B. Knegt

ABSTRACT The wind-pollinated, dioecious shrub Juniperus communis L. is declining in Dutch heathlands, mainly because recruitment is scarce. Aside from ecological factors, inbreeding associated with reduced population size and isolation in the currently fragmented landscape might explain this decline. However, the breeding system of this species largely prevents inbreeding. We assessed variation in 18 allozyme loci in 12 Dutch juniper populations to investigate population structure and to test the hypothesis that small populations have less genetic variation and show more inbreeding than large populations. Variation was high for a species with a fragmented distribution, but similar to values observed in other juniper species. The proportion of polymorphic loci (P) ranged between 0.72 and 0.83, expected heterozygosity (He) ranged between 0.16 and 0.27 and the mean effective number of alleles per locus (A) ranged between 2.5 and 3.3. Population size was not correlated with genetic variation or with sex ratio. Deviations from Hardy–Weinberg expectations, mainly heterozygote deficiencies, were mostly observed in larger populations, which appear to have been broken up into smaller subunits. Differentiation among populations was small (FST = 0.026) and there was a significant departure from random mating (FIS = 0.174). Geographic and genetic structures were not related. Our data suggest that gene flow in this wind-pollinated shrub was high during the establishment phase of the current populations. Gene exchange by pollen and seeds may be less extensive in the present-day landscape. However, juniper decline is not likely to be caused by genetic factors. Alternative hypotheses include changed heathland disturbance regimes, pollen limitation, seed predation and age-related fecundity reductions. Implications for the restoration of juniper populations are discussed.

ABSTRACT Many habitats and species have their existence threatened, especially in densely populated areas such as Western Europe. To stop the decline of biodiversity, the Natura 2000 network is being set-up. The ultimate objective is to get all habitat types (of Annex I of the Habitats Directive) and species (of Annexes II, III and IV of the Habitats Directive and Annex I of the Birds Directive) in a favourable conservation status. In the Netherlands a national ecological network has been set up for this purpose which includes the designated Natura 2000 sites. The current amount of atmospheric nitrogen deposition, acidification and desiccation were compared with limit values per habitat type for nitrogen deposition load, soil pH and spring groundwater table respectively and subsequently presented together in one map. Fragmentation was tested for 80 species.For two-third of the examined natural surface the critical load for nitrogen deposition is exceeded, desiccation is present in over 90% of the area of groundwater dependent nature. Problems with acidification are less pronounced. Fragmentation is present causing regional problems for up to six species. When the four pressures are combined, about two third of the areas suffer from at least one pressure. Many areas suffer from a combination of nitrogen deposition and desiccation.We conclude that environmental and spatial conditions are insufficient to meet the biodiversity target set by the European Union for the Natura 2000 network, habitat types and species.

... De reeks 'Werkdocumenten' bevat tussenresultaten van het onderzoek van de uitvoerende instellingen voor de unit Wettelijke Onderzoekstaken Natuur & Milieu (WOT Natuur & Milieu)De reeks is een intern communicatiemedium en wordt niet buiten de context van de WOT ...

... LEI – Wageningen UR Postbus 29703, 2502 LS Den Haag Tel: (070) 335 83 30; fax: (070) 361 56 24; e-mail: informatie.lei@wur.nl ... Rene Verburg Marlies Sanders Joep Dirkx Bart de Knegt Tom Kuhlman Page 8. Page 9. Inhoud Woord vooraf 5 Samenvatting 9 1 Inleiding 13 ...

Leefomgeving (PBL, 2011a) een inspiratiebron zijn voor het kabinet en de Tweede Kamer, voor het op de langere termijn gerichte natuur-en landschapsbeleid. Aan hoeveel, waar en wat voor type natuur heeft de samenleving in 2040 behoefte? Hoe zou Nederland eruitzien met die natuur? De Natuurverkenning 2011 draait derhalve om het maken van ruimtelijke keuzes. Als men met een methodische blik het terrein van de evaluatie van ruimtelijk beleid beschouwt dan is vanuit de economie de techniek van de maatschappelijke ...

... e O n d e rz o e k s ta k e n N a tu u r & M ilie u B. de Knegt, J. Clement, PW Goedhart, H. Sierdsema, CAM van Swaay & P. Wiersma Natuurkwaliteit van het agrarisch gebied Page 2. Page 3. ... agrarisch gebied B. de Knegt J. Clement PW Goedhart H. Sierdsema CAM van Swaay ...