Movement of the Conifer Belt on the Sierra Nevada over the Last 140 Years
Thorne, J. H., Thrasher, S.L., Bjorkman, J.S.
University of California, Davis
Vegetation dynamics at the landscape scale can be uniquely informed by historical vegetation maps. Albert Wieslander led the Vegetation Type Map
(VTM) Project, a remarkable effort to survey the forests of California in the 1930s (Wieslander 1935a, 1935b, 1985; Griffin and Critchfield 1972). His
crews surveyed over 150,000 km2, drawing detailed vegetation maps, and taking 3000 photos and 17,000 vegetation plots (Colwell 1977). In
addition, they drew maps of the estimated extent of coniferous forest from pre-settlement conditions (1850) for El Dorado County. We rendered
these maps to a GIS for El Dorado County, a county that encompasses a large elevational gradient on the west slope of the Sierra Nevada. The
lower edge of the continuous coniferous forest was found to have moved upslope by over 500 meters, and eastwards over 26 km.
A. Convert vegetation map to
Ponderosa/Non-ponderosa
B. Lay 1 km2 grid over map
We hypothesized that this shift in the retreating edge of ponderosa pine forest is due at least in part to climate change. Adult ponderosa pine in the region are
known to have been heavily logged. Therefore, the disappearance of the adult trees is due to human perturbation. However, the lack of regeneration of forest,
as quantified by little measurable regeneration of ponderosa pine in deforested zone in the 1996 map, means there has been no successful recruitment of
seedlings (Table 2). This component is potentially tied to climate change. With the warming trend reported below, summer drought periods begin sooner, leading
to a longer dry period and greater drought stress.
Minimum monthly temperatures from Sierra Nevadan weather stations in the study area increased for all month of the year at all sites, and indicated a average
warming in the region of approximately 30 C over the past 100 years. The monthly trends are reported in Appendix A, Table 5. The yearly trends are presented
here. Sacramento, below the ponderosa shift zone, had the lowest rate of warming, at 0.0120 C/yr-1 (r2= 0.56 for 10 year smoothed data). The station at
Placerville, in the center of the shift zone for ponderosa pine forest, is experiencing minimum monthly temperature increases at the rate of 0.0550 C/yr-1 since
1942 (N = 57 years, R2= 0.83, 10 year smoothed data). The Placerville station went from having two months a year which were frozen every night (December
and January) to none. Yosemite Valley is minimum monthly temperatures are warming at the rate of 0.0410C/yr-1 (r2= 0.88, 10 year smoothed data) and the
Tahoe station is recording a rate of 0.0440C/yr-1 (r2= 0.84, 10 year smoothed data).
C. Take all cells with 75% or
more cover of Ponderosa
Figure 7. Steps showing how the horizontal shift between fronts was calculated.
East-west transects were taken every 250 meters so that distance between fronts
could be measured.
These results are consistent with other observations that the Sierra Nevada are warming: Lake Tahoe, a 500 km2 integrator of surrounding conditions is
warming (Coats et al. 2006); snowmelt throughout western North America is experiencing earlier snowmelt (Stewart et al. 2005) and fires in the western US are
occurring more frequently (Westerling et al. 2006).
Figure 8. Vector lines showing the Ponderosa pine
leading edge boundaries for each time period
INTRODUCTION
Terrestrial species may respond to warming climate by moving to higher latitudes or elevations (Root et al. 2003; Parmesan & Yohe 2003), a process
here termed a leading edge dynamic. Several studies of conifer response to climate change have examined high elevation or latitude leading, or
advancing, forest edges (Millar et al 2004; Kullman 2001; Chapin et al 2005), and found that establishment of new stands of trees beyond their
historic range could be linked to changes in temperature within the last 100 years.
Potentially confounding dominant vegetation and land cover types occupied 40% of the 540 km2 zone of ponderosa pine forests retreat. The remaining 60% is
the portion of change that we were not able to discount by confounding factors available in the digital maps (Figure 11).
DISCUSSION
1996 Boundary
The historical VTM maps provided an opportunity to detect a major shifts in ponderosa pine forest edges. The trailing edge phenomenon shown here is
driven by increasing drought stress on recruitment, and related competition effects, that are due to advancing spring and subsequent summer drought
conditions. Advancing spring leads to a longer summer drought period, and in areas below the snowline, this drought begins as soon as soils dry out
following the last rain of the season. In Placerville, the weather station in the middle of the deforested zone, there are no longer any months of the year that
are frozen, a loss of 90 days of frozen conditions over the past 60 years. We think this is related to the lack of regeneration of ponderosa pine seedlings
here evidenced in the vegetation map time series. Management implications for the Sierra Nevada are that clear cutting at the lower edge of the conifer
zone will likely be associated with poor subsequent conifer establishment, and the potential loss of snow retention capacity. Since water is such a valuable
commodity in the west, these findings set up a potential conflict between managing forests for water resources and managing them for timber.
Trailing edge dynamics are characterized by unstable ecosystem states, wherein the conditions under which a forest stand established are no longer
in effect; when a stand-removing disturbance occurs at this ecotone, the same vegetation type is unlikely to be able to re-establish itself. This
dynamic can occur either because the underlying conditions have so changed that the original vegetation is no longer physiologically suited to the
location, or because the time required for vegetative succession to pre-disturbance condition has been extended long enough that other disturbances
are likely to preclude re-establishment of the original vegetation.
Conifer forest dynamics at the lower, trailing, edge are potentially difficult to study because patterns of attrition among long-lived species require a
record of forest extent over periods of time sufficient to record the loss of forest. Conifer response to climate change at the lower end of physiological
tolerance manifests itself as increased sensitivity to perturbations such as drought (Breshears et al 2005; Allen and Breshears 1998) or beetle
infestation (Powers et al. 1999).
1850 Boundary
Ponderosa pine is limited at its lower edge by an interaction of temperature and moisture (Barton 1993; Oliver and Ryker 1990), physiologically
defined as water deficit (Stephenson 1990), and potentially through competition ponderosa seedlings reduce their metabolic rates when drought
stressed, while many of their competitors do not (Lopushinsky and Klock 1974; Shainsky & Radosevich 1986).
We hypothesized that a decrease in water availability, driven by air temperature increases measured at a series of weather stations, would result in
conditions unfavorable to the establishment of young Ponderosa pine, and that this dynamic could be measured by an upslope retreat of Montane
coniferous forest.
The focus on average temperatures in much climate work misses where larger changes are already happening, in the wings of the temperature
distributions. In arid ecosystems like the lower Sierra Nevada, minimum monthly temperatures are likely a more biologically relevant metric than average
temperatures. In areas below snowline, drying conditions are different than in areas associated with snow melt, and more research is needed to identify the
speed with which soils are dried following the last rain of the season. Biotic response to climate will likely vary by ecoregion, and biologically important climate
parameters will likely vary—the different rate of warming in the Central Valley of California suggests that ecoregion may be under a different dynamic than the
nearby mountains. Finally, terrestrial carbon sequestration efforts need to be informed by changing species range locations, so that trees no longer adapted to
certain regions are not planted there.
1934 Boundary
Figure 12. Map showing possible confounding factors
such as grasslands (yellow), urban areas (black) or
lands affected by fire (orange).
Figure 9. Ponderosa pine land cover area in El Dorado county for each time period
Figure 10. Map showing location of weather
stations. Red stars indication the following from
West to East: Sacramento, Placerville, Tahoe
City, Yosemite Valley.
We examined change in the trailing edge of Montane coniferous forest on the western slope of the Sierra Nevada Mountains, California. Maps of El
Dorado County from 1850, 1934, and 1996 were used to derive the western, lower extent of ponderosa pine dominated forests for each time step.
The area studied covered 56 km north-south, approximately 10% of the total western forest edge of the mountain range. Differences in Montane
coniferous forest lower extents between years were measured on an east-west transect every 250 m from south to north and horizontal, elevational,
and area shift were recorded, permitting quantification of the change over 146 years. The resulting pattern was examined in light of climate records
and potential confounding effects.
Weather Stations
Minimum Monthly
temperatures
meters
Years
Sacramento
6
>100
Placerville
625
43
Yosemite V. 1250
84
1875
83
Tahoe City
Ponderosa western extent
1996 - CalVeg
Sacramento rate
0.013 C/yr
Overlay Plot
18
Figure 1. Forest Extent Map for 1850
Forest cover map of El Dorado County from 1934, with GIS overlay of vegetation types. The map was made by
the director of the Wieslander VTM project in 1934, and used old stump fields, and remnant stands of pine as
the basis for map boundaries. The brown in this map of El Dorado County represents the area (144,943 acres)
in Ponderosa Pine forest which was lost between 1850 and 1934. The brown color represents the western
extent of Ponderosa pine in the county in 1850. Light green represents areas which had been logged, which
contained second-growth Ponderosa Pine (155,277 acres) at that same time. Dark green areas (38,520 acres)
represent virgin timber below 6,000 feet (Weeks 1934).
Figure 2. The study area, El Dorado County, outlined in red. The
colored maps show areas where1934 VTM maps have been
digitized and areas highlighted in purple have historic maps that
have not yet been digitized. These were the second time step in our
study.
4
14
Figure 3. 1934 VTM Vegetation Map (and detail). Once
scanned into digital format, the maps are
georeferenced, the polygons digitized and then
attributed. Species combinations from the maps can be
cross-walked into modern vegetation classifications.
To determine the distance and elevation differences between these three fronts, we used GIS to create east-west transects every 250 meters from
the south to the north end of El Dorado County (Figures 8,9). We then sampled the latitude, longitude, and elevation of each front along each
transect, generating paired coordinates and elevations (1850 vs. 1934, 1850 vs. 1996, and 1934 vs. 1996). Horizontal shift between fronts was
calculated as the average straight-line east to west distance in kilometers between the fronts. Elevational shift was calculated as the average
difference in elevation between all paired sample points.
Finally, the front from each time period was digitized to a vector line (Figure 8). This permitted the creation of polygons representing zones of
conversion between 1850-1934 , 1934-1996, and 1850-1996. The areas and composition of dominant vegetation types inside these zones were then
assessed (Figure 9). Dominant vegetation types that might represent confounding factors to Ponderosa pine seedling mortality due to climate were
identified: grasslands, urban and places that had burned within the zone vacated by Ponderosa Pine Forest were removed from the spatial extents.
These types were hypothesized to affect the recruitment of ponderosa seedlings in the following ways: in grasslands either cattle feeding or
competition from non-native grasses may affect ponderosa pine seedlings; urban settings may prohibit their establishment; and fire may also destroy
seedlings.
WEATHER
Minimum monthly average records from four weather stations forming an elevational transect in the region were examined. Three stations had 100 +
year records (Sacramento, Yosemite Valley, and Tahoe City; National Climate Data Center 2006); the remaining stations (Placerville, in the middle of
the deforested region) had a 60 year record (Western Regional Climate Center, 2006) (Figure 10). Monthly minimum temperatures were arrayed by
month across years, and a 10 year moving average taken to smooth the data (Figure 11). A regression of each month’s temporal record was taken,
as well as the yearly average of monthly minimums. Rates of warming were calculated and the months which changed from a below 0o C to above,
were noted for each site.
Figure 4. Fully converted VTM quad which can be queried by
species or vegetation type
RESULTS
In total, the western, lower, extent of continuous Ponderosa Pine Forest moved eastwards an average of 26.4 km with an elevational change of on
average 526 meters (Table 1). The 1850s map was the least certain of our data, however the trend was also evident using only the 1934-1996 time
period, during which time the western edge moved an average of 7.1 km (+/- 300 m, the potential horizontal spatial registration error when
intersecting the 1934 map with the 1996 one); accompanied by an upward shift of on average 193 meters. The overall area affected by the shift from
1850 to 1996 was 562 km2.
6
5
5
4
4
3
3
12
3
2
10
2
8
1
4
1
0
2
1
0
-1
-1
-2
-2
-3
-3
0
2
1860
1880
1900
1920
1940
1960
1980
2000
-2
MAR
SEP
APR
OCT
18901900 1910 1920 1930 1940 1950 1960 1970 1980 1990 20002010
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
1950
FEB
AUG
-5
-5
2020
YEARS
JAN
JUL
-4
-4
-1
0
Y
We chose CalVeg, a 1996 vegetation map produced by the US Forest Service, for the modern vegetation map (Schwind & Gordon 2001). The 1996
map is less species-specific and is coded by Wildlife Habitat Relationship Type (WHR Types; California Department of Fish and Game 2004).
Ponderosa pine cover was estimated by calculating the total cover of all WHR Types in which ponderosa pine is defined as an element. These
included the cover types Ponderosa Pine (PPN), Sierra Mixed Conifer (SMC), Jeffrey Pine (JPN), and Douglas-fir (DFR). By including all of these
WHR Types, our classification was inclusive and thus represents a liberal estimate of the spatial extent of ponderosa pine within El Dorado County in
1996.
To determine the lower elevation front of ponderosa pine forest for 1934 and 1996, we used GIS sampling tools to determine the edge where
ponderosa pine cover was 75% or more with a 1 km2 sampling grid. For the 1934 map, we calculated the cover of all polygons for which ponderosa
pine had been identified as present in the 1934 surveys. The 1996 map is less species specific and is coded by Wildlife Habitat Relationship Type
(WHR Types; California Department of Fish and Game 2004). Ponderosa pine cover was estimated by calculating the total cover of all WHR Types
in which ponderosa pine is defined as an element. These included the cover types Ponderosa Pine (PPN), Sierra Mixed Conifer (SMC), Jeffrey Pine
(JPN), and Douglas-fir (DFR). By including all of these WHR Types, our classification was inclusive and thus represents a liberal estimate of the
spatial extent of ponderosa pine within El Dorado County in 1996.
6
5
16
6
We digitized the historic vegetation maps for use in a GIS. The first step in digitizing the VTM maps was to create high-resolution digital images of
both the base maps and VTM tiles (Figure 2). Once the base topographic map and VTM tiles for a particular quadrangle were scanned, the base
map was first registered into its native map projection using ERDAS Imagine, a remote sensing computer program, and then the VTM tiles were
registered onto the base map. This process resulted in a georeferenced VTM map with a minimal amount of introduced distortion (Figure 3). Next,
the vegetation polygons on the maps were digitally traced in ArcMap software and the species codes in the maps were transcribed into a
standardized table (Figure 4). The species combinations assigned to vegetation types according to the Manual of California Vegetation (Sawyer and
Keeler-Wolf, 1995) and the Wildlife Habitat Relationships Model (WHR) developed by the California Department of Fish and Game (2004).
Tahoe rate
0.0440 C/yr
R2= 0.96 P<0.001
Yosemite Valley
rate 0.0420 C/yr
R2= 0.75 P<0.001
DEGREES C
METHODS—
MAPS
We used a 146-year record of lower elevation treeline of Ponderosa Pine Forests (Pinus ponderosa), and 60 to 100-year-long monthly records from
three weather stations that form an elevational transect. We used the two historical maps and a modern map to measure the dynamics of the lower
edge of Ponderosa Pine forests in California. Figures 5-9 show the steps used to derive a quantitative measure of forest front. We used three timesteps portrayed in maps to conduct the analysis: 1850, 1934, and 1996. The first two maps were surveyed by Wieslander, the earlier one based on
stump fields and the 1934 based on existing vegetation. For the 1850 map the entire vegetation cover delineated as deforested or replanted was
treated as the ponderosa pine forest edge (Figure 1).
Placerville rate
0.0890 C/yr
R2= 0.71 P<0.001
Y
Ponderosa western extent
1934 - Wieslander
DEGREES C
Ponderosa western extent
1850 - Wieslander
10 yr
moving
average
DEGREES C
This poster documents the steps used in the production and analysis of historic and modern vegetation maps to document upslope movement of
Ponderosa pine (Pinus ponderosa), and presents corroborating evidence that this shift is due to changes in minimum monthly temperatures, which
have increased by over 3 degrees C at three weather stations forming an elevational transect in the mountains. This change ushers in an earlier
summer, with associated drought, that we hypothesize drives seedling mortality in this system through both direct and indirect effects. We
documented the extent of mapped possible confounding factors: grazed grasslands, urban, and fire perimeters, and found those occupied 40% of
the 540 km2 region that had been deforested. Preliminary results indicate Ponderosa pine at the lower edge was replaced by Montane hardwoods.
Human interactions with Montane conifer types have shown the forest to be more sensitive to perturbation than was previously understood, when
considering landscape dynamics over multiple decades on a large elevational gradient.
Albert Weislander
Figure 6. Portion of the VTM map
showing the leading edge of
Ponderosa pine
Figure 5. The steps taken to determine the edge of Ponderosa pine for the 1934 and 1996
fronts.
ABSTRACT
MAY
NOV
JUN
DEC
1960
1970
1980
1990
2000
2010
YEARS
YEARS
Year
Y
JAN
Y
FEB
DEC
Y
MAR
APR
NOV
MAY
OCTOBER
JUNE
Figure 11. Monthly minimum temperatures from four weather
stations in central Sierra Nevada.
E-W
Shift
km
Total Area 1850 Forest (980 km2) reported by
Weeks (1934)
Horizontal Vertical
Rate of
Rate of
Shift Vertical Elevation Change Change
m
t-test
m/yr
m/yr
Remaining Forest
Deforested in 1850
and still deforested in
next time step
1850 - 1996
26.4
526.7
0.00000
18
4
Extent 1850
440 km2
540 km2
1850 - 1930s
19.6
374.7
0.00000
23
7
Extent 1934
360 km2
473 km2
1930s - 1996
7.1
193.1
0.00000
11
2.9
Extent 1996
94 km2
525 km2
Table 1. Area change and elevational change of the lower boundary of Ponderosa pine.
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