Ecological Succession and Community Dynamics - Ecological Systems - page 51

albedo associated with a documented shift in forest type (larch to dark conifer)
indicates that warming temperatures may lead to a positive feedback response:
a warmer climate accelerates the natural succession from larch to dark-conifer
forest; the resultant albedo change promotes additional warming.
Dynamic vegetation models, specifically forest gap models, are ideally suited
to an exploration of the impacts that climate change may have on the structure and
composition of boreal forests and the existence of a climate/land cover feedback
in this region. The FAREAST [ 74 ] model was run at a total of 2,083 sites across
the former USSR. FAREAST uses monthly climate parameters derived from
historical station data to compute daily temperature and update soil water [ 75 ].
In particular, at each site, the model's climate inputs are drawn from a statistical
distribution of monthly values for minimum and maximum mean temperature and
precipitation which is derived from 60 years of data recorded at local weather
stations [ 76 ]. The model also uses values for soil field capacity and soil carbon and
nitrogen [ 77 ].
The birth, growth, and eventual death of individual trees are determined in
response to local site parameters such as soil moisture and nutrient availability,
which are updated annually with bio-environmental conditions, soil moisture, and
available nutrients. Individual trees compete for light and nutrients with stochastic
processes governing the birth and death of trees in a circular twelfth hectare plot,
which approximates the size of a mature tree crown. Forty-four individual tree
species are included in FAREAST simulations. These species represent the genera
which dominate Northern Eurasian forests. Each species is characterized by 25
parameters which describe the species' fundamental silvics and determine which
species has an advantage in terms of competition for light or nutrients, or tolerance
to lack of water. At each of the 2,083 sites, 200 independent twelfth hectare plots
were simulated and then the modeled biomass values were averaged for each
species in each year of the model run.
The overall response of Russian boreal forests to climate change when the
effects of changes in temperature and precipitation are separated show higher
average precipitation leads to increased biomass ( Fig. 3.5a ), lower average
precipitation results in decreased biomass ( Fig. 3.5b ), and warming causes
decreases in biomass for certain regions, though in parts of Siberia, where average
temperatures are extremely low, warming induces an increase in forest biomass
( Fig. 3.5c ). There are also genera specific patterns in the shifts in biomass that
occur across Russia. Specifically, there are different patterns of change for Larix
spp. and Pinus spp. in response to temperature warming. Both genera display
a decrease in biomass in western and southwestern Russia and the Russian Far
East. The number of sites that experience a biomass decrease for Larix spp. is
larger than the number of sites that show a decline in Pinus spp. In particular, the
sites that show a decline in Larix spp. extend further northward in both European
Russia and the Russian Far East. A more detailed analysis is required to deter-
mine whether these patterns are the result of a replacement of Larix spp. with
Pinus spp.