P Van Hecke

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.

Diurnal changes in the response of canopy photosynthetic rate to elevated CO2 in a coupled temperature-light environment

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.

A reference value for the interior-to-edge ratio of isolated habitats

Isolated habitats, the consequence of the fragmentation process, are the object of external disturbance. This divides the patch area into two zones: interior and edge. The interior-to-edge ratio quantifies the potential disturbance impact. A method is presented to calculate a reference value for the interior-to-edge ratio, based upon the minimum edge for a given interior. The method is based on pixel geometry features and mathematical morphology. A corrected interior-to-edge ratio is defined using the reference value. The method is illustrated for woodlot fragments in the Belgian Kempen region.

Land-cover change: Quantification metrics for perforation using 2-d gap features

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.

Evolution of cacao-pests over a six year period

During a six year period (1991–1996) 284 cacao samples have been inspected for pests. Twenty six different taxa have been observed, but five of them represent 92% of the total population:Ahasverus advena, Carpophilus obsoletes, Cryptolestes ferrugineus, Ephestia cautella andTribolium castaneum. Over the total period, a trend cannot be shown neither in the number of pests nor in the number of taxa. The change of the composition of the population is mainly due to the substitution ofA. advena byC. ferrigineus and later byC. obsoletes. Both Hill’s indices are increasing during the first five years.