SAP FLOW-SCALED STAND TRANSPIRATION AND CANOPY STOMATAL CONDUCTANCE IN AN ACACIA MANGIUM FOREST

doi:10.17521/cjpe.2006.0086

Abstract

Abstract:

Background and Aims Canopy stomatal conductance is considered as a sensitive parameter of forest ecosystem in responding to environmental changes and can be used to monitor the impacts of water stress, air pollution, trace gas and other environmental factors on forest function. Sap flow of an Acacia mangium forest was measured to scale up stand transpiration and canopy stomatal conductance. It aimed at understanding water use of A. mangium forest in different environmental water conditions and its responses to soil moisture change at an ecosystem level.
Methods The Granier's thermal dissipation probes were applied to monitor sap flow of 14 sample trees in an Acacia mangium forest in hilly land of South China. The sap flow data were used to calculated whole-tree and stand transpiration, and to estimate mean canopy stomatal conductance (G_c) by combing with synchronous measurement of environmental factors.
Key Results Sap flux density (J_s) and whole-tree transpiration (E_t) were obviously affected by tree morphological features. Even though there were less individual trees with large diameter at breast height (DBH) in the sample plot, they comprised relatively larger proportions of total sapwood area and transpiration of the forest. Daily variations of J_s and E_t were mainly controlled by photosynthetically active radiation (Q_o) and vapor pressure deficit (D). Soil moisture (θ) had greater effects on J_s and E_t of trees with larger DBH than on those trees with smaller DBH. The differences of J_s and E_t across individuals reduced with decreasing θ. The highest stand transpiration (E) occurred in July when the radiation and hydrothermal condition were favorable, whereas the reduced water supply in soil during the time period of September-November resulted in lower E value and its sensitivity to D. G_c responded to major environmental factors in a similar way with E. Long-term lower θ would significantly decreased G_c, which would cause reduction in E.
Conclusions Mature A. mangium forest can maintain a vigorous transpiration under sufficient radiation and hydrothermal conditions, but could not stand well under long-term stress of soil moisture. Granier's probes have the potential to accurately estimate stand transpiration, canopy conductance and their responses to environmental factor, extending the temporal and spatial scales in studying the ecological process of forest ecosystem.

Key words: Acacia mangium, Sap flow, Mean canopy stomatal conductance, Atmospheric vapor pressure deficit, Granier's probe

ZHAO Ping, RAO Xing-Quan, MA Ling, CAI Xi-An, ZENG Xiao-Ping. SAP FLOW-SCALED STAND TRANSPIRATION AND CANOPY STOMATAL CONDUCTANCE IN AN ACACIA MANGIUM FOREST[J]. Chin J Plant Ecol, 2006, 30(4): 655-665.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2006.0086

https://www.plant-ecology.com/EN/Y2006/V30/I4/655

Figures/Tables 11

Fig.1 Tree distribution within the sample plot of Acacia mangium forest ● Sap flow measurement ○Non-sap flow measurement

Fig.2 Correlation between diameter at breast height (DBH) and sapwood area (As)

Table 1 Tree characteristics of Acacia mangium

序号 No.	树高 Height (m)	枝下高 Stem length (m)	冠幅 Canopy size (m×m)		胸径 DBH (m)	边材厚度 Sapwood depth (m)	边材面积 Sapwood area (m²)
1	19.3	3.1	6.7	3.3	0.292 9	0.025 4	0.020 1
2	17.9	2.7	4.6	3.0	0.238 8	0.021 7	0.013 7
3	22.8	9.0	6.1	8.4	0.328 9	0.027 9	0.024 8
4	19.5	2.8	7.5	8.1	0.375 1	0.031 1	0.031 6
5	15.6	1.6	3.8	2.7	0.181 5	0.017 8	0.008 3
6	14.5	6.0	2.6	4.1	0.171 9	0.017 1	0.007 5
7	12.0	7.3	3.3	4.1	0.168 9	0.016 9	0.007 2
8	19.5	6.5	3.3	3.3	0.238 8	0.021 7	0.013 7
9	18.2	9.2	3.7	2.4	0.213 3	0.020 0	0.011 1
10	18.7	10.9	3.0	4.1	0.254 7	0.022 8	0.015 5
11	19.5	12.0	3.9	1.5	0.203 8	0.019 3	0.010 2
12	20.0	9.3	4.6	4.3	0.207 0	0.019 5	0.010 5
13	12.0	3.7	3.6	4.7	0.133 7	0.014 5	0.004 7
14	19.5	4.7	2.6	2.7	0.181 5	0.017 8	0.008 3

Fig.3 Frequency distribution of diameter at breast height (DBH) and sum of sapwood area of all trees at different DBH ranks 1: DBH<0.15 m 2: 0.15 m≤DBH<0.20 m 3: 0.20 m≤DBH<0.25 m 4: 0.25 m≤DBH<0.30 m 5: 0.30 m≤DBH

Fig.4 Daily variation of photosynthetically active radiation (Qo), air temperature (T), vapor pressure deficit (D),sap flux density (JS) and whole-tree transpiration (Et)

Fig.5 Daily variation of total transpiration of different diameter at breast height (DBH) rank in different seasons Inserted graphics show the percentage of transpiration of different DBH in the total stand transpiration Eci: Total transpiration trees at DBH rank of I

Fig.6 Change of stand transpiration (E), photosynthetically active radiation (Qo), air temperature (T), vapor pressure deficit (D) and soil moisture (θ) in year of 2004 Missing data were due to equipment failure as a result of power off or lightning

Fig.7 Stand transpiration (Ec) in response to vapor pressure deficit (D) in different soil moisture levels θ:Soil moisture

Fig.8 Daily variation of mean canopy stomatal conductance (Gc)

Fig.9 Yearly course of mean canopy stomatal conductance (Gc)

Fig.10 Mean canopy stomatal conductance (Gc) in correlation with soil moisture (θ)

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