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RESUMEN

Perforation or gap formation in a vegetation is a major process in landscape transformation. The occurrence of gaps profoundly alters the microclimatical conditions in a vegetation. A method is proposed to quantify perforation by using the three main 2-D characteristics of the gaps: area, number and boundary length. New measures are developed by normalizing the observed values to the reference status of minimum and maximum perforation. As minimum perforation status, the presence of one single gap with area equal to the map resolution is assumed. The new measures are combined using a 3-D Euclidean distance to visualize the process and to detect changes. The method is exemplified using a field case of gaps in a tropical terra firme rainforest at Tiputini, Ecuador.

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RESUMEN

A method is proposed to quantify disturbance impact on isolated habitats. For every landscape patch, the breakpoint distance, defined as the penetration distance for which equality of interior and edge habitat is observed, can be calculated. Disturbance with equal impact at all patch sides is assumed. Effects of patch compactness, size, convolution, and perforation are discussed. The potential use of the measure for nature reserve design is discussed. The breakpoint distance follows the reserve design guidelines for individual patches, based on island biogeography and is consistent with the form and function principle. A large breakpoint distance is preferred for natural habitats. Small size, small compactness, intense convolution, and the occurrence of many gaps depress the breakpoint distance.

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RESUMEN

The relative increase with elevated CO2 of canopy CO2 uptake rate (A), derived from continuous measurements during the day, was examined in full-cover vegetative Lolium perenne canopies after 17 days of regrowth. The stands were grown at ambient (358±50 µmol mol(-1)) and increased (626±50 µmol mol(-1)) CO2 concentration in sunlit growth chambers. Over the entire range of temperature and light conditions (which were strongly coupled and increased simultaneously), A was on average twice as large in high compared to ambient CO2. This response (called M=A in high CO2/A in ambient CO2) could not be explained by changes in canopy conductance for CO2 diffusion (GC). In spite of interaction and strong coupling between temperature and light intensity, there was evidence that temperature rather than light determined M. Further, high CO2 treatment was found to alleviate the afternoon depression in A observed in ambient CO2. A temperature optimum shift or/and a larger carbohydrate sink capacity through altered root/shoot ratio are proposed in explanation.

RESUMEN

The relationship between leaf photosynthetic capacity (p n, max), net canopy CO2- and H2O-exchange rate (NCER and E t, 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 E t 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 p n, 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.

RESUMEN

Effects of rising atmospheric CO2 concentrations on gas exchange, growth and productivity were investigated on an important grassland species, Trifolium repens L. cv. Blanca. Pure stands of this species were cultivated over an entire growing season in small acrylic greenhouses with an artificial atmosphere of ±367 or ±620 ppm CO2, respectively. Effects on growth and development were examined in a functional growth analysis, while consequences for gas exchange were determined by photosynthesis and transpiration measurements on canopy level. The stands were regularly clipped for production assessment. Canopies grown at high CO2 levels showed an average increase in productivity of almost 75%. Growth analysis indicated development of a larger foliage area as the major cause, particularly in the first days of regrowth after cutting. The growth advantage that began in this stage was maintained or bettered during the following weeks. The difference between gas exchange measurements expressed per unit leaf area and per unit ground area suggested that changes in net photosynthesis and respiration did not contribute to the increase in total yield. Transpiration declined under high CO2 if expressed on a leaf area basis but total canopy transpiration was at least as large as in ambient CO2 due to the larger leaf area. Water-use efficiency calculations on the summer data indicated a 35% improvement with a doubling of CO2 concentration.

RESUMEN

No significant differences were found between four mathematical equations describing the response of CO2 exchange rate to photosynthetic photon flux density in seven poplar clones under laboratory conditions. Choice of an optimal equation for poplar may be based on the contemplated aims. High significant differences (at p<0.001) were found among the clones.

RESUMEN

The internal resistance r i, to carbon dioxide diffusion was separated into the mesophyll and the carboxylation components, and it was shown that the mesophyll and the carboxylation resistances r m and r x had similar order of magnitude, suggesting the importance of both in the photosynthesis of Hevea brasiliensis.

RESUMEN

Studies were made of resistance to gaseous exchange between large sunflower leaves and the bulk air in a crop canopy. Two components of the diffusive pathway for mass and sensible heat were evaluated; A) the resistance from the interior of the leaf to the leaf surface, and B) the resistance from the surface of the leaf through the leaf boundary air layer to the bulk air.IT WAS FOUND THAT: A) leaf resistance not only displays diurnal trends but shorter fluctuations, and B) boundary air layer resistance was significantly smaller than predicted from classical boundary layer formulae.

RESUMEN

At several heights and times of day within a crop of Zea mays, internal leaf diffusion resistance (r(i)) and external boundary layer diffusion resistance (r(a)) were evaluated by measuring the temperature of a transpiring and a non-transpiring leaf (simulated by covering both sides of a normal leaf with strips of poly-ethylene tape), and by measuring the immediate air temperature, humidity and windspeed.Both r(a) and r(i) increased with depth into the crop. However, r(a) generally was less than 10% of r(i).Profiles of latent-heat flux density and source intensity of transpiration showed that transpiration corresponded roughly to foliage distribution (with an upward shift) and were not similar to the profile of radiation absorption.The data were compared with heat budget data. The 2 approaches yielded quite similar height distributions of transpiration per unit leaf area and total transpiration resistance.The total crop resistance to transpiration was computed as 0.027 min cm(-1). This compares to Monteith's values of 0.017 to 0.040 min cm(-1) for beans (Phaseolus vulgaris L.), and Linacre's values of 0.015 to 0.020 min cm(-1) for turf.