Northern European review would judge old-growth by 20 old trees a hectare and 5% deadwood

Deadwood written as a share of growing stock behaves differently from deadwood written in cubic metres per hectare. Double a stand's growing stock and the volume of deadwood it must hold to pass the proposed test doubles with it ^[17]. A fixed volume per hectare asks the same amount of a high-volume stand and a low-volume one. "Deadwood volume varies considerably between forest types and geographical zones, as well as according to the forest's disturbance history, making it difficult to establish precise volume thresholds," said Mikko Mönkkönen, professor of applied ecology at the University of Jyväskylä ^[5].

Boreal stands keep a mix of ages as they develop. Structure varies considerably in naturally developed boreal forests, and younger trees can occupy much of the canopy in a forest that also holds many old trees and shows old-growth processes ^[3]. An average computed over the dominant stand scores that forest as young. The unit of assessment matters as well: the review proposes a minimum stand area of 0.5 hectares, a threshold commonly used internationally ^[11].

"The key to identifying old-growth forests is to view the forest as a whole and consider the overall picture formed from a range of characteristics," Mönkkönen said ^[5].

The Commission's rule already tolerates partial failure. The floor is five of the seven indicators ^[16], and a stand can miss two of the four complementary ones and still qualify ^[18].

The definition is legally binding because RED III prohibits logging in primary and old-growth forests ^[7]. The EU Restoration Regulation prohibits the deterioration of habitat types in good condition as defined in the Habitats Directive, without explicit mention of old-growth and primary forests ^[8]. By a rough estimate, only around 3% of Europe's forests meet primary or old-growth criteria, and the extent continues to decline rapidly in Northern Europe ^[1].

The review recommends indicators; it does not count how many stands would be classified differently under Finland's mandatory criteria and the proposed set ^[19]. The direction of the mismatch is an argument from how boreal forests develop. On the asymmetry of the risk the researchers do commit: overly strict or unscientific national criteria could leave a large proportion of the remaining old-growth forests unprotected and vulnerable to logging, and underestimating the extent of what remains means long-term biodiversity losses and higher future restoration costs ^[14]. They also grant that some uncertainty is inevitable, because natural variation between forests is considerable ^[15].

What makes these stands matter for biodiversity is their structure: dead and old trees provide habitat for fungi, lichens, mosses, deadwood-dependent insects and cavity nesters ^[2].