The Water Use Strategies Of Different Plant Groups In A Tropical Cloud Forest

Tropical cloud forest is an ecosystem sensitive to global change.The horizontal precipitation formed by cloud affects the forest environment and plant distribution.The environmental conditions and species composition of the tropical cloud forest are obviously different from that of the tropical forest at lower altitude,such as higher frequently fog,stronger wind,lower temperature;higher relative humidity;smaller diameter breast height and area,the leaves are leathery,and the epiphytes and endemic species are abundant.Therefore,the water use sources and strategies of tropical cloud forest may be unique.In this paper,we measured and assessed variation of 4 stable isotope traits responding to water use resource and water use efficiency for vascular epiphytes and trees across dry and wet seasons in a tropical cloud forest,13 leaf functional traits responding to water condition and bivariate relationships between the functional traits in water-photosynthesis-nutrient.The main results are as follows:1)We used the method of stable isotope and wilcoxon rank test to measured and assessed the water use source and water use efficiency of two plant groups in dry and wet seasons.Results showed that 99%of the water resources of trees came from soil water,while nearly 99%of the water resources of vascular epiphytes（include epiphytic angiosperms and pteridophytes）came from the absorption of rain water by leaves in wet season.In dry season,the water resources of vascular epiphytes and trees mainly came from fog water（52%of trees,47%of vascular epiphytes,55%for epiphytic angiosperms,47%for epiphytic pteridophytes）and rain water（46%of trees,53%of vascular epiphytes,44%for epiphytic angiosperms,52%for epiphytic pteridophytes）.There was no significant difference in water use efficiency between vascular epiphyte and trees in dry season,but there was significant difference in wet season.The results showed that fog water was the key water resource of the plants in the tropical cloud forest.In the case of no increase in the soil available water,stimulating the leaves to absorb the fog water will help the vascular epiphytes and trees to adapt to the seasonal drought resistance.2)Two-way anova,One-way anova,PCA and correlation analysis were used to measured and assessed variation of 13 leaf functional traits responding to water use strategies for vascular epiphytes and trees across dry and wet seasons.Results showed that the stomatal and anatomical traits of the vascular epiphytes and trees in dry season were significantly lower than those in wet season,which indicated that water use strategies of vascular epiphytes and trees were different in dry and wet seasons,and plants tended to adapt to dry season by reducing water loss.Compared with vascular epiphytes,trees have lower stomatal length and higher stomatal density,which indicated that trees adopt rapid water use strategy,while vascular epiphyte tended to adopt conservative water use strategy.It also indicated that epiphytes were more susceptible to water shortage than trees,and it was easy to die in the extreme drought events.3)Assessed of water-photosynthesis-nutrient bivariate relationship of two different plant groups by using SMA method.Results showed that the bivariate relationships between the functional traits in water-photosynthesis-nutrient of vascular epiphytes and trees was not only convergent but also divergent.The SLA-δ¹³C,Aarea-LWC,Nmass-δ¹³C,Nmass-SLA and Pmass-Aarea of two plant groups were convergent;while LWC-SLA,δ¹³C-Aarea,LWC-Nmass,LWC-Pmass,δ¹³C-Pmass and SLA-Pmass were divergent.In generally,the investment of vascular epiphytes in water was higher than trees,which was a"slow investment income"strategy（conservative strategy）,while the investment of trees in photosynthesis and nutrients was higher,which was a"fast investment income"strategy（open strategy）.