Biodiversity, carbon storage and dynamics of old northern ... - BPAN.fi

Although natural disturbance factors and their resulting forest dynamics
will vary across the forest landscape with differences in terrain, climate
and hydrology, with a low frequency of severe, stand-replacing disturbances,
most forests under natural dynamics will be formed by a combination
of less severe disturbance factors. This results in structurally
complex forests with small-scale spatial variation and a landscape with
continuous presence and high proportion of old, large trees, reaching
their biological age of 200–400 years (Kuuluvainen 2009). A few modelling
studies considering the effects of the various natural disturbance regimes
on the forest age structure (Angelstam & Andersson 1997, Pennanen
2002) as well as historical studies of selected old forest sites in
northern Sweden (Östlund et al. 1997) indicate that biologically old forests
(far older than the current harvestable age) may have constituted
40–50% of the forest landscape, varying from 20% under predominance
of extremely severe fires to 80% under low-severity fire regimes. Such
natural forest dynamics also result in a large, more or less continuous
supply of dead wood in the form of logs and snags, covering a range of
sizes, wood types, decay stages and mortality agents. Estimates of the
total amount of dead wood in forests under natural dynamics vary from
20–120 m 3 ha -1 in boreal forests in Fennoscandia to 130–250 m 3 ha -1 for
European temperate forests (Jonsson & Siitonen 2012: table 12.4, Christensen
et al. 2005: table 1). Siitonen (2001) specified that the amount of
coarse woody debris (CWD) varies from 90–120 m 3 ha -1 in the southern
and middle boreal zone, 50–80 m 3 ha -1 in the northern boreal zone, and
to about 20 m 3 ha -1 in the northernmost timberline forests. Such variations
in the natural supply of dead wood can also be expected on a local
scale, with the highest production at more productive sites. Decay rates
also vary with climatic factors, being faster in warm and moist than in
cool and dry climates.
Considering that stand-replacing disturbances like severe fires seem
to be less frequent in natural boreal forests than previously thought
(Kuuluvainen 2009), it is relevant to consider how long-term absence of
major disturbances will affect key ecosystem properties. Wardle et al.
(2012) have recently synthesized their studies of ecosystem processes
for a number of boreal forest lake islands in Northern Sweden, where
time since the last major disturbance by fire varies from 60 to more than
5000 years. As time since the last major disturbance has progressed,
islands have gone through a succession where key nutrients like nitrogen
(N) and phosphorus (P) become less accessible for plant growth and
primary production and decomposition rates decrease. This results in
lower aboveground biomass production, but lower decomposition leads
to higher belowground and overall carbon stocks. Total ecosystem carbon
sequestration rates appear to be similar for younger and older islands,
at about 4.5 g m -2 yr -1 , over several thousand years. Species richness
for vascular plants and aboveground (but not belowground) invertebrates
increases with time since disturbance.
Biodiversity, carbon storage and dynamics old northern forests 31