BOTANY, VOLUME 4 - ECOLOGY - 2007

13. PLANTS IN THEIR HABITAT

13.5. Water relations

Plants can only survive on land with the help of efficiently functioning roots, a non-compressible capillary transport system (xylem, see 3.2.4.2), a combination of dynamic (Stomata) and static (cuticle) adaptations protecting against evaporation, and turgent Cells vacuolated to varying degrees (Water potential, see 6.1.4.2, 6.3.2.1). When water is scarce, a plant can enhance both Water uptake and mechanisms that defend against water loss and its consequences. Since the Structure AND Functions of the components responsible for regulating the water regime were described above, we will now focus on responses to moisture deficiency. We will demonstrate how plants harmoniously control water potential, regulate water fluxes, and manage transport resistance mechanisms, while simultaneously influencing the water regime of the ecosystem as a whole.

13.5.1. Water potential and Transpiration

The water potential (see 6.1.4.2) at a specific point within a plant is the net result of water flow and the resistance to its movement. From a thermodynamic perspective, it indicates a reduced availability of water at the measurement point relative to pure free water, which can also be described as tension, suction, or negative pressure. The water potential becomes more negative as it drops further. Until it crosses a critical threshold (which ranges from -1.5 to -2.0 MPa in leaves, depending on the species), the water potential is considered to be only slightly above the plant's actual water supply level. Under sufficient soil moisture, the leaf water potential is lower (more negative) the higher the transpiration rate. This decrease in potential with increasing throughput is comparable to water pressure in a pipe, which drops more significantly the wider the tap is opened. If the soil suffers from a water deficit, even very minimal transpiration (or even complete stomatal closure) cannot prevent the water potential from plummeting to very low values. Depending on the Hydration status, equally low leaf water potentials can occur either under very intense transpiration or when transpiration has almost completely ceased. Without data on the simultaneous transpiration stream, it is impossible to accurately interpret water potential and determine whether the plant is truly experiencing a water deficit (at least not at values above -2 MPa).

The key to assessing a plant's water supply is the potential-transpiration relationship diagram (Fig. 13.11). Using generally linear relationships, this diagram employs diagonal lines indicating increasing hydraulic resistance to interpret its magnitude. It also allows the equilibrium potential with the soil in the early morning hours (predawn water potential) to be determined through the regression of potential at zero transpiration. By numbering and connecting the points on the diagram recorded chronologically over the course of a day, one can observe that the connecting line forms an instructive hysteresis loop (in the afternoon hours, values return to zero transpiration at a lower water potential level along a different path than the one the values took in the morning under increasing transpiration). This indicates a degradation of hydraulic conditions during the day, caused either by xylem cavitation (air blockage, gas embolism) or by transport resistance within The ROOT System.

Class="center">Fig. 13.11. Relationship between leaf water potential and transpiration rate. The sum of all hydraulic resistances in the soil-plant-atmosphere continuum is typically described by a linear relationship (diagonal lines a, b, and c indicate progressively increasing translocation resistance). The potential at zero transpiration represents The equilibrium state between the plant and the soil in the early morning hours (PD, predawn water potential). A — normal decrease without hysteresis; B — daytime hysteresis (time points plotted on the graph), indicating an increasing impairment of water transport (xylem cavitation or soil dehydration near fine roots). In moist soil, a steeper regression line slope also reflects pathological Changes in the xylem or damage to fine roots (e.g., fungal infection or root rot).

Leaf water potential is generally lower than the water potential in the xylem of SHOOT axes or the trunk. The equilibrium pressure of a cut shoot or leaf petiole, measured using a pressure chamber (see Fig. 6.39), is a composite value for all distal Tissues—primarily leaf tissues—which is why the terms 'leaf' or 'shoot' water potential are appropriate (rather than xylem water potential). Under high transpiration rates, There is a sharp drop in potential from the shoot axis, through the leaf petiole, out toward the blade.

A non-linear relationship exists between the reciprocal of water potential and water saturation deficit (WSD, see 6.3.6)—known as the pressure-volume curve. This curve makes it possible to determine Cell wall elasticity, osmotic pressure at full turgor, and the water potential at the turgor loss point; consequently, such characteristic curves are widely applied to numerous outdoor plants. Water deficits can actually arise because of cell wall elasticity (in 'concrete-like' rigid cells, moisture deficits could not occur because water does not stretch them). Cell wall elasticity is also a measure of how directly (and strongly) the atmospheric suction force during evaporation is transmitted to the root systems in the soil. Therefore, in regions with periodic droughts, the proportion of plants with very rigid, inelastic cell walls increases. This is also one of the reasons for the prevalence of sclerophylly in regions with dry seasons.

An ecologically crucial phenomenon is xylem cavitation. When capillaries are subjected to high tension during intense transpiration, air seeding and The formation of gas bubbles (gas embolism, see 6.3.5) can occur, rapidly disrupting the hydraulic continuum. Because cavitation is essentially the rule for trees under favorable weather conditions (its acoustic onset can be detected in the ultrasonic range), it is suspected that cavitation is not always detrimental to trees (perhaps helping to protect against overload). Consequently, the water potential in leaves drops faster than in the xylem (due to the limited cohesion-tension continuum). There is preliminary evidence that late-afternoon embolisms can be repaired just as rapidly as they form at noon, with xylem parenchyma and phloem playing an active role in this process. When the phloem is removed, xylem 'repair' is noticeably delayed. These findings belong to a developing field of research where subtle analyses of xylem physiology may yet yield surprises. At the center of this research are not debates over the cohesion theory (which is fundamentally virtually indisputable), but rather the dynamics of cavitation, its impact on actual tension states within the xylem, the function of pits and wood parenchyma, and the mechanisms by which the xylem is able to rapidly reverse cavitation.

Pronounced differences in the critical water potential thresholds that trigger initial embolisms (determined via ultrasonic measurements on drying branches) across various species can correlate with conduit diameter (wider conduits carry a higher probability of embolism). Conversely, this may be linked to specific root penetration depths (reaching accessible soil water) and seasonal development rhythms (wet and dry periods). In Mediterranean mixed stands consisting of large-pored, drought-deciduous species and small-pored evergreens, the latter—possessing deeper root systems compared to the former—are less active (though over a longer duration) and less susceptible to embolism during dry periods. These xylem traits are even more pronounced in woody plant species inhabiting arid environments.

13.5.2. Responses to water deficit

Plant water relations progress through several stages depending on the severity of the water deficit and include 'preventive' responses. In nature, six such stages can be distinguished, though the occurrence of the first and, above all, the Second Stage has been 'proven' for only a limited number of species.

Stage 1. Stomatal response to the transpiration rate (even under high soil moisture), first discovered in the 1970s by O. Lange and R. Lösch in Würzburg and initially attributed to air humidity sensitivity. This discovery transformed our understanding of the mechanics regulating plant water balance (Fig. 13.12) when it was revealed that stomata respond to high transpirational demand proactively—reducing aperture width without experiencing stress (i.e., without a significantly lowered water potential). It has since been demonstrated that the actual trigger is not air humidity itself, but the resulting rate of transpiration. It is hypothesized that the

transpiration-dependent accumulation of Abscisic acid (ABA) in the apoplast of guard cells and subsidiary cells acts as the chemical messenger.

Fig. 13.12. Stomatal responses to air humidity. Many plants, particularly perennials, constrict their stomatal pores (leaf diffusive conductance) even when soil moisture is high if the air becomes dry. In this way, they slow down soil desiccation. An example of this ' feed-forward ' response is based on Eucalyptus pauciflora from southeastern Australia (data shown only for water potential values <1.5 MPa).

At the ecosystem level, the consequence of this behavior is that soil water reserves are not excessively depleted by the time high water availability ends. Such conservative water consumption is naturally advantageous only for long-lived plants or those 'banking' on drought. In fact, this behavior is most pronounced in trees—especially long-lived ones—and perennial desert plants. Critical water vapor pressure deficits in the air that trigger this response range from 8 hPa (for plants adapted to humid climates) to 15 hPa (for those adapted to arid conditions). At a Temperature of 20 °C, this corresponds to a relative air humidity of 65% and 35%, respectively; at 30 °C, it corresponds to 80% and 65%. Exceeding this threshold causes stomatal diffusive conductance to drop in a nearly linear relationship with increasing water vapor pressure deficit, which dictates the stomatal behavior of many plants (especially trees) in the late morning hours. This curbs transpiration and stabilizes water potential. Studies on desert plants have shown that on overcast days with lower air dryness, leaf water potentials actually drop more severely due to delayed stomatal responses than on clear days, when vapor pressure deficits reach high levels and stomata curtail transpiration disproportionately fast. This behavior, also known as a feed-forward response, likely serves to limit the risk of cavitation, which is why it is more strongly expressed in woody plants (see 13.5.1).

Stage 2. Stomatal response to root-sourced signals indicating that moisture is becoming scarce in the immediate vicinity of fine roots. If experimental potted plants are allowed to dry out slowly in a way that prevents leaf water potential from deviating significantly from the soil water potential (with pots placed inside a pressurized chamber, Fig. 13.13), turgor loss is prevented. Even under high air humidity, this results in a sustained reduction in stomatal aperture width that closely correlates with decreasing soil moisture content. This phenomenon is attributed to increased ABA production in fine roots as a response to the increasing dryness of their surrounding environment. This reaction also leads to a restriction of stomatal-controlled transpiration without ever reaching a critical leaf water potential threshold. Because the pathway between the roots and the tree crown is often lengthy, these responses are delayed. Evidence suggests that the concentration of ABA in the xylem sap is the decisive factor, rather than its passive accumulation in the apoplast of transpiring leaves.

Fig. 13.13. Stomatal response to a root-derived signal: A — A root signal (abscisic acid) prompts stomata to respond to progressive soil drying while leaves are artificially maintained in a turgid state (water potential equals zero). This condition is simulated by applying elevated pressure (p) to the root zone, while a water-filled capillary inserted into the shoot axis indicates a balanced zero pressure. By moving the meniscus toward the axis (suction), pressure in the root chamber can be adjusted; B — demonstrated using wheat (Triticum aestivum), showing that below a critical water content in the growth substrate, leaf diffusive conductance (stomatal opening) decreases linearly despite the turgid state of the shoot.

Proving this mechanism in nature is extremely difficult. Like the air-humidity response, it exhibits a feed-forward character that, on the one hand, ensures nutrient supply and, on the other, protects against the sudden onset of acute stress (turgor loss). It is well established that trees growing in dry soils exhibit elevated levels of ABA in their xylem sap. Nevertheless, it remains an open question whether this is a passive consequence of a generally slowed transpiration stream or an active physiological process.

Stage 3. Stomatal responses to turgor loss. If the leaf water potential approaches the osmotic potential of The Cell sap—despite the preventive measures of stages 1 and 2—the counter-pressure exerted by the cell walls on the protoplast drops to zero. Soft leaves wilt even before this point is reached. The critical water potential threshold at which stomata close passively due to dehydration also lies close to this turgor loss point. A chronically low turgor (low 'predawn water potential') increases the sensitivity of stomata to dry air.

In nature, the loss of turgor occurs relatively rarely. There are even cases (in species of the genera Piper and Helianthus) where stomata remain open despite noticeable wilting, which emphasizes that stomata generally begin to respond to water potential signals very late. This peculiar behavior is explained by the fact that plants, by wilting (lowering their leaf blades), avoid midday solar radiation and thereby protect themselves from overheating and, ultimately, from water loss.

From the foregoing, it can be concluded that the capacity of plants to maintain a favorable water balance through immediate physiological reactions is exhausted. Further survival depends on desiccation tolerance. Recurring water deficit leads to a drop in osmotic potential (accumulation of osmotic pressure, osmotic adaptation, osmoregulation), which delays the loss of turgor and increases resistance to dehydration.

Stage 4. Biomass allocation. Plants subjected to prolonged and repeated moisture stress respond by enhancing root growth at the expense of leaf biomass. As a result, the balance shifts in favor of water uptake (exploiting deeper soil layers). If the mechanisms applied in stages 1–4 fail to achieve their purpose, stage 5 ensues.

Stage 5. Leaf shedding and the consequent reduction in LAI. If water deficit is still not halted by this, stage 6 ultimately follows.

Stage 6. Thinning of the plant cover and replacement of the established species composition by new drought-tolerant species. These may include species with specific phenorhythms (escapers —

from English escape), exceptionally deep roots, pronounced succulence (avoidance strategy — from English avoidance), or true drought resistance.

The most effective Methods of "avoiding" water loss are obligate leaf shedding during the dry period and/or surviving drought in the form of seeds, bulbs, or underground shoots. Succulents and deep-rooted plants escape physiological stress, with the latter being more successful in surviving prolonged drought. Succulents require small amounts of regularly supplied moisture, as they are forced to rely solely on the reserves stored within their tissues. When the soil is dry, the roots of succulents lose contact with it and become isolated from the substrate. The extent of tissue water loss varies widely, but most tissues can lose up to 50% of their water. True drought resistance, up to the point of complete desiccation, is rare among vascular plants (resurrection plants), but quite common among aerial Lichens, mosses, and Algae (so-called poikilohydric plants, in contrast to homiohydric plants whose desiccation-tolerance strategies were described earlier).

In nature, the response to moisture deficit corresponding to stage 6 is of the greatest importance. A well-adapted species composition hardly needs the reactions of stages 1–5, meaning that these species do not impair the photosynthetic machinery, which would otherwise have to be temporarily slowed down or even interrupted. This explains why, along the natural moisture gradient at the leaf level, relatively minor differences in water regime parameters are observed. These differences remain moderate even when 1 m² of soil projects 0.01 m² of transpiring leaves (desert) or 5 m² (beech forest). Desert plant leaves transpire (except during extreme droughts) in much the same way as mesic plants, but they produce very few yet fully functional leaves. This investment economy in functional leaves is closely linked to their mineral supply and, consequently, to the mineral budget of the ecosystem (see 13.6.3).

Discoveries made predominantly over the past two decades have shown that much of what was previously evaluated as negative phenomena in the leaf water regime (in terms of stress symptoms) actually represents immediate, counteracting, systemic warning responses (stomatal reactions in stages 1 and 2), or is the consequence of high activity rather than an expression of inflicted damage (low water potential); in any case, these are not direct stress impacts over a large area. Agricultural research on crops regionalized in arid zones has also made a significant contribution to this issue. For example, H. Passioura (Canberra) was able to demonstrate that in very dry agricultural regions of Australia, wheat varieties with "poor" root xylem yield better harvests. This paradox is based on the observation that regular water deficits in the leaves (due to poorly conductive xylem) during vegetative development, although slightly inhibiting growth, ensure that soil water remains available by the time of flowering and grain ripening. Very similarly, the aforementioned "feed-forward" reactions can be evaluated; although they lead to what initially seem like unjustified restrictions, they significantly promote the long-term prosperity and survival of long-lived plants.

Through partial leaf removal, F. Meinzer (Hawaii) and other authors were able to demonstrate that stomata also respond in a compensatory manner to the water demands of the entire plant community, not only adjusting stomatal diffusion accordingly, but also exhibiting regulatory behavior that incorporates the aerodynamic resistance of the boundary layer. Ultimately, this also comes down to the method of measuring transpiration; According to the aforementioned authors, chemical signals would act too slowly, which is why hydraulic signals are hypothesized. These Examples further demonstrate that the previously used distinction between autecology and synecology hinders a convincing interpretation of processes. Almost all the reactions discussed above can only be understood based on their effects at the ecosystem level.

13.5.3. Stomatal behavior in nature

Along with the already described effects of the water regime, the width of the stomatal pores is determined primarily by light resources and thus by The rate of photo

synthesis (and the CO₂ level in leaf tissues). Photosynthesis extracts CO₂ from the air and delivers it to the leaf, where it diffuses inward through the stomata (see 6.5.7). As long as the water status permits, stomata adjust to a diffusive conductance that maintains the internal leaf CO₂ concentration (C_i) at about 70% of the ambient air concentration (c_a). The higher the photosynthetic rate (A), the greater the diffusive conductance must be to maintain a stable intercellular concentration (the C_i/c_a ratio). This is also closely linked to CO₂ gas exchange and water vapor diffusion, which implies a tight correlation between the maximum rate of photosynthesis at normal CO₂ levels (A_max) and maximum stomatal diffusive conductance (g_max). The higher the photosynthetic intensity, the higher the maximum stomatal conductance to water vapor (Fig. 13.14).

Fig. 13.14. Relationship between maximum photosynthetic rate and transpiration status in C₃ plants. Under a favorable water supply, there is a linear correlation between the maximum diffusive conductance of the leaf epidermis (stomata; g_max) and the maximum photosynthetic rate Am_ax per unit of leaf area (A_cap), regardless of life form (dots indicate conifers, deciduous trees, shrubs, herbaceous perennials, grasses, succulents, etc.). This means that The ratio of the intercellular CO₂ concentration to the external concentration (C₁/C_a) in all C₃ plants falls within the same range (0.7 — 0.8), which is considered the result of evolutionary optimization between water loss and CO₂ uptake through the pore system

The comparison shown in Fig. 13.14 includes extreme cases—fast-growing herbaceous plants and inactive, very slow-growing species. It obscures the fact that the mean values for dominant tree species of the Earth's major biomes in these standard parameters in natural habitats hardly differ if a sufficiently large number of species is included in the comparison. The global value of g_max relative to the projected leaf area is (218 ± 24) mmol H₂O m^-2 • s^-1 for 151 woody plant species. Herbaceous plants and crops have values that are on average almost twice as large, whereas CAM plants have significantly lower values. The minimum diffusive conductance of water vapor (g_min) by leaves is largely determined by the presence of apertures in closed stomata, which is why referring to cuticular diffusive conductance is inaccurate. The variation of g_min relative to g_max ranges from 1/20 (herbaceous shade plants) to 1/300 or even less in succulents (for trees and shrubs, from 1/40 to 1/60, i.e., 3 to 6 mmol • m^-2 • s^-1).

Fig. 13.15. Schematic representation of the daily course of leaf diffusive conductance. Under favorable weather conditions and moderately moist soil (leaf water potential before sunrise > 0.2 MPa), the leaf diffusive conductance g (equal to stomatal conductance, as cuticular transpiration is very low) follows the increase in light availability in the morning (phase I) up to the maximum value g. Once the critical vapor pressure deficit (vpd) of the air is reached (approximately 10 hPa), the value of g decreases in parallel with a further increase in vpd (phase II). In the afternoon, under the same climatic conditions as in the morning, the value of g becomes progressively lower and drops almost to zero (time-independent factors, accumulation of end products in leaves, passively increasing ABA content, active increase in ABA content as a root signal regarding the dehydration of the immediately adjacent soil, xylem cavitation, etc. — phase III). Since the determinants of g succeed one another throughout the day, performing a statistical analysis to determine factor dependencies without separating the phases is impossible. Curve 1: high air and soil humidity; curve 2: high soil humidity but reduced air humidity (vpd>10 hPa); curve 3: soil water deficit situation. The arrow indicates increasing drought stress

Stomatal or leaf diffusive conductance are two interchangeable terms, as the difference between water transpiration through stomatal pores and through stomata combined with the intervening cuticle is negligible. Diffusive conductance is the reciprocal of diffusive resistance r. Both quantities are derived, by analogy with Ohm's law, from areal transpiration Tr (the "transpiration stream") and the water vapor gradient of the leaf–air system, ∆w ("tension"; Fick's law of diffusion). Here, it is assumed that the air inside the leaf is saturated with water vapor, with the vapor pressure within the leaf being solely a function of leaf temperature. Two dimensions are used. If the characteristic (gH₂O m^-2 • s^-1)/(gH₂O • m^-3) is used for g = Tr/∆w, g is obtained in m • s^-1. If ∆w is expressed in (kPa • H₂O, kPa^-1 of air), it turns out that due to the dimensionless Description of the air humidity gradient, g shares the same dimensions as Tr, i.e., in g • m^-2 • s^-1, or in mmol • m^-2 • s^-1. Importantly, in this case, ∆w is derived from the ratio: water vapor pressure inside the leaf minus water vapor pressure outside the leaf (total air pressure). Hence, The values of g and the transpiration rate calculated from the actual value will be largely independent of air pressure, which affects molecular diffusion (data from various altitudinal belts). This is one of the reasons why this dimension is increasingly being applied.

The interaction of humidity, solar radiation, and temperature results in a characteristic daily pattern of stomatal aperture width (diffusive conductance; Fig. 13.15). In the morning hours, stomatal opening is determined primarily by light and photosynthesis. At a certain photon flux density (PFD) (at 20–25% of full sunlight in sun-loving plants), g_max is reached. If the critical vapor pressure deficit is exceeded (see 13.5.2, at least about 8 hPa, or about 65% relative humidity at 20 °C), this becomes the decisive controlling factor for most tree species, and g decreases (in fine weather, from the late pre-noon hours onwards). Under identical climatic parameters, afternoon values are generally lower than pre-noon values. Such time-dependent responses are explained by the accumulation of photosynthetic products and elevated ABA levels. The daily course of g in woody plants is represented by a single-peaked curve with a maximum before noon. Herbaceous plants react to the weather mostly in the afternoon, meaning a second maximum of g may appear when transpiration tension decreases again. In overcast, damp weather, the sole determining factor is PFD. Diffusive conductance also governs the diffusion of harmful gases (e.g., ozone), meaning that dry air and relative water stress will result in minor damage.

During further pore opening, changes in aperture width at a given air humidity are almost proportional to changes in transpiration, yet this has a relatively minor effect on photosynthesis. Therefore, plants can regulate transpiration over a wide range without exerting a proportional impact on photosynthesis. Only when pore widths are very small does the effect on photosynthesis become pronounced. This Asymmetry means that leaf stomata constitute the only significant controllable mechanism for the transpiration stream, whereas CO₂ uptake, In addition to gas diffusion, faces another resistance in the mesophyll cells that is about 4 to 5 times larger when stomata are fully open (in C₃ plants; in C₄ plants, mesophyll resistance is significantly lower). Thus, stomatal resistance represents (primarily in C₃ plants) only a minor fraction of all obstacles to CO₂ uptake.

When stomata are widely open, the resistance to gas diffusion in the aerodynamic leaf boundary layer becomes highly significant. Leaf size and pubescence increase boundary-layer resistance, which acts analogously to stomatal resistance, thereby reducing the efficiency of stomatal regulation at wide apertures. At narrow pore apertures and under drought stress, these aerodynamic factors are relatively insignificant for water diffusion, but by counteracting thermal convection and leaf heat reflection—and thereby mitigating water vapor pressure—they can indirectly help maintain the water balance. Incidentally, A large number of plants in arid regions are sparsely pubescent or entirely glabrous, which contradicts the widespread belief that pubescence provides direct protection against transpiration. Increased boundary-layer resistance (which ranks alongside stomatal resistance) minimizes the effectiveness of stomatal regulation in any case.

13.5.4. Water regime of ecosystems

Under high soil moisture, a forest or meadow in Central Europe on a fine summer day releases 4–5 mm of water evaporated into the atmosphere (1 mm = 1 L per 1 m²). The reserves of plant-available soil water decrease by this exact amount. How often this can be repeated (during a spell of fine weather) depends on the water content of the root-zone soil profile (see Table 13.3 in Section 13.7.5.1). The potential available moisture reserve is determined by the profile depth, potential "dead zones" of the soil Skeleton (stones), and the volume fraction of medium-sized pores. As a rough estimate for well-developed, non-sandy soils, a total pore volume of 50% of the total volume can be assumed (higher in upper soil layers, significantly lower in deep horizons). Roughly half of this volume, about 250 mm of water per 1 m of skeleton-free profile, can be considered readily available. The remaining soil pore volume consists of rapidly draining large voids (e.g., earthworm burrows) or very fine pores whose water is inaccessible to plants. After thorough wetting, such a model profile could maintain an adequate moisture supply for about a month and a half at the aforementioned daily evaporation rates. In a shallower profile with a high stone content or in sandy soil with lower water-holding capacity, the period during which the ecosystem can survive without precipitation is shortened. A pronounced feed-forward stomatal response prolongs this period (see 13.5.2 — stages 1 and 2). Atmospheric water that runs off or remains on the leaf canopy as wetting moisture is lost to the ecosystem.

These relationships are described by the water balance equation:

N = E + Tr + I + A + dB,

where N is precipitation, equal to the sum of soil surface evaporation (E), plant transpiration (Tr), interception loss (wetting moisture) (I), water outflow (surface runoff and infiltration water percolating to depth) (A), and changes in available soil moisture storage (dB). Over a long period (e.g., an entire year), The change in soil moisture storage is zero. E, Tr, and I can be combined into METABOLISM/2.html">THE CONCEPT OF evapotranspiration, abbreviated as ET. Thus, N = ET + A.

In a closed plant community, the decisive factor of the water regime is precipitation interception. A vast amount of water is retained by leaves, especially during frequent but light rains (1–2 mm of water goes into wetting the entire canopy). In the Amazon basin, nearly a quarter of total precipitation (about 2000 mm per year) is accounted for by interception losses, which approach the discharge volume of the Amazon River itself. Interception volume is influenced by leaf arrangement patterns and LAI. Dense coniferous forests intercept twice as much water as a meadow. When forests are uprooted or mowed, moisture interception is eliminated, and runoff increases.

In Central Europe, the annual amount of evaporated water (ET) is about 500 mm (up to 650 mm at low elevations in the southern part). Approximately 70% of all evaporated water lost by Europe escapes through leaf stomata, which highlights the paramount role of the plant cover in the water regime. Plants particularly promote evaporation during dry periods because, via their root systems, they tap into water reserves that would otherwise not reach the evaporation stage without plant cover (see Table 13.3 in Section 13.7.5.1, as well as 13.1.2). On the other hand, vegetation promotes the formation of soils with a high water-storage capacity and impedes surface runoff. Closed, well-watered plant communities evaporate more water per unit area than open bodies of water (such as lakes) due to their large leaf area, often higher temperatures, and favorable aerodynamic coupling with the atmosphere.

Water sources feeding vegetation can be traced using deuterium, a stable heavy isotope of hydrogen (in heavy water). Heavy water evaporates somewhat slower than normal water, which is why water vapor (and consequently clouds and precipitation) worldwide always contains slightly less D₂O than groundwater. Plants with a relatively low deuterium content in their tissues (verified via mass spectrometry) draw moisture from surface "fresh" soil water, in contrast to plants whose roots are in contact with groundwater. This is analogous to The behavior of the heavy and relatively stable oxygen isotope ¹⁸O compared to "normal" ¹⁶O. H₂¹⁸O water evaporates more slowly and therefore accumulates in the seas, in whose sediments ¹⁸O is found in quantities proportional to the evaporation rate and thus temperature, a fact utilized in paleoecology for climate reconstruction. Under high transpiration, the heavy oxygen isotope also saturates plant tissues more strongly, which, alongside ¹³C discrimination (see 13.7.4), makes it possible to reconstruct patterns of water and carbon regime changes (e.g., using samples from old tree growth rings).

Plant Growth and water consumption are closely intertwined. To produce 1 kg of plant biomass, 500 to 1,000 L of water is required, whereas for C₄ plants the figure is 250–400 L. In arid regions, CAM plants require remarkably little water; the "price" paid for exclusively nocturnal gas exchange is a very slow growth rate. The concept of water use efficiency (WUE) lacks a single unambiguous definition. The classical definition originated in agriculture, where WUE denotes The amount of useful yield (e.g., grain, hay) per unit of water consumed that reaches the soil surface (grams of dry matter per liter of water). If, for instance, about 50% of the produced plant biomass in cereal crops falls on the grain (harvest index), the water consumption per unit of yield, like that per unit of biomass, doubles. The inevitable water loss due to soil surface evaporation and interception losses is also factored in here (which is important for irrigated agriculture). Much later, WUE was adopted as a term in gas exchange physiology and now defines the ratio of leaf photosynthesis to transpiration (mmol/mol). The parallel existence of these two Definitions has caused considerable confusion and requires a clear specification of the applied concept in each case. Moreover, the word "efficiency" is ambivalent because, in an ecological sense, a distinction must once again be drawn between mass production and resilience (through "success," see 12.1). As W. Larcher suggests, the neutral term "coefficient" should be applied instead.