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Le projet majeur africain de la Grande Muraille Verte

Abdoulaye Dia
Robin Duponnois

Partie I. Espèces végétales de la Grande Muraille Verte

Isohydric and Anisohydric Water Use Considerations in Species Selection for the Great Green Wall of Africa

Roger Kjelgren


The hydric model of plant water use behavior is potentially useful in selecting appropriate species in arid to semi-arid condition shelterbelts plantings like the Great Green Wall. Anisohydric species have high “use it or lose it” use at the cost of negative internal water potential, out-competing other plants but possibly inefficiently using water. Isohydric species “save it for a rainy day” strategy conserves water in non-competitive situations but may limit carbon gain and growth. Empirical testing of this model was conducted in two studies involving subtropical herbaceous and tropical woody plants. In Queenland, Australian, container grown Orthosiphon aristatus, Dianella revolata ‘Breeze’, and Ptilotus nobilis plants were stressed through cyclical dry downs or kept well watered while stomatal conductance, predawn water potential, and osmotic adjustment were measured. O. aristatus, a rainforest species, fit the anisohydric model of high water use and more negative water potential during soil drying until stomatal closure and leaf wilting. Deep rooted D. revolata native to Eucalyptus scrub forests fit the isohydric model of reduced transpiration under mild water stress. By contrast, shallow-rooted P. nobilis native to Australia’s very dry interior became water stress by anisohydric behavior to achieve osmotic adjustment that allowed subsequent isohydric behavior. In Bangkok, Thailand, stomatal conductance and predawn water potential was measured in three container-grown, well watered and water stressed monsoonal dry forest species, Pterocarpus indicus, Lagerstroemia loudonii, and Swietiena macrophylla. The deciduous Lagerstroemia loudoni, had very high water use consistent with other deciduous species, but isohydrically closed stomata to moderate internal water potential. The obligate evergreen Swietenia macrophylla fit the isohydric model even more closely, moderating transpiration and water potential under a wide range of dry soil and air. P. indicus, nominally evergreen but deciduous when water stressed, fit the anisohydric model more closely, allowing internal water potential to become more negative during soil drying. Anisohydric species may be more suited for planting in deeper soils with high water holding capacity, while isohydric species may be more suitable for shallower and sandier soils in noncompetitive plantings.

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1Selecting well-adapted species for establishing non or minimal irrigation tree plantings in arid-to-semi-arid habitats like the Great Green Wall is critical. Planting a given site with a species accession collected from that site ensures adaptation and improves but does not guarantee success. Physical or climatic conditions of a site, particularly land degradation, may have changed enough to fall outside the range of species adaptation. In the extreme, for geriatric stands of a given species that established under a wetter-but-vanished climate, replanting locally-sourced material into a drier climate may not be successful.

2Success in establishing shelterbelt plantings in arid-to-semi-arid habitats where soil or climate conditions may have changed depends on identifying the key soil, biological, and climate conditions of a given site then matching with suitable plant material. Satellite or aerial spectral imaging and subsequent geographic information systems analysis is a critical tool for characterizing soil and biological conditions of a site. Key climate factors, rainfall and air temperature while more challenging, can be obtained from both remotely sensed and ground observations.

3Matching plant material that meets other socio-economic criteria to a given site is a two-step process. The first is identifying species found is habitats similar to those being planted. If a given site to be planted has soil and vegetation degradation or changed climate, then habitats with similar or harsher conditions are candidates for selecting suitable species. The second and more difficult is identifying essential phenological, morphological, and physiological traits that can either enhance or diminish survival, establishment, and adequate growth.

4Substantial variation in plant water use strategies among species can be exploited to enhance establishment success. In seasonal monsoonal regions drought deciduous species are often mixed with dry evergreen species. Drought deciduous species often are deep-rooted in fertile soils and drought avoiding, maximizing photosynthesis during the wet season, with the greatest gas exchange rates among woody species (Wullschleger et al., 1998). By contrast, monsoonal evergreen species are more drought tolerant and better adapted to infertile soils, with overall lower gas exchange rates that are sustained through the dry season (Choat et al., 2005). How suited a dry deciduous or evergreen species may depend on isohydric versus anisohydric water use strategy (Schultz, 2003).

5Iso-/aniso-hydric water use strategy is a conceptual model describes species that moderate internal water potential in dry soil and atmosphere by stomatal closure (isohydric) versus those that maintain open stomata as far as possible under dry soil and air at the cost of more negative internal water potential (anisohydric). Iso-/aniso-hydric strategies are not necessarily associated with habitat of a given species. Anisohydric “use-it-or-lose-it” strategy is found in both dry and wet habitats where a species has a fall-back water source such as frequent rainfall or deep rooting (West et al., 2008). Isohydric “save-it-for-a-rainy-day” strategy is common in many temperate as well as arid habitat woody species where moderating internal water potential and soil water depletion is not a competitive disadvantage.

6Inferential profiling can inform selection of potential woody species with a given set of planting site conditions up to a point. In a shallow soil a deciduous species may not be suitable, but in a deep soil one with anisohydric water use may grow less due to more rapid soil water depletion than one with isohydric use that grows better due to greater water use efficiency. This paper reports on studies of iso-/aniso-hydric water use in monsoonal dry tree species in tropical Thailand and herbaceous perennial species in subtropical Australia.

Material and Methods

Thailand Tree Study

7This study was conducted at the Faculty of Forestry building located on the Kasetsart University (KU) campus in Bangkok, Thailand. We investigated water use of four species, Cassia fistula L. (golden shower tree), Pterocarpus indicus Willd. (Malay padauk), Lagerstroemia loudonii T. & B. (Loudon’s crape myrtle) and Swietenia macrophylla King (Honduras mahogany). We conducted two studies, the first in late January-early February, and then late March-early April, 2006, the hottest period in Bangkok. For both studies trees 30 mm in trunk diameter and 2-3 meter height were harvested balled-and-burlapped from a local field nursery and potted into 20 liter containers with composted bark growing substrate. Trees were allowed to establish for three weeks prior to data collection under shade and high frequency irrigation, at which time inspection showed that they had completely rooted into the growing medium. Three replicate trees were used in the first study, and four in the second.

8Both studies were conducted on the roof of the KU Forestry building where a weather station (Weatherhawk Inc., Logan UT) was also located to collect incoming solar radiation, relative humidity, air temperature, and wind speed to calculate local reference evapotranspiration (ETo) as a turfgrass reference (Walter et al., 2000), as well as rainfall. The weather station air temperature sensor was calibrated to a separate sensor located over nearby turf to control for higher air temperature from greater longwave radiation and sensible heat effects from the tiled roof surface. Trees were arranged in a completely randomized design, spaced approximately 1.5 m from each other in a grid pattern.

9Prior to each study period tree root zones were well watered the previous night and allowed to drain. The difference between daily gravimetric container weight yielded volumetric tree water. Total tree leaf area was measured at end of each study by harvesting all leaves from each tree, determining dry specific leaf area of a 20-leaf sub sample, then calculating the product of total leaf dry weight and specific leaf area. Half the trees of each species were progressively allowed to dry at 75%, 50%, and 25% of local ET. During the progressive drying daily measurements were stomatal conductance with a porometer (Decagon, Pullman WA) and predawn water potential with a pressure chamber (Soil Moisture Inc, Santa Barbara, CA).

Australia Perennial Study

10This experiment compared plant responses during successive dry downs, conducted at The University of Queensland’s Gatton plant nursery unit in a subtropical regioncharacterized by dry austral winters (June-August) and high precipitation monsoonal summers (December-January). We used Orthosiphon aristatus (cats whiskers; Lamiaceae), a tropical rainforest species (Shibuya et al., 1999) likely to have little drought tolerance; Dianella revoluta ‘Breeze’ (flax lily; Liliaceae), a species indigenous to dry schlerophyll woodlands in South Eastern Australia (Cunningham, 2000); and, Ptilotus nobilis (mulla mulla; Amarantaceae) collected in arid Western Queensland (Lee et al., 2007).

11Rooted cuttings of each species were transplanted in early October 2006 into 200-mm diameter (7 liter) pots containing a red ferrosol clay-loam soil (Biggs et al., 2001). This clay-loam soil has a high water holding capacity (0.2 mm water /mm soil at field capacity), thereby reducing potentially confounding effects from rapid root zone drying in organic potting substrate. Temperatures during the experiment in mid spring were initially hot (30 - 36o C maximum) with humidity from 20-30%. A frontal system with clouds then moved through and cooled temperatures down to 21 - 25o C for several days. For the last 4 days of the experiment, temperatures rose to near 30o C, with similar low humidity.

12Two treatments were imposed, well-watered (control) and water withheld variably over a 12-day stress cycle during November 2006. Each watering-treatment-byspecies combination involved 10 single-plant replicates, for a total of 60 plants. The experiment was laid out on a concrete pad in a completely randomized design. All plants were irrigated daily for fifteen minutes with two 4 L. h-1 drip emitters per container during their establishment in October as they acclimated to full sun conditions.

13All plants were watered by hand at the start of the dry-down cycle in addition to routine drip irrigation to ensure growing substrate at field capacity. Drip emitters were then withdrawn from plants in the no water treatment, which was allowed to deplete soil water until visible water stress symptoms (viz. wilting or leaf curling) were evident in at least half of the individual plants within the stress treatment. At this point, all plants within that species were re-watered to return the substrate to field capacity. Thus, each species had dry-down periods of different duration.

14Stomatal conductance was measured as described above either each day or every other day during dry-down periods. Substrate water status was recorded concurrently using a time-domain reflectometer (HydroSense CS620, Campbell Scientific, Logan UT) probe with 200-mm long prongs that reached from the substrate surface to close to the container bottom. Internal plant water status was measured with a pressure chamber during the dry-down periods just prior to re-watering of the species then currently exhibiting the most drought-stressed symptoms on a four-plant subsample from each species x treatment combination.

15Osmotic adjustment in terms of accumulation of organic solutes was estimated using the pressure-volume-curve analysis method as described by Robichaux (1984) at the end of the last drying cycle. Pressure volume curves were run over three days, one species per day starting with Orthosiphon and ending with Dianella. One leaf from each plant (well watered the night before) was excised early the next morning, weighed, immediately covered in plastic wrap and aluminum foil to eliminate confounding transpiration effects on leaf water potential, and then sealed into the pressure chamber for measurement of balance pressure. Leaves were then uncovered and allowed to bench dry under paper to minimize drying too quickly.

16After pressure-volume data measurements were completed, all leaves were harvested from each of the plants, a 10-leaf sub-sample removed, and the remaining leaves dried at 60 C for 24 hours and then weighed. The sub-sample was measured for average leaf area then dried. We used the leaf area: weight ratio (specific leaf area) to calculate total leaf area from total leaf mass. The root system of each plant was also harvested, the substrate washed away, drying, then the roots were dried at 60 °C for 24 hours and weighed.

Results and Discussion

17Weather conditions got progressively hotter and drier during the monsoonal dry season due to longer days increasing the evaporation rate (Figure 1). During the first dry down period in early February, all species, including the deciduous Lagerstroemia loudonii exhibited similar transpiration rates. Lagerstroemia and Swietenia macrophylla showed no signs of water stress. Pterocarpus indicus had somewhat higher conductance rates compared to the other two species, but did close stomata in response to progressive drying, but did not have lower predawn water potentials. During the second study later in the dry season when conditions where hotter, we reduced water application to 50% more rapidly, resulting in water stress and stomatal closure in all three species. More consistent with being a drought deciduous species (Ishida et al., 2006), Lagerstroemia increased conductance compared to the first study, probably because it had put on a new set of leaves after drought-induced dormancy earlier in the dry season.

18Lagerstroemia is the only species with wet water potential became more negative in close alignment with the dry trees, a decline that suggested that Lagerstroemia leaves may be adjusting osmotically, and advantage in extracting more water from the root zone.

Figure 1

Figure 1

Stomatal conductance and predawn leaf water potential, and air temperature, of three subtropical herbaceous perennial species grown in containers either well watered (wet) or water stressed (dry) where plants were not irrigated. Asterisks indicate significant differences between wet and dry treatments.

19Pterocarpus conductance was marginally higher, but conductance of the dry trees was lower than the first study, possibly due to hotter conditions, while Swietenia conductance was the same, although closing in response to water stress.

20An isohydric model was generally consistent with the water use strategy of these species. As stomatal conductance fell in response to declining substrate water content, predawn water potential remained relatively constant down to about 30% stomatal closure relative to well-watered levels (Figure 2). Pterocarpus was the exception, as its stomatal were closed at 15% ratio, allowed predawn water potential to fall to 2.5 times, of the wet plants.

21This fall may be in part from thinner Pterocarpus leaves that allowed cuticular transpiration even when stomata were closed, causing further decreases in water potential. Pterocarpus is actually a brevi-deciduous species, as it does drop all its leaves when water stresses, but is otherwise evergreen. Inability to regulate water potential through stomatal closure would explain it brevi-deciduous behavior, as severely stressed leaves would more likely senesce.

22In the Australia study, O. aristatus the rainforest species was as expected more drought intolerant than either D. revolata or P. nobilis, but the latter two species appeared to differ in their drought tolerance mechanisms. Both Ptilotus and Orthosiphon showed signs of leaf curling (Ptilotus) or wilting (Orthosiphon) after several hot days days without water. While stomata of all three species were nearly closed at this point, Dianella showed no visible signs of water stress and no differences in midday water potential, and thus was allowed to continue drying for two days further. The remaining days of the study were cooler and somewhat cloudy, progressively increasing in air temperature maxima from 21 to 29° C. Cooler temperatures and lower evapotranspiration were seen in reduced conductance which would have in turn reduced plant water use, and fewer visible signs of water stress.

Figure 2

Figure 2

Ratio of dry to wet water potential related to dry to wet ratio of stomacal conductance.

Figure 3

Figure 3

Ratio of dry to wet water potential related to dry to wet ratio of stomacal conductance.

23Interestingly, conductance of Dianella that had been dried down did not fully recover even after re-watering, and water potential differences between the stressed versus control treatments were negligible. By contrast, Ptilotus conductance increased to near well-watered levels after re-watering, but with a large water potential difference, and then declined, moderating water potential to levels similar to the well-watered plants.

24By normalizing stomatal conductance and water potential of stressed relative to well-watered plants, stomatal conductance appeared to control leaf water potential in an isohydric pattern for Ptilotus and Dianella (Fig. 3). Dianella moderated leaf water potential to within 40% of well-watered (wet) values through nearly complete stomatal closure. Ptilotus stomata did not initially moderate leaf water potential as much, as it fell to three times more negative as well watered plants as stomata closed during the first drying cycle. Otherwise partial stomatal closure in Ptilotus maintained water potential within 40% of well watered levels. Orthosiphon stomata maintained less control in a more anisohydric pattern. Leaf water potential fe efll to four times more negative than well watered plants as stomata completely closed during the first drying cycle. Partial stomatal closure still allowed Orthosiphon water potential to decline to nearly twice as negative as well watered plants.

25The results from these two studies indicate that both woody and herbaceous species show iso-/aniso-hydric water use strategies. The monsoonal dry forest species regulated internal water potential, even though they differed widely in their water use rates. Consistent with other reports (Choat et al., 2005; Ishida et al., 2006), the deciduous Lagerstroemia had higher conductance and transpiration rates that would allow it to maximize photosynthesis and growth during the monsoonal wet season, as well as progressively more negative water potential even in the well-watered plants. Swietenia was a more conservative, very isohydric pattern, of stomatal control, and with more negative water potential, hydraulic restrictions appear to be a major factor (Brodribb et al., 2003). However, stomatal sensitivity to soil water depletion and high vapor pressure deficits and evaporative demand are likely to increase with age when these trees are larger (Ishida et al., 2006). Gravity will exert a greater pull, increasing the pull on internal water potential such that stomata will close at lower vapor pressure deficits to reduce the pull on internal water. In an arid zone shelterbelt planting an evergreen species such as Swietenia with a deep root system would be well suited. Monsoonal deciduous species, Lagerstroemia for example, would work in the higher rainfall zones of the Great Green Wall with a longer monsoonal wet season to maximize growth, but would probably not be suited for the drier areas.

26Dianella and Ptilotus differed in the drought tolerance mechanis. Dianella did not osmotically adjust, appearing to follow a hydraulically controlled isohydric model more closely, and had a deeper root system that would extract water from a larger soil volume (Stewart et al., 2004). These adaptations would suit Dianella’s slightly more mesic habitat compared to Ptilotous for survival between rainfall in a somewhat more non-competitive situation (Cunningham, 2000). Ptilotus did not fit the iso-and anisohydric models as well. Initial soil drying in the first study did not moderate water potential to the same degree as Dianella, but did appear to trigger osmotic adjustment (Davies et al., 2005), consistent with the observed more negative midday leaf water potentials, and allowing more aggressive water extraction (Lenz et al, 2006) with an observed finer and more shallow root system. This less conservative, moderately isohydric behavior would be consistent with its native range in the driest areas of Australia where extracting as much water, particularly surface water from infrequent rainfall, as possible while tolerating desiccation would be key to survival (Lee et al., 2007). Ptilotus represents a model strategy that combines anisohydric and isohydric strategies to maximize osmotic adjustment that allows more opens stomata in drier soil.


27The iso-/anisohydric model of plant water adds a useful element to consider when selecting species for shelterbelts in semi-to-arid conditions such as the Great Green Wall. Initial screening based on clear morphological traits that reduces transpiration such as small, thick, bluish, hairy foliage is a significant step. Another morphological factor is evergreen-ness. Deciduous species that tend towards ani-isohydric behavior may have very high transpiration rates that can be advantage in deep soils where photosynthesis and growth are maximized during the rainy season, but too long a dry season deciduousness could result in consumed water without enough compensating growth. Conversely, evergreen isohydric species that are particularly sensitive to dry air and soil may have very limited chances for optimal growth. Ptilotus offers a water use strategy if combined with deep rooting that rapidly achieves osmotic adjustment through anisohydric water use, but then maintains a more isohydric approach during soil.


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Dept. Plants, Soils, and Climate Utah State University, Logan, UT 84322

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