Ecophysiology of desert plants

The southern African Deserts, especially the Succulent Karoo, are characterized by a high biodiversity of succulent plants, in particular taxa in the families of the Mesembryanthemaceae and the Crassulaceae. These succulents have developed various integrated and co-adapted morphological and ecophysiological features that maximise their chances of surviving the detrimental conditions in arid habitats. Most of these succulents are characterised by the development of Crassulacean acid metabolisms (CAM). A case study in the winter rainfall region of southern Africa showed that around 35% of the species and up to 70% of the total vegetation performed the CAM pathway. CAM can be expressed in various ways. Its expression may range from a C3–type pattern of photosynthesis to nearly exclusive night-time CO2 uptake. This range of extremes may be found within one species during progressing plant development and/or as a response to changes in environmental conditions. In CAM-plants transpiration losses is reduced because of nocturnal CO2 uptake and daytime stomata closure, but on the other hand they show low CO2 uptakes in comparison to most C3 plants. However, it is assumed because of the higher water use efficiency that CAM is an important water saving strategy under dry environmental conditions. From the results of numerous long-termed field investigations on the ecophysiology of plants of the Namib desert and the Karoo it could be proposed that the capability to store water is very important adaptational strategy to survive the desert conditions. Several comparative field and lab studies on the ecophysiology CAM- and C3 succlents were carried out in the South African Deserts.

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  • Variability of CAM in leaf-deciduous succulentsfrom the Succulent Karoo (South Africa)

Seasonal or drought-induced deciduousness is a common adaptational strategy in aboveground persistent succulent species native to the Succulent Karoo (South Africa). This feature may appear together with crassulacean acid metabolism (CAM) in stems and/or leaves. Due to differences in morphology or life cycle, different species may exhibit different degrees of flexibility in the expression of CAM. The potential for CAM plasticity has not been investigated in leaf-deciduous succulents of the Succulent Karoo. Diurnal gas exchange and nocturnal organic acid accumulation were measured to investigate whether the expression of CAM may vary in Tylecodon paniculatus (L.F.) Toelken (Crassulaceae), Monilaria moniliformis (Mesembryanthemaceae) and Ceraria fruticulosa H. Pearson & Stephens (Portulacaceae), a selection of leaf-deciduous succulents. These species differ both in leaf and stem morphology. In the seasonal deciduous Tylecodon paniculatus only obligate CAM was found, regardless of the plant water status. When droughted Monilaria moniliformis switches directly from CAM-cycling (C3-like diurnal gas exchange patterns combined with nocturnal acid accumulation) to CAM-idling (gas exchange completely ceased while acid accumulation still continued). This seasonal deciduous species has also both succulent leaves and stem. A highly flexible CAM expression (CAM-cycling, full-CAM and CAM-idling) was observed in the drought deciduous Ceraria fruticulosa (non-succulent stem showing no CAM) in response to changes in water availability. The ecological significance of the observed diversity in the flexibility of CAM expression was discussed in terms of morphological traits, and the relative ecological significance of both metabolic adaptation and leaf-deciduousness.

Veste, M.,  Herppich, W.B., von Willert, D.J., Basic and Applied Ecology 2, 283-288, 2001. (more…)


  • Diversity, flexibility and phylogeny of photosynthetic types in the succulent flora of southern Africa
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CAM is a common feature in various southern African succulent plant families and especially widespread in the winter-rainfall Succulent Karoo. General differences between the C3 and CAM modes of photosynthesis are outlined. CAM is not always a constant feature, but may be induced by drought or irreversibly induced during ontogenesis. CAM may be expressed in different ways such as CAM-cycling (Monilaria moniliformis, Aizoaceae) with a C3-like gas exchange pattern but nocturnal malate and citrate accumulation accompanied by fixation of respiratory CO2, or flexible shifts between CAM, CAM cycling, and CAM-idling (Delosperma tradescantioides, Aizoaceae and Portulacaria, Portulacaceae). Within the Aizoaceae, the evolutionary pathway C3 → CAM is correlated with phylogenetic patterns and plant functional types; similar findings exist for Kalanchoe and Macaronesian Semperviva (Crassulaceae). The physiological and ecological importance of CAM and its different expressions (flexible CAM, CAM-cycling) is still not fully understood.

Veste, M., Thiede, J., In: Breckle.S,.W., Schweizer, B., Fangmeier, A. (Hrsg.), Ergebnisse weltweiter Forschung, Beiträge des 2. Schimper-Symposiums, pp. 361-370, 2004. (more…)


  • CAM variations in the leaf-succulent Delosperma tradescantioides (Mesembryanthemaceae), native to southern Africa

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides, a succulent that grows in drought-prone microenvironments in summer rainfall and all-year rainfall regions of southern Africa. When well-watered, this species exhibited Crassulacean acid metabolism (CAM)-cycling, but its carbon fixation pattern changed during the development of drought, shifting to either low-level CAM or to CAM-idling. The rate and pattern of this change depended on environmental conditions, duration of water stress and leaf age. At the onset of drought, diurnal malate fluctuation increased, but was strongly depressed (by ca 70%) as drought continued, and when leaf water content and water potential were low (ca 35 and 50% of the initial levels, respectively). When rewatered, rates of growth and photosynthesis, gas exchange and water status recovered fully to pre-stressed values within two days. Whole-shoot carbon uptake rates suggested that leaf growth had continued unabated during a short-term (≅ one week) drought. This emphasises that CAM-idling allows the maintenance of active metabolism with negligible gas exchange when soil water is limiting. It is possible that old or senescent leaves may provide water for the expansion of developing leaves during initial periods of drought. Regardless of the water regime and environmental conditions, leaf nocturnal malate accumulation and water content were positively correlated and increased with leaf age. Thus the gradual loss of water from older mature leaves may induce CAM-idling, which reduces water loss. An important ecological consequence of this combination of CAM modes is the potential to switch rapidly between fast growth via C3 gas exchanges when well-watered to water-conserving CAM-idling during drought.

Herppich, W.B., Midgley, G., von Willert, D.J., Veste, M., Physiologia Plantarum 98: 485-492, 1996 (more…)


  • Environmental variability and allocation trade-offs maintain species diversity in a process-based model of succulent plant communities
Numees, Richtersveld 2

Ecological theory suggests that environmental variability can promote coexistence, provided that species occupy differential niches. In this study, we focus on two questions: (1) Do allocation trade-offs provide a sufficient basis for niche differentiation in succulent plant communities? (2) What is the relative importance of different forms of environmental variability on species diversity and community composition? We approach these questions with a generic, individual-based simulation model. In our model, plants compete for water in a spatially explicit environment. Species differ in their size at maturity and in the allocation of carbon to roots, leaves and storage tissue. The model was fully specified with independent literature data. Model output was compared to characteristics of a species-rich community in the semi-arid Richtersveld (South Africa). The model reproduced the coexistence of plants with different sizes at maturity, the dominance of succulent shrubs, and the level of vegetation cover. We analyzed the effects of three forms of environmental variability: (a) temporal fluctuations in precipitation (rain and fog), (b) spatial heterogeneity of water supply due to run-on and run-off processes and (c) ‘rock pockets’ that limit root competition in space. The three types of variability had differential effects on diversity: diversity exhibited a strong hump-shaped response to temporal variation. Spatial variability increased diversity, with the strongest increase occurring at intermediate levels of temporal variability. Finally, rock pockets had the weakest effect, but contributed to diversity by providing refuges for small species, particularly at low temporal variability. The model thus shows that spatio-temporal variation of resource supply can maintain diversity over long time scales even in small systems, as is the case in the Richtersveld succulent communities. Trade-offs in allocation provide the basis for necessary niche differentiation. By describing resource competition between individual plants, our model provides a mechanistic basis for the link from species traits to community composition at given environmental conditions. It thereby contributes to an understanding of the forces shaping plant communities. Such an understanding is critical to reduce the threats environmental change poses to biodiversity and ecosystem services.

Reineking, B., Veste, M., Wissel, C., Huth, A., Ecological Modelling 199 (4), 468-504, 2006. (more…)


  • Ions and water relations of Brownanthus pseudoschlichtianus (Aizoaceae) in the Richtersveld (Succulent Karoo)

Brownanthus pseudoschlichtianus (Aizoaceae) is a succulent chamaephyte of the Richtersveld (NorthernCape Region, South Africa) and occurs here mainly in plains habitats characterised by sandy, silty to loamy topsoils.A survey of ion characteristics of Brownanthus pseudoschlichtianus growing on different soil types along the gradient from coast to inland is presented. B. pseudoschlichtianus showed a high accumulation of Na and Cl, which is typical also for halophytic species in the Aizoaceae and other plant families. The ion pattern showed no differences between the soil types. Based on these results it can be concluded that Brownanthus arenosus,B.marlothii B. pubescens and B. pseudoschlichtianus developed specific genetically fixed ion pattern.

Veste, M., Gembler, K., Jürgens, N., Schumania 4, 127-132, 2004. (more…)


  • Parasitic flowering plants on Euphorbia in South Africa and Namibia. 

By definition parasitic plants get nutrients, carbohydrates, ions and water from their host plants and they develop special morphological features to survive on the living tissue of other plants. !e haustorium connects the parasite with its host and allows the transportation of water, inorganic and organic compounds into the parasite. Holoparasites are unable to produce chlorophyll for photosynthesis and are totally dependent on their hosts. A second group are the semi-parasites; their leaves produce chlorophyll and they are able to photosynthesis. From South Africa and Namibia 67 parasitic flowering plants are known, 23 are stem parasites and 44 are root parasites. Mainly woody species are potential hosts for the parasites, but also succulents like Aloe dichotoma, Cotyledon and Lampranthus are listed as hosts.

Veste, M., Euphorbia World 3 (3), 5-9, 2007. (more…)


  • Influence of halophytic hosts on their parasites – the case of Plicosepalus acaciae 
Mistel

Halophytes develop various morphological and physiological traits that enable them to grow successfully on saline substrates. Parasitic plants on halophytic hosts may also encounter salt stress. We investigated the mistletoe Plicosepalus acaciae (syn: Loranthus acacia; Loranthaceae), which occurs on five halophytic and at least ten non-halophytic hosts in the Southern Arava-Valley (Israel). P. acaciae is a common parasite north of Eilat to the Dead Sea area and in the Jordan Valley. Morphological and physiological responses of P. acaciae to salinity were investigated by comparison of plants on halophytic with those on non-halophytic hosts. Ion patterns of different host-parasite associations were determined as was the development of leaf succulence at different growth stages. The leaf water content of P. acaciae increased and leaves developed succulence when growing on halophytic hosts, especially on Tamarix species, where leaf water content was 3 times higher than on non-halophytic hosts and the leaf volume increased 4 – 5 times. The reason for increased succulence was a higher ion-concentration of, and osmotic adjustment with, Na+ and Cl-. P. acaciae showed a high morphological and ecophysiological plasticity enabling to cope with salt stress and can be classified as a facultative eu-halophyte, which increases its halo-succulence according to the host. Host-parasite-associations are a model system for investigation of halophytes under different salt stress conditions.

Veste, M., Todt, H., Breckle, S.-W., Annals of Botany Plants (2015) (more..)


  • The mistletoe Loranthus acaciae (Loranthaceae) on halophytic and non-halophytic hosts in the southern Arava-Valley (Israel)
Loranths

The mistletoe Loranthus acaciae Zucc. (Loranthaceae) occurs on five halophytic and ten non-halophytic hosts in the southern Arava-Valley (Israel). Six new mistletoe-host associations were reported for Loranthus acaciae. Adaptation of Loranthus growing on halophytic hosts were investigated. Water content and succulence of the mistletoe increased on halophytic hosts especially on Tamarix species. Leaf water content was 3 times  higher on halophytic hosts and the leaf volume increased 4 – 5 times. Loranthus acaciae can develop morphological adaptation when growing on halophytic hosts and can be as classified as a facultative eu-halophyte.

Todt, H., Breckle, S.-W., Veste, M., In: Breckle.S,.W., Schweizer, B., Arndt, U. (Hrsg.), Ergebnisse weltweiter Forschung, Verlag Günter Heimbach, Stuttgart, pp. 475-480, 2000



  • Spatial and temporal variability of soil water in drylands: plant water potential as a diagnostic tool
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Arid and semi-arid regions are characterized by low rainfall and high potential evaporative demand. Here, water is the major limiting factor for plant growth and productivity. Soil and surface hydrology properties (e.g. field capacity, infiltration rates) effectively control the water re-distribution in the ecosystem, a fact that is aggravated in arid environments. Information of the spatial and temporal accessibility of soil water in desert ecosystems is limited. The purpose of the studies is the application of plant water potential to estimate the spatial and temporal variations of soil water availability in different arid ecosystems of the Negev (Israel) and southern Morocco. As model plants the evergreen shrubs Retama raetam, Thymelaea hirsuta and trees (Acacia tortilis) were chosen. Seasonal and spatial variations of the pre-dawn water potential (ψ pd) were examined as diagnostic tool to determine water availability on the landscape level. The seasonal differences in the pre-dawn water potential were less pronounced on the dune compared to the interdune. This showed a better water availability on the dune slope. Also in the investigated wadis systems spatial differences of the water potential could be detected and related to the vegetation pattern.


  • Determination of actual evapotranspiration and transpiration in desert sand dunes (Negev Desert) using different approaches
Niz1

In an arid environment, especially in sandy areas where surface runoff is of no practical importance in the hydrological budget, it is rainfall, dewfall and evapotranspiration that become the most important variables. To assess actual evapotranspiration, several methods (flux-gradient, BREB, eddy correlation) were applied to data from the Nizzana experimental site in the northwestern Negev Desert. Additionally, a model specifically designed for arid environments is introduced in this paper. This zero plane model shows the most reasonable results compared with the other methods, which overestimate evapotranspiration to a large degree. It is shown that plant transpiration is the dominant process in total evapotranspiration while advective processes do not play a major role in the near-ground boundary layer, although the study area is influenced by a sea breeze. Actual transpiration of Artemisia monosperma was measured in a field experiment to validate the calculated evapotranspiration. The vegetation contributed 41% of the calculated total evapotranspiration in a single month.


  • Evapotranspiration, transpiration and dewfall
Gas exchange measurements of Artemisia monosperma, Yevul

Actual evapotranspiration is the most critical parameter in hydrological water balance models. The knowledge of atmospheric, plant and soil interactions in drylands is limited. Many evaporation models have been developed, but mainly for agricultural crops. The best known is the single-source Penman-Montheith evaporation model. This model assumes that canopies can be regarded as one uniform surface or big, single leaf. Most of the models have been largely successfully used for estimating evapotranspiration from vegetation which is not drought-stressed and relatively uniform, such as in agricultural fields. However, arid and semi-arid regions are characterized by patchy vegetation and larger open spaces. In an arid environment, especially in sandy areas where surface runoff is of no practical importance in the hydrological budget, it is rainfall, dewfall, and evaporation and plant transpiration which constitute the most relevant parameters. Dewfall may become an input variable at least seasonally even more important than rainfall. In the Negev sand dunes, especially dew has an important ecological implication for the activity and distribution of biological soil crusts and lichens.n In this paper, we will apply and compare some of the most common approaches to compute actual evapotranspiration from field measurements in the sandy north-western Negev Desert in Israel, and introduce a practical model specifically applicable to arid environments. In this context, dewfall will be inferred as a reversal of the evaporative process

Littmann, T., Veste, M., Ecologcial Studies 200, 183-200, 2008 (more…)


  • Temporal and spatial variability of plant water status and leaf gas exchange
Gas exchange and photosynthesis measurments

Arid and semi-arid regions are characterised by low rainfall as well as high potential and actual evaporative demand. Consequentially, water is the major limiting factor for plant growth and productivity. Besides precipitation, hydrological soil properties are most important for soil water availability in the Nizzana sand dunes. The vegetation pattern in these sand dunes reflects the spatial differences in soil water availability. Detection of spatial heterogeneity requires a high number of soil sensors to evaluate water availability on the landscape level. Unfortunately, the use of tensiometers is limited mainly by excessively low soil water contents in the upper layers. As alternative, phanerophytes are good indicators of water resources in these heterogeneous ecosystems. Desert perennials develop extensive root systems and are able to exploit soil water from deeper horizons. Especially shrubs and trees depend on sufficient water resources during the entire year. Water uptake by roots depends on gradients of water potential in the soil—plant—atmosphere continuum. The leaf water potential can be easily and rapidly determined by means of pressure chambers or by thermocouple psychrometers. Commonly used parameters for plant water stress characterisation are the minimum water potential (ψmin) and the predawn water potential (ψpd). During the night, the water potential of a nontranspiring plant will equilibrate with the “wettest” water potential of the substrate around the roots, and ψsoil becomes ψpd of. Therefore, ψpd in many cases will be a good estimate of the soil's water availability.

Veste, M., Ecologcial Studies 200, 367-375, 2008 (more…)


  • Root growth and water uptake in a desert sand dune ecosystem
Minirhizotronsystem

In desert sand dominated areas are often prominent. Sandy ecosystems exhibit a pattern of different soil types. Sometimes, under special conditions, a crypotogamic crust develops which influences the hydrological conditions. Root systems  of psammophytes are normally very long and far.reaching. The water uptake and water relations of desert shrubs strictly related to the structure of the root system and to the water availaibility in the soil. The use of the minirhizotron technique in deserts for data sampling is discussed.

Veste, M., Breckle, S.-W., Acta Phytogeographica Suecica 81, 59-64, 1996. (more…)




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  • Root respiration of Pancratium sickenbergeri - a desert  geophyte

For the calculation of carbon budgets on the whole plant level the knowledge of carbon flux is an important parameter. the root respiration and photosynthesis of Pancracium sickenbergerii was measured under greenhouse and desert conditions. the use of a new gas exchange chamber for root respiration measurements under laboratory conditions is shown. The root respiration of Pancracium is similar to other desert plants, e.g. Opuntia and Agave.

Veste, M., Breckle, S.-W., Acta Phytogeographica Suecica 81, 68-70, 1996. (more…)






  • Welwitschia mirabilis - an ecopysiological view
Welwitschia mirabilis, Brandberg, Namibia - Arbeitskopie 2

One of the most extraordinary and best-known desert plant of the Namib is undoubtedly Welwitschia mirabilis, colonized large parts of the central Namib Desert north of the Kuiseb Rivers to the Nicolau-Rivier in southern Angola. Especially in the barren landscape of the central Namib it is with her size very noticeable. Since its discovery by the Austrian physician and naturalist Friedrich Welwitsch in southern Angola and by Thomas Baines in Swakop Rivier have grown up around this plant many scientific mysteries and myths. In order to better understand the amazing adaptability of this bizarre plant and to gain more detailed information specifically about their water and carbon budget, we conducted a long-term research project led the working group of Prof. Dr. DJ Willert of the Westfälichen Wilhelms-Universität Münster with extensive ecophysiological measurements along a coastal-inland transect of the Welwitschia Vlakte to Brandberg through.

Veste, M., Herppich, W.B., Naturwissenschaftliche Rundschau 61, 620-625, 2008. (more..)


  • Spatial patterns of soil water and vegetation in arid ecosystems and their applications for shelterbelt design – examples from the Negev desert
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In arid and semi-arid ecosystems water is the major limiting factor for plant growth and determined the vegetation pattern. It is a well-known fact from large-scale comparisons that standing biomass and rainfall show a close positive relationship (e.g. Shmida 1985). Vegetation cover and biomass show the same decrease from humid to arid climates as does mean annual rainfall, which is supposedly the controlling factor. On the other hand, soil and surface hydrology properties (e.g. field capacity, infiltration rates) effectively control the water distribution in the ecosystem, a fact that is aggravated in arid environments. Information of the spatial and temporal accessibility of soil water in desert ecosystems is limited. The purpose of the studies is the application of plant water potential to estimate the spatial and temporal variations of soil water availability in different arid ecosystems of the Negev (Israel). As model plants typical evergreen shrubs, e.g. Artemisia monosperma, Retama raetam, Thymelaea hirsuta were chosen. Pre-dawn water potential (Ψpd) is measured by a pressure-probe  and is used in this context as diagnostic tool to determined water availability on the landscape level. The pattern will be linked to the hydrological processes in the different ecosystem and their implication for vegetation patterns. 

Veste, M., Littmann, T., In: Proceeding of the First International Forum on Ecological Construction of the Western Beijing, Beijing:  74-86, 2005. (more…)

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