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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.

Marginal water-use efficiency plays a critical role in plant carbon–water coupling relationships. We investigated the ecosystem marginal water-use efficiency (λ) of a tropical seasonal evergreen forest to (1) determine the general pattern of λ across time, (2) compare different models for calculating λ, and (3) address how λ varies with soil water content during different seasons. There was a U-shaped diurnal pattern in λ, which was higher in the early morning and late afternoon. At other times of the day, λ was lower and remained constant. Ecosystem λ was higher in the wet season than in the dry season. All three models successfully captured the diurnal and seasonal patterns of λ but differed in the calculated absolute values. The idea that λ is constant on a subdaily scale was partly supported by our study, while a constant λ was only true when data from the early morning and late afternoon were not included. The λ increases with soil water content on a seasonal scale, possibly because early morning λ remained low in dry conditions when the soil water content was low.

We tested for a tradeoff across species between plant maximum photosynthetic rate and the ability to maintain photosynthesis under adverse conditions in the unfavorable season. Such a trade-off would be consistent with the observed trade-off between maximum speed and endurance in athletes and some animals that has been explained by cost-benefit theory. This trend would have importance for the general understanding of leaf design, and would simplify models of annual leaf carbon relations. We tested for such a trade-off using a database analysis across vascular plants and using an experimental approach for 29 cycad species, representing an ancient plant lineage with diversified evergreen leaves. In both tests, a higher photosynthetic rate per mass or per area in the favorable season was associated with a stronger absolute or percent decline in the unfavorable season. We resolved a possible mechanism based on biomechanics and nitrogen allocation; cycads with high leaf toughness (leaf mass per area) and higher investment in leaf construction than in physiological function (C/N ratio) tended to have lower warm season photosynthesis but less depression in the cool season. We propose that this trade-off, consistent with cost-benefit theory, represents a significant physio-phenological constraint on the diversity and seasonal dynamics of photosynthetic rate.

According to the different engineering mechanical states of top coal caving and normal stoping of gaseous loose thick coal seams, the dialectical relation between this caving method and dynamic disasters was analyzed by simulating the change of stress states in the process of top coal initial caving with different mining and caving ratios based on the ANSYS10.0. The variation of elastic energy and methane expansion energy during first top coal caving was analyzed by first weighting and periodic weighting and combining with coal stress and deformation distribution of top coal normal stoping as well as positive and negative examples in top coal caving of outburst coal seam. The research shows that the outburst risk increases along with the increase of the caving ratio in the initial mining stage. In the period of normal stoping, when the mining and caving ratio is smaller than 1:3 and hard and massive overlying strata do not exist (periodic weighting is not obvious), it is beneficial to control ground stress leading type outburst. Thus, it is unreasonable to prohibit top coal caving in dangerous and outburst prone areas.