[Water use of re-vegetation pioneer tree species Schima superba and Acacia mangium in hilly land of South China].

The xylem sap flows of two pioneer tree species, i.e., Acacia mangium and Schima superba, in degraded hill lands of South China, were continually monitored with Granier' s thermal dissipation probes during 2004-2007 and 2008-2012, respectively, and their seasonal transpiration changes at different tree age levels were compared. The results showed that the annual transpiration of both species increased with tree ages, and S. superba demonstrated a higher value than A. mangium. The average annual whole-tree transpiration of S. superba (7014.76 kg) was higher than that of A. mangium (3704.97 kg). A. mangium (511.46-1802.17 kg) had greater seasonal variation than S. superba (1346.48-2349.35 kg). The standard regression coefficients (beta) of transpiration (Eh), photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) for both species increased with soil moisture, suggesting the increase of soil moisture generated a greater sensitivity of plants to environmental factors. Partial correlation analysis revealed that soil moisture played an important role in the seasonal variation of transpiration of both species. The optimum soil moistures of S. superba and A. mangium were 0.22-0.40 and 0.29-0.30 (V/V), respectively, indicating the native pioneer species S. superba better adapted to water deficit compared with exotic pioneer species A. mangium.

[Seasonal differences in the leaf hydraulic conductance of mature Acacia mangium in response to its leaf water use and photosynthesis].

In this study, measurements were made on the leaf water potential (psi1), stomatal conductance (g(s)), transpiration rate, leaf area index, and sapwood area of mature Acacia mangium, aimed to understand the relationships of the leaf hydraulic conductance (K1) with the leaf water use and photosynthetic characteristics of the A. mangium in wet season (May) and dry season (November). The ratio of sapwood area to leaf area (A(sp)/A(cl)) of the larger trees with an average height of 20 m and a diameter at breast height (DBH) of 0.26 m was 8.5% higher than that of the smaller trees with an average height of 14.5 m and a DBH of 0.19 m, suggesting that the larger trees had a higher water flux in their leaf xylem, which facilitated the water use of canopy leaf. The analysis on the vulnerability curve of the xylem showed that when the K1 decreased by 50%, the psi1 in wet season and dry season was -1.41 and -1.55 MPa, respectively, and the vulnerability of the xylem cavitation was higher in dry season than in wet season. The K1 peak value in wet season and dry season was 5.5 and 4.5 mmol x m(-2) x s(-1) x MPa(-1), and the maximum transpiration rate (T(r max)) was 3.6 and 1.8 mmol x m(-2) x s(-1), respectively. Both the K1 and T(r max), were obviously higher in wet season than in dry season. Within a day, the K1 and T(r), fluctuated many times, reflecting the reciprocated cycle of the xylem cavitation and refilling. The leaf stomatal closure occurred when the K1 declined over 50% or the psi1 reached -1.6 MPa. The g(s) would be maintained at a high level till the K1 declined over 50%. The correlation between the hydraulic conductance and photosynthetic rate was more significant in dry season than in wet season. The loss of leaf hydraulic conductance induced by seasonal change could be the causes of the decrease of T(r) and CO2 gas exchange.

[Water recharge through nighttime stem sap flow of Schima superba in Guangzhou region of Guangdong Province, South China: affecting factors and contribution to transpiration].

To understand the nighttime water recharge of tree through its sap flow is beneficial to the precise estimation of total transpiration and canopy stomatal conductance, and to the further understanding of the time lag between canopy transpiration and stem sap flow. By using Granier's thermal dissipation probe, this paper measured the stem sap flow of Schima superba, and synchronously measured the main environmental factors including air temperature, relative humidity, photosynthetically active radiation, and soil moisture content, and also analyzed the water recharge through nighttime stem flow of S. superba at daily and seasonal scales. The sap flow density of S. superba was lower at night than at daytime, and the nighttime sap flow density had a larger variation in dry season than in wet season. The water recharge at night generally started from sunset when radiation was approaching zero, and lasted up to midnight (18:00-22:00). No significant difference was observed in the nighttime water recharge among seasons, and no significant correlations were found between the nighttime water recharge and environmental factors, but the nighttime water recharge was well regressed with the diameter at breast height, tree height, tree canopy size, stem biomass, and canopy biomass, suggesting that tree form features and biomass could better explain the nighttime water recharge. The contribution of nighttime water recharge to the total transpiration varied significantly with seasons, and was obviously higher in dry season than in wet season.

[Application of thermal dissipation probe in the study of Bambusa chungii sap flow].

Based on the validation of Granier's empirical formula for calculating tree stem sap flux density, a comparative study was conducted on the measurement of Bambusa chungi sap flow by using different lengths of thermal dissipation probe (TDP), aimed to approach the applicability of TDP in measuring the sap flow of B. chungii. The difference in the daily change of the sap flow between B. chungii and nearby growing Schima superb was also analyzed. Because of the thinner bamboo wall and the heterogeneous anatomy, the sap flux density of B. chungii measured by 10 mm long probe could be underestimated, but that measured by 8 and 5 mm long probes could be relatively accurate. The comparison of the sap flow between B. chungii and nearby growing S. superba revealed that both the mean sap flux density and its daily change pattern' s skewness of B. chungii were higher than those of S. superba, but the nighttime sap flow of B. chungii was less than that of S. superba, indicating that the water recharge of B. chungii during nighttime was less active than that of S. superba. It was suggested that using TDP to investigate the sap flow of bamboo would be feasible, but careful calibration would be required before the TDP was put into application on different bamboo species.