Fred Kockelbergh

Microenvironmental and vegetational heterogeneity induced by phytogenic nebkhas in an arid coastal ecosystem

The nebkhas of woody plants represent distinct habitats in arid and semiarid ecosystems. Nebkhas are mounds composed of wind-borne sediment within or around shrub canopies. We studied the effects of widely spaced nebkhas of Retama raetam shrub on their microenvironment and associated herbaceous vegetation in the Mediterranean coast of Sinai Peninsula. Our measurements included nebkha size (height and width) and shrub size (canopy height and diameter). We identified four distinct microsites at each nebkha: crest, mid-slope, edge, and internebkha space. We measured soil temperature and moisture, photosynthetically active radiation (PAR), and soil properties. The plant species grown at each microsite were identified and their densities were measured. Average soil temperature and PAR were highest at internebkha space and lowest at nebkha crest. The maximum diurnal temperature and PAR of internebkhas exceeded that of nebkhas. Soil moisture and nutrient concentrations showed a gradient of spatial heterogeneity and were highest at the nebkha edge. Regression analysis indicated that total herbaceous plant density was significantly related to nebkha size, and to shrub canopy diameter and area. Detrended correspondence analysis indicated that patterns of species composition were correlated with the spatial variability in soil moisture and nutrient content along the gradient of increasing distance from the nebkha crest. It is assumed that shrub canopy and its nebkha interact in governing ecosystem functioning in this environment.

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.

Increased Turnover but Little Change in the Carbon Balance of High-Arctic Tundra Exposed to Whole Growing Season Warming

Abstract Tundra ecosystems constitute large stocks of carbon and might therefore, if climate warming releases CO2, induce positive feedback and amplify temperature increase. We studied the effect of a 2.5°C temperature increment, induced by controlled infrared irradiation, on various components of the carbon balance of a High Arctic tundra ecosystem at Zackenberg in Northeast Greenland (74°N, 21°W) over the 1999 growing season. Gross photosynthesis (Pgross), belowground respiration (Rsoil), and canopy respiration (Rcanopy) were regularly determined with closed dynamic CO2 exchange systems, and the whole-growing season C-balance was reconstructed by relating these components to potentially controlling factors (green cover, soil moisture, radiation, soil and canopy temperature, and thawing depth). Thawing depth and green cover increased in heated plots, while soil moisture was not significantly affected. Pgross increased 24.2%, owing to both a green cover and a physiological influence of warming. Belowground respiration was enhanced 33.3%, mainly through direct warming impact and in spite of lower Q10 in the heated plots; the factors controlling Rsoil were day of the year and soil moisture. Rcanopy did not differ significantly between treatments, although green cover was higher in the heated plots. This tundra ecosystem acted as a relatively small net sink both under current (0.86 mol CO2 m−2) and heated (1.24 mol CO2 m−2) conditions. Nevertheless, turnover increased, which was best explained by a combination of direct and indirect temperature effects, and delayed senescence.

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.

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

Both seed germination and seedling mortality increase with experimental warming and fertilization in a subarctic tundra

Climate change is expected to cause (sub)arctic plant species to move polewards to track their climatic niche. However, rapid migration requires recruitment from seed, which is rare in arctic regions where most plants reproduce vegetatively. Here, we examined whether recruitment from seed would improve under warmer and more fertile future conditions. We found that seedling establishment was little affected by warming and fertilization, suggesting that (sub)arctic species may experience difficulties in tracking their climatic niche. Predictions of future species distributions in arctic regions solely based on abiotic factors may therefore overestimate species’ ranges.