There are plant ecologists or botanists such as Steve Simon of Asheville, NC, USA who do this mapping of PNV.  There are others too who do the analysis and produce these maps.  Since much of your areas natural vegetation was probably forests wifh some natural history of forest fires, I would look there first, plant ecologists and a composite of naturalist history and records.  Having access to a soil scientist/geologist as well as forester/botanist seems to be important.  Possibly a hydrologist and aquatic biologist  too, as land use changes have also altered hydrology and riparian/aquatic habitat.  When you talk of ecosystem change, there are probably other technical specialists to consult, archeologists, wildlife biologists, etc.

Thank you Mr. Hansen for your reply, now I am wondering if all the PNV values are for forests, even if the location (my case study) now is considered as a grassland? 

There are some natural grasslands, such as wet meadows and areas that burn too frequently to support forests.  Grasslands often have soils high in organic accumulation, unless overgrazed and eroded.  Alot may depend on how far back you go in time.  Your area has a wealth of history, but also land uses I would suppose. 

Sehr geehrter Her Zurba,

als Technischer Chemiker bin ich vielleicht nicht die richtige Adresse?

Viele Grüße

Martin Bertau

https://www.bfn.de/0302_de.html

PNV maps are available countrywide at 1:0.5m and at finer scale of many areas. In German only, to the best of my knowledge.

Sehr geehrter Her Prof. Bertau, 

Vielen Dank für die Frage, Eigentlich an diesem Punkt sind Sie richtig, ich bin ein bisschen weit von den technischen Teil.

VIele Grüße,

Kamal

Thank you Mr. Van Gils for the link, next week I will get the maps, the same book in the link you provided, and I hope to find the needed things.

Best wishes,

Dear Kamal,

since you mentioned not only Germany but Europe as well, you may be interested in the recently published multilayer PNV of Hungary, which is now available for researchers.

Related website: http://www.novenyzetiterkep.hu/english/node/potveg_en

Related publication: Article Implementation and application of multiple potential natural...

HTH,

Ákos