Drought Strategies Research Articles

An ecohydrological perspective on drought‐induced forest mortality

AbstractRegional‐scale drought‐induced forest mortality events are projected to become more frequent under future climates due to changes in rainfall patterns. The occurrence of these mortality events is driven by exogenous factors such as frequency and severity of drought and endogenous factors such as tree water and carbon use strategies. To explore the link between these exogenous and endogenous factors underlying forest mortality, a stochastic ecohydrological framework that accounts for random arrival and length of droughts as well as responses of tree water and carbon balance to soil water deficit is proposed. The main dynamics of this system are characterized with respect to the spectrum of anisohydric‐isohydric stomatal control strategies. Using results from a controlled drought experiment, a maximum tolerable drought length at the point where carbon starvation and hydraulic failure occur simultaneously is predicted, supporting the notion of coordinated hydraulic function and metabolism. We find qualitative agreement between the model predictions and observed regional‐scale canopy dieback across a precipitation gradient during the 2002–2003 southwestern United States drought. Both the model and data suggest a rapid increase of mortality frequency below a precipitation threshold. The model also provides estimates of mortality frequency for given plant drought strategies and climate regimes. The proposed ecohydrological approach can be expanded to estimate the effect of anticipated climate change on drought‐induced forest mortality and associated consequences for the water and carbon balances.

Co-ordination of growth, gas exchange and hydraulics define the carbon safety margin in tree species with contrasting drought strategies

Gas exchange, growth, water transport and carbon (C) metabolism diminish during drought according to their respective sensitivities to declining water status. The timing of this sequence of declining physiological functions may determine how water and C relations compromise plant survival. In this paper, we test the hypothesis that the degree of asynchrony between declining C supply (photosynthesis) and C demand (growth and respiration) determines the rate and magnitude of changes in whole-plant non-structural carbohydrates (NSC) during drought. Two complementary experiments using two tree species (Eucalyptus globulus Labill. and Pinus radiata D. Don) with contrasting drought response strategies were performed to (i) assess changes in radial stem growth, transpiration, leaf water potential and gas exchange in response to chronic drought, and (ii) evaluate the concomitant impacts of these drought responses on the temporal patterns of NSC during terminal drought. The three distinct phases of water stress were delineated by thresholds of growth cessation and stomatal closure that defined the 'carbon safety margin' (i.e., the difference between leaf water potential when growth is zero and leaf water potential when net photosynthesis is zero). A wider C safety margin in E. globulus was defined by an earlier cessation of growth relative to photosynthesis that reduced the demand for NSC while maintaining C acquisition. By contrast, the narrower C safety margin in P. radiata was characterized by a synchronous decline in growth and photosynthesis, whereby growth continued under a declining supply of NSC from photosynthesis. The narrower C safety margin in P. radiata was associated with declines in starch concentrations after ∼ 90 days of chronic drought and significant depletion of starch in all organs at mortality. The observed divergence in the sensitivity of drought responses is indicative of a potential trade-off between maintaining hydraulic safety and adequate C availability.

Functional traits predict drought performance and distribution of Mediterranean woody species

Water availability is one of the key environmental factors that affect plant establishment and distribution. In many regions water availability will decline with climate change, exposing small seedlings to a greater likelihood of drought. In this study, 17 leaves, stem, root, and whole-plant traits of ten woody Mediterranean species were measured under favourable growing conditions and seedling drought survival was evaluated during a simulated dry-down episode. The aims of this study were: i) to assess drought survival of different species, ii) to analyse which functional traits predict drought survival time, and iii) to explain species distribution in the field, based on species drought survival and drought strategies. Drought survival time varied ten-fold across species, from 19 to 192 days. Across species, drought survival was positively related to the rooting depth per leaf area, i.e., the ability to acquire water from deeper soil layers while reducing transpiring leaf area. Drought survival time was negatively related to species ability to grow quickly, as indicated by high relative growth and net assimilation rates. Drought survival also explained species distribution in the field. It was found that species were sorted along a continuum, ranging between two contrasting species functional extremes based on functional traits and drought performance. One extreme consisted of acquisitive fast-growing deciduous species, with thin, soft metabolically active leaves, with high resource use and vulnerability to drought. The opposite extreme consisted of conservative slow-growing evergreen species with sclerophyllous leaves, deep roots, a low transpiring area, and low water use, resulting in high drought survival and drought tolerance. The results show that these drought strategies shape species distribution in this Mediterranean area.

Drought response strategies define the relative contributions of hydraulic dysfunction and carbohydrate depletion during tree mortality

Plant survival during drought requires adequate hydration in living tissues and carbohydrate reserves for maintenance and recovery. We hypothesized that tree growth and hydraulic strategy determines the intensity and duration of the 'physiological drought', thereby affecting the relative contributions of loss of hydraulic function and carbohydrate depletion during mortality. We compared patterns in growth rate, water relations, gas exchange and carbohydrate dynamics in three tree species subjected to prolonged drought. Two Eucalyptus species (E.globulus, E.smithii) exhibited high growth rates and water-use resulting in rapid declines in water status and hydraulic conductance. In contrast, conservative growth and water relations in Pinus radiata resulted in longer periods of negative carbon balance and significant depletion of stored carbohydrates in all organs. The ongoing demand for carbohydrates from sustained respiration highlighted the role that duration of drought plays in facilitating carbohydrate consumption. Two drought strategies were revealed, differentiated by plant regulation of water status: plants maximized gas exchange, but were exposed to low water potentials and rapid hydraulic dysfunction; and tight regulation of gas exchange at the cost of carbohydrate depletion. These findings provide evidence for a relationship between hydraulic regulation of water status and carbohydrate depletion during terminal drought.

Adaptation to Climate Change in European Water Law and Policy

Climate change exacerbates the challenges that water management nowadays has to deal with. This highlights the need for a legal framework that promotes adaptation by addressing the ecological value of water and the risks of flooding and drought. This paper analyzes to what extent the European legal framework meets this need. This is evaluated by analyzing to what extent the Water Framework Directive, the Floods Directive and the Water Scarcity and Drought Strategy build resilience. Resilience is an important concept in the adaptation to climate change discourse. It refers to the capacity of a social-ecological system to absorb disturbance and reorganize while undergoing change so as to still retain essentially the same function, structure, identity and feedbacks. This paper uses five criteria to measure this: (1) the flexibility of rules and (2) the adaptiveness of such rules (3) openness, public participation and access to the courts (4) multilevel governance at the bioregional scale and (5) effectiveness. The European legal framework partly meets these criteria. It offers a river basin approach and both the setting of goals and objectives and their achievement is a multilevel, cyclical process, which includes public participation. However, the achievement of the goals is not easily enforceable and coordination within international river basins is legally weak. This detracts from the expected effectiveness of the EU legal framework in promoting adaptation by protecting the aquatic ecosystem and reducing flood and drought risks.

China's drought strategies in rural areas along the Lancang River

Given that climate change is likely to exacerbate drought in certain parts of the world, a critical issue is how appropriate context-relevant drought policy can be developed and implemented. This paper examines how China is developing policies to help local communities in the Lancang River Basin to cope with the problems of climate variability and change. The research process includes (a) content analysis of relevant policies at different levels of administration and (b) stakeholder interviews to examine how effective policies are at the local level along the Lancang River in Yunnan province. The paper concludes that China needs to provide more emphasis on (a) enhancing the adaptive capacity of institutions and individuals, (b) drought risk rather than disaster management, (c) enhancing coordination between different departments and levels of government, (d) decentralization in policymaking as local governments can make context-relevant comprehensive policies more easily and are less hierarchical in their relationships, (e) involving farmers or other stakeholders in decision making, (f) building up grass root organizations to support policy implementation to compensate for the retreat of governmental organizations from the local level and (g) adopting market mechanisms which are still in a nascent stage now.

The costs of drought: the 2007/2008 case of Barcelona

Water scarcity and drought are social and economic problems in large parts of the world that will intensify due to climate change. The existing literature on the costs of drought is scarce, fragmented and heterogeneous and there is a need for comprehensive cost estimations to help design an effective policy response. This is particularly the case in Europe, which is currently developing its drought strategy. The severe drought that affected Barcelona in 2007/2008 is used here as a case study to illustrate the costs of this type of extreme event. We assess direct losses of the affected economic sectors, indirect costs to the rest of the economy and non-market welfare losses caused by environmental damage and household water restrictions. Additionally, we also look at the cost of the measures implemented to address the drought and discuss the implications in terms of their cost-effectiveness. Our results indicate a total cost of this drought event of 1,605 million Euros (0.48% of the regional GDP) and highlight the need for more accurate cost estimations at the European level. The study also points to the need to promote water saving measures and to increase the structural adaptive capacity of systems subjected to water scarcity.

Morphological Adaptations to Drought and Reproductive Strategy of the Moss Syntrichia caninervis in the Gurbantunggut Desert, China

The biological soil crusts (BSCs) in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China, feature moss-dominated BSCs, which play an indispensable role in sand fixation. Syntrichia caninervis Mitt. (S. caninervis) serves as one of the most common species in BSCs in the desert. In this study we examined the morphological structure of S. caninervis from leafy gametophyte to protonema using light and scanning electron microscopy (SEM). We also examined the relationships between the morphological structure of S. caninervis and environmental factors. We found that: (1) this moss species is commonly tufted on the sand surface, and its leaves are folded upwards and twisted around the stem under dry conditions; (2) the cells on both upper and lower leaf surfaces have C-shaped dark papillae, which may reflect sunlight to reduce the damage from high temperature; (3) the leaf costa is excurrent, forming an awn with forked teeth; and (4) the protonema cells are small and thickset with thick cell walls and the cytoplasm is highly concentrated with a small vacuole. In addition, we also found that the protonema cells always form pouches on the tip of the mother cells during the process of cell polarization. Our results suggest that S. caninervis has, through its life cycle, several morphological and structural characteristics to adapt to dry environmental conditions. These morphological features of S. caninervis may also be found in other deserts in the world due to the world-wide distribution of the species.

Drought and Underdevelopment in Bundelkhand: A Public Policty Analysis

This paper is an attempt to analyse the public policy for drought management in India in general and Bundelkhand region of Central India in particular. Bundelhand has been in news for farmers’ suicide, starvation deaths and increasing level of poverty. Despite having adequate water and natural resources the region has been suffering from acute poverty and hunger. This study argues that the Ministry of Agriculture’s present policy for drought, based upon the principle of ‘one-size fits all,’ is narrow in nature. In measuring drought only two factors are taken into account – first, the precipitation level during the Monsoon season and second, net sown area during a particular season. In addition to this, it focuses only on short term strategies for drought mitigation. It ignores the long term issues like environmental sustainability and inclusive development. In case of Bundelkhand it is visible that ignoring these issues has serious implications not only for the economy of the region but also on the society and climate. Above all it has serious impact on the political and civil unrest, which has never been analysed while understanding drought. This paper argues that there is an immediate need of rethinking on government of India’s drought management strategies. It should rather adopt a holistic framework. It should focus, on the one hand, region specific drought strategies taking into account country’s diverse geographic terrain, variety of types of soil, uneven rainfall, and economic diversity. On the other hand it should look into various implications of drought including economic, social, environmental and political.

Seedling root morphology and biomass allocation of 62 tropical tree species in relation to drought‐ and shade‐tolerance

Summary Water availability is the main determinant of species’ distribution in lowland tropical forests. Species’ occurrence along water availability gradients depends on their ability to tolerate drought. To identify species’ traits underlying drought‐tolerance we excavated first year seedlings of 62 dry and moist forest tree species at the onset of the dry season. We evaluate how morphological seedling traits differ between forests, and whether functional groups of species can be identified based on trait relations. We also compare seedling traits along independent axes of drought and shade‐tolerance to assess a hypothesized trade‐off. Seedlings of dry forest species improve water foraging capacity in deep soil layers by an increased below‐ground biomass allocation and by having deep roots. They minimize the risk of cavitation by making dense stems, and reduce transpiration by producing less leaf tissue. Moist forest seedlings have large leaf areas and a greater above‐ground biomass, to maximize light interception, and long, cheap, branched root systems, to increase water and nutrient capture. Associations among seedling traits reveal three major drought strategies: (i) evergreen drought‐tolerant species have high biomass investment in enduring organs, minimize cavitation and minimize transpiration to persist under dry conditions; (ii) drought‐avoiding species maximize resource capture during a limited growing season and then avoid stress with a deciduous leaf habit in the dry season; (iii) drought‐intolerant species maximize both below‐ and above‐ground resource capture to increase competitiveness for light, but are consequently precluded from dry habitats. We found no direct trade‐off between drought‐ and shade‐tolerance, because they depend largely on different morphological adaptations. Drought‐tolerance is supported by a high biomass investment to the root system, whereas shade‐tolerance is mainly promoted by a low growth rate and low SLA. Synthesis. We conclude that there are three general adaptation strategies of drought‐tolerance, which seemingly hold true across biomes and for different life forms. Drought‐ and shade‐tolerance are largely independent from one another, suggesting a high potential for niche differentiation, as species’ specialization can occur at different combinations of water and light availability.

The economic impacts of drought on the economy of Iran: An integration of linear programming and macroeconometric modelling approaches

In this paper, we provide economy-wide estimates of the costs of drought in the cropping sector of the Iranian economy, using a linear programming model to estimate the direct costs on agriculture, and a macroeconometric model to trace the indirect impacts on the rest of the economy. The results indicate that a severe drought such as the one that occurred in the crop year 1999–2000 imposes a direct cost of 1605 million USD, equivalent to 30.3% of the total value added of the cropping sector in Iran. This, in turn, leads to a 12.7% reduction in the value added of other agricultural sub-sectors (livestock, fisheries and forestry). In the rest of the economy, the manufacturing and service sectors experience value added declines of 7.8 and 3.7%, respectively. In addition, there is a substantial decrease in investment in the agricultural, manufacturing and service sectors. Thus, such a drought reduces overall GDP by about 4.4%, and it would also result in decreased non-oil exports, increased food imports, and a rise in inflation. The results of some drought mitigation simulations are reported in brief. Such estimates strengthen the case for increased attention to drought strategies and management in agriculture in Iran and elsewhere.

Climate-related variability in carbon and oxygen stable isotopes among populations of Aleppo pine grown in common-garden tests

The Aleppo pine (Pinus halepensis Mill.) is found in the Mediterranean under a broad range of moisture and thermal conditions. Differences in severity and duration of water stress among native habitats may act as selective forces shaping the populations’ genetic make-up in terms of contrasting drought strategies. We hypothesised that these strategies should translate into intraspecific variation in carbon isotope composition (δ13C, surrogate of intrinsic water-use efficiency, WUEi) of wood holocellulose, and such variation might be linked to changes in oxygen isotope composition (δ18O, proxy of stomatal conductance) and to some climatic features at origin. Thus, we evaluated δ13C, δ18O, growth and survival for 25 Aleppo pine populations covering its geographic range and grown in two common-garden tests. We found intraspecific variability for δ13C and growth, with high-WUEi populations (which showed 18O-enriched holocellulose) having low growth. These results suggest stomatal regulation as common control for δ13C and productivity. We also detected sizeable relationships between δ13C and climate factors related to the magnitude and timing of drought such as the ratio of summer to annual rainfall. The main climate variable associated with δ18O was minimum temperature, but only in the coldest trial, suggesting differences in growth rhythms among sources. Overall, slow growing populations from highly-seasonal dry areas of the western Mediterranean exhibited a conservative water-use, as opposed to fast growing sources from the northernmost distribution range. The particular behaviour of the Mediterranean Aleppo pine as compared with other conifers demonstrates different selective roles of climate variables in determining intraspecific fitness.

Diversity of Tropical Tree Seedling Responses to Drought

ABSTRACTDrought is an important seedling mortality agent in dry and moist tropical forests, and more severe and frequent droughts are predicted in the future. The effect of drought on leaf gas exchange and seedling survival was tested in a dry‐down experiment with four tree species from dry and moist forests in Bolivia. Seedlings were droughted and wilting stage and gas exchange were monitored. Drought led to a gradual reduction of photosynthesis and stomatal conductance over time, whereas respiration and photosynthetic water‐use efficiency initially increased with drought and then declined. Seedlings gradually went through the different wilting stages, until they eventually died, but the trajectory differed for the four species. The strong relationship between wilting stage and photosynthesis means that simple field observations can provide valuable information on plant physiological performance. Three different drought strategies were identified. Dry forest species Ceiba samauma shed its leaves and survived. The moist forest species Cariniana ianeirensis postponed drought stress by having low rates of transpiration and high water‐use efficiency. Dry forest Astronium urundeuva and moist forest Triplaris americana followed an opportunistic strategy; they are early successional species that can quickly grow to maturity but periodic drought can be lethal. Strikingly, dry and moist forest species did not differ clearly in their drought tolerance strategies.

Cavitation, stomatal conductance, and leaf dieback in seedlings of two co-occurring Mediterranean shrubs during an intense drought.

Seedling shrubs in the Mediterranean semi-arid climate are subjected to intense droughts during summer. Thus, seedlings often surpass their limits of tolerance to water stress, resulting in the loss of hydraulic conductivity due to xylem cavitation. The response in terms of stomatal conductance, vulnerability to cavitation, leaf dieback, and survival were analysed in two co-occurring seedlings of mastic tree (Pistacia lentiscus L.) and kermes oak (Quercus coccifera L.) during an intense drought period. Both species reacted to drought with steep decreases in stomatal conductance before the critical water potential brought about the onset of cavitation events. Q. coccifera showed wider safety margins for avoiding runaway embolism than P. lentiscus and these differences could be related to the particular drought strategy displayed by each species: water saver or water spender. The limits for survival, resprout capacity and leaf dieback were also analysed in terms of loss of conductivity. By contrast with previous studies, the species showing higher seedling survival in the presence of drought also showed higher susceptibility to cavitation and operated with a lower safety margin for cavitation. Both species showed a leaf specific conductivity (LSC) threshold below which leaf biomass had to be regulated to avoid runaway embolism. However, each species displayed a different type of response: P. lentiscus conserved total leaf area up to 100% loss of LSC, whereas Q. coccifera continuously adjusted leaf biomass throughout the drought period in order to maintain the LSC very close to the maximum values recorded without loss of conductivity. Both species maintained the capacity for survival until the loss of conductivity was very nearly 100%.

Drought strategies in East Africa: The climatologist's role

The structures of the links in the chain drought, harvest failure, famine are examined. Possible interventions are feasible at the link between drought and harvest failure, to lessen or avert the impact. Such interventions, based upon knowledge of the climatology of drought-prone regions, may be discussed under three headings: short-term predictions, long-term predictions, and the analysis of spatial patterns of drought.

Drought relief schemes for the 'Pastoral Zone'.

In this article I have attempted to firstly provide a consensus view of graziers to sound drought strategies; secondly, outline Government policies or action directed towards assisting graziers affected by drought; and finally, address the subject of drought policy as it relates to conservation of the rangeland resource. Drought strategies discussed include pre-drought (e.g. fodder reserves, conservative stocking), longer term (e.g. increasing property size, spatial diversification of grazing blocks) and in-drought (e.g. reduce stock numbers early in drought). Grounds for Government intervention and aid for drought affected producers are analysed with regard to both the individual farmer's needs and the impact nationally of low return from a drought-affected primary industry. Aspects discussed include provision of better infrastructure (e.g, new roads), taxation concessions, a National Drought Fodder Reserve, land tenure policy, the Rural Adjust- ment Scheme and credit and freight concessions. From the conservation viewpoint, it is pointed out that officially declared 'droughts' occur too frequently and there are no incentives for graziers to either act early before a drought becomes firmly established or to delay re-stocking after the drought has broken. It is concluded that a balance between in-drought assistance and long term assistance needs to be struck, and that drought policies should be directed towards 'good' management strategies. If this can be achieved then primary producers should be less dependent on relief schemes.

EXPECTED VALUES OF COSTS AND REVENUES DEPENDENT ON DROUGHT†

In earlier papers on drought, sequences of wet and dry months were treated as realisations of binomial trials with periodic probabilities of failure (dry months), and the cost‐revenue outcomes of alternative drought strategies were handled probabilistically. When costs and revenues may be represented by a set of second degree polynomials over a set of adjacent subranges of the time scale, their expectation may be expressed as a simple closed formula in the twelve probabilities of dry months. As an illustration of the use of these new formulae, means and standard deviations of drought duration calculated in this manner are presented for three typical localities in Queensland. Previously known closed formulae for the mean and the variance in the case where all calendar months have equal probability of being dry emerge as special cases and are verified.