Abstract
Trees on the northern boundary of Asian tropics experience hot-humid and cool-dry seasons, but little is known about their seasonal dynamics in canopy physiology. We used a canopy crane to reach the canopy of nine tropical tree species and measured canopy leaf gas exchange, water status, and trunk sap flux during the hot-humid and cool-dry seasons in Xishuangbanna, China. We found that most tree species exhibited significant reductions in maximum photosynthetic rate (Amax), stomatal conductance (gsmax), predawn and midday leaf water potentials, and maximum sap flux density in the cool-dry season. Compared to the hot-humid season, Amax declined by 19–60%, and maximum water flux declined by −14% (an increase) to 42%. The cool-dry season decline in Amax of four species can be partly explained by an increased stomatal limitation (decreased gsmax and intercellular CO2 concentrations). Therefore, a predicted increase in drought in this region may decrease the carbon sequestration and productivity of these forests. We did not find a tradeoff between performance (Amax in the hot-humid season) and persistence through the cool-dry season; species with higher Amax in the hot-humid season did not show higher percent seasonal declines in the cool-dry season. Amax was significantly and positively associated with the trunk sap flux for both seasons, but the association was weaker in the cool-dry season. Thus, our results suggest that some tradeoffs and trait associations are environment dependent. Our results are important for understanding carbon and water fluxes of seasonal tropical forests and their responses to environmental changes.
Highlights
The dynamics of tree carbon and water fluxes are driven by environmental conditions such as temperature, solar radiations, and relative humidity (Fauset et al 2019; Dusenge, and Way 2017; Way et al, 2015)
The main objectives of the present study were: 1) to quantify the seasonal changes in canopy photosynthesis and water flux of trees at the northern limit of Asian tropics; 2) to test whether the potential cool-dry season declines in Amax of some species of this region is due to increased stomatal limitation, and whether the seasonal changes in environmental conditions shift the coordination between water flux and photosynthesis; and 3) to test whether there is a tradeoff between maximum photosynthetic performance (Amax in the hot-humid season) and persistence through the cool-dry season across species
The studied trees showed a high variation in seasonal dynamics of canopy leaf gas exchange at the northern limit of Asian tropics
Summary
The dynamics of tree carbon and water fluxes are driven by environmental conditions such as temperature, solar radiations, and relative humidity (Fauset et al 2019; Dusenge, and Way 2017; Way et al, 2015) Changes in these environmental conditions lead to changes in canopy leaf physiology which can be species-specific (Aragao et al, 2014; Chen and Cao 2015; Siddiq et al 2017). E.g., those at the northern edge of Asian tropics, are characterized by a seasonality in temperature and rainfall, which results in a hot-humid season and a cool-dry season These seasonal changes will change the canopy leaf physiology which has not been well-studied until now. These forests are strong carbon sinks and contribute significantly to the global carbon cycle (Zhang et al, 2006, Tan et al, 2012; Cristiano et al, 2014; Zhang et al, 2016), but the physiological mechanisms explaining their high carbon sink function and seasonal dynamics are not well-understood
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