In natuurgebied de Groote Heide is acht jaar geleden de knikkende schroeforchis ontdekt. De soort verspreidt zich erg snel, zowel door miljoenen stofzaden als door uitlopers van de wortels. In het huidige tempo zijn er zorgen over de negatieve effecten op inheemse soorten en verdere verspreiding naar andere natuurgebieden. Hebben we te maken met een nieuwe invasieve exoot?

Opkomst van de knikkende schroeforchis in de Nederlandse natuur.

The invasive Australian swamp stonecrop,Crassula helmsii, is a perennial amphibious herboriginating from Australia and New Zealand. In freshwater wetlands of North-western Europe, this alien plant species is invasive due to its efficient colonization of empty niches. The establishment of dense C. helmsii growth is threatening native biodiversity and functioning of freshwater ecosystems, especially oligotrophic wetlands with high disturbance and nutrient enrichments. As the effects of these potential drivers of ecosystem degradation are generally difficult to determine in the field, we tested the competitive strength of C. helmsii in a greenhouse experiment with two native competitor species of the same habitat type, Pilularia globulifera and Littorella uniflora. In addition, the effect of nutrient enrichment by water bird feces on competition was studied by adding waterfowl droppings.

Reducing nutrient availability and enhancing biotic resistance limits settlement and growth of the invasive Australian swamp stonecrop (Crassula helmsii)

Limiting the spread and impacts of invasive alien species (IAS) on biodiversity and ecosystems has become a goal of global, regional and national biodiversity policies. Evidence based management of IAS requires support by risk assessments, which are often based on expert judgment. We developed a tool to prioritize potentially new IAS based on their ecological risks, socio-economic impact and feasibility of management using multidisciplinary expert panels.

Lessons learned from rapid environmental risk assessments for prioritization of alien species using expert panels

Worldwide, coastal freshwater wetlands arefacingsalinizationat anincreasingrate duetolarge-scaleland use change, freshwater extraction, climate-drivensea level rise, droughts and land subsidence. Although itis known that increased surface water salinity doesinfluence wetland functioning, effects on nutrientdynamics reported in literature are contradictory andevidence from controlled, long-term field experiments isscarce. We therefore tested the effects of 4 levels ofincreased surface water salinity, from oligohaline tomesohaline conditions (0.9, 2.25, 4.5, 9 PSU), onbiogeochemical and physicochemical processes in thesediment of a formerly brackish freshwater wetland.

Salinization lowers nutrient availability in formerlybrackish freshwater wetlands; unexpected resultsfrom a long-term field experiment

Mires and peatlands in general are heavilyinfluenced by anthropogenic stressors like acidification,eutrophication, desiccation and fragmentation.Groundwater-fed mires are, in contrast to rainwater-fedmires, often well protected against desiccation due toconstant groundwater discharge. Groundwater-fedmires can however be influenced by groundwater pol-lution such as groundwater nitrate enrichment, a threatwhich has received minor attention in literature. Thepresent case study demonstrates how groundwater ni-trate enrichment can affect the biogeochemical function-ing and vegetation composition of groundwater-fedmires through direct nitrogen enrichment and indirectnitrate-induced sulphate mobilisation from geologicaldeposits.

Effects of Groundwater Nitrate and Sulphate Enrichmenton Groundwater-Fed Mires: a Case Study

Zakkende veengronden worden een steeds groter probleem voor de bestaande huizen in veengebieden, maar ook de waterhuishouding voor de landbouw wordt problematisch. Bovendien komt veel CO2 vrij door ontwatering.
Om de zakking van de bodem te vertragen of te stoppen worden technische oplossingen bedacht die weliswaar duur zijn, maar waardoor het huidige landgebruik niet zou hoeven te wijzigen. Deze technische benadering wordt omarmt door vooral agrariërs en belangenorganisaties. In dit artikel wordt onderbouwd waarom de gepropageerde technische maatregelen -onderwaterdrainage en/of drukdrainage- geen oplossing zijn voor zowel de veenzakking als het beperken van de CO2 uitstoot van de veengebieden. Een discussie over een landgebruik waarbij de waterpeilen sterk worden verhoogd moet worden gevoerd!

Onderwaterdrainage in veenweidegebieden

Sod cutting is used as measure in heathlands affected by increased nitrogen deposition, but species recovery is often limited. This paper provides experimental evidence for barriers to successful restoration introduced by sod cutting: soil acidity and P limitation. Having important implications for managing ecosystems under high nitrogen load, this advocates a sound N emission reduction instead of N removal policy.

Barriers to restoration: Soil acidity and phosphorus limitation constrain recovery of heathland plant communities after sod cutting.