Dipterocarps are keystone species in the forest of Southeast Asia. As useful timbers of high commercial value, Dipterocarp stands have been logged on a large scale, leading in some areas to a serious decline of forest resources. Therefore, it becomes urgent to protect its germplasm. With 34 Dipterocap species protected, XTBG boasts the largest collection of its kind in China.
Based on the Dipterocap collection within XTBG, Prof. Cao Kunfang’s research team has made important research advances in the physiological ecology of Dipterocarp species. Their research observations have been published in such prestigious peer-reviewed journals as Tree Physiology and Functional Ecology.
1. J.-L. Zhang, L.-Z. Meng and K.-F. Cao* Sustained diurnal photosynthetic depression in uppermost-canopy leaves of four dipterocarp species in the rainy and dry seasons: does photorespiration play a role in photoprotection? Tree Physiology 29(2):217-228 (2009). doi:10.1093/treephys/tpn018
Abstract
Diurnal and seasonal changes in gas exchange and chlorophyll fluorescence of the uppermost-canopy leaves of four evergreen dipterocarp species were measured on clear days. The trees, that were growing in a plantation stand in southern Yunnan, China, had canopy heights ranging from 17 to 22 m. In the rainy season, Dipterocarpus retusus Bl. had higher photosynthetic capacity (Amax) than Hopea hainanensis Merr. et Chun, Parashorea chinensis Wang Hsie and Vatica xishuangbannaensis G.D. Tao et J.H. Zhang (17.7 versus 13.9, 11.8 and 7.7 µmol m–2 s–1, respectively). In the dry season, Amax in all species decreased by 52–64%, apparent quantum yield and dark respiration rate decreased in three species, and light saturation point decreased in two species. During the diurnal courses, all species exhibited sustained photosynthetic depression from midmorning onward in both seasons. The trees were able to regulate light energy allocation dynamically between photochemistry and heat dissipation during the day, with reduced actual photochemistry and increased heat dissipation in the dry season. Photorespiration played an important role in photoprotection in all species in both seasons, as indicated by a continuous increase in photorespiration rate in the morning toward midday and a high proportion of electron flow (about 30–65% of total electron flow) allocated to oxygenation for most of the day. None of the species suffered irreversible photoinhibition, even in the dry season. The sustained photosynthetic depression in the uppermost-canopy leaves of these species could be a protective response to prevent excessive water loss and consequent catastrophic leaf hydraulic dysfunction.
Keywords: chlorophyll fluorescence, electron flow, gas exchange, heat dissipation, hydraulic conductivity
2. Jiao-Lin Zhang and Kun-Fang Cao* Stem hydraulics mediates leaf water status, carbon gain, nutrient use efficiencies and plant growth rates across dipterocarp species Functional Ecology doi:10.1111/j.1365-2435.2009.01552.x
Abstract
Stem vascular system strongly influences structure and functioning of leaves, life-history, and distribution of plants. Xylem structure and hydraulic conductivity of branches, leaf functional traits, and growth rates in 17 dipterocarp species in a mature plantation stand were examined to explore the functional relationships between these traits.
Maximum hydraulic conductivity on the bases of both sapwood and leaf area (kL) were positively correlated with midday leaf water potential in the rainy season, stomatal conductance, area-based maximum photosynthetic rate, photosynthetic N (PNUE) and P use efficiencies (PPUE), and mean height and diameter growth rates. Moreover, kL was positively correlated with mesophyll thickness and mass-based maximum photosynthetic rate. These results revealed the mechanistic linkage between stem hydraulics and leaf photosynthesis through nutrient use efficiency and mesophyll development of leaves.
A detrended correspondence analysis (DCA) using 37 traits showed that the traits related to stem hydraulics and leaf carbon gain were loaded on the first axis whereas traits related to light harvesting were loaded on the second axis, indicating that light harvesting is a distinct ecological axis for tropical canopy plants. The DCA also revealed a trade-off between photosynthetic water use efficiency and hydraulic conductivity along with PNUE and PPUE.
The congeneric species were scattered fairly close together on the DCA diagram, indicating that the linkages between stem hydraulics, leaf functional traits, and plant growth rates are phylogenetically conserved.
These results suggest that stem hydraulics mediates leaf water status, carbon gain, nutrient use efficiencies, and growth rates across the dipterocarp species. The wide variation in functional traits and growth rates among these dipterocarp species along with the trade-offs mentioned above provide a possible explanation for their co-existence in tropical forest communities. |
