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13.3.2.2 Range Shifts
The replacement of “cooler or wetter” ecological systems and their indicator spe-
cies by “warmer or drier” systems and their indicator species during climate change
is called range shift. Theoretical bioclimatic envelope modeling (Rehfeldt et al.
2006 ; Bradley 2009a) and one field study (Kelly and Goulden 2008 ) provided
widely conflicting conclusions on the speed of range shifts. Thus, in this modeling
example, the percentage of the area shifting from A. tridentata spp. vaseyana to A.
tridentata spp. wyomingensis systems over 100 years was first set at 10 %. We itera-
tively determined that a rate (probability per year) of 0.0604 year−1 (i.e., 604 virtual
pixels shifted per 10,000 pixels per year) matched the 10 % range shift over 100
years. We further set values of 87 % and 13 % for the total area of A. tridentata spp.
vaseyana that would be replaced by A. tridentata spp. wyomingensis and A. nova
A. Nelson (black sagebrush), respectively, based on current sagebrush community
proportions as found by Provencher et al. ( 2013 ). For simplicity, we only tracked
the range shift between A. tridentata spp. vaseyana and A. tridentata spp. wyomin-
gensis for this chapter. Moreover, we gradually introduced the range shift by setting
the initial value of range shift to zero under the assumption of no climate change at
year zero, and then linearly increased temporal multipliers for range shifts to a value
of two by year 75 of the simulation. Therefore, the average rate of range shift of
0.0604 year−1 had a value of one over the 75 years of the time series to maintain rate
integrity. In comparison, a control simulation without climate change would have a
range shift temporal multiplier series equal to zero for all time intervals.
13.3.2.3 Climate Variability Effects on Ecological Processes
Temporal multipliers act as forcing factors of ecological processes in the STSM and
also reflect hypotheses about the effects of climate variability on ecological pro-
cesses. One temporal multiplier is a non-dimensional number ≥ 0 in a yearly time
series that multiplies a base disturbance rate in the STSM. For example, for a given
year, a temporal multiplier of one implies no change in a disturbance rate, whereas
a multiplier of zero is a complete suppression of the disturbance rate, and a multi-
plier of three triples the disturbance rate. A temporal multiplier can be obtained
from time series data or theoretically derived. In the current case, multipliers vary
for scenarios with or without climate change. Temporal multipliers are determined
by dividing each yearly value of the time series (for example, area burned) by the
temporal average of the time series, thus creating a non-dimensional time series
with an average of one. Division by the time series’ average would remove the
hypothesis of altered levels of the ecological process being modeled under climate
change scenarios; thus each raw value of the new time series (e.g., future area
burned) with climate change must be divided by the average of the time series not
experiencing climate change.
L. Provencher et al.