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Change of water vapor exchange properties of 3 tree species under drought
Journal of Arid Environments (2002) 51: 423–435 doi:10.1006/jare.2001.0964, available online at http://www.idealibrary.com on
Change of water vapor exchange properties of 3 tree species under drought
Chen Xiongwen* Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, People’s Republic of China (Received 7 November 2000; and accepted 16 November 2001) The water vapor exchange properties of Fraxinus mandshurica Rupr., Quercus mongolica Fish. et Turcz. and Acer mono Maxim. at the relative water content of soil (RWCS) of 100%, 70%, 50% and 30% were studied by analysis of water loss, transpiration, water use efficiency (WUE) and stomatal conductance. Seedlings did cost a lot of water during the growing season, and the total water loss of each kind of seedling decreased differently under low RWCS conditions. The dynamics of water loss for 3 species under different RWCS were different. At RWCS of 100%, F. mandshurica and A. mono had higher average leaf transpiration rate than that of Q. mongolica, respectively. Leaf diurnal transpiration of 3 species decreased considerably at RWCS of 30%. But the diurnal WUE was not sensitive to the change of RWCS for these species. Only the average stomatal conductance of A. mono changed significantly under lower RWCS. The implication of these results was that tree species adapted to lower RWCS by changing its water vapor exchange properties, and it would be possible to decrease water loss from forests by selecting proper tree species. # 2002 Elsevier Science Ltd. Keywords: Relative water content of soil; stomatal conductance; transpiration; water loss; water use efficiency
Introduction North-east forest area is the largest and most important forest area in China, but the climate in the region is likely to be altered by global climate change, and predictions show that in an decrease in precipitation and increase in temperature will take place and drought will be inevitable. Furthermore, the increasing urbanization in this area needs more water resources. Then, the whole area is facing water shortage. In fact, water shortage often occurred in the large cities of this area during the recent decades. However, forests cost a large amount of water by photosynthesis and transpiration of trees. If trees can maintain their growth with less water, forests can reduce water consumption, then, water shortage can be alleviated to some extent. Therefore, a *Corresponding author. E-mail: [email protected] 0140-1963/02/030423 + 13 $35.00/0
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proper understanding about water consumption of tree species and change in water vapor exchange properties of tree species under drought condition is necessary. On the other hand, this research also has implications for the project of western developing that the Chinese government is carrying out to plant trees and restore grass at large scale in the western arid and semi-arid land to improve its environment (Jiang, 2000; Zhang et al., 2000). But how much water should be supplied in order to grow trees in the arid and semi-arid area is not known, and it is also controversial as to whether trees will exhaust the limited underground water. Water availability affects trees in several ways and the plant response to drought is complex. Usually plants adapt to the dry environment by drought avoidance and drought tolerance, which includes whole plant mechanisms that provide plants with the ability to respond and survive drought (Shantz, 1927; Levitt, 1980; Laffray & Louguet, 1990). Hydraulic properties of plants have been related to the evolution and adaptation of plants to environmental constraints (Tyree & Ewers; 1991). Many researches have been done about the ecophysiological effects of drought on trees, but the research of the water use strategies and mechanisms of water loss of different tree species under drought conditions is limited (Borghetti et al., 1998; Palmroth et al., 1999; Sobrado 2000). In the North-east forest area of China Quercus mongolica Fish. and Turcz. is a pioneer tree species, and it tends to grow during drought and on unfertilized soils and show higher tolerance to drought (Abrams, 1990; Breda et al., 1993; Epron & Dreyer, 1993). Fraxinus mandshurica Rupr. is one of the good tree species for wood, and Acer mono Maxim. is an important co-occurring species in the forest. The aim of this research is to study the change of water vapor exchange properties for 3 species under different water contents of soil. The water vapor exchange properties of seedlings in the research include water loss of whole plant, seasonal change of leaf transpiration, diurnal dynamics of leaf transpiration, water use efficiency and stomatal conductance. The underlying hypotheses of the research are that (i) seedlings will decrease the water loss, leaf transpiration and stomatal conductance under lower water content of soil. (ii) There exists threshold of soil water for tree growth. (iii) Leaf WUE will increase under drought.
Material and methods Seedling Seedlings of F. mandshurica, A. mono and Q. mongolica about 2-years old and of similar size (about 10–25 cm in height and 0?2–0?4 cm in stem diameter) were transplanted from the nursery at Changbaishan Ecological Research Station (41?211N, 128?101E) of the Chinese Academy of Sciences (CAS) to the greenhouse of Institute of Botany (39?561N, 116?171E) of the CAS at Beijing in June, 2000. The vegetation of the former site is temperate mixed coniferous and broadleaved forest, and the vegetation of the latter site is temperate deciduous broadleaved forest. Because the mean air temperature at Beijing was about 4–101C higher than it was at Changbaishan during the growing season, it would be helpful to study tree response under drought condition and climatic change scenario. All the leaves of seedlings were cut at the beginning of the experiment. Each plastic pot was planted by one seedling, and the pot size was about 20 cm in diameter and 15 cm in height. These seedlings were grown in the moisture loamy soil for several weeks, when seedlings had 2–3 leaves the relative water content of soil (RWCS) began to decrease. The top of each pot was covered tightly by two plastic sheets and only the stem of the seedling was left out so that the water evaporation from soil would be decreased. There were 8 seedlings for each treatment, and the total number of seedling is about 96. Because the greenhouse was semi-opened (free air and
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sun-light could enter but rain could not enter), the environmental conditions, such as temperature and radiation regimes, varied with the local climate. Water treatment The drought was controlled by RWCS. There were about RWCS of 100%, 70%, 50% and 30%. Then, each pot (including seedling, soil and pot) had its prescriptive weight by its treatment. Every morning or another each pot was weighed and watered to its prescriptive weight by scale if it was below that. Then, the water loss (evapotransipiration) of the seedling during this time period could be figured out by the following formula: Water loss ¼ weight of last time weight of this time The total water loss of each species was the sum of water loss that occurred every day. Transpiration and stomatal conductance measurements The rate of photosynthesis and transpiration and stomatal conductance were measured at the same time for the top leaf, in which the leaf area was bigger than the leaf chamber, at least 3 times by Portable Photosynthesis System of LCA-4 (Analytical Development Corporation, U.K.) every hour. Every 2 or 3 weeks the diurnal photosynthesis, transpiration and stomatal conductance were measured for 3 species of each treatment from 8 a.m. to 5 p.m. The data of every treatment were summarized by calculating the average of 8 replicates at one time. The seasonal data are just time-series of the average of every day. Water use efficiency (WUE) Leaf water use efficiency (WUE) was calculated by the photosynthetic rate/ transpiration rate. Statistical analysis The average values of 8 replicates were analysed by t-test procedure of SAS, and the dynamics of each index was analysed by (i) ANOVA at p ¼ 0?01 and 0?05 level of error. and (ii) visual sense. Results Total water loss Seedlings did cost a lot of water during their life times. When there was no water stress (RWCS of 100%) F. mandshurica cost about 8.7 kg water during the growing season, and it was the water loss among 3 tree species. The total water loss decreased when seedlings were under low RWCS (Fig. 1). At RWCS of 50% A. mono cost more water than F. mandshurica and Q. mongolica, respectively. When RWCS decreased to 30% A. mono had more water loss than F. mandshurica, but the unexpected thing was that after 34 days Q. mongolica at RWCS of 30% died. Dynamics of water loss Three tree species decreased their water loss when they were under low RWCS, but their responses were different. The water loss for F. mandshurica during the growing
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Tree species of different treatments Figure 1. The total water loss for each seedling of 3 species in the growing season. Fm100: Fraxinus mandshurica at the relative water content of soil (RWCS) of 100%. Fm70: Fraxinus mandshurica at RWCS of 70%. Fm50: Fraxinus mandshurica at RWCS of 50 %. Fm30: Fraxinus mandshurica at RWCS of 30%. Am100: Acer mono at RWCS of 100%. Am70: Acer mono at RWCS of 70%. Am50: Acer mono at RWCS of 50%. Am30: Acer mono at RWCS of 30%. Qm100: Quercus mongolica at RWCS of 100%. Qm50: Quercus mongolica at RWCS of 50%.
season among RWCS of 100%, 70%, 50% and 30% were significantly different (po0?05) (Fig. 2). For A. mono the water loss between RWCS of 100% and 30%, 70% and 30%, 50% and 30% were significantly different, respectively, and the average water loss between all treatments were significantly different except between RWCS of 70% and 50%, 70% and 30% (Fig. 3). For Q. mongolica, the water loss between RWCS of 100% and 50%, 100% and 30%, 50% and 30% were significantly different (Fig. 4), respectively, although there were no treatment of RWCS of 70% and seedlings under RWCS of 30% died after 34 days. At RWCS of 100% the dynamics of water loss of 3 species in the growing season was different, the difference between F. mandshurica and Q. mongolica, A. mono and Q. mongolica was significant (po0?01), respectively. However, at RWCS of 50% the seasonal water loss dynamics of the 3 species were similar, and at RWCS of 30% the water loss of A. mono was significantly higher than that of F. mandshurica. Seasonal transpiration dynamics For F. mandshurica the seasonal leaf transpiration rate at RWCS of 100% was significantly lower than that at RWCS of 30% (po0?05), the dynamics of
Figure 2. The dynamics of water loss for Fraxinus mandshurica under RWCS of 100%, 70%, 50% and 30%: ( ) 100%, ( ) 70%; ( ) 50%; ( ) 30%.
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Figure 3. The dynamics of water loss for Acer mono under RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
transpiration rate between other treatments were not significantly different (Fig. 5). For A. mono and Q. mongolica the seasonal transpiration between all treatments was not significantly different for each species, respectively (Figs 6 & 7). At RWCS of 100% F. mandshurica and A. mono had higher average leaf transpiration rate (ALTR) than that of Q. mongolica, respectively (po0?01). ALTR was not significantly different between the three species at RWCS of 50% and 30%, respectively. Diurnal transpiration For F. mandshurica, the dynamics of the diurnal leaf transpiration rate was significantly different between RWCS of 100% and 30% (Fig. 8). For A. mono the dynamics of the diurnal transpiration rate between 100% and 50%, 100% and 30%, 70% and 50% were significantly different, respectively (Fig. 9). But for Q. mongolica the dynamics of diurnal transpiration rate between RWCS of 100% and 50% was not significantly different (Fig. 10). The diurnal leaf transpiration rate of 3 species at different RWCS increased in the morning, and reached the maximum at noon, then decreased in the afternoon. At RWCS of 100% there was one peak of diurnal leaf transpiration rate, but there were usually several small peaks at RWCS of 70%, 50% and 30%. Diurnal WUE The dynamics of diurnal WUE for F. mandshurica and Q. mongolica were not significantly different between different RWCS, respectively (Figs 11&12). However,
Figure 4. The dynamics of water loss for Quercus mongolica under RWCS of 100%, 50% and 30% (legends are the same as in Fig. 2).
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Figure 5. The dynamics of leaf transpiration for Fraxinus mandshurica at RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
the dynamics of diurnal WUE for A. mono under different RWCS were significantly different except between RWCS of 50% and 30% (Fig. 13). The dynamics of diurnal WUE between F. mandshurica, Q. mongolica and A. mono at each RWCS were not significantly different (p40?05).
Diurnal average stomatal conductance The variances of stomatal conductance during a day at certain RWCS were considerable, so only the average stomatal conductance (ASC) at different RWCS were compared (Fig. 14). ASC of F. mandshurica was not significantly different by statistics among different RWCS (p40?05), but ASC was higher at RWCS of 70% and 50%. Also ASC of Q. mongolica was not significantly different between RWCS of 100% and 50%. However, ASC of A. mono at RWCS of 70% was significantly higher than that at RWCS of 50% and 30%, respectively, and ASC at RWCS of 100% was significantly higher than that at RWCS of 50%.
Figure 6. The dynamics of leaf transpiration for Acer mono at RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
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Figure 7. The dynamics of transpiration rate for Quercus mongolica at RWCS of 100%, and 50% (legends are the same as in Fig. 2).
Discussion Total water loss
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Seedlings did cost a lot of water during their life times. When there was no water stress (RWCS of 100%) F. mandshurica cost about 8.7 kg water during the growing season, and it was the highest water loss among 3 tree species (Fig. 1). It could be assured that the adult trees would cost far more water. If forest reduced water loss, the water shortage in North-east China could be alleviated to some extent. The total water loss of Q. mongolica was lowest among 3 tree species when there was no water stress, from this it could be inferred that Q. mongolica was able to grow in more drought condition. However, the water cost of Q. mongolica at RWCS of 50% was only slightly lower than it was at RWCS of 100%. Therefore, Q. mongolica could be planted to reduce water loss from forest, but it could not be planted at drought area because it cost relatively more water and also it was hard to survive in drought condition. For each species, the total water loss decreased when seedlings were under RWCS of 70%, 50% and 30% except A. mono at RWCS of 50% and 70%. So the first hypothesis was verified, but it did not mean that the water loss always decreased linearly as RWCS decreased. At
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Figure 8. The dynamics of diurnal leaf transpiration for Fraxinus mandshurica at RWCS of 100% ( ); 70% ( ); 50% ( ) and 30% ( ).
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RWCS of 50%, A. mono cost more water than F. mandshurica and Q. mongolica, respectively. When the RWCS decreased to 30% A. mono had more water loss than F. mandshurica, but the unexpected thing was that after 34 days Q. mongolica died. The possible explanation for this was that Q. mongolica could not stand the drought. Therefore, at same drought condition F. mandshurica cost less water than A. mono. The result verified the expectation that if the composition of tree species was changed, forests would have less water loss under drought condition. The drought-tolerant species would be those that cost less water but could also grow under drought condition. This result had implications for planting trees at arid and semi-arid areas. Dynamics of water loss
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Generally, the 3 tree species decreased their water loss when they were under low RWCS conditions, but their responses were different. The ‘spiking’ in these figures were related with soil rehydration and local weather change because in different weather conditions the water losses were quite different. For F. mandshurica the dynamics of water loss during the growing season was significantly different between 4 levels of RWCS, it was sensitive to the alternation of RWCS. The patterns of water loss at RWCS of 100%, 70% and 50% were similar, but at RWCS of 30% it varied substantially (Fig. 2). For A. mono the dynamics of water loss between water content
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Figure 10. The dynamics of diurnal leaf transpiration for Quercus mongolica at RWCS of 100% ( ) and 50% ( ).
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Time (h) Figure 11. The dynamics of diurnal WUE for Fraxinus mandshurica at RWCS of 100% ( ); 70% ( ); 50% ( ) and 30% ( ).
of 100% and 30%, 70% and 30%, 50% and 30% were significantly different (po0.05). The average water loss between all treatments was significantly different except between RWCS of 70% and 50%, 70% and 30% (Fig. 3). For Q. mongolica the dynamics of water loss between RWCS of 100% and 50%, 100% and 30%, 50% and 30% were significantly different although there was no treatment of RWCS of 70% and seedlings under RWCS of 30% died after 34 days (Fig. 4). Without water stress the dynamics of water loss of 3 species in the growing season were different, the difference between F. mandshurica and Q. mongolica, A. mono and Q. mongolica were significant (po0.01). However, at RWCS of 50% the water loss progression of the 3 species were similar, and at RWCS of 30% the water loss of A. mono was significantly higher than that of F. mandshurica. Therefore, RWCS of 30% was threshold for seedlings of F. mandshurica, A. mono. and Q. mongolica. Seasonal transpiration dynamics For the 3 species, the dynamics of leaf transpiration during the growing period was different (Figs 5–7). For F. mandshurica the leaf transpiration of seedlings at RWCS of 30% was significantly higher than that at RWCS of 100%, but the dynamics of transpiration between other treatments were not significant. For A. mono and Q. mongolica the transpiration dynamics between all treatments had no significant difference. Then, it seemed that RWCS had no definite relation with leaf transpiration of 3 species seedlings. Therefore, this result partly disagreed with the first hypothesis that the transpiration would decrease under drought, and it also contrasted with the evidence of many species that transpiration decreased as soil dried (Tyree & Ewers,
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1991, 1996, Thomas & Eamus, 1999). However, results from Zhang et al. (1997), Read & Farquhar (1994) and Palmroth et al. (1999) also indicated that ‘plants in dry habitats have a conservative water use strategy’ might not be generally true. The vulnerability of Q. mongolica at RWCS of 30% could be derived from a loss of hydraulic conductance in the soil–plant–atmosphere continuum (Tyree & Sperry, 1989; Hacke & Sauter, 1995; Cochard et al. 1996). At RWCS of 100% F. mandshurica and A. mono had higher average leaf transpiration rate (ALTR) than that of Q. mongolica, respectively (po0.01). This was consistent with the total water loss of these species. ALTR was not significantly different among the three species at RWCS of 50% and 30%, respectively, although at RWCS of 50% and 30% A. mono. cost more water than F. mandshurica and Q. mongolica, respectively. The main reason for the contrast between total water loss and ALTR might be leaf area. Diurnal transpiration
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The leaf transpiration rate of 3 species increased in the morning (Figs 8–10, and reached the maximum at noon, then decreased in the afternoon. There was one peak for diurnal leaf transpiration without water stress, but there were usually several small peaks at RWCS of 70%, 50% and 30%. The ranges of leaf transpiration for different
Figure 14. Stomatal conductance of Fraxinus mandshurica, Acer mono and Quercus mongolica at different RWCS (Abbreviatons are the same as in Fig. 1).
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species under different water contents were different. For F. mandshurica the dynamics of the diurnal leaf transpiration were not significantly different between the treatments at RWCS of 100% and 70%, 100% and 50%, respectively, but the diurnal leaf transpiration was significantly different between RWCS of 100% and 30%. It was similar to A. mono. Therefore, RWCS of 30% would be the threshold for leaf transpiration. Under low RWCS in early morning the leaf transpiration of F. mandshurica was very low and it approached close to 0, but the leaf transpiration rate increased more quickly for the seedlings under RWCS of 50% and 30%. It reached its maximum at noon. For A. mono the dynamics of the diurnal transpiration between RWCS of 100% and 50%, 100% and 30%, 70% and 50% were significantly different, respectively. Therefore, for F. mandshurica and A. mono the diurnal leaf transpiration would increase under low RWCS, this result seemed to disagree with the first hypothesis. The possible explanation of high transpiration at low RWCS might be the increased leaf temperature, and the leaf temperature of seedlings of 3 species under low RWCS condition was about 0.5–11C higher than those at RWCS of 100% during the growing season. Schulze & Hall (1982) pointed out that the high evaporative demand was a characteristic peculiarity of species less tolerant to drought. It indicated the setting up of a feed-forward response that would allow the species to avoid large water deficits (Farquhar, 1978; Schulze, 1986; Schulze et al., 1987; Aranda et al., 2000). But for Q. mongolica the dynamics of diurnal transpiration between RWCS of 100% and 50% was not significantly different, and the seedlings died at RWCS of 30%. Therefore, the leaf transpiration rate would not always decrease under low RWCS for all plant species, drought–tolerant species should have lower transpiration rate and lower water loss at low RWCS. Diurnal WUE There were not significant differences between the dynamics of diurnal WUE of F. mandshurica , Q. mongolica and A. mono at each RWCS (Figs 11–13). It indicated that 3 species had similar WUE at the same RWCS. For F. mandshurica and Q. mongolica the dynamics of diurnal WUE were not significantly different between different RWCS. But for A. mono the behaviors of diurnal WUE at RWCS of 50% and 100% were slightly different, the seedlings at RWCS of 100% had peak at around 9 a.m. and 13 a.m., on the contrary, the seedlings at RWCS of 50% had one peak at around 11 a.m. From 10 a.m. to 12 a.m. the photosynthetic rate of seedlings at RWCS of 100% was very low and close to 0, but it reached its maximum for seedlings under lower RWCS. The dynamics of diurnal WUE of A. mono under different RWCS were significantly different except under RWCS of 50% and 30%, but their averages had no significant difference. Plants that possessed drought tolerance could well establish their photosynthesis which were resistant to water shortage (Brestic et al., 1995; Franca et al., 2000). Therefore, the change of RWCS would not significantly change WUE for the three species, but Lawlor (1995) indicated that there was no unified concept of the events which increased or reduced photosynthetic efficiency. This result would disagree with the second hypothesis that WUE would increase under drought stress. Diurnal average stomatal conductance Plants, which are drought tolerant, generally have lower leaf stomatal conductance than those that are not. However, the variances of stomatal conductance during a day at different RWCS were considerable, so only the stomatal conductance at different RWCS of one typical day (August 11) were compared. If the diurnal maximum stomatal conductance of each species at different RWCS were compared, there were
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no significant differences among different RWCS for each species. Then, the diurnal average stomatal conductance (ASC) were compared (Fig. 14). ASC of F. mandshurica were not significantly different by statistics among different RWCS (p40.05), but ASC were higher at RWCS of 70% and 50%. Also, ASC of Q. mongolica were not significantly different between RWCS of 100% and 50%. However, ASC of A. mono at RWCS of 70% was significantly higher than that at RWCS of 50% and 30%, respectively. And ASC at RWCS of 100% was significantly higher than that at RWCS of 50%. Then, ASC of A. mono was sensitive to RWCS. ASC at different RWCS could be used to select drought–tolerant species. In conclusion, this research showed that the water vapor exchange properties of different tree seedlings would be changed except WUE under different RWCS. Trees did cost a lot of water during their life times, so it would be possible to decrease water loss from forests by changing tree species. Tree species which had drought tolerance and low water cost were useful for this activity. But the challenge was how to select this kind of tree species. Water loss during the growing period was a good index, however, it was hard to obtain without experiment. Leaf transpiration and average stomatal conductance had no direct relation with water loss because water loss also depended on the total leaf area of trees. Similar research on adult tree should be done. However, low leaf transpiration, low average stomatal conductance and leaf area, and high WUE were alternative index to select tree species that could decrease water loss and also grow in arid and semi-arid areas. Financial support for this research was provided jointly by NKBRSF project (G1999043407) and KSCX2-1-07.
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Laffray, D. & Louguet, P. (1990). Stomatal responses and drought resistance. Bulletin de la Societe Botanique de France, 137: 47–60. Lawlor, D.W. (1995). The effects of water deficit on photosynthesis. In: Smirnoff, N. (Ed.), Environment and Plant Metabolism Flexibility and Acclimation, pp. 129–160. Oxford: BIOS. 270 pp. Levitt, J. (1980). Response of Plants to Environmental Stresses, Vol. II. Water, Radiation, Salt and other Stresses. London: Academic Press. 657 pp. Palmroth, S., Berninger, F., Nikinmaa, E., Lloyd, J., Pulkkinen, P. & Hari, P. (1999). Structural adaptation rather than water conservation was observed in Scots pine over a range of wet to dry climates. Oecologia, 121: 302–309. Read, J. & Farquhar, G. (1994). Comparative studies in Nothofagus. Leaf carbon isotope discrimination. Function Ecology, 5: 684–695. Schulze, E.-D. & Hall, A.E. (1982). Stomatal responses, water loss and CO2 assimilation rates of plants in contrasting environments. In: Pirson, A. & Zimmermann, M. H. (Eds), Encyclopedia of Plant Physiology, NS. Physiological Plant Ecology II, Vol. 12B, pp. 181–230. Berlin, Heidelberg, New York: Springer. Schulze, E.D. (1986). Carbon dioxide and water vapour exchange in response to drought in the atmosphere and in the soil. Annual Review of Plant Physiology, 37: 247–274. Schulze, E.-D., Turner, N.C., Gollan, T. & Shackel, K.A. (1987). Stomatal responses to air humidity and to soil drought. In: Zeiger, E., Farqhuar, G.D. & Cowan, I. R. (Eds.) Stomatal Function, pp. 311–322. Stanford, CA: Stanford University Press. 503 pp. Shantz, H. L. (1927). Drought resistance and soil moisture. Ecology, 8: 145-157. Sobrado, M.A. (2000). Relation of water transport to leaf gas exchange properties in three mangrove species. Trees, 14: 258–262. Thomas, D.S. & Eamus, D. (1999). The influence of predawn leaf water potential on stomatal responses to atmospheric water content at constant Ci and on stem hydraulic conductance and foliar ABA concentrations. Journal of Experimental Botany, 50: 243–251. Tyree, M.T. & Sperry, J.S. (1989). Vulnerability of xylem to cavitation and embolism. Annual Review of Plant Physiology and Plant Molecular Biology, 40: 19–38. Tyree, M.T. & Ewers, F.W. (1991). Tansley review 34. The hydraulic architecture of trees and other woody plants. New Phytologist, 119: 345–360. Tyree, M.T. & Ewers, F.W. (1996). Hydraulic architecture of woody tropical plants. In: Mulkey, S.S., Chazdon, R.L. & Smith, A.P.S. (Eds), Tropical Forest Plant Ecophysiology, pp. 217–243. New York: Chapman & Hall. 675 pp. Zhang, J. W., Feng, Z., Cregg, B.M. & Schumann, C.M. (1997). Carbon isotopic composition, gas exchange, and growth of three populations of ponderosa pine differing in drought tolerance. Tree Physiology, 17: 461–466. Zhang, P., Shao, G., Zhao G., Master, D. C. L., Parker, G. R., Dunning, J. B. & Li, Q. (2000). China’s forest policy for the 21st century. Science, 288: 2135-2136.
Change of water vapor exchange properties of 3 tree species under drought
Chen Xiongwen* Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, People’s Republic of China (Received 7 November 2000; and accepted 16 November 2001) The water vapor exchange properties of Fraxinus mandshurica Rupr., Quercus mongolica Fish. et Turcz. and Acer mono Maxim. at the relative water content of soil (RWCS) of 100%, 70%, 50% and 30% were studied by analysis of water loss, transpiration, water use efficiency (WUE) and stomatal conductance. Seedlings did cost a lot of water during the growing season, and the total water loss of each kind of seedling decreased differently under low RWCS conditions. The dynamics of water loss for 3 species under different RWCS were different. At RWCS of 100%, F. mandshurica and A. mono had higher average leaf transpiration rate than that of Q. mongolica, respectively. Leaf diurnal transpiration of 3 species decreased considerably at RWCS of 30%. But the diurnal WUE was not sensitive to the change of RWCS for these species. Only the average stomatal conductance of A. mono changed significantly under lower RWCS. The implication of these results was that tree species adapted to lower RWCS by changing its water vapor exchange properties, and it would be possible to decrease water loss from forests by selecting proper tree species. # 2002 Elsevier Science Ltd. Keywords: Relative water content of soil; stomatal conductance; transpiration; water loss; water use efficiency
Introduction North-east forest area is the largest and most important forest area in China, but the climate in the region is likely to be altered by global climate change, and predictions show that in an decrease in precipitation and increase in temperature will take place and drought will be inevitable. Furthermore, the increasing urbanization in this area needs more water resources. Then, the whole area is facing water shortage. In fact, water shortage often occurred in the large cities of this area during the recent decades. However, forests cost a large amount of water by photosynthesis and transpiration of trees. If trees can maintain their growth with less water, forests can reduce water consumption, then, water shortage can be alleviated to some extent. Therefore, a *Corresponding author. E-mail: [email protected] 0140-1963/02/030423 + 13 $35.00/0
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proper understanding about water consumption of tree species and change in water vapor exchange properties of tree species under drought condition is necessary. On the other hand, this research also has implications for the project of western developing that the Chinese government is carrying out to plant trees and restore grass at large scale in the western arid and semi-arid land to improve its environment (Jiang, 2000; Zhang et al., 2000). But how much water should be supplied in order to grow trees in the arid and semi-arid area is not known, and it is also controversial as to whether trees will exhaust the limited underground water. Water availability affects trees in several ways and the plant response to drought is complex. Usually plants adapt to the dry environment by drought avoidance and drought tolerance, which includes whole plant mechanisms that provide plants with the ability to respond and survive drought (Shantz, 1927; Levitt, 1980; Laffray & Louguet, 1990). Hydraulic properties of plants have been related to the evolution and adaptation of plants to environmental constraints (Tyree & Ewers; 1991). Many researches have been done about the ecophysiological effects of drought on trees, but the research of the water use strategies and mechanisms of water loss of different tree species under drought conditions is limited (Borghetti et al., 1998; Palmroth et al., 1999; Sobrado 2000). In the North-east forest area of China Quercus mongolica Fish. and Turcz. is a pioneer tree species, and it tends to grow during drought and on unfertilized soils and show higher tolerance to drought (Abrams, 1990; Breda et al., 1993; Epron & Dreyer, 1993). Fraxinus mandshurica Rupr. is one of the good tree species for wood, and Acer mono Maxim. is an important co-occurring species in the forest. The aim of this research is to study the change of water vapor exchange properties for 3 species under different water contents of soil. The water vapor exchange properties of seedlings in the research include water loss of whole plant, seasonal change of leaf transpiration, diurnal dynamics of leaf transpiration, water use efficiency and stomatal conductance. The underlying hypotheses of the research are that (i) seedlings will decrease the water loss, leaf transpiration and stomatal conductance under lower water content of soil. (ii) There exists threshold of soil water for tree growth. (iii) Leaf WUE will increase under drought.
Material and methods Seedling Seedlings of F. mandshurica, A. mono and Q. mongolica about 2-years old and of similar size (about 10–25 cm in height and 0?2–0?4 cm in stem diameter) were transplanted from the nursery at Changbaishan Ecological Research Station (41?211N, 128?101E) of the Chinese Academy of Sciences (CAS) to the greenhouse of Institute of Botany (39?561N, 116?171E) of the CAS at Beijing in June, 2000. The vegetation of the former site is temperate mixed coniferous and broadleaved forest, and the vegetation of the latter site is temperate deciduous broadleaved forest. Because the mean air temperature at Beijing was about 4–101C higher than it was at Changbaishan during the growing season, it would be helpful to study tree response under drought condition and climatic change scenario. All the leaves of seedlings were cut at the beginning of the experiment. Each plastic pot was planted by one seedling, and the pot size was about 20 cm in diameter and 15 cm in height. These seedlings were grown in the moisture loamy soil for several weeks, when seedlings had 2–3 leaves the relative water content of soil (RWCS) began to decrease. The top of each pot was covered tightly by two plastic sheets and only the stem of the seedling was left out so that the water evaporation from soil would be decreased. There were 8 seedlings for each treatment, and the total number of seedling is about 96. Because the greenhouse was semi-opened (free air and
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sun-light could enter but rain could not enter), the environmental conditions, such as temperature and radiation regimes, varied with the local climate. Water treatment The drought was controlled by RWCS. There were about RWCS of 100%, 70%, 50% and 30%. Then, each pot (including seedling, soil and pot) had its prescriptive weight by its treatment. Every morning or another each pot was weighed and watered to its prescriptive weight by scale if it was below that. Then, the water loss (evapotransipiration) of the seedling during this time period could be figured out by the following formula: Water loss ¼ weight of last time weight of this time The total water loss of each species was the sum of water loss that occurred every day. Transpiration and stomatal conductance measurements The rate of photosynthesis and transpiration and stomatal conductance were measured at the same time for the top leaf, in which the leaf area was bigger than the leaf chamber, at least 3 times by Portable Photosynthesis System of LCA-4 (Analytical Development Corporation, U.K.) every hour. Every 2 or 3 weeks the diurnal photosynthesis, transpiration and stomatal conductance were measured for 3 species of each treatment from 8 a.m. to 5 p.m. The data of every treatment were summarized by calculating the average of 8 replicates at one time. The seasonal data are just time-series of the average of every day. Water use efficiency (WUE) Leaf water use efficiency (WUE) was calculated by the photosynthetic rate/ transpiration rate. Statistical analysis The average values of 8 replicates were analysed by t-test procedure of SAS, and the dynamics of each index was analysed by (i) ANOVA at p ¼ 0?01 and 0?05 level of error. and (ii) visual sense. Results Total water loss Seedlings did cost a lot of water during their life times. When there was no water stress (RWCS of 100%) F. mandshurica cost about 8.7 kg water during the growing season, and it was the water loss among 3 tree species. The total water loss decreased when seedlings were under low RWCS (Fig. 1). At RWCS of 50% A. mono cost more water than F. mandshurica and Q. mongolica, respectively. When RWCS decreased to 30% A. mono had more water loss than F. mandshurica, but the unexpected thing was that after 34 days Q. mongolica at RWCS of 30% died. Dynamics of water loss Three tree species decreased their water loss when they were under low RWCS, but their responses were different. The water loss for F. mandshurica during the growing
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Tree species of different treatments Figure 1. The total water loss for each seedling of 3 species in the growing season. Fm100: Fraxinus mandshurica at the relative water content of soil (RWCS) of 100%. Fm70: Fraxinus mandshurica at RWCS of 70%. Fm50: Fraxinus mandshurica at RWCS of 50 %. Fm30: Fraxinus mandshurica at RWCS of 30%. Am100: Acer mono at RWCS of 100%. Am70: Acer mono at RWCS of 70%. Am50: Acer mono at RWCS of 50%. Am30: Acer mono at RWCS of 30%. Qm100: Quercus mongolica at RWCS of 100%. Qm50: Quercus mongolica at RWCS of 50%.
season among RWCS of 100%, 70%, 50% and 30% were significantly different (po0?05) (Fig. 2). For A. mono the water loss between RWCS of 100% and 30%, 70% and 30%, 50% and 30% were significantly different, respectively, and the average water loss between all treatments were significantly different except between RWCS of 70% and 50%, 70% and 30% (Fig. 3). For Q. mongolica, the water loss between RWCS of 100% and 50%, 100% and 30%, 50% and 30% were significantly different (Fig. 4), respectively, although there were no treatment of RWCS of 70% and seedlings under RWCS of 30% died after 34 days. At RWCS of 100% the dynamics of water loss of 3 species in the growing season was different, the difference between F. mandshurica and Q. mongolica, A. mono and Q. mongolica was significant (po0?01), respectively. However, at RWCS of 50% the seasonal water loss dynamics of the 3 species were similar, and at RWCS of 30% the water loss of A. mono was significantly higher than that of F. mandshurica. Seasonal transpiration dynamics For F. mandshurica the seasonal leaf transpiration rate at RWCS of 100% was significantly lower than that at RWCS of 30% (po0?05), the dynamics of
Figure 2. The dynamics of water loss for Fraxinus mandshurica under RWCS of 100%, 70%, 50% and 30%: ( ) 100%, ( ) 70%; ( ) 50%; ( ) 30%.
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Figure 3. The dynamics of water loss for Acer mono under RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
transpiration rate between other treatments were not significantly different (Fig. 5). For A. mono and Q. mongolica the seasonal transpiration between all treatments was not significantly different for each species, respectively (Figs 6 & 7). At RWCS of 100% F. mandshurica and A. mono had higher average leaf transpiration rate (ALTR) than that of Q. mongolica, respectively (po0?01). ALTR was not significantly different between the three species at RWCS of 50% and 30%, respectively. Diurnal transpiration For F. mandshurica, the dynamics of the diurnal leaf transpiration rate was significantly different between RWCS of 100% and 30% (Fig. 8). For A. mono the dynamics of the diurnal transpiration rate between 100% and 50%, 100% and 30%, 70% and 50% were significantly different, respectively (Fig. 9). But for Q. mongolica the dynamics of diurnal transpiration rate between RWCS of 100% and 50% was not significantly different (Fig. 10). The diurnal leaf transpiration rate of 3 species at different RWCS increased in the morning, and reached the maximum at noon, then decreased in the afternoon. At RWCS of 100% there was one peak of diurnal leaf transpiration rate, but there were usually several small peaks at RWCS of 70%, 50% and 30%. Diurnal WUE The dynamics of diurnal WUE for F. mandshurica and Q. mongolica were not significantly different between different RWCS, respectively (Figs 11&12). However,
Figure 4. The dynamics of water loss for Quercus mongolica under RWCS of 100%, 50% and 30% (legends are the same as in Fig. 2).
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Figure 5. The dynamics of leaf transpiration for Fraxinus mandshurica at RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
the dynamics of diurnal WUE for A. mono under different RWCS were significantly different except between RWCS of 50% and 30% (Fig. 13). The dynamics of diurnal WUE between F. mandshurica, Q. mongolica and A. mono at each RWCS were not significantly different (p40?05).
Diurnal average stomatal conductance The variances of stomatal conductance during a day at certain RWCS were considerable, so only the average stomatal conductance (ASC) at different RWCS were compared (Fig. 14). ASC of F. mandshurica was not significantly different by statistics among different RWCS (p40?05), but ASC was higher at RWCS of 70% and 50%. Also ASC of Q. mongolica was not significantly different between RWCS of 100% and 50%. However, ASC of A. mono at RWCS of 70% was significantly higher than that at RWCS of 50% and 30%, respectively, and ASC at RWCS of 100% was significantly higher than that at RWCS of 50%.
Figure 6. The dynamics of leaf transpiration for Acer mono at RWCS of 100%, 70%, 50% and 30% (legends are the same as in Fig. 2).
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Figure 7. The dynamics of transpiration rate for Quercus mongolica at RWCS of 100%, and 50% (legends are the same as in Fig. 2).
Discussion Total water loss
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Seedlings did cost a lot of water during their life times. When there was no water stress (RWCS of 100%) F. mandshurica cost about 8.7 kg water during the growing season, and it was the highest water loss among 3 tree species (Fig. 1). It could be assured that the adult trees would cost far more water. If forest reduced water loss, the water shortage in North-east China could be alleviated to some extent. The total water loss of Q. mongolica was lowest among 3 tree species when there was no water stress, from this it could be inferred that Q. mongolica was able to grow in more drought condition. However, the water cost of Q. mongolica at RWCS of 50% was only slightly lower than it was at RWCS of 100%. Therefore, Q. mongolica could be planted to reduce water loss from forest, but it could not be planted at drought area because it cost relatively more water and also it was hard to survive in drought condition. For each species, the total water loss decreased when seedlings were under RWCS of 70%, 50% and 30% except A. mono at RWCS of 50% and 70%. So the first hypothesis was verified, but it did not mean that the water loss always decreased linearly as RWCS decreased. At
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RWCS of 50%, A. mono cost more water than F. mandshurica and Q. mongolica, respectively. When the RWCS decreased to 30% A. mono had more water loss than F. mandshurica, but the unexpected thing was that after 34 days Q. mongolica died. The possible explanation for this was that Q. mongolica could not stand the drought. Therefore, at same drought condition F. mandshurica cost less water than A. mono. The result verified the expectation that if the composition of tree species was changed, forests would have less water loss under drought condition. The drought-tolerant species would be those that cost less water but could also grow under drought condition. This result had implications for planting trees at arid and semi-arid areas. Dynamics of water loss
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Generally, the 3 tree species decreased their water loss when they were under low RWCS conditions, but their responses were different. The ‘spiking’ in these figures were related with soil rehydration and local weather change because in different weather conditions the water losses were quite different. For F. mandshurica the dynamics of water loss during the growing season was significantly different between 4 levels of RWCS, it was sensitive to the alternation of RWCS. The patterns of water loss at RWCS of 100%, 70% and 50% were similar, but at RWCS of 30% it varied substantially (Fig. 2). For A. mono the dynamics of water loss between water content
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Figure 10. The dynamics of diurnal leaf transpiration for Quercus mongolica at RWCS of 100% ( ) and 50% ( ).
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of 100% and 30%, 70% and 30%, 50% and 30% were significantly different (po0.05). The average water loss between all treatments was significantly different except between RWCS of 70% and 50%, 70% and 30% (Fig. 3). For Q. mongolica the dynamics of water loss between RWCS of 100% and 50%, 100% and 30%, 50% and 30% were significantly different although there was no treatment of RWCS of 70% and seedlings under RWCS of 30% died after 34 days (Fig. 4). Without water stress the dynamics of water loss of 3 species in the growing season were different, the difference between F. mandshurica and Q. mongolica, A. mono and Q. mongolica were significant (po0.01). However, at RWCS of 50% the water loss progression of the 3 species were similar, and at RWCS of 30% the water loss of A. mono was significantly higher than that of F. mandshurica. Therefore, RWCS of 30% was threshold for seedlings of F. mandshurica, A. mono. and Q. mongolica. Seasonal transpiration dynamics For the 3 species, the dynamics of leaf transpiration during the growing period was different (Figs 5–7). For F. mandshurica the leaf transpiration of seedlings at RWCS of 30% was significantly higher than that at RWCS of 100%, but the dynamics of transpiration between other treatments were not significant. For A. mono and Q. mongolica the transpiration dynamics between all treatments had no significant difference. Then, it seemed that RWCS had no definite relation with leaf transpiration of 3 species seedlings. Therefore, this result partly disagreed with the first hypothesis that the transpiration would decrease under drought, and it also contrasted with the evidence of many species that transpiration decreased as soil dried (Tyree & Ewers,
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1991, 1996, Thomas & Eamus, 1999). However, results from Zhang et al. (1997), Read & Farquhar (1994) and Palmroth et al. (1999) also indicated that ‘plants in dry habitats have a conservative water use strategy’ might not be generally true. The vulnerability of Q. mongolica at RWCS of 30% could be derived from a loss of hydraulic conductance in the soil–plant–atmosphere continuum (Tyree & Sperry, 1989; Hacke & Sauter, 1995; Cochard et al. 1996). At RWCS of 100% F. mandshurica and A. mono had higher average leaf transpiration rate (ALTR) than that of Q. mongolica, respectively (po0.01). This was consistent with the total water loss of these species. ALTR was not significantly different among the three species at RWCS of 50% and 30%, respectively, although at RWCS of 50% and 30% A. mono. cost more water than F. mandshurica and Q. mongolica, respectively. The main reason for the contrast between total water loss and ALTR might be leaf area. Diurnal transpiration
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The leaf transpiration rate of 3 species increased in the morning (Figs 8–10, and reached the maximum at noon, then decreased in the afternoon. There was one peak for diurnal leaf transpiration without water stress, but there were usually several small peaks at RWCS of 70%, 50% and 30%. The ranges of leaf transpiration for different
Figure 14. Stomatal conductance of Fraxinus mandshurica, Acer mono and Quercus mongolica at different RWCS (Abbreviatons are the same as in Fig. 1).
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species under different water contents were different. For F. mandshurica the dynamics of the diurnal leaf transpiration were not significantly different between the treatments at RWCS of 100% and 70%, 100% and 50%, respectively, but the diurnal leaf transpiration was significantly different between RWCS of 100% and 30%. It was similar to A. mono. Therefore, RWCS of 30% would be the threshold for leaf transpiration. Under low RWCS in early morning the leaf transpiration of F. mandshurica was very low and it approached close to 0, but the leaf transpiration rate increased more quickly for the seedlings under RWCS of 50% and 30%. It reached its maximum at noon. For A. mono the dynamics of the diurnal transpiration between RWCS of 100% and 50%, 100% and 30%, 70% and 50% were significantly different, respectively. Therefore, for F. mandshurica and A. mono the diurnal leaf transpiration would increase under low RWCS, this result seemed to disagree with the first hypothesis. The possible explanation of high transpiration at low RWCS might be the increased leaf temperature, and the leaf temperature of seedlings of 3 species under low RWCS condition was about 0.5–11C higher than those at RWCS of 100% during the growing season. Schulze & Hall (1982) pointed out that the high evaporative demand was a characteristic peculiarity of species less tolerant to drought. It indicated the setting up of a feed-forward response that would allow the species to avoid large water deficits (Farquhar, 1978; Schulze, 1986; Schulze et al., 1987; Aranda et al., 2000). But for Q. mongolica the dynamics of diurnal transpiration between RWCS of 100% and 50% was not significantly different, and the seedlings died at RWCS of 30%. Therefore, the leaf transpiration rate would not always decrease under low RWCS for all plant species, drought–tolerant species should have lower transpiration rate and lower water loss at low RWCS. Diurnal WUE There were not significant differences between the dynamics of diurnal WUE of F. mandshurica , Q. mongolica and A. mono at each RWCS (Figs 11–13). It indicated that 3 species had similar WUE at the same RWCS. For F. mandshurica and Q. mongolica the dynamics of diurnal WUE were not significantly different between different RWCS. But for A. mono the behaviors of diurnal WUE at RWCS of 50% and 100% were slightly different, the seedlings at RWCS of 100% had peak at around 9 a.m. and 13 a.m., on the contrary, the seedlings at RWCS of 50% had one peak at around 11 a.m. From 10 a.m. to 12 a.m. the photosynthetic rate of seedlings at RWCS of 100% was very low and close to 0, but it reached its maximum for seedlings under lower RWCS. The dynamics of diurnal WUE of A. mono under different RWCS were significantly different except under RWCS of 50% and 30%, but their averages had no significant difference. Plants that possessed drought tolerance could well establish their photosynthesis which were resistant to water shortage (Brestic et al., 1995; Franca et al., 2000). Therefore, the change of RWCS would not significantly change WUE for the three species, but Lawlor (1995) indicated that there was no unified concept of the events which increased or reduced photosynthetic efficiency. This result would disagree with the second hypothesis that WUE would increase under drought stress. Diurnal average stomatal conductance Plants, which are drought tolerant, generally have lower leaf stomatal conductance than those that are not. However, the variances of stomatal conductance during a day at different RWCS were considerable, so only the stomatal conductance at different RWCS of one typical day (August 11) were compared. If the diurnal maximum stomatal conductance of each species at different RWCS were compared, there were
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no significant differences among different RWCS for each species. Then, the diurnal average stomatal conductance (ASC) were compared (Fig. 14). ASC of F. mandshurica were not significantly different by statistics among different RWCS (p40.05), but ASC were higher at RWCS of 70% and 50%. Also, ASC of Q. mongolica were not significantly different between RWCS of 100% and 50%. However, ASC of A. mono at RWCS of 70% was significantly higher than that at RWCS of 50% and 30%, respectively. And ASC at RWCS of 100% was significantly higher than that at RWCS of 50%. Then, ASC of A. mono was sensitive to RWCS. ASC at different RWCS could be used to select drought–tolerant species. In conclusion, this research showed that the water vapor exchange properties of different tree seedlings would be changed except WUE under different RWCS. Trees did cost a lot of water during their life times, so it would be possible to decrease water loss from forests by changing tree species. Tree species which had drought tolerance and low water cost were useful for this activity. But the challenge was how to select this kind of tree species. Water loss during the growing period was a good index, however, it was hard to obtain without experiment. Leaf transpiration and average stomatal conductance had no direct relation with water loss because water loss also depended on the total leaf area of trees. Similar research on adult tree should be done. However, low leaf transpiration, low average stomatal conductance and leaf area, and high WUE were alternative index to select tree species that could decrease water loss and also grow in arid and semi-arid areas. Financial support for this research was provided jointly by NKBRSF project (G1999043407) and KSCX2-1-07.
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