U53A: 707
HIGH-ELEVATION RESPONSE OF CONIFERS TO CLIMATE CHANGE IN THE SIERRA NEVADA AND
WESTERN GREAT BASIN, USA: TREELINE ELEVATION IS NOT THE PRIMARY EFFECT
1 USDA
Constance I Millar1, Robert D Westfall1, Diane L Delany1, John C King2, Harry A Alden3
Forest Service, PSW Research Station, Box 245, Berkeley, CA; 2 Lone Pine Research, Bozeman, MT; 3 The Smithsonian Institution, Suitland, MD
INTRODUCTION
Movement of treeline up and downslope is traditionally assumed to be the primary response of high-elevation forests to past climate change (LaMarche 1974, Lloyd and Graumlich 1997), and an anticipated major effect of future global warming. Considerable effort, thus, has focused on
discriminating among climatic factors that would cause forests to move up or down slope. In the Sierra Nevada and Great Basin ranges, however, complex and fluctuating changes in forest demography, slope aspect and position, tree growth, crown form, species composition, and site conversions
independent of upper treeline shifts are occurring that have not been investigated for possible correlations with climate.
We have been investigating forest response to historic climate at timescales including decadal (20th-century) to millennial (the past 3500 years) in varied locations and forest types of the high Sierra Nevada and SW Great Basin ranges. We present overviews of four independent studies that reflect
individually and collectively on the issue of climate effects on high-elevation forests. In each study, we used standard tree-ring methods to assess ages of trees and to cross-date deadwood using replicated standard chronologies confirmed with local marker events.
STUDY 1:
LIMBER PINE POPULATIONS DURING THE PAST 3500 YEARS EXTIRPATE & RECOLONIZE DIVERSE SLOPES &
AND CORRELATE WITH CENTURY-SCALE DROUGHTS
Limber pine (Pinus flexilis) grows in highly restricted woodland populations on N/NE aspects from 26653222 m in the Wassuk Mtns, NV (fig 1). Deadwood of limber pine occurs mixed with live trees, but also
occurs widespread and abundantly on NW, W, SW, S, SE, and E aspects throughout the Wassuk range, where
no live trees currently exist (fig 2). Similar conditions exist in neighboring SW Great Basin ranges, and in the
E Sierra Nevada.
ASPECTS
Fig 4
Stand
GR
RC
DC
LP
CN
LG
GR
NC
CC
Aspect
N/NE
SE/E/NE
N/NE
W/NW/N
W/NW
W/NW
S/SW
S/SE
NE/E
Dead
High
Live
High
Dead
Low
Live
Low
3190
3200
3086
3151
3178
3109
3139
3210
2774
3222
3178
2990
no live
no live
no live
no live
3033
2774
2944
2700
2819
3048
2993
3004
2915
2621
2665
2682
2700
2819
no live
no live
no live
no live
2621
2665
QUESTIONS: What are the ages of live and deadwood whitebark
pine at upper treeline, when do upright branches (flags) release and
how long do they persist, have horizontal branch growth-rates
changed over the 20th-century, and, if so, do trends correspond with
climate?
Fig 2
Fig 3
UPPER TREELINE IN SIERRA NEVADA WHITEBARK PINE IS STABLE FOR MORE THAN 1700 YEARS;
CHANGES IN CROWN FORM & GROWTH CORRESPOND TO DECADAL & CENTURY CLIMATE TRENDS
The zone of krummholz (stunted) whitebark pine (Pinus albicaulis) in the Sierra Nevada extends 100-500 m above upright tree-form populations to
form upper treeline at 3500 m (fig 9). Well-preserved deadwood is scattered among live krummholz patches at upper treeline, but is not found above
current live trees, suggesting that treeline may not be sensitive to climate in this species. By contrast, apical branches occasionally release to extend
above wind-sheared krummholz crowns and repeat vegetative layering events in krummholz pine suggest that changes in crown structure at treeline may
be the primary response to climate variability. Further, repeat photos from the early 1900s compared to recent conditions show increases in patch size
of whitebark pine krummholz, suggesting that overall growth may have responded to 20th-century climate.
Fig 1
QUESTIONS: What are the ages of deadwood and living
populations, and do age, aspect, and/or elevation correlate with known
climate phases?
RESULTS: The oldest limber pine deadwood we have dated in the
Wassuk range is 3585 years old. Abundance of deadwood varies over
time, with many slopes and aspects having periods with no wood for
100-300 years. Some of these slopes subsequently regain wood, then
lose deadwood representation again (fig 3). Pulses of abundance are
aspect-dependent, with increased abundance on N/NE slopes 35003000 ybp, 2400-1700 ybp, 1200-900 ybp, and 600-0 ybp, and
decreases or absences during intervening times. On S/SW slopes, by
contrast, abundances increase 2600-2100 ybp, 1800-1600 ybp, and
1400-800 ybp. From 800 ybp to present, deadwood or live trees occur
on N/NE slopes. A single live stand on S exposure is dying. Upper
and lower elevations of deadwood overall in the range do not exceed
live populations but vary by aspect and age (fig 4). Periods of
deadwood absence correspond to centuries of known paleodrought (fig
5).
STUDY 3:
Fig 5
Bars indicate treering series of live
and deadwood
0
500
1000
1500
2000
2500
Years BP (2000 CE)
3000
3500
4000
Fig 9
RESULTS: Live whitebark pine krummholz pines at upper treeline
in the central Sierra Nevada persist more than 1700 years old, which
is 3-4 times the typical longevity of upright whitebark pine in lower
elevations, and appears to result from the capacity of krummholz pine
to vegetatively layer (branch rooting events) episodically. Deadwood
scattered within live treeline whitebark krummholz populations in
Yosemite National Park date to more than 1700 years ago (fig 10).
No deadwood was found above current treeline, however, in these
locations, indicating that treeline has been stable through climate
variability of the last 2000 years. Ages of upright branches (flags)
cluster in the multi-decadal period of the 20th-century, from 1945 to
1980 (fig 11), which corresponds to a strong negative period of the
Pacific Decadal Oscillation. Relative to early & late 20th-century,
temperatures were depressed during this period, precipitation was
lower, and interannual variability was very low (fig 13). Horizontal
growth of krummholz whitebark pine in these locations more than
doubled over the 20th-century (fig 12), and corresponded to trends in
warming climates during the past century (fig 13).
Bars indicate
tree-ring
series of
deadwood
Fig 10
YEAR CE
Six Sites
Five Sites
mean, SD
mean, SD
Drought Periods, YBP, identified by proxies other than limber pine:
600-800:
Stine, Walker, Pyramid, Mono, Owens, Tree Rings, Springs, Pinyon
1300-1400: Walker, Pyramid, Pinyon
1400-2100:
Pyramid, Springs, Pinyon
2100-2700: Walker
2800-2900:
Pyramid
Fig 13
n = 126
n =799
Fig 11
STUDY 2:
COMPLEX CHANGES IN FOREST COMPOSITION ON WHITEWING MTN, E SIERRA NEVADA, CA, DURING PAST
FOLLOW CENTENNIAL CLIMATE PHASES
Well-preserved downed deadwood logs litter otherwise barren slopes above local upper treeline at 2990-3051 m on Whitewing Mtn, E Sierra
Nevada (fig 6). Only a few scattered live krummholz (stunted) individuals of whitebark pine currently grow among rock outcrops near the summit.
Forests at lower elevation in the region include western white pine, lodgepole pine, Jeffrey pine, and mountain hemlock.
1100 YEARS
STUDY 4:
20th-CENTURY INVASIONS BY PINES INTO SUBALPINE MEADOWS & FORMER SNOWFIELD OPENINGS
CORRELATE WITH PHASES OF THE PACIFIC DECADAL OSCILLATION & 20th-CENTURY WARMING
In the Sierra Nevada subalpine meadows and barren areas formerly covered by persistent
snowfields are converting to forests of whitebark pine (Pinus albicaulis), western white pine (P.
monticola), and lodgepole pine (P. contorta) (fig 14, 15).
Fig 6
QUESTIONS: What is the species composition represented by the deadwood forest, when did the trees grow on Whitewing Mtn, and does the
timing of population occurrence correlate with climate?
QUESTIONS: What are ages of pines invading meadows and former snowfield openings, is
colonization progressive or episodic, and does time of invasion correlate with climate and/or
recent human land uses?
RESULTS: Anatomical analyses of deadwood indicate that five species of pine and one species of hemlock are represented by deadwood on
Whitewing Mtn (fig 7). Of these, only whitebark pine is native and local at the Whitewing summit elevation currently; four of the other species are
native to the eastern Sierra Nevada but have current upper limits ranging from 250 to 500 m below Whitewing Mtn summit. The final deadwood
species, sugar pine, is not currently native to the eastern Sierra Nevada, but occurs at this latitude west of the Sierra range crest, where it grows in
mid-elevation mixed-conifer forests with an upper elevation limit 600 m below Whitewing Mtn. Tree-ring dating of deadwood indicates that the
Whitewing summit forest and adjacent San Joaquin Ridge grew from 900-1350 CE (fig 8). There is no evidence of trees on Whitewing Mtn prior to
900 CE nor from 1350-1740 CE. The occasional live stunted whitebark pines now growing near the summit date from 1740 CE to present. At the
time of the summit forests, growing conditions were favorable, with annual growth exceeding 2 ring/cm, long, straight stems (mean = 4.1 m) and
large diameters (mean = 26.9 cm). The mixed conifer forest corresponds to the warm, winter-wet Medieval Climate Anomaly; the period of no
trees corresponds to the Little Ice Age; and the period of krummholz whitebark pine corresponds to warming, wetter climates of the 20th century.
(SEE POSTER U53A #708)
RESULTS: Of the meadows we studied (elev 2315 – 3050 m), pines invaded episodically
between 1945-1976 with a small pulse also in the late 1990s (fig 16). There was no trend to age
of invasion within these periods, nor was there a relationship between distance to forest edge and
meadow location of invading pine. Of snowfield openings studied (elev 2730-3105 m), pines
invaded in two major 20th-century pulses, between 1935-1945 and 1990-2000 (fig 17). Invasion
dates in both situations were not significantly correlated with history of livestock grazing or fire,
or with elevation, but correlated significantly in direct and interaction effects with Pacific
Decadal Oscillation (PDO) indices, minimum temperature, and interannual variability in moisture
(fig 18). Invasion responses to different phases of PDO reflect the slope, aspect, and hydrology
differences of snowfield versus meadow locations & topography.
Bars indicate tree-ring series of live and
deadwood on Whitewing Mtn and San
Joaquin Ridge, E Sierra Nevada, CA
N (deadwood) = 57; N (live whitebark pine
krummholz) = 27
Fig 12
Fig 14
Fig 15
Whitewing Mtn
REFERENCES:
King, J. C. and Graumlich, L. J., 1998: Stem-layering and genet longevity
in whitebark pine. A final report on cooperative research with the National
Park Service (CA 8000-2-9001).
LaMarche, V. 1974. Paleoclimatic inferences from long tree-ring records.
Science 183: 1043-1048.
Lloyd, A. H. and Graumlich, L. J., 1997: Holocene dynamics of treeline
forests in the Sierra Nevada. Ecology, 78: 1199-1210.
Millar, C.I., R.D. Westfall, D.L. Delany, J.C. King, and L.C. Graumlich.
2004. Response of subalpine conifers in the Sierra Nevada, California,
U.S.A., to 20th-century warming and decadal climate variability. Arctic,
Antarctic, and Alpine Research 36 (2): 181-200.
Rogers, D. L., Millar, C. I. and Westfall, R. D., 1999: Fine-scale genetic
structure of whitebark pine (Pinus albicaulis): associations with watershed
and growth form. Evolution, 53(1): 74-90.
Westfall, R.D. and C.I. Millar. 2004. Genetic consequences of forest
population dynamics influenced by historic climate variability in the western
USA. Forest Ecology and Management 197: 157-168.
N
20
Species ID
Whitebark Pine
20
9
7
4
3
Fig 7
W White Pine
Lodgepole Pine
Jeffrey Pine
Mountain Hemlock
Sugar Pine
Ten Meadows
_____
Current Upper Treeline Elev
Pinus albicaulis
krummholz, 3200 m
(straight stem)
P. monticola
2600 m
P. contorta
2700 m
P. jeffreyi
2550 m
Tsuga mertensiana
2800 m
P. lambertiana
W Sierra Nevada, 2450 m
Six Snowfields
(1959, 21.6)
mean, SD
mean, SD
San Joaquin
n = 897
800
1000
1200
1400
1600
n = 608
1800
2000
Fig 8
YEAR CE
Fig 16
Fig 17
Fig 18
Contour intervals are in units of ecological response. Main axis units are SD’s from mean of the
variable. Scatter of dots is the set of recorded points from 90-yr weather record, large dots indicate
positive PDO and small dots negative PDO
CONCLUSIONS
Considering the four studies individually and collectively, we found that episodic, threshold, and reversible changes in subalpine ecosystems are more common responses to climate variability than
simple linear or gradual changes. Further, type conversions (meadow & other open areas converting to forest) and complex changes in population abundance, population aspect, forest species
composition, and species-specific elevation shifts are equally if not more important than simple shifts in elevation of upper treeline elevation.