Seasonal Dynamics Of Radial Growth And Sap Flow And Their Responses To Environmental Factors In A Tropical Karst Habitat

Karst landforms are widely distributed in southwest China,and the area of karst landforms in Xishuangbanna region of Yunnan count for 19 percent.Due to the shallow soil layer and poor water retention capacity of karst landforms,many plants grow on bare rocks.Even in wet season at this relatively humid area,the plants are subject to frequent drought stress.The increasing temperature and shifting precipitation distributions under climate change may affect the growth rate and seasonal phenology of trees.Studying seasonal dynamics of radial growth of trees in karst habitat and their response to environmental factors can provide a reference for understanding the growth strategies of karst habitat plants and their adaptation to seasonal drought.In this study,two deciduous tree species(Melia azedarach and Garuga floribunda)and one evergreen tree species(Celtic philippensis)in the tropical karst habitat of Xishuangbanna,Yunnan were selected.We used high resolution point dendrometer,micro-coring and thermal dissipation probes to monitor daily-to-monthly variations stem radius and sap flow for two consecutive years.We correlated radial growth and sap flow with environmental factors monitored simultaneously.Furthermore,we measured seasonal variations of leaf photosynthesis and leaf water potential analyzed their associations with tree radial growth.The result shows:(1)On the daily scale,there are obvious diurnal cycles in the stem radius variation of the three studied tree species.On the interannual scale,the three research tree species showed seasonal changes in 2018 and 2019.The three research tree species began to grow later in 2019 than in 2018,which may be related to the high temperature and drought encountered in the early 2019 growing season.The annual growth of deciduous tree species in 2019 is slightly lower than in 2018,while the annual growth of evergreen tree species in 2019 is higher than in 2018.(2)Daily growth of Melia azedarach correlated positively with daily rainfall and relative humidity,indicating that the growth of Melia azedarach is mainly restricted by moisture;however,the daily growth of Garuga floribunda net increase is not sensitive to environmental factors.Daily growth of Celtic philippensis correlated positively with daily maximum temperature,photosynthetically active radiation,vapor pressure deficit and wind speed,indicating that the growth of Celtic philippensis is mainly affected by temperature.(3)In areas with seasonal aridity,there are "inactive" dormant periods and active periods on cambium activities.The start of Garuga floribunda cambial cells earlier than Melia azedarach,but the maximum cambial cells of the two tree species were similar.For the same tree species,due to the interannual climate differences,the time and number of cambial also differed.(4)Melia azedarach and Garuga floribunda have similar seasonal dynamics of woody partialization,but there are differences in differentiation time.The annual growth of Garuga floribunda is greater than Melia azedarach,but the maximum growth rate is less than Melia azedarach,and the maximum growth rate appears later than Melia azedarach.In different years of the same tree species,there are differences in the time of wood partialization.In 2018,Melia azedarach started to grow earlier than 2019 and ended later than 2019.The growing season is longer and the annual growth is greater than 2019,but the maximum growth rate less than 2019.(5)The sap flux density of the three studied tree species correlated positively with photosynthetically active radiation,daily maximum temperature,and water vapor pressure deficit,and correlated negatively with relative humidity.In terms of two deciduous tree species,Melia azedarach is more drought-tolerant than Garuga floribunda.And on a daily scale,the sap flux density has a significant correlated positively with the maximum daily shrinkage,but the sap flux density is not synchronized with the daily dynamics of the stem radial variation,with a certain time lag.(6)The cambium activities and stem radial growth of tropical Karst forest trees growth is regulated by photosynthesis and water relations.When the deciduous tree species had the highest growth rate,the net photosynthetic rate also peaked.The net photosynthetic rate of the evergreen tree species Celtic philippensis did not fluctuate throughout the year,and no significant correlation was found with radial growth.There was a negative correlation between the water deficit of the studied tree species and the water potential before pre-dawn and midday.The greater the water potential at noon,the more severe the water stress.