Phenology, Leaf Structure And Function, And Seasonal Variation In Photosynthesis Of Woody Plants In A Dry-Hot Valley Of Yuanjiang, Southwestern China

Due to the high mountains blocking moist winds and causing rain-shadow effect, most of the valleys in eastern Himalayas region of China are dominated by dry and hot/warm climate and host savanna vegetation. However, till now, the phenology and ecophysiology of the woody plants in these valleys have been hardly investigated. In this study, the phenological and ecophysiological characteristics of more than 30 woody plants in a valley of Yuanjiang, which is one of typical dry-hot valleys in southwestern China, were characterized. Shoot elongation patterns, leaf dynamics, flowering and fruiting events, and fruit types in 32 woody species were observed over three years. Leaf anatomy and morphology, nutrient status, photosynthetic capacity and thermotolerance in 34 woody species were measured. The relationships between leaf structure and function across these woody plants were explored. Additionally, the water status and photosynthesis of six representative woody species were monitored over 3 years between 2003 and 2005. These six species are contrasted with three different phenological types, i.e., an evergreen, Cyclobalanopsis helferiana, two winter-deciduous (WD), Buchanania latifolia and Symplocos racemosa, and three drought-deciduous (DD) species, Terminthia paniculata, Wendlandia tinctoria and Woodfordia fruticosa. The results are as follows:Phenological observations showed that deciduous species dominated the Yuanjiang dry-hot valley. 72% woody species observed were deciduous, among which the majority were winter-deciduous (19 out of 23 species), and other four species were drought-deciduous. Shoot extension of the woody plants was not consistent with the seasonal fluctuation in rainfall. Among the species observed, six species can flush in the late dry season.22 out of 32 species extended their shoots successively from the start of rainy season and ceased the shoot elongation in late rainy season. Shoot dieback in late dry season was observed in 9 out of these 22 species. Other six species elongated shoots to the maximum length once a year within a very short period of time (mostly within two weeks). In the late rainy season, 30 out of 32 species began to shed their leaves, and the evergreen species fell 1/3 to 1/2 current-year leaves. Although flowering and fruiting events can be seen throughout the year, the flowering of these savanna plants was concentrated in dry season and early rainy season. Fruit/seed ripening of these plants took place from the late rainy season to the end of dry season. Their fruits were mostly the drupe-type.The woody species in the Yuanjiang valley exhibited xeromorphic characteristics, i.e. small leaf size, well-developed palisade mesophyll, high leaf density and stomatal density. The mean foliar concentrations of nitrogen (N) and phosphorous (P) were lower than those of the tropical rainforest plants, which were correlated with each other and with specific leaf area (SLA). The mean foliar N/P ratio of the 26 species was 15.3, and 12 out of 26 species (46%) had foliar N/P ratio higher than 16, probably indicating P limitation. Across species, mass-based maximum photosynthetic rate had positive correlation with SLA and stronger correlation with foliar P than with N concentrations. These species had high photosynthetic thermotolerance, which was correlated negatively with thickness of palisade mesophyll and positively with foliar N, indicating that thicker leaves and leaves with higher N concentration have stronger thermotolerance.At the end of a prolonged drought, predawn leaf water potential (Ψpd) in C. helferiana and S. racemosa dropped to c.–0.8MPa, whileΨpd in B. latifolia was still close to zero and DD species were leafless at this time. The evergreen and WD species responded to drought by closing stomata and thus maintained constant relative water contents (RWC), which is a typical drought avoidance strategy, but at the expense of carbon gain. DD species maintained high photosynthetic capacity with a decrease in stomatal conductance and RWC until the driest period of time, and then shifted from the drought-tolerance strategy to the avoidance mechanism by shoot dieback. There was no significant difference in the means ofδ¹³C across the phenological groups. Foliarδ13C in the woody species from the Yuanjiang valley savanna were higher than those in tropical rainforest plants in the same region, indicating that the former had higher long-term water use efficiency than the later ones. The evergreen and WD species had stronger heat dissipation than the DD species in dry seasons. All species increased reflectance of photosynthetically active radiation in dry season probably due to degradation of chlorophyll as indicated by another leaf reflectance index, which probably reduced light harvesting as a result. All species showed strong increase in the ratio of red to green spectral reflectance of leaves in a dry season, indicating the accumulation of anthocyanin which may contribute to screening sunlight and scavenging reactive oxygen species.Diverse phenological performances, xeromorphic leaf traits and different responses to drought in the savanna woody species with different leaf phenology may facilitate partitioning of resource utilization and hence co-existence of woody plants in dry-hot valley savannas.