Linking Restoration and Ecological Succession (Springer ... - Inecol

56 David A. Wardle and Duane A. Peltzer
from the soil, and thus enhance NPP. This emphasizes the importance of understanding
feedbacks between the aboveground and belowground subsystems
when attempting to predict the ecological consequences of ecosystem restoration.
Studies on these islands have also shown that the importance of species effects
on ecosystem properties changes during successional time. An ongoing
experiment set up in 1996 (first 7 years reported by Wardle and Zackrisson
2005) involved experimental removals of various combinations of understory
plant species on each of 30 islands across the sequence. This work revealed that
two of the dominant understory species (V. myrtillus and V. vitis-idaea) drove
belowground processes on the large islands but not the small ones. Removals of
these species significantly reduced litter decomposition, soil microbial biomass,
and soil respiration on the large islands to levels more characteristic of small
islands. In contrast, species removals on the small islands had no detectable
effects on these properties, probably because of the increasing relative importance
of abiotic drivers. This points to species’ effects (and consequences
of species losses) in ecosystems being context-dependent and of diminishing
importance as retrogressive succession proceeds, as well as to the role of understory
species in governing the effects of successional status on the functioning
of the belowground subsystem. Any consequences of restoration effort for both
the aboveground and belowground subsystems will therefore depend in a large
part upon how the relative abundances of these understory species are influenced
by frequency of fire regime.
The island system provides evidence that reducing fire frequency, and the
ecosystem retrogression that results, greatly influences ecosystem C sequestration
(Wardle et al. 2003). As retrogressive succession proceeds (i.e., as islands
become smaller), standing plant biomass and NPP are both impaired,
in both the tree and understory shrub layers. This results in less C storage
aboveground. Further, decomposition rates of plant litter are reduced, for three
reasons: (1) because plant species that produce poorer litter quality (Picea and
Empetrum) begin to dominate; (2) because of phenotypic plasticity within plant
species (i.e., plants of a given species producing poorer quality litter during retrogression);
and (3) because of lower activity of the decomposer microflora.
Further, during retrogression, decomposition is impaired before NPP, which
results in a net gain of C to the soil. As such, the largest islands store less
than 5 kg C/m 2 in the soil while some of the smallest ones store over 35
kg C/m 2 . Because the belowground (rather than the aboveground) component
stores the majority of C in these forests, there is net ecosystem C sequestration
over time, in the order of 0.45 kg C/m 2 for every century without a
major fire. These results point to fire suppression in forest ecosystems contributing
significantly to C sequestration, and if these patterns are widespread
elsewhere then this may play an important part in the global carbon cycle. They
also point to the fact that ecosystem restoration involving the reintroduction
or encouragement of natural fire regimes (and consequent reversion of retrogressive
succession), would in turn alter the balance between aboveground and
belowground processes, and thereby greatly influence total ecosystem carbon
storage.
While there has been substantial fire suppression in the boreal forests of northern
Scandinavia over the past couple of centuries, there is recent recognition
that the use of fire in these systems may have beneficial consequences from a