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). Tree leaf photosynthesis (A) and water fluxes are sensitive to changes in environmental conditions, and reach their maximum values under optimum conditions (Tucci et al. 2010; Yang et al. 2012; Zhang et al. 2014a; Gitelson et al, 2014). In the tropics, the optimum conditions are observed during moderate atmospheric temperature and humidity, which create the suitable driving force (vapor pressure deficit) for water fluxes (Siddiq et al., 2017) and suitable temperature for photosynthesis (Cao et al., 2006; Kumagai et al., 2006; Slot et al., 2017; Slot and Winter 2017). In tropical areas with a seasonality in temperature and/or rainfall, (e.g. marginal tropics) the reduction of temperature and/or rainfall during the cool and/or dry season can result in reduced carbon and water fluxes (Vongcharoen et al., 2018; Frenne et al., 2019; Santanoo et al., 2019). Forests at marginal tropics, 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.
The marginal tropical rainforests in Xishuangbanna, China, which is on the northern boundary of Asian tropics, are typical Asian tropical rainforests in terms of species composition, phenology, and an important component of the Indo-Burma diversity hotspot (Myers et al. 2000, Cao et al. 2006; Hua 2013). They are also strong carbon sinks (Zhang et al., 2006) contributing significantly to the global carbon cycle. The tropical forests of this region are under the threat of degradation due to global warming, increasing drought, decreasing fog persistence, and the introduction of exotic species for commercial uses (Singh et al., 2019; Zhang et al., 2014a; Qiu 2010; Li et al., 2006). All these changes may significantly alter the water and carbon cycles of the region. For instance, the carbon fixation of the forests was significantly reduced in this region due to a drought in 2010 (Zhang et al., 2012). An understanding of water and carbon fluxes of trees from this region under different environmental conditions will help to predict their response to climate change including an increase in climate variability and develop effective management strategies.
Although there are some studies reporting the seasonal changes in photosynthesis of crops and small trees (Zhang et al., 2014a) and ecosystem-level carbon fluxes of the marginal Asian tropical forests (Zhang et al., 2006), more mechanistic studies are needed to understand their canopy physiology in responding ambient seasonal environmental changes. For instance, temperate plants are found to follow a general tradeoff between maximum photosynthesis in the favorable season, and persistence through the unfavorable season; species with higher maximum photosynthetic performance (Amax) in the favorable season show higher percent seasonal declines in Amax during the cold or dry season (Zhang et al., 2017). However, it is unknown whether trees from the marginal tropics with less seasonality compared to the temperate regions follow the same tradeoff. Understanding tree physiology and its seasonal dynamics of marginal tropical forests will also help to predict the response of temperate forests that are adjacent to them to future warming, and the response of tropical forests to a predicted increase in climate variability (e.g. seasonal drought or dry spells). Further, a more physiological understanding of these forests can improve the performance of the global land surface models, which are used to understand and predict the global water and carbon fluxes in a changing climate. Marginal tropical and subtropical forests are under-represented in these models (Pan et al., 2020; Gentine et al., 2019; Li et al., 2018).
It has been observed that photosynthetic carbon gain and water flux are coupled (Cowan & Farquhar 1977; Santiago et al., 2004, Brodribb and Feild, 2000; Fauset et al., 2019; Siddiq et al., 2019) because both processes are regulated by the stomata. A large water flux enabled by a high transport capacity will result in a high leaf water potential (less negative) during active transpiration at a given evaporative demand, which can potentially facilitate photosynthetic gas exchange (Landsberg et al., 2017). However, environmental conditions of the habitat can shift the coupling between water transport and leaf gas exchange (Sack et al., 2005), and therefore this coupling can also be potentially changed due to seasonal changes in environmental conditions. The evaporative cooling strategies will adjust according to seasonal changes in temperature. In the cool season, the needs for cooling through canopy transpiration are less, while in the hot-humid season the canopy needs a significant amount of evaporative cooling to avoid heat damage. Additionally, water flux and stomatal conductance may not be the major limiting factors on photosynthesis in the cool season as tropical trees can be sensitive to chilling induced photodamage (Levitt 1980; Dungan et al., 2003; Huang et al., 2010; Zhang et al., 2014b; Yang et al., 2017). Therefore, water flux and photosynthesis are not necessarily coupled in the unfavorable season. The other factors such as leaf phenology and leaf age that influence leaf photosynthesis (Kitajima et al., 1997; 2002) can also alter the coupling between water flux and carbon gain in the cool-dry season, as these forests have species with a range of leaf life spans including both evergreen and deciduous species. Generally, how this coupling responds to environmental changes, and how it shifts in different seasons are not well-understood.
Here we accessed the canopy of tropical trees in Xishuangbanna with a canopy crane, and measured canopy leaf carbon assimilation and water fluxes in the hot-humid and cool-dry seasons. 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 (less percent decline in Amax) across species. We hypothesized that the species with high rates of carbon fixation during the hot-humid season have higher seasonal declines in the cool and dry season according to the performance vs endurance tradeoff (Zhang et al., 2017). It was also hypothesized that most tree species will show significant declines in photosynthesis and water use, mainly caused by an increased stomatal limitation due to decreased water availability. We also hypothesized that the coordination between photosynthesis and water flux will be weaker during the cool-dry season due to the increased limitation of factors other than water transport (e.g. photochemistry) on photosynthesis.