Water and Carbon Balances of Deciduous and Evergreen Broadleaf Trees from a Subtropical Cloud Forest in Southwest China

Evergreen and deciduous trees are conspicuous growth forms across most forest ecosystems around the world. The competition or coexistence between deciduous and evergreen tree species is an old but important ecological topic. The relative advantages of being deciduous or evergreen and the environmental constraints on tree and ecosystem level carbon assimilation in subtropical forests are poorly understood. Evergreen broadleaf trees dominate the forests from elevations of 1000 to 2600 m in the subtropical area of Southwest China, while the subtropical forests from Southeast China at similar elevations are dominated by deciduous trees. The eco-physiological mechanism in explaining this distribution pattern is an interesting topic that has not yet been studied. The objectives of this dissertation were (1) to understand the difference in daily water use and photosynthesis between subtropical evergreen and deciduous tree species, (2) to understand the environmental constraints on the leaf and ecosystem level carbon assimilation of the subtropical forests in Southwest China, (3) to understand the tradeoffs in water relations and carbon assimilation between being deciduous and evergreen in the subtropics, and (4) to determine the annual carbon balance of evergreen and deciduous trees in the subtropics of Southwest China, which may partially explain the dominance of evergreen trees at the high elevations of this region. Tree hydraulic and photosynthetic traits, diurnal and seasonal dynamics in the water use and carbon assimilation of evergreen and deciduous trees from a subtropical montane cloud forest in Southwest China were studied from 2008 to 2011. Environmental conditions (temperature, rainfall, daytime fog, photosynthetic photon flux density etc.) and ecosystem level water and carbon exchange were also recorded. These forests are characterized by a summer with high precipitation and a dry mild winter. The long duration of fog and frequent rain during the summer is an important characteristic of the forest. I found that (1) the deciduous species had significantly higher stem hydraulic conductivity, greater stem capacitance, higher midday stem water potential, and higher midday stomatal conductance than the evergreen species, (2) the evergreen cloud forest buffered the effects of seasonal water deficits and rainfall anomalies by using soil water storage and ground water at depth, and by lowering the leaf and canopy water loss rates, (3) the evergreen species maintained high carbon assimilation during the winter, and the net ecosystem carbon gain was higher in the winter than in the summer, (4) the most common deciduous species in the forest extended their leaf life spans and used red senescing leaves to assimilate a considerable amount of carbon during part of the winter season. I concluded that (1) the considerable carbon gain by the evergreen broadleaf trees in the winter/dry season results in a higher yearly carbon gain than for the deciduous species, which partially explains the dominance of evergreen trees in the subtropical forests of Southwest China; (2) the low temperatures in the winter do not limit, while the longtime duration of cloud cover and leaf wetness during the summer strongly constrains, the tree and ecosystem level carbon gain, which weakens the advantages of deciduous species by having higher photosynthetic rates compared to the evergreen species during the summer; (3) extending the carbon assimilation period of a deciduous species into the winter months suggests that being evergreen is more competitive than being deciduous in subtropical forests of Southwest China. Therefore, the results of my dissertation provide a potential explanation for the dominance of evergreen trees in the subtropical forests in Southwest China. Substantial carbon uptake during the winter/dry season also allows this subtropical cloud forest to be one of the largest carbon sinks among old-growth forests in the world, suggesting the importance of subtropical forests in the global carbon cycle. The presence of cloud forests in the subtopics of China is not known or very little known to the international academic community. This dissertation puts the Chinese cloud forests in the world map and provides a mechanistic understanding of the selective pressures operating in subtropical cloud forests. The results of this dissertation also reveal the hydraulic and photosynthetic adaptations of evergreen and deciduous tree species growing in subtropical cloud forests