Kakšna je trajnostna populacija dreves na drevesno-pašnem podsistemu?

Wood-pastures are important for their open-ground biodiversity and for the old trees they contain. However, younger trees to replace the current generation of old trees are often scarce, a potential threat to the future of the habitat mosaic and of species dependent on the trees. A simple model was used to illustrate how many younger trees might be expected under different assumptions of desired final density of old trees and rates of loss as trees age for an oak-dominated wood-pasture. From these the overall canopy cover of the landscape was estimated under an active pollarding regime and where the trees grow to full crown size. Canopy cover was often five times as great under full crown as under a pollarding regime; much of the canopy cover was in the younger (often missing) cohorts. The openness of current wood-pastures is in part a consequence of the absence of a sustainable tree population structure. Some protected sites may be too small to allow space for the missing generation of trees and at the same time retain current levels of openness. Analogies between current wood-pasture structures and ‘natural wood-pastures’ of the pre-Neolithic era must take account of the missing generations of trees.Pašniki z drevesi so pomembni za biodiverziteto zaradi njihove odprtosti in zaradi prisotnih starih dreves. Vendar so mlada drevesa, ki naj bi nadomestila trenutno generacijo starih dreves redka, kar predstavlja potencialno grožnjo mozaičnemu habitatu in vrstam, ki so odvisne od prisotnosti dreves. Uporabili smo preprost model, ki predstavlja koliko mladih dreves lahko pričakujemo v različnih predvidevanih gostotah starih dreves in različnih stopnjah odmiranja starih dreves zaradi starosti na pašniku, kjer dominirajo hrasti. Iz tega smo ocenili skupno pokrovnost krošenj v krajini, če se drevesa aktivno obrezuje ali le-ta zrastejo do polne velikosti krošnje. Pokrovnost krošenj je pogosto do petkrat večja, če drevesa zrastejo do polne velikosti, kot če jih obrezujemo. Večina pokrovnosti krošenj je zaradi mlajših dreves (ki pa pogosto manjkajo). Trenutna odprtost pašno-drevesnih podsistemov je delno posledica odsotnosti trajnostne strukture drevesne populacije. Določene zaščitene površine so verjetno premajhne, da bi se lahko manjkajoče generacije dreves obnovile in da bi se obenem ohranila trenutna stopnja odprtosti pašnika. Pri analogiji med sedanjimi pašno-drevesni podsistemi in naravnimi pašniki z drevesi iz obdobja pred Neolitikom moramo upoštevati manjkajoče generacije dreves