Abstract: [Objective] Growth of trees is strongly influenced by climatic factors. The changes of xylem cells of trees record more detailed information compared to the tree-ring widths, and could enrich the present knowledge extracted from the traditional dendroclimatological or dendroecological studies. Our aim is to provide a reference for studying the global change using the characteristics of trees xylem cells. [Method] This paper summarizes the process of xylem formation, the research methods of studying the responses of xylem growth to climatic factors and the effect of climatic change (temperature, precipitation, solar radiation, photoperiod and increasing atmospheric CO₂) on trees cambial activity and xylem growth. [Result] At present, there are two methods to study the response of xylem growth to climatic factors at cell level: pinning and microcoring. Pinning was mainly used to study the response of cambium to injury or records the xylem growth increment of tropical trees, and microcoring can be used to observe the changes of the number of cambium cells in different stages and the xylem formation process. Recent research demonstrated that the formation of xylem went through four stages: cambial reactivation, cell radial enlargement, secondary wall thickening and lignification, the formation of mature xylem cells, and each stage was affected by climatic factors. The changes of xylem growth of trees were significantly correlated with some climatic factors, such as temperature, precipitation, solar radiation, photoperiod and increasing atmospheric CO₂. Temperature mainly affects the cambium activity at early stage of tree growth, and changes the time of reactivation of the cambium, cell division rate and active period. But temperature will not change the sequence of xylem cell differentiation and seasonal growth pattern. Precipitation mainly affects the rate of the cambium cell division, the duration of cambium cell division in the growing season and the tracheid diameter. Inadequate precipitation could reduce the rate of cell division, shorten the time of cell differentiation, and result in a decrease in the duration of cambium activity. Meanwhile, precipitation shortage could inhibit cell stretch, and the lumen diameter become small. Light intensity affects indirectly the activity of cambium cells through affecting photosynthesis. Photoperiod mainly controls the maximum growth rate of xylem cells and the end of growing period. Up to now, no consistent conclusions was reported about the relationship between the growth of xylem cells and the increasing atmospheric CO₂. Some studies suggested that elevated CO₂ concentrations affect the rate of the cell division and cell expansion rather than the secondary cell wall thickening, and result in a significant increase in the width of early wood growth ring. Some experimental evidences also showed that elevated CO₂ concentrations did not cause larger changes of early wood cell structure. Conversely, the width of late wood growth ring increased obviously. [Conclusion] With the development of research theory and technology, the growth characteristics of xylem cells under extreme climate, the growth responses of xylem cells in different tree species to climate change and the comprehensive effect of a variety of climate factors on trees xylem growth will become the main direction of future research.

Key words: climate change, cambium activity, xylem growth