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Drought: The most important physical stress of terrestrial ecosystems
Acta Ecologica Sinica 34 (2014) 179–183
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Drought: The most important physical stress of terrestrial ecosystems q Bin He a,b,⇑, Xuefeng Cui a,b, Honglin Wang b, Aifang Chen b a b
College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Revised 18 November 2013 Accepted 8 May 2014
Keywords: Drought Stress Terrestrial ecosystems Wildfire Tree mortality
a b s t r a c t Drought is projected to become more prevalent in the future due to climate change, and its impact on the fate of terrestrial ecosystems has aroused great concern in the scientific community over the past decade. Mounting evidence suggests that drought may be the most important physical stress of terrestrial ecosystems: drought limits vegetation growth, increases wildfires, and induces tree mortality, among other impacts. Drought not only weakens the carbon sink function of terrestrial ecosystems but also may interfere directly or indirectly with biosphere–atmosphere interactions, further exacerbating climate change. This paper reviews the current evidence of the impacts of drought on terrestrial ecosystems, with particular emphasis on the ways in which drought alters the biological, biogeophysical and biogeochemical processes underlying the interaction between the biosphere and the atmosphere. Ó 2014 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History drought situation and future projection . . . . . . . . Drought reduces terrestrial ecosystem productivity . . . . . Drought increases wildfire. . . . . . . . . . . . . . . . . . . . . . . . . . Drought induces tree mortality . . . . . . . . . . . . . . . . . . . . . . The role of drought in the biosphere–atmosphere system Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Terrestrial ecosystems play a key role in global carbon cycling [43], as they currently sequester 20–30% of global anthropogenic CO2 emissions [46]. Conversely, global terrestrial NPP is also a major driving force of the interannual CO2 growth rate [67]. Over the past several decades, climate change seems to have had a generally positive impact on terrestrial ecosystems [24,8], a phenomenon known as ‘‘the green trend’’ [3,41]. However, an increasing
q Fundings: This work was supported by the National Science and Technology Support Program (Grant No. 2012BAH29B02), National Basic Research Development Program of China (Grant Nos. 2011CB952001 and 2012CB95570001), and National Natural Foundation of China (Grant No. NSFC41301076). ⇑ Corresponding author at: College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China. E-mail address: [email protected] (B. He).
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number of studies have shown that the reduction of net primary production in the global terrestrial ecosystem due to warming and drought has greatly weakened this ‘‘positive feedback’’ [67,24,17,31]. For example, severe drought in moist tropical forests should provoke large carbon emissions by increasing forest flammability and tree mortality and by suppressing tree growth [38]. The benefits of climate change for middle and high latitude terrestrial ecosystems are also being weakened by severe drought [64,32]. Drought frequency and severity are predicted to increase across numerous continental interiors [21], and the consequences of these changes for the dominant plant species are largely unknown [36]. A comprehensive knowledge of drought impacts and related disturbances is critical for understanding the interactions between terrestrial ecosystems and the atmosphere. Drought could cause terrestrial ecosystems to act as carbon sources to the atmosphere in several ways, such as by suppressing tree growth, reducing both autotrophic and heterotrophic
http://dx.doi.org/10.1016/j.chnaes.2014.05.004 1872-2032/Ó 2014 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
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B. He et al. / Acta Ecologica Sinica 34 (2014) 179–183
respiration, causing plant mortality and disease or increasing rates of fire intensity. All of these impacts could significantly reduce terrestrial ecosystem productivity [67,54]. Drought impacts on terrestrial ecosystems have received relatively little scientific attention and may therefore be greatly underestimated. The existing studies do provide substantial evidence for the effects of drought on terrestrial ecosystems, but the possible consequences of these effects remain poorly understood. This paper reviews the primary direct and indirect impacts of drought on terrestrial ecosystems, as well as the biological and chemical processes of interaction between drought, terrestrial ecosystems and climate change. This study aims to further the understanding of the role drought playing in terrestrial ecosystems.
2. History drought situation and future projection As a normal part of climate variability, drought has occurred many times over the past 1000 years across many parts of the world [20]. Since the 20th century, however, drought has increased on both the regional and global scales [20]. The global area impacted by drought has increased sharply since the 1970s, a rise attributed, in part, to global warming [19,28]. Some recent research proposes that the increase in global drought conditions has been overestimated due to the inherent deficiencies of the drought index and that little change in global drought has occurred over the past 60 years [51]. However, an analysis of global drought trends using the SPEI index develop by Vicente-Serrano [60] suggests that areas impacted by severe and extreme drought have increased dramatically. Although the effects of this drought may have been overestimated, the severity of the drought has actually intensified. The drought induced temperature extremes had been reported in several studies [26,37], and the basic mechanism behind this phenomenon is soil moisture variations affecting the partitioning of sensible and latent heat [37]. This issue can be traced back to the relationship between precipitation and temperature. In early studies, more stresses were play on the impacts of temperature on precipitation. For example Isaac and Stuart [27] computed temperature-precipitation relationships for Canadian stations using daily data. For the east and west coasts and northern Canada, more precipitation accompanies warm conditions in winter and cool conditions in summer, thereby reversing the correlation with seasons. Therefore, we think that it is still hard to determine the causal relationship between temperature and water deficiency. This also implies the great importance for the further study of the relationship between drought and temperature, which is crucial for attributes analysis for drought event and the reduction of the uncertainty of future drought projection. Global warming and regional droughts are predicted to intensify and become more frequent during this century due to
anthropogenic climate change [15]. Future drought trends have also been projected using a multi-model under multi-scenario method [21]. Nearly all simulation results suggest that, as a result of decreased precipitation and/or increased evaporation, future droughts will become increasingly serious [21,50,14]. Although, current studies indicates that the future rise in drought and its impacts is potentially dramatic, these projections should be further improved based on more in-depth understanding of the contribution of the natural factors and human activities to drought, respectively. 3. Drought reduces terrestrial ecosystem productivity Research suggests that frequent drought and heat have great impacts on terrestrial carbon cycling by reducing ecosystem productivity, although there remain some uncertainties regarding their effects [17]. Several studies have proposed that dry conditions may actually boost tropical productivity through increases in available solar radiation [45,9]. The terrestrial biosphere could contribute significant amount of CO2 emission during major drought events, such as those evoked by the 1997–1998 El Niño event [43]. These impacts occurred on both the global and regional scales. Table 1 displays a number of major drought events and their impacts on the carbon cycles of their respective terrestrial ecosystems. Among those areas presently studied, the Amazon rainforest is particularly important due to its special status in the global carbon cycle and climate change. This area can process 18 Pg C annually, more than twice the amount of annual anthropogenic fossil fuel emissions [33]. During the 2005 Amazon drought, the total loss of carbon biomass carbon was 1.2–1.6 Pg C [42]. In 2010, the Amazon experienced a second 100 year drought, and the carbon impact of the 2010 drought may eventually exceed 5 billion tons of CO2 released [31]; this figure accounts for 16.6% of global industrial emissions (30.1 Pg C) in 2010. The research warns that, if extreme droughts like these events become more frequent, the Amazon rainforest may lose its ability to act as a natural buffer for anthropogenic carbon emissions [31]. Two other critical ecosystems to Earth’s climate, the boreal forests and the temperate forests, are facing the same fate, their carbon sinks weakened by increasingly frequent drought events [32,57]. At the global scale, a reduction in terrestrial NPP of 0.55 petagrams over the past decade (2000–2009) has been estimated using the global MODIS NPP algorithm. Furthermore, the drying trend of the Southern Hemisphere has decreased the NPP in that area, counteracting the increase in NPP observed over the Northern Hemisphere [67]. Although a number of studies have examined the influences of drought on plant productivity and the terrestrial carbon cycle, the physiological mechanism targeted by drought remain unclear. Drought has generally been considered to reduce photosynthesis
Table 1 Main drought events and their impacts on the carbon cycle of terrestrial ecosystems. ID
Study area
Years of drought
Reduction of carbon
Refs.
1
Northern Hemisphere midlatitude regions
1998–2002
0.9 Pg C yr
1
2
European
2003
0.5 Pg C yr
1
3
Amazon forest
1997–1998 2001 2005 2010
Reduced by 20–30% 0.2 Pg C 1.2–1.6 Pg C More than 5 Pg C
[44] [38] [42] [31]
4
China
Droughts over the period 1901–2002
Most droughts generally reduced NPP and NEP in drought-effect area
[63]
5
Canada’s boreal forests
Drought over 1963–2008
Drought reduced the biomass carbon sink
[32]
6
Southwestern China
2010
100–200 g C m
7
Global
2000–2009
0.5 Pg C
2
yr
1
[64] [17]
[66] [67]
B. He et al. / Acta Ecologica Sinica 34 (2014) 179–183
or impact the autotrophic and heterotrophic respiration of the ecosystem to reduce the GPP [54]. Drought stress reduces the capacity of plants to provide water to their leaves for photosynthesis by disturbing the water transport system ultimately leading to desiccation and mortality [16]. Although different tree species display a wide range of drought sensitivity, recent studies have shown that all forest biomes are equally vulnerable to hydraulic failure without consideration of their current rainfall environment, as their hydraulic safety margins are usually independent of mean annual precipitation [16]. The time scale of drought could be playing a key role in defining the sensitivity of land biomes to drought, arid and humid biomes respond to drought at short time-scales, but the semiarid and subhumid biomes tend to respond to drought at long time-scales [61]. Furthermore, drought events in different seasons may impact vegetation differently. Drought in summer reduces transpiration and limits photosynthesis; while water stress in spring can also directly reduce photosynthesis [66], it can also inhibit canopy development and peak leaf area, thereby leading to the decline in annual net carbon uptake [66,39]. An in-depth understanding of terrestrial ecosystem responses to drought requires long-term, large-scale field monitoring. Future droughts could reduce the terrestrial carbon sink or even convert it into a source, which would send positive feedback to the global climate system and accelerate future global warming [32,63]. Without reliable data for below-ground NPP and a better understanding of the interactions between the biosphere and the atmosphere, however, it is difficult project the fate of the terrestrial ecosystem under future droughts.
4. Drought increases wildfire Fire is one of the most important disturbances in terrestrial ecosystems on a global scale [56] and contributes significantly to the budgets of several trace gases and aerosols [2]. It is generally asserted that anthropogenic climate change will lead to widespread and more frequent fires [15] and that this rise will be related to changes in rainfall and increased temperatures, especially as they lead to drought conditions in areas with abundant fuel loads [62]. The relationship between drought and wildfire has been discussed for many years. Although the connection between drought and fire is intuitive, the mechanisms behind regional patterns of drought are very complex [48]. It is difficult to determine how many fires are directly caused by drought, several studies have proven that drought is an important driving force for fire [25,56,59]. Drought-induced wild fires have been associated with global circulation anomalies such as the El Niño–Southern Oscillation [55] and, more recently, the Pacific Decadal Oscillation [25] and the SSTS in the North Atlantic [18]. In several case studies in southern Borneo, a strong coupling between regional drought intensity and fire emissions has been found by van Der Werf [56]; forest loss rates and areas of vulnerable peatland both increase in drought years. Data collected from tree-ring analyses were used to reconstruct the PDSI and forest fire events over the period 1700–1975 in the U.S. southwest, further supporting the correlation of drought and fire [48]. Studies in California [30], Washington [25], and the American Rocky Mountains [48], have all arrived at similar conclusions. Fire influences the climate system through the release of carbon and atmospheric aerosols and through changing the surface albedo. Although wildfires are usually contained to a tiny area of the globe, they can have huge impacts on the global carbon cycle [47]. Currently, all sources of fire create CO2 emissions almost equal to 50% of those coming from fossil-fuel combustion (2–4 Pg C year 1 versus 7.2 Pg C year 1) [11]. During the period of
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1997–1998 ENSO-related drought event, Indonesian peat fires released an estimated 0.8–2.6 Pg C [40], and during 2000–2006, the average fire emissions from this region increased to 128 ± 51 (1r) Tg C year 1 [56]. The already strong relationship between fire emissions and drought is likely to strengthen due to positive feedback, because increased greenhouse gas concentrations may lead to more frequent or severe drought events [23]. Forest fires increase atmospheric aerosol concentrations, leading to regional and global impacts on the solar heating of the surface and atmosphere as well as the hydrological cycle [5]. Emissions of aerosols could lead to either warming or cooling on a regional scale, which are depending on factors such as aerosol composition, the albedos of both the Earth’s surface and clouds, and so on. Although considerable uncertainty remains, the risk of Amazonian drought may increase due to decreasing aerosol pollution and increased greenhouse gases [18]. A strong relationship between dry-season aerosol optical depth (AOD) and dry-season precipitation also suggests a positive feedback mechanism between aerosols and drought, this may contribute to intensified drought under climate change [5]. Climate modeling studies suggest that these aerosols may lengthen or intensify periods of drought in the Amazon [55] and in Indonesia [18]. Drought may also produce aerosols directly, such as the dust aerosols dramatically increased during drought years in the Sahel [35]. The interactions between drought, fire, aerosols and greenhouse gases are quite complex. Whereas greenhouse gases release the emission of thermal radiation to space, thus warming the surface, aerosols should reflect and absorb solar radiation (the aerosol direct effect) and modify cloud properties (indirect effect), thereby cooling the surface. These disturbances on the radiation balance are very different and need different research approaches [29].
5. Drought induces tree mortality Drought also influences terrestrial ecosystems by increasing tree mortality [65]. Increased tree mortality in response to climate change and intensive drought has been monitored and welldocumented at both the regional and global scale [12], often in combination with other abiotic and biotic factors [16], and the available examples span different forest types and climatic zones [1]. The observed drought-induced mortality at the global level suggests that forests will be at risk if the climate becomes drier; tree die-offs may become more severe and extensive under future global climate change [12]. The relationship between tree mortality and drought is complicated for the following reasons: (1) Drought acts on trees both as a direct physiological stress and also as a factor for other mortality agents such as insects, diseases and fire suppression. Potential interactions of biotic mortality agents often make it much difficult to determine the proximate reason of tree death [12,22,7]. (2) The response of tree mortality to the timing and length of droughts has been shown to be non-linear [7]. Mortality responses at a lag of several years to decades after drought events complicate the investigation of cause-effect relationships [6]. (3) Significant differences exist in the drought responses of different tree species [52], tree sizes [58], ages [53], growth rates [52], locations [52], and distribution regions; even the intensity of disturbance by human activities, measured by the level of intact forest, may determine resistance to drought impact [52]. Projects analyzing drought-induced mortality under future climatic conditions require a better understanding of the fundamental mechanisms underlying tree mortality and survival during drought. Three main hypotheses to explain the physiology of drought-related tree mortality have been developed based on theoretical modeling and experiment: hydraulic-failure, carbon
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starvation and biotic agents [34]. Due to the tempo-spatial mismatch between field observations and laboratory simulations, it is difficult to determine which mechanism is the main driving factor [13]. Before understanding the mechanisms underlying climate change and forest, the quantitative knowledge about the physiological thresholds of individual tree death under chronic or acute water stress should be required [1]. Notably, the relationship between the threshold for tree mortality and drought-induced water stress, which may also underlie the effects of biotic agents such as bark beetles [49], remains uncertain. More severe drought, as produced by climate change, has the potential to trigger or aggravate widespread tree die-off [57,34,13]. The consequences of drought-induced forest die-off should be manifold for ecosystems, and the climate system [65].Widespread global and regional scale die-off events could change forest structure and composition, consequently impacting the stability of land-surface interactions such as carbon sequestration and possibly causing feedback for atmospheric CO2 and climate [57,34,4]. Multi-year drought has been shown to initiate prolonged growth decreases that increase a tree’s long-term risk of death [6] for two reasons: (1) Tree death occurs generally several years or even several decades after the drought [6]. (2) Reductions in tree density following mortality events failed to buffer sites against additional mortality during subsequent droughts, so the ‘release effect’ of tree mortality did not increase the ability of the surviving trees to endure the next severe drought [36]. Furthermore, extended periods of warm, dry conditions could weaken the ability of trees to withstand insect attacks [1,22], which may lead to more severe tree mortality. More researches are still needed on micro-climatic, hydrological, and biogeochemical consequences of and feedbacks to mortality [65].
6. The role of drought in the biosphere–atmosphere system It has been recognized that biological processes can control the earth system in a significant way [24]. The world’s forests influence climate system through physical, chemical, and biological processes that impact planetary energetics, the hydrologic cycle, and atmospheric composition [10]. The role of drought in the biosphere–atmosphere system is summarized in Fig. 1, in which it is shown that drought is a primary driver of feedback interactions
Terrestrial ecosystem
Severe and frequent drought
Reducing productivit y growth
Releasing CO2
Caused tree mortality
Increasing fire
Changing surface albedo
Other disturbances
Releasing Aerosol
Exacerbating climate change
Fig. 1. The role of drought in the biosphere–atmosphere.
between the biosphere and the atmosphere. The consequences of drought will go beyond weakening the ability of the terrestrial ecosystem to function as a carbon sink function; drought will interfere with biosphere–atmosphere interactions both directly and indirectly, thus further exacerbating climate change. In certain regions, such as the Amazon forest, relatively small changes in terrestrial ecosystem dynamics may strongly affect the concentration of atmospheric CO2 and thus influence the rate of climate change itself [42]. Several impacts of drought on terrestrial ecosystems may occur simultaneously, making the process even more complex. For example, drought- and fire-related tree mortality could further increase forest flammability by a positive fire feedback loop that should exacerbate forest impoverishment [38]. There is an urgent need to understand the impacts of drought from the view of the earth-atmosphere system. 7. Conclusion Drought is projected to become more prevalent in the future, and its impact on terrestrial ecosystems is still difficult to assess. It is necessary to understand the role of drought in the interactions of the atmosphere and biosphere to project its effect on terrestrial ecosystems. Field measurements in combination with model simulations are critical to improve assessments of ecological and land surface changes due to drought impacts, which could occur in response to global warming. References [1] C.D. Allen, A.K. Macalady, H. Chenchouni, et al., A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests, For. Ecol. Manage. 259 (2010) 660–684. [2] M.O. Andreae, P. Merlet, Emission of trace gases and aerosols from biomass burning, Global Biogeochem. Cycles 15 (2001) 955–966. [3] A. Angert, S. Biraud, C. Bonfils, et al., Drier summers cancel out the CO2 uptake enhancement induced by warmer springs, Proc. Nat. Acad. Sci. USA 102 (2005) 10823–10827. [4] M.E. Assessment, Ecosystems and Human Well-Being: our Human Planet: Summary for Decision-Makers, Island Pr, 2005. [5] S.L. Bevan, et al., Proc. of the ‘2nd MERIS/(A)ATSR User Workshop’, Frascati, Italy, 22–26 September 2008 (ESA SP-666, November 2008), 2008. [6] C. Bigler, O.U. Braker, H. Bugmann, et al., Drought as an inciting mortality factor in Scots pine stands of the Valais, Switzerland, Ecosystems 9 (2006) 330–343. [7] C. Bigler, D.G. Gavin, C. Gunning, et al., Drought induces lagged tree mortality in a subalpine forest in the Rocky Mountains, Oikos 116 (2007) 1983–1994. [8] C. Boisvenue, S.W. Running, Impacts of climate change on natural forest productivity – evidence since the middle of the 20th century, Global Change Biol. 12 (2006) 862–882. [9] D. Bonal, A. Bosc, S. Ponton, et al., Impact of severe dry season on net ecosystem exchange in the Neotropical rainforest of French Guiana, Global Change Biol. 14 (2008) 1917–1933. [10] G.B. Bonan, Forests and climate change: forcings, feedbacks, and the climate benefits of forests, Science 320 (2008) 1444–1449. [11] D.M.J.S. Bowman, J.K. Balch, P. Artaxo, et al., Fire in the earth system, Science 324 (2009) 481–484. [12] D.D. Breshears, N.S. Cobb, P.M. Rich, et al., Regional vegetation die-off in response to global-change-type drought, Proc. Nat. Acad. Sci. USA 102 (2005) 15144–15148. [13] D.D. Breshears, O.B. Myers, C.W. Meyer, et al., Tree die-off in response to global change-type drought: mortality insights from a decade of plant water potential measurements, Front. Ecol. Environ. 7 (2008) 185–189. [14] E.J. Burke, S.J. Brown, N. Christidis, Modeling the recent evolution of global drought and projections for the twenty-first century with the hadley centre climate model, J. Hydrometeorol. 7 (2006) 1113–1125. [15] I.P.O.C. Change, Climate change 2007: the physical science basis, Agenda 6 (2007) 07. [16] B. Choat, S. Jansen, T.J. Brodribb, et al., Global convergence in the vulnerability of forests to drought, Nature 491 (2012) 752. [17] P. Ciais, M. Reichstein, N. Viovy, et al., Europe-wide reduction in primary productivity caused by the heat and drought in 2003, Nature 437 (2005) 529– 533. [18] P.M. Cox, P.P. Harris, C. Huntingford, et al., Increasing risk of Amazonian drought due to decreasing aerosol pollution, Nature 453 (2008) 212–U217. [19] A. Dai, K.E. Trenberth, T. Qian, A global dataset of palmer drought severity index for 1870–2002: relationship with soil moisture and effects of surface warming, J. Hydrometeorol. 5 (2004) 1117–1130.
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Contents lists available at ScienceDirect
Acta Ecologica Sinica journal homepage: www.elsevier.com/locate/chnaes
Drought: The most important physical stress of terrestrial ecosystems q Bin He a,b,⇑, Xuefeng Cui a,b, Honglin Wang b, Aifang Chen b a b
College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China
a r t i c l e
i n f o
Article history: Received 5 September 2013 Revised 18 November 2013 Accepted 8 May 2014
Keywords: Drought Stress Terrestrial ecosystems Wildfire Tree mortality
a b s t r a c t Drought is projected to become more prevalent in the future due to climate change, and its impact on the fate of terrestrial ecosystems has aroused great concern in the scientific community over the past decade. Mounting evidence suggests that drought may be the most important physical stress of terrestrial ecosystems: drought limits vegetation growth, increases wildfires, and induces tree mortality, among other impacts. Drought not only weakens the carbon sink function of terrestrial ecosystems but also may interfere directly or indirectly with biosphere–atmosphere interactions, further exacerbating climate change. This paper reviews the current evidence of the impacts of drought on terrestrial ecosystems, with particular emphasis on the ways in which drought alters the biological, biogeophysical and biogeochemical processes underlying the interaction between the biosphere and the atmosphere. Ó 2014 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History drought situation and future projection . . . . . . . . Drought reduces terrestrial ecosystem productivity . . . . . Drought increases wildfire. . . . . . . . . . . . . . . . . . . . . . . . . . Drought induces tree mortality . . . . . . . . . . . . . . . . . . . . . . The role of drought in the biosphere–atmosphere system Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Terrestrial ecosystems play a key role in global carbon cycling [43], as they currently sequester 20–30% of global anthropogenic CO2 emissions [46]. Conversely, global terrestrial NPP is also a major driving force of the interannual CO2 growth rate [67]. Over the past several decades, climate change seems to have had a generally positive impact on terrestrial ecosystems [24,8], a phenomenon known as ‘‘the green trend’’ [3,41]. However, an increasing
q Fundings: This work was supported by the National Science and Technology Support Program (Grant No. 2012BAH29B02), National Basic Research Development Program of China (Grant Nos. 2011CB952001 and 2012CB95570001), and National Natural Foundation of China (Grant No. NSFC41301076). ⇑ Corresponding author at: College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China. E-mail address: [email protected] (B. He).
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number of studies have shown that the reduction of net primary production in the global terrestrial ecosystem due to warming and drought has greatly weakened this ‘‘positive feedback’’ [67,24,17,31]. For example, severe drought in moist tropical forests should provoke large carbon emissions by increasing forest flammability and tree mortality and by suppressing tree growth [38]. The benefits of climate change for middle and high latitude terrestrial ecosystems are also being weakened by severe drought [64,32]. Drought frequency and severity are predicted to increase across numerous continental interiors [21], and the consequences of these changes for the dominant plant species are largely unknown [36]. A comprehensive knowledge of drought impacts and related disturbances is critical for understanding the interactions between terrestrial ecosystems and the atmosphere. Drought could cause terrestrial ecosystems to act as carbon sources to the atmosphere in several ways, such as by suppressing tree growth, reducing both autotrophic and heterotrophic
http://dx.doi.org/10.1016/j.chnaes.2014.05.004 1872-2032/Ó 2014 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
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B. He et al. / Acta Ecologica Sinica 34 (2014) 179–183
respiration, causing plant mortality and disease or increasing rates of fire intensity. All of these impacts could significantly reduce terrestrial ecosystem productivity [67,54]. Drought impacts on terrestrial ecosystems have received relatively little scientific attention and may therefore be greatly underestimated. The existing studies do provide substantial evidence for the effects of drought on terrestrial ecosystems, but the possible consequences of these effects remain poorly understood. This paper reviews the primary direct and indirect impacts of drought on terrestrial ecosystems, as well as the biological and chemical processes of interaction between drought, terrestrial ecosystems and climate change. This study aims to further the understanding of the role drought playing in terrestrial ecosystems.
2. History drought situation and future projection As a normal part of climate variability, drought has occurred many times over the past 1000 years across many parts of the world [20]. Since the 20th century, however, drought has increased on both the regional and global scales [20]. The global area impacted by drought has increased sharply since the 1970s, a rise attributed, in part, to global warming [19,28]. Some recent research proposes that the increase in global drought conditions has been overestimated due to the inherent deficiencies of the drought index and that little change in global drought has occurred over the past 60 years [51]. However, an analysis of global drought trends using the SPEI index develop by Vicente-Serrano [60] suggests that areas impacted by severe and extreme drought have increased dramatically. Although the effects of this drought may have been overestimated, the severity of the drought has actually intensified. The drought induced temperature extremes had been reported in several studies [26,37], and the basic mechanism behind this phenomenon is soil moisture variations affecting the partitioning of sensible and latent heat [37]. This issue can be traced back to the relationship between precipitation and temperature. In early studies, more stresses were play on the impacts of temperature on precipitation. For example Isaac and Stuart [27] computed temperature-precipitation relationships for Canadian stations using daily data. For the east and west coasts and northern Canada, more precipitation accompanies warm conditions in winter and cool conditions in summer, thereby reversing the correlation with seasons. Therefore, we think that it is still hard to determine the causal relationship between temperature and water deficiency. This also implies the great importance for the further study of the relationship between drought and temperature, which is crucial for attributes analysis for drought event and the reduction of the uncertainty of future drought projection. Global warming and regional droughts are predicted to intensify and become more frequent during this century due to
anthropogenic climate change [15]. Future drought trends have also been projected using a multi-model under multi-scenario method [21]. Nearly all simulation results suggest that, as a result of decreased precipitation and/or increased evaporation, future droughts will become increasingly serious [21,50,14]. Although, current studies indicates that the future rise in drought and its impacts is potentially dramatic, these projections should be further improved based on more in-depth understanding of the contribution of the natural factors and human activities to drought, respectively. 3. Drought reduces terrestrial ecosystem productivity Research suggests that frequent drought and heat have great impacts on terrestrial carbon cycling by reducing ecosystem productivity, although there remain some uncertainties regarding their effects [17]. Several studies have proposed that dry conditions may actually boost tropical productivity through increases in available solar radiation [45,9]. The terrestrial biosphere could contribute significant amount of CO2 emission during major drought events, such as those evoked by the 1997–1998 El Niño event [43]. These impacts occurred on both the global and regional scales. Table 1 displays a number of major drought events and their impacts on the carbon cycles of their respective terrestrial ecosystems. Among those areas presently studied, the Amazon rainforest is particularly important due to its special status in the global carbon cycle and climate change. This area can process 18 Pg C annually, more than twice the amount of annual anthropogenic fossil fuel emissions [33]. During the 2005 Amazon drought, the total loss of carbon biomass carbon was 1.2–1.6 Pg C [42]. In 2010, the Amazon experienced a second 100 year drought, and the carbon impact of the 2010 drought may eventually exceed 5 billion tons of CO2 released [31]; this figure accounts for 16.6% of global industrial emissions (30.1 Pg C) in 2010. The research warns that, if extreme droughts like these events become more frequent, the Amazon rainforest may lose its ability to act as a natural buffer for anthropogenic carbon emissions [31]. Two other critical ecosystems to Earth’s climate, the boreal forests and the temperate forests, are facing the same fate, their carbon sinks weakened by increasingly frequent drought events [32,57]. At the global scale, a reduction in terrestrial NPP of 0.55 petagrams over the past decade (2000–2009) has been estimated using the global MODIS NPP algorithm. Furthermore, the drying trend of the Southern Hemisphere has decreased the NPP in that area, counteracting the increase in NPP observed over the Northern Hemisphere [67]. Although a number of studies have examined the influences of drought on plant productivity and the terrestrial carbon cycle, the physiological mechanism targeted by drought remain unclear. Drought has generally been considered to reduce photosynthesis
Table 1 Main drought events and their impacts on the carbon cycle of terrestrial ecosystems. ID
Study area
Years of drought
Reduction of carbon
Refs.
1
Northern Hemisphere midlatitude regions
1998–2002
0.9 Pg C yr
1
2
European
2003
0.5 Pg C yr
1
3
Amazon forest
1997–1998 2001 2005 2010
Reduced by 20–30% 0.2 Pg C 1.2–1.6 Pg C More than 5 Pg C
[44] [38] [42] [31]
4
China
Droughts over the period 1901–2002
Most droughts generally reduced NPP and NEP in drought-effect area
[63]
5
Canada’s boreal forests
Drought over 1963–2008
Drought reduced the biomass carbon sink
[32]
6
Southwestern China
2010
100–200 g C m
7
Global
2000–2009
0.5 Pg C
2
yr
1
[64] [17]
[66] [67]
B. He et al. / Acta Ecologica Sinica 34 (2014) 179–183
or impact the autotrophic and heterotrophic respiration of the ecosystem to reduce the GPP [54]. Drought stress reduces the capacity of plants to provide water to their leaves for photosynthesis by disturbing the water transport system ultimately leading to desiccation and mortality [16]. Although different tree species display a wide range of drought sensitivity, recent studies have shown that all forest biomes are equally vulnerable to hydraulic failure without consideration of their current rainfall environment, as their hydraulic safety margins are usually independent of mean annual precipitation [16]. The time scale of drought could be playing a key role in defining the sensitivity of land biomes to drought, arid and humid biomes respond to drought at short time-scales, but the semiarid and subhumid biomes tend to respond to drought at long time-scales [61]. Furthermore, drought events in different seasons may impact vegetation differently. Drought in summer reduces transpiration and limits photosynthesis; while water stress in spring can also directly reduce photosynthesis [66], it can also inhibit canopy development and peak leaf area, thereby leading to the decline in annual net carbon uptake [66,39]. An in-depth understanding of terrestrial ecosystem responses to drought requires long-term, large-scale field monitoring. Future droughts could reduce the terrestrial carbon sink or even convert it into a source, which would send positive feedback to the global climate system and accelerate future global warming [32,63]. Without reliable data for below-ground NPP and a better understanding of the interactions between the biosphere and the atmosphere, however, it is difficult project the fate of the terrestrial ecosystem under future droughts.
4. Drought increases wildfire Fire is one of the most important disturbances in terrestrial ecosystems on a global scale [56] and contributes significantly to the budgets of several trace gases and aerosols [2]. It is generally asserted that anthropogenic climate change will lead to widespread and more frequent fires [15] and that this rise will be related to changes in rainfall and increased temperatures, especially as they lead to drought conditions in areas with abundant fuel loads [62]. The relationship between drought and wildfire has been discussed for many years. Although the connection between drought and fire is intuitive, the mechanisms behind regional patterns of drought are very complex [48]. It is difficult to determine how many fires are directly caused by drought, several studies have proven that drought is an important driving force for fire [25,56,59]. Drought-induced wild fires have been associated with global circulation anomalies such as the El Niño–Southern Oscillation [55] and, more recently, the Pacific Decadal Oscillation [25] and the SSTS in the North Atlantic [18]. In several case studies in southern Borneo, a strong coupling between regional drought intensity and fire emissions has been found by van Der Werf [56]; forest loss rates and areas of vulnerable peatland both increase in drought years. Data collected from tree-ring analyses were used to reconstruct the PDSI and forest fire events over the period 1700–1975 in the U.S. southwest, further supporting the correlation of drought and fire [48]. Studies in California [30], Washington [25], and the American Rocky Mountains [48], have all arrived at similar conclusions. Fire influences the climate system through the release of carbon and atmospheric aerosols and through changing the surface albedo. Although wildfires are usually contained to a tiny area of the globe, they can have huge impacts on the global carbon cycle [47]. Currently, all sources of fire create CO2 emissions almost equal to 50% of those coming from fossil-fuel combustion (2–4 Pg C year 1 versus 7.2 Pg C year 1) [11]. During the period of
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1997–1998 ENSO-related drought event, Indonesian peat fires released an estimated 0.8–2.6 Pg C [40], and during 2000–2006, the average fire emissions from this region increased to 128 ± 51 (1r) Tg C year 1 [56]. The already strong relationship between fire emissions and drought is likely to strengthen due to positive feedback, because increased greenhouse gas concentrations may lead to more frequent or severe drought events [23]. Forest fires increase atmospheric aerosol concentrations, leading to regional and global impacts on the solar heating of the surface and atmosphere as well as the hydrological cycle [5]. Emissions of aerosols could lead to either warming or cooling on a regional scale, which are depending on factors such as aerosol composition, the albedos of both the Earth’s surface and clouds, and so on. Although considerable uncertainty remains, the risk of Amazonian drought may increase due to decreasing aerosol pollution and increased greenhouse gases [18]. A strong relationship between dry-season aerosol optical depth (AOD) and dry-season precipitation also suggests a positive feedback mechanism between aerosols and drought, this may contribute to intensified drought under climate change [5]. Climate modeling studies suggest that these aerosols may lengthen or intensify periods of drought in the Amazon [55] and in Indonesia [18]. Drought may also produce aerosols directly, such as the dust aerosols dramatically increased during drought years in the Sahel [35]. The interactions between drought, fire, aerosols and greenhouse gases are quite complex. Whereas greenhouse gases release the emission of thermal radiation to space, thus warming the surface, aerosols should reflect and absorb solar radiation (the aerosol direct effect) and modify cloud properties (indirect effect), thereby cooling the surface. These disturbances on the radiation balance are very different and need different research approaches [29].
5. Drought induces tree mortality Drought also influences terrestrial ecosystems by increasing tree mortality [65]. Increased tree mortality in response to climate change and intensive drought has been monitored and welldocumented at both the regional and global scale [12], often in combination with other abiotic and biotic factors [16], and the available examples span different forest types and climatic zones [1]. The observed drought-induced mortality at the global level suggests that forests will be at risk if the climate becomes drier; tree die-offs may become more severe and extensive under future global climate change [12]. The relationship between tree mortality and drought is complicated for the following reasons: (1) Drought acts on trees both as a direct physiological stress and also as a factor for other mortality agents such as insects, diseases and fire suppression. Potential interactions of biotic mortality agents often make it much difficult to determine the proximate reason of tree death [12,22,7]. (2) The response of tree mortality to the timing and length of droughts has been shown to be non-linear [7]. Mortality responses at a lag of several years to decades after drought events complicate the investigation of cause-effect relationships [6]. (3) Significant differences exist in the drought responses of different tree species [52], tree sizes [58], ages [53], growth rates [52], locations [52], and distribution regions; even the intensity of disturbance by human activities, measured by the level of intact forest, may determine resistance to drought impact [52]. Projects analyzing drought-induced mortality under future climatic conditions require a better understanding of the fundamental mechanisms underlying tree mortality and survival during drought. Three main hypotheses to explain the physiology of drought-related tree mortality have been developed based on theoretical modeling and experiment: hydraulic-failure, carbon
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starvation and biotic agents [34]. Due to the tempo-spatial mismatch between field observations and laboratory simulations, it is difficult to determine which mechanism is the main driving factor [13]. Before understanding the mechanisms underlying climate change and forest, the quantitative knowledge about the physiological thresholds of individual tree death under chronic or acute water stress should be required [1]. Notably, the relationship between the threshold for tree mortality and drought-induced water stress, which may also underlie the effects of biotic agents such as bark beetles [49], remains uncertain. More severe drought, as produced by climate change, has the potential to trigger or aggravate widespread tree die-off [57,34,13]. The consequences of drought-induced forest die-off should be manifold for ecosystems, and the climate system [65].Widespread global and regional scale die-off events could change forest structure and composition, consequently impacting the stability of land-surface interactions such as carbon sequestration and possibly causing feedback for atmospheric CO2 and climate [57,34,4]. Multi-year drought has been shown to initiate prolonged growth decreases that increase a tree’s long-term risk of death [6] for two reasons: (1) Tree death occurs generally several years or even several decades after the drought [6]. (2) Reductions in tree density following mortality events failed to buffer sites against additional mortality during subsequent droughts, so the ‘release effect’ of tree mortality did not increase the ability of the surviving trees to endure the next severe drought [36]. Furthermore, extended periods of warm, dry conditions could weaken the ability of trees to withstand insect attacks [1,22], which may lead to more severe tree mortality. More researches are still needed on micro-climatic, hydrological, and biogeochemical consequences of and feedbacks to mortality [65].
6. The role of drought in the biosphere–atmosphere system It has been recognized that biological processes can control the earth system in a significant way [24]. The world’s forests influence climate system through physical, chemical, and biological processes that impact planetary energetics, the hydrologic cycle, and atmospheric composition [10]. The role of drought in the biosphere–atmosphere system is summarized in Fig. 1, in which it is shown that drought is a primary driver of feedback interactions
Terrestrial ecosystem
Severe and frequent drought
Reducing productivit y growth
Releasing CO2
Caused tree mortality
Increasing fire
Changing surface albedo
Other disturbances
Releasing Aerosol
Exacerbating climate change
Fig. 1. The role of drought in the biosphere–atmosphere.
between the biosphere and the atmosphere. The consequences of drought will go beyond weakening the ability of the terrestrial ecosystem to function as a carbon sink function; drought will interfere with biosphere–atmosphere interactions both directly and indirectly, thus further exacerbating climate change. In certain regions, such as the Amazon forest, relatively small changes in terrestrial ecosystem dynamics may strongly affect the concentration of atmospheric CO2 and thus influence the rate of climate change itself [42]. Several impacts of drought on terrestrial ecosystems may occur simultaneously, making the process even more complex. For example, drought- and fire-related tree mortality could further increase forest flammability by a positive fire feedback loop that should exacerbate forest impoverishment [38]. There is an urgent need to understand the impacts of drought from the view of the earth-atmosphere system. 7. Conclusion Drought is projected to become more prevalent in the future, and its impact on terrestrial ecosystems is still difficult to assess. It is necessary to understand the role of drought in the interactions of the atmosphere and biosphere to project its effect on terrestrial ecosystems. Field measurements in combination with model simulations are critical to improve assessments of ecological and land surface changes due to drought impacts, which could occur in response to global warming. References [1] C.D. Allen, A.K. Macalady, H. Chenchouni, et al., A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests, For. Ecol. Manage. 259 (2010) 660–684. [2] M.O. Andreae, P. Merlet, Emission of trace gases and aerosols from biomass burning, Global Biogeochem. Cycles 15 (2001) 955–966. [3] A. Angert, S. Biraud, C. Bonfils, et al., Drier summers cancel out the CO2 uptake enhancement induced by warmer springs, Proc. Nat. Acad. Sci. USA 102 (2005) 10823–10827. [4] M.E. Assessment, Ecosystems and Human Well-Being: our Human Planet: Summary for Decision-Makers, Island Pr, 2005. [5] S.L. Bevan, et al., Proc. of the ‘2nd MERIS/(A)ATSR User Workshop’, Frascati, Italy, 22–26 September 2008 (ESA SP-666, November 2008), 2008. [6] C. Bigler, O.U. Braker, H. Bugmann, et al., Drought as an inciting mortality factor in Scots pine stands of the Valais, Switzerland, Ecosystems 9 (2006) 330–343. [7] C. Bigler, D.G. Gavin, C. Gunning, et al., Drought induces lagged tree mortality in a subalpine forest in the Rocky Mountains, Oikos 116 (2007) 1983–1994. [8] C. Boisvenue, S.W. Running, Impacts of climate change on natural forest productivity – evidence since the middle of the 20th century, Global Change Biol. 12 (2006) 862–882. [9] D. Bonal, A. Bosc, S. Ponton, et al., Impact of severe dry season on net ecosystem exchange in the Neotropical rainforest of French Guiana, Global Change Biol. 14 (2008) 1917–1933. [10] G.B. Bonan, Forests and climate change: forcings, feedbacks, and the climate benefits of forests, Science 320 (2008) 1444–1449. [11] D.M.J.S. Bowman, J.K. Balch, P. Artaxo, et al., Fire in the earth system, Science 324 (2009) 481–484. [12] D.D. Breshears, N.S. Cobb, P.M. Rich, et al., Regional vegetation die-off in response to global-change-type drought, Proc. Nat. Acad. Sci. USA 102 (2005) 15144–15148. [13] D.D. Breshears, O.B. Myers, C.W. Meyer, et al., Tree die-off in response to global change-type drought: mortality insights from a decade of plant water potential measurements, Front. Ecol. Environ. 7 (2008) 185–189. [14] E.J. Burke, S.J. Brown, N. Christidis, Modeling the recent evolution of global drought and projections for the twenty-first century with the hadley centre climate model, J. Hydrometeorol. 7 (2006) 1113–1125. [15] I.P.O.C. Change, Climate change 2007: the physical science basis, Agenda 6 (2007) 07. [16] B. Choat, S. Jansen, T.J. Brodribb, et al., Global convergence in the vulnerability of forests to drought, Nature 491 (2012) 752. [17] P. Ciais, M. Reichstein, N. Viovy, et al., Europe-wide reduction in primary productivity caused by the heat and drought in 2003, Nature 437 (2005) 529– 533. [18] P.M. Cox, P.P. Harris, C. Huntingford, et al., Increasing risk of Amazonian drought due to decreasing aerosol pollution, Nature 453 (2008) 212–U217. [19] A. Dai, K.E. Trenberth, T. Qian, A global dataset of palmer drought severity index for 1870–2002: relationship with soil moisture and effects of surface warming, J. Hydrometeorol. 5 (2004) 1117–1130.
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