the intracellular calcium ion concentration by regulating the calcium pool in vivo and controlling the absorption and transport of calcium.

Organic acids and mycorrhiza secreted by roots can help vegetation obtain nutrients in the soil to cope with nutrient deficiency in the soil. The organic acid content secreted by the roots of karst plants is usually higher than that of non-karst plants, and the increase of organic acid content can help plants to absorb trace elements. Arbuscular mycorrhizal fungi (AM) and ectomycorrhizal fungi (ECM) help plants adapt to nutrient deficiency by absorbing mineral nutrients from the soil.

(3) The vegetation succession in the karst area is similar to the general stage of vegetation succession in normal landforms, but the vegetation succession in the karst area has the particularity of self-generation. The vegetation succession from the middle to the top of the mountain takes a longer time. Vegetation succession changed physical and chemical properties and soil quality. In the process of succession, soil bulk density gradually decreases, and porosity gradually increases over time. At the same time, soil nutrients accumulate with the positive succession of vegetation. During the positive succession process of karst vegetation, the soil water and fertilizer retention capacity is enhanced, the stability is increased, and the survival probability of species and the species diversity are also increased. The special rock geochemical background in the karst area leads to the obvious changes in the spatial distribution of soil resources. High soil heterogeneity promotes the formation of plant community species diversity. For example, the tropical karst area in Xishuangbanna, Yunnan Province covers an area of 3600km², accounting for 19% of the land area. The forest survey results in the karst area show that there are 153 families, 640 genera and 1394 species of vascular plants, accounting for 77.7% of the total floristic families, 56.1% of the genera and 37.9% of the species, respectively.

The karst plant community is the result of the long interaction between the plant and the environment, the continuous adaptation to the environment and the growth and reproduction. The karstification of plants, the adaptation mechanism of plants under the typical karst soil environment such as drought, high calcium stress and lack of nutrient elements, as well as the succession characteristics of karst vegetation shown in the adaptation process and the biodiversity of vegetation are a whole interaction. The site growth of vegetation promotes the weathering of carbonate rocks and the formation of soil, creating conditions for their own growth. Meanwhile, soil area and soil thickness are positively correlated with plant diversity. The succession of vegetation changes the physical and chemical properties of soil and improves the quality of soil, and the karstification of carbonate rocks promotes the growth of vegetation. The adaptive mechanism of vegetation affects the distribution and growth of plants and promotes the succession process of plant communities and the formation of plant diversity. Due to the spatial heterogeneity of the karst environment and the diversity of plant habitats, plant diversity in karst areas is manifested as few genera, few species and endemic species. However, the internal relationship between plant diversity and the stability of the community ecosystem, as well as the comparative characteristics with non-karst areas still need to be further studied and explored.