Ludger Kappen

Some aspects of the great success of lichens in Antarctica

The terrestrial vegetation in Antarctica is restricted to small, very isolated, ice-free areas on the continent and on the islands adjacent to the Peninsula region. Lichens, a symbiosis between a fungus and an alga or cyanobacterium, form the most prominent component in the vegetation. They have the greatest number of species compared with other cryptogamic and unicellular organisms. They are able to colonize all types of solid substrata and have a low mineral nutrient demand. Their physiological vigour is derived from high freezing tolerance and the ability to be photosynthetically activated by water vapour uptake from snow at temperatures as low as c. -20°C. Long-term monitoring in their natural habitats demonstrates that lichens are photosynthetically active at suboptimal temperatures. Locally, however, they can benefit from meltwater on insolated rocks, but only for short periods in early summer. Although at this time they may be exposed to substrate temperatures of > 20°C and strong light they do not suffer from photostress. If, in winter, lichens are covered and kept dark by snow with temperatures close to 0°C, lichens enter a negative carbon balance. This effect may be relevant to lichens under conditions of global or regional warming.

Ecophysiological Investigations on Lichens of the Negev Desert: VI. Annual Course of the Photosynthetic Production of Ramalina macifarmis (Del.) Bory1)

Summary Diurnal courses oflight, temperatures, water content, and CO 2 exchange of the thalli of Ramalina maciformis were measured in its natural habitat in the Negev desert from March to September 1971. The measurements of water content of the lichen thalli by means of a Hiltner dew balance were extended over a period of about two years. Water content was the most prominent factor controlling the CO 2 -gas exchange of the lichen. The different types of water uptake by rain-, dew-, or water-vapour moistening and the corresponding daily courses of net photosynthesis are presented. The experimental data, formed the basis of a correlation model for net photosynthesis of R. maciformis by means of which the daily photosynthetic production due to dew and water-vapour uptake was calculated for a, whole year according to the registrations of the thallus water contents, temperatures of a neighbouring weather station, and photoperiod. The net photosynthesis due to rain-water imbibition and the respiration during the night were separately calculated. The photosynthetic CO 2 gain which was yielded by dew on c. 200 days was as high as that calculated for the 29 rain days of the extremely rainy annual period of 1971/72. In years with precipitation near the long term average, as was the subsequent period 1972/73, rain induced production of R. maciformis was only 2/3 of that yielded by dewfall and may be only a small fraction in dry years, whereas dewfall maintains almost always its regular occurrence. Rainfall in the Negev, although being very effective for high photosynthetic productivity, is too scarce to provide life of R. maciformis . The existence of many lichens in the Negev is only possible because of the high frequency and regularity of dew falls. It is discussed whether the calculated amount of the annual photosynthetic gain is representative for the natural production and growth.

Long-term effects of drought on wild and cultivated plants in the Negev desert: I. Maximal Rates of Net Photosynthesis

SummaryThe relation between daily maximal rates of net photosynthesis and plant water status was studied during a dry season on irrigated and non-irrigated, naturally growing, perennial wild plants.Species were examined which differ in phenology, leaf anatomy and morphology: Hammada scoparia, Artemisia herba-alba, Zygophyllum dumosum, and Reaumuria negevensis. Prumus armeniaca which was growing in the run-off farm at Avdat and which has mosomorphic leaves was included in the comparison. All plants differed in their seasonal change in plant water status, and in their seasonal change in daily maximal net photosynthesis. Rates of CO2 uptake were not uniquely related to simultanously measured leaf water potentials. Daily maximal rates of net photosynthesis of non-irrigated plants, and the difference between maximal CO2 uptake of irrigated and non-irrigated plants were examined in relation to pre-dawn water potential. Maximal net photosynthesis rates decreased very rapidly with decrease in pre-dawn water potential or, for Hammada scoparia, they decreased even with a constant level of pre-dawn water potential. Consequently, it was considered necessary to include both time and water potential in a parameter “bar day” describing the accumulated drought stress of the plants. All species showed the same relation between relative maximal net photosynthesis and drought experience as determined by cumulative daily addition of pre-dawn water potentials for the non-irrigated plants since the last rain.

Aspects of carbon and nitrogen cycling in soils of the Bornhöved Lake district II. Modelling the influence of temperature increase on soil respiration and organic carbon content in arable soils under different managements

Based on field measurements in two agriculturalecosystems, soil respiration and long-term response ofsoil organic carbon content (SOC) was modelled. Themodel predicts the influence of temperature increaseas well as the effects of land-use over a period ofthirty years in a northern German glacial morainelandscape. One of the fields carried a maizemonoculture treated with cattle slurry in addition tomineral fertilizer (“maize monoculture”), the otherwas managed by crop rotation and recieved organicmanure (“crop rotation”). The soils of both fieldswere classified as cambic Arenosols. The soilrespiration was measured in the fields by means of theopen dynamic inverted-box method and an infrared gasanalyser. The mean annual soil respiration rates were 268 (maizemonoculture) and 287 mg CO2 m-2 h-1(crop rotation). Factors controlling soil respirationwere soil temperature, soil moisture, root respirationand carbon input into the soil. Q10-valuesof soil respiration were generally higher in winterthan in summer. This trend is interpreted as anadaptive response of the soil microbial communities.In the model a novel mathematical approach withvariable Q10-values as a result oftemperature and moisture adjustment is proposed. Withthe calibrated model soil respiration and SOC werecalculated for both fields and simulations over aperiod of thirty years were established. Simulationswere based on (1) local climatic data, 1961 until1990, and (2) a regional climate scenario for northernGermany with an average temperature increase of 2.1 K.Over the thirty years period with present climateconditions, the SOC pool under “crop rotation” wasnearly stable due to the higher carbon inputs, whereasabout 16 t C ha-1 were lost under “maizemonoculture”. Under global warming the mean annualsoil respiration for both fields increased and SOCdecreased by ca. 10 t C ha-1 under “croprotation” and by more than 20 t C ha-1 under“maize monoculture”. It was shown that overestimationof carbon losses in long-term prognoses can be avoidedby including a Q10-adjustment in soilrespiration models.

Stomatal acclimation influences water and carbon fluxes of a beech canopy in northern Germany

Summary The response of beech leaves to leaf-to-air vapour pressure deficit (VPD) is not constant throughout the growing season. In situ measurements of leaf gas exchange showed that this intra-annual variation of stomatal sensitivity to VPD cannot be described adequately by a simple annual course of phenology alone. Reduction in stomatal aperture in dry air was negatively correlated with the sum of photosynthetic photon flux density (PPFD) of the previous three weeks, but was positively correlated with the mean VPD of the previous month. These acclimation effects contribute significantly to explaining variances in stomatal sensitivity across the growing season. The observed mid-term reaction to PPFD is in agreement with recent findings regarding the blue light perception of stomatal guard cells. A mathematical formulation representing these mid-term variations in stomatal sensitivity was incorporated into an ecosystem flux model describing the carbon and water exchange between forests and the atmosphere. Model results were in good agreement with independent eddy covariance measurements above the beech canopy. The model, therefore, is useful for investigating the importance of stomatal acclimation for ecosystem carbon and water balances. At an annual scale, acclimation appears to influence the fluxes moderately (less than 10%). The model suggests, however, that stomatal acclimation optimises the seasonal course of carbon gain and water loss of beech forests significantly with respect to the varying availabilities of water and energy. Die Reaktion von Buchenblattern auf das Wasserdampfdefizit zwischen Blatt und Luft ist nicht das ganze Jahr uber konstant. Die in-situ gemessene intraannuelle Variabilitat der stomataren Sensibilitat gegenuber der Luftfeuchte kann daher mit einer einfachen phanologischen Kurve nicht adaquat beschrieben werden. Daten, die uber drei klimatisch sehr unterschiedliche Jahre an Buchenblattern im Hauptforschungsraum des Bornhoved-Projektes aufgenommen wurden, zeigten vielmehr, das das Ausmas, in dem die stomatare Leitfahigkeit als Reaktion auf trockene Luft verringert wird, einerseits negativ korreliert ist mit der mittleren Tagessumme der photosynthetisch aktiven Quantenstromdichte der letzten drei Wochen, andererseits positiv korreliert ist mit dem mittleren Wassersattigungsdefizit der Luft des jeweils letzten Monats. Eine Berucksichtigung dieser als stomatare Akklimatisation interpretierten Befunde erklart die Varianz der stomataren Sensibilitat signifikant besser. Die beobachtete mittelfristige Reaktion der Stomata auf die Witterung stimmt mit kurzlich publizierten biochemischen Beobachtungen uberein, nach denen die Blaulichtrezeptoren in Schlieszellen von Spaltoffnungen ebenfalls auf das Lichtklima reagieren. Eine mathematische Formulierung fur die mittelfristige Anpassung der stomataren Sensibilitat wurde in ein Okosystemflus-Modellsystem, das die Wasser- und CO2-Flusse zwischen Waldern und der Atmosphare berechnet, integriert. Die Ergebnise von Simulationslaufen mit dem erweiterten Modell zeigten gute Ubereinstimmung mit Eddy Kovarianz Messungen, die unabhangig uber dem Buchenwald durchgefuhrt wurden. Weitere Simulationlaufe zeigten, das die Akklimatisation die jahrlichen Bilanzen zwar nur moderat (weniger als 10%) beeinflust, aber tagliche Abweichungen von bis zu 60% auftraten. Es konnte gezeigt werden, das die stomatare Akklimatisation zur Optimierung des Kohlenstoffgewinns der Baume bei wechselnder Verfugbarkeit von Wasser und Energie beitragt.

Seasonal and Diurnal Courses of Water Relations of the Arido-Active Plant Hammada scoparia in the Negev Desert

SummaryThe photosynthesizing branches of Hammada scoparia, one of the typical dwarf shrubs of the Negev desert, undergo a seasonal change from succulent to xeromorphic anatomy. This trend is accompanied by a marked decrease of water content and of total water Ψplant and osmotic Ψπplant potential. Irrigated plants do not show such transitions. The daily courses of Ψplant and Ψπplant showed minima around noon and a tendency for maxima before sunrise. Turgor pressure Ψpplant reached minima around noon and became negative (until ca.-10 bars). Generally, Ψplant decreases with increasing water vapour concentration difference between plant and air (WD) in the first half of the day, and in the second half the reversal of this trend occurs. Mostly smaller increments of Ψplant were correlated with larger increases in WD which lead to the conclusion that stomates closed enough to maintain transpiration at a constant value. Non-irrigated and irrigated plants showed different hysteresis loops of relation between Ψplant and WD. Regulatory reduction of transpiration appears largely independently of Ψplant which is in spring and with irrigated plants on a high level, with non-irrigated plants in summer on a low level. In summer the continous but decreasing drop of Ψplant with increasing WD was interpreted as caused by a change in soil or root resistance. Independent of the seasonal state and of the Ψplant level, H. scoparia regulates its water status within limited ranges of Ψpplant changes: the irrigated plants on a higher level, the non-irrigated on a lower level of Ψpplant. The water contents of the tissues of H. scoparia are linearily related to Ψplant as well as Ψpplant. Steeper slopes with non-irrigated plants in summer than with spring palnts and with irrigated plants during the whole season signify that in the latter a certain increment in turgor pressure corresponds to a large gain in water content while in the non-irrigated summer plants it varies only little for an identical change in Ψpplant. This behaviour of non-irrigated wild plants apparently is due to the change of the elastic properties of the tissues in the assimilating branches.

Environmental Indication: A Field Test of an Ecosystem Approach to Quantify Biological Self-Organization

In this study, we take an ecosystem approach to examine the degree of biological self-organization at the ecosystem level. An integrated set of indicators is derived from a theoretical framework and tested by field data from an ecosystem research project focusing on the Bornhöved Lake district in northern Germany. This field test is based on a comparison of the self-organized phenomena that comprise the carbon, water, and energy budgets of two adjacent edaphically and climatically similar ecosystems, that have vastly different levels of human interference—a crop field and a beech forest. In terms of biomass storage, biologically incorporated nitrogen and phosphorus, species number, total ecosystem respiration per total biomass (qCO2), total ecosystem assimilation per available nutrients, and transpiration per total evapotranspiration, we found clear differences between the systems. Ecosystem surface temperature and Rn/K* were found to be of limited utility in characterizing the two systems. The study is rooted in the concept of ecological integrity, an influential idea at the interface of ecological and environmental debate that has acquired a number of different meanings. Among other interpretations, it can be viewed as a guiding principle for sustainable land use that aims at long-term protection of ecological life-support systems. Effective use of any interpretation of this concept requires a theoretically consistent and applicable set of indicators. Therefore, we also discuss the integration of the indicator set and its potential use in monitoring programs.

Photosynthesis of Desert Plants As Influenced by Internal and External Factors

One of the main aims of current research in physiological ecology is to investigate and causally interpret the photosynthetic productivity of plants and of plant communities in different habitats. Efforts have been made to develop mathematical models to compute the net fixation of CO2 by plants from meteorological parameters and to predict their productivity (e.g., Lommen et al, 1971; Chapter 4, by Hall and Bjorkman). One basic requirement for the realization of such models is a detailed knowledge of the functional relationships between the photosynthetic efficiency of a plant and the external conditions characteristic for its particular habitat. Special attention must be paid to the responses of the different morphological types, considering the variability of their physiological state and their capacity for regulative adaptations. The more sophisticated models recently proposed have made apparent large gaps in our knowledge about the influence of important internal and external factors on the CO2 exchange of plants. Therefore, during our work on productivity and water relations of desert plants, we first focused our interest on a functional analysis of the photosynthetic responses of the plants in their natural habitat, using two approaches. On the one hand, we investigated net photosynthesis and transpiration in relation to the natural conditions in which the different plant types were growing. On the other, we analyzed the importance of single environmental factors by carrying out experiments under artificially changed conditions in the field.

The possible role of aerosols on stomatal conductivity for water vapour

Summary Stomatal conductance, which controls both the exchange of water vapour and CO 2 , is thought to be strictly determined by stomatal aperture. However, water transport processes might be modified by deposition of hygroscopic salts on the leaf surface. For this reason, simultaneous measurements of gas exchange and stomatal aperture were performed on Sambucus nigra leaves before and after treatment with sub-micrometer NaNO 3 aerosol. Aerosol treatment of the leaves led to higher transpiration, which was particularly pronounced at small apertures, leading to an enhancement of minimum leaf conductance between 45 and 90%. CO 2 -uptake of Sambucus leaves was not enhanced by aerosol treatment, and consequently water use efficiency decreased at small stomatal apertures. Artificial leaves consisting of small water reservoirs tapped with filter membranes also showed an increase in evaporation after aerosol treatment, which indicates a mainly physical explanation for the findings with Sambucus leaves. Absorption and desorption of water by salt deposits due to changing thickness of the laminar sublayer and entering turbulent elements -‘bursts’- is suggested to be a possible explanation of the results. The cyclic elementary process, condensation and evaporation of water on the leaf surface, was observed microscopically under changing boundary layer conductance at 50% relative humidity (RH). Hygroscopic particles might therefore act as intermediate short-term storage or as primary evaporation sites after liquid water transport along films of salt solution through stomatal pores. A dynamic model describes the effects of cycles of short-term storage of salts. Possible consequences for plant water relations are discussed. Wasserdampf- und CO 2 -Austausch von Blattern werden durch die stomatare Leitfahigkeit reguliert, welche bislang als eine Funktion alleine der Spaltoffnungsweite gesehen wurde. Hygroskopische Salze auf der Blattoberflache konnten diese eindeutige Beziehung aufgrund ihrer Wechselwirkung mit Wasser(dampf) beeinflussen. Aus diesem Grund wurden gleichzeitig Gasaustausch und Spaltoffnungsweiten an Blattern von Holunder ( Sambucus nigra ) gemessen. Die Ergebnisse dieser Messungen vor und nach Bespruhung der Blatter mit NaNO 3 -Aerosol wurden verglichen. Nach der Aerosolbehandlung war die Transpiration signifikant erhoht. Die relative Zunahme war bei kleinen Offnungsweiten besonders stark und die mimimale Leitfahigkeit erhohte sich um 4590%. Dagegen stieg die CO 2 -Aufnahme der Sambucus -Blatter durch die Aerosolbehandlung nicht an, wodurch sich bei kleinen Spaltoffnungsweiten eine geringere Wassernutzungseffizienz ergab. Auch fur Modellblatter (kleine mit Membranfiltern verschlossene Wassergefase) wurde nach Aerosolbehandlung ein Verdunstungsanstieg gemessen, was auf eine uberwiegend physikalische Ursache der Ergebnisse an Sambucus hindeutet. Die Ursache konnte in der wiederholten Ab- und Desorption von Wasser an den Salzablagerungen liegen und auf Veranderungen der Grenzschichtdicke und das Eindringen von Turbulenzelementen (“bursts”) in die Grenzschicht zuruckgehen. Der elementare Zyklus, Kondensation und Evaporation von Wasser auf der Blattoberflache bei sich andernder Grenzschichtleitfahigkeit, wurde bei 50% relativer Feuchte mikroskopisch beobachtet. Analog konnten hygroskopische Partikel als kurzzeitige Zwischenspeicher fungieren oder, nach dem Transport flussigen Wassers entlang von Filmen aus Salzlosung durch die Spaltoffnungen, als primare Verdunstungsorte wirken. Ein dynamisches Modell beschreibt die Wirkung wiederholter kurzzeitiger Speicherung durch das Salz. Die mogliche Bedeutung der Ergebnisse fur den pflanzlichen Wasserhaushalt wird diskutiert.

Carbon dioxide fluxes of soils and mosses in wet tundra of Taimyr Peninsula, Siberia: controlling factors and contribution to net system fluxes

Knowledge of the environmental controls of carbon dioxide fluxes is essential for understanding the dynamics of carbon exchange between ecosystems and atmosphere. In this study we investigated soil respiration and moss photosynthesis as well as their contribution to the net carbon dioxide flux of two different wet tundra systems. During two summers, in situ carbon dioxide fluxes were measured in a tussock tundra and in a low-centre polygonal tundra on Taimyr Peninsula, central Siberia. Measurements were carried out by means of a multichannel gas exchange system. Results show pronounced differences in soil respiration rates as related to microscale topography, mainly due to differences of soil water table and soil temperatures. Modelling of soil respiration for individual microsites revealed differences of process performance with respect to both factors. The wet microsites showed the highest potential regarding an increase of soil respiration rates in warmer and drier climate change scenarios. Another important process compensating the COz release from the soil was the photosynthesis of the moss layer, assimilating as much as 51% to 98% of the daily amount of carbon dioxide released from wet tundra soils. This result demonstrates the importance of mosses in the context of tundra ecosystem processes. The magnitude of net system fluxes of the whole system at the depression of the polygonal tundra was strongly influenced by changes in soil water table. Consequently, any changes of the hydrology, as anticipated in the context of global change, would effectively alter the carbon balance of wet tundra systems.