I. Impens

Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration

The relationship between leaf photosynthetic capacity (pn, max), net canopy CO2- and H2O-exchange rate (NCER and Et, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 μl· l-1) and elevated (631±43 μl·l-1) CO2 concentrations. An open system for continuous and simultaneous regulation of atmospheric CO2 concentration and NCER and Et measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCERmax of full-grown canopies was 49% higher at elevated CO2 level, stimulation of pn, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO2 (approx. 10% in one growing period examined). A larger amount of CO2-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO2 effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO2 level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCERmax difference. While total water use was the same under high and low CO2 levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO2 concentration.

Climate Warming Postpones Senescence in High Arctic Tundra

Abstract Lengthening of the growing season at high latitudes, observed by satellites with the Normalized Difference Vegetation Index (NDVI), has been ascribed to climate warming. To test this assumption, and to verify whether changes in vegetation greenness are quantitative or qualitative, we experimentally warmed patches of High Arctic tundra with infrared heating in Northeast Greenland. By analyzing digital images of the vegetation, changes in cover were distinguished from changes in senescence. During the season, experimental warming significantly increased green cover, for example, at the time of peak cover, the total green cover was enhanced from 59.1 to 67.3%. The dominant wavelength (hue) reflected by our tundra plots shifted from yellow-green to yellow. Experimental warming with 2.5°C delayed this hue-shift by 15 d. The results demonstrate that higher summer temperatures do not only promote plant growth at these latitudes but also retard and/or postpone the senescence process, contrary to indications from previous research that late-season phenology in the High Arctic is governed by photoperiod.

Simulation of climate change with infrared heaters reduces the productivity of Lolium perenne L. in summer

Abstract Field-grown perennial ryegrass was subjected to climate warming and elevated CO2 concentration during summer in free air conditions (no enclosure of the vegetation). Increased foliage temperature (2.5°C above fluctuating ambient) was induced by heating the stand with infrared radiation sources, modulated by an electronic control device (FATI, Free Air Temperature Increase). Enhanced CO2 was produced by a FACE system (Free Air CO2 Enrichment). Exposure to simulated climate warming drastically reduced above-ground harvestable dry matter (52% loss). The nitrogen allocated to the leaf fraction was thus concentrated into less dry matter, which enhanced the nitrogen concentration on a mass basis (+17%) but also per unit leaf area (+47%). As a consequence, CO2 assimilation rates were not affected in these slower growing plants in the +2.5°C treatment, and the photochemical efficiency of non-cyclic electron transport of photosystem II was also unaffected. Although the plants were grown in the field without root restrictions, long-term exposure to elevated CO2 concentration induced noticeable acclimation of the photosynthetic apparatus (40% loss of fixation potential), which largely outweighed the direct stimulation in this summer period. Part of the reduced rates could be attributed to lower N concentration on a leaf area basis. The results are compared with responses of this species in sunlit conditioned greenhouses, which indicates that experiments in enclosures may underestimate effects in the field. This also emphasizes the need to validate other plant responses to climate warming and CO2 enrichment in free air conditions.

The ratio UV-B/photosynthetically active radiation (PAR) determines the sensitivity of rye to increased UV-B radiation

Abstract To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of rye (Secale cereale L.) to increased UV-B levels, plants were grown under eight different light treatments. In the control series (at ambient levels of UV-B), UV-B and visible light were decreased in parallel, resulting in four different total irradiation treatments with the same UV-B/photosynthetically active radiation (PAR) ratios. A second series with a 30% increase in UV-B irradiation at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in significant differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. A 30% increase in UV-B resulted in equal reductions in total dry weight (from 20.0 to 28.6%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the leaves was stimulated by PAR and even more by UV-B, except at the highest UV-B irradiation, where a small decrease was noted. These results indicate that rye plants are able to adapt to changes in the natural light environment as long as the ratio UV-B/PAR is constant.

Effects of plant canopy structure on light interception and photosynthesis

Abstract A set of different plant canopies with vertically different and horizontally clumped leaf areal density was generated on a mainframe computer. Three-dimensional transfer of direct light in these canopies was numerically calculated and the irradiance of light on inclined leaf surfaces was predicted. Using this information the daily gross photosynthesis of the canopy was estimated. The effects of spatial distribution of leaf areal density and of leaf orientation on light interception and photosynthesis were analyzed. The distribution of leaf areal density affects the spatial distribution of sunflecked leaf area, while leaf orientation affects both the spatial distribution of sunflecked leaf area and the distribution of light irradiance on leaf surfaces. The results show that the influence of spatial distribution of leaf areal density is only about 1–8% of daily total canopy photosynthesis, and that leaf orientation has much greater effects on canopy photosynthesis. At summer solstice and lower latitudes, erectophile leaves could produce up to 25% higher daily photosynthesis in the canopy than planophile leaves.

Measurement of gap fraction of fractal generated canopies using digitalized image analysis

Abstract A fractal based computer graphical model was used to generate poplar tree stands. From the model, the downward cumulative leaf area index (L) and leaf normal inclination distribution were determined. By graphically projecting downward geometrical layers of the simulated poplar stands on a horizontal plane, a set of pictures were obtained. The gap fractions of the projection pictures were analyzed using a digitalized image analysis system (DIAS). These measured gap fractions and the corresponding downward cumulative leaf area indices were used to test the two most often used canopy light penetration models, namely the exponential model and the negative binomial model. Results showed that at high values of L neither the exponential nor the negative binomial model could adequately describe the relationship between the gap fraction and L. These two theoretical models generally underestimated gap fraction, especially at high L. Viewed from zenith, at L = 6 ∼ 8 , the theoretical models predicted a gap fraction about 1–12%, whereas the gap fraction measured by DIAS was about 15–22%.

Influence of High Temperature on End-of-Season Tundra CO2 Exchange

The high-arctic terrestrial environment is generally recognized as one of the worlds most sensitive areas with regard to global warming. In this study, we examined the influence of an isolated warm period on net ecosystem carbon dioxide (CO2) exchange at high latitude during autumn. Using the Free Air Temperature Increase (FATI) technique, we manipulated air, soil, and vegetation temperatures in late August in a tundra site at Zackenberg in the National Park of North and East Greenland (74°N 21°W). The consequences for gross canopy photosynthesis, canopy respiration, and belowground respiration of increasing these temperatures by approximately 2.5°C were determined with closed dynamic CO2 exchange systems. Under current temperatures, the ecosystem acted as a net CO2 source, releasing 19 g CO2-C m−2 over the 14-day study period. Warm soils and senescing vegetation in autumn were unequivocally responsible for this efflux. Heating enhanced CO2 efflux to 29 g CO2-C m−2. This effect was attributed to a 39% increase in belowground respiration, which was the main component of the carbon (C) budget. Gross photosynthesis, on the other hand, was not affected significantly by the simulated warming. Although the aftereffects of an isolated warm period on the C balance in early winter could be significant, simulations with a simple C budget model suggest that soil carbon pools are not affected to a great extent by such a climatic disturbance. The influence on atmospheric carbon, however, appears to be significant.

Competition in a Global Change Environment: The Importance of Different Plant Traits for Competitive Success

Plant traits of both structural and physiological nature have been reported to play an important role in the outcome of a competitive interaction. In this experiment a survey of the importance of characteristics such as height, leaf area and vertical distribution, leaf inclination, light transmission through a leaf and through the canopy together with leaf photosynthesis rates, was made for two grasses: Lolium perenne L. and Festuca arundinacea Schreb. Both species were grown in a competitive set-up with minimal belowground interactions under four different global change conditions of CO2 concentration and air temperature (ambient, elevated CO2 concentration 700 ,umol mol-1, increased air temperature + 4?C, combined) in naturally sunlit air-conditioned half-open plastic greenhouses. Plants were grown for a whole growing season with regular (every 4 weeks) clippings at 3.5 cm height. Results demonstrate that Lolium has a much greater competitive ability when compared to Festuca (aggressivity = 0.465). This may be a consequence of the slightly higher leaf photosynthesis rates (+ 15%) in combination with a

Twist number statistics as an additional measure of habitat perimeter irregularity.

Individual habitats are altered by the surrounding landscape matrix. Quantitative analysis of habitat boundary is therefore necessary. A shape index is proposed based on the perimeter twist number; twists divide the patch perimeter in seperate segments. Using Principal Components Analysis, the shape index is compared with fractal dimension (per-patch calculation based on area and perimeter) and with four shape (compactness) indices based upon pixel geometry, next to area and perimeter statistics. The analysis is executed with simulated data based on percolation maps. The proposed index can be used as a complementary shape descriptor because of (i) its independence of fractal dimension, (ii) its restricted correlation with only one compactness index and (iii) its ability to identify habitats characterized by limes divergens and a high interior-edge ratio.

Climate-warming simulation in tundra: enhanced precision and repeatability with an improved infrared-heating device.

Climate-warming experiments in tundra require an accurate and repeatable technique for temperature increase, as temperatures are low and vegetation often heterogeneous. However, remote locations have constrained most studies to using uncontrolled, passive enclosures. Here we present an improved prototype of our Free Air Temperature Increase (FATI) system, an apparatus to elevate vegetation temperature in the open field by electronically modulated infrared irradiation. Three modifications are introduced to enhance precision and repeatability: (1) noncontact sensing of canopy temperature (Tcanopy) with semiconductors, (2) an optimizing device to attune individual FATI-units, and (3) high-speed data logging. Performance tests in the High Arctic tundra of northeast Greenland during autumn 1998 demonstrated more reproducible warming compared with the original system: average increments of canopy temperature (ATcanopy) obtained in different patches of vegetation varied no more than 0.16?C. Most fluctuation in ATcanopy was also eliminated, with instantaneous values within ?0.5?C of the target increment of +2.5?C for at least 87% of the time. Average warming in the upper