Pinus contorta, Flowery Range, 1537 m.
Low-Elevation Riparian Environments:
Warm-Climate Refugia for Conifers in the Great Basin, USA?
Constance I.
1
Millar ,
David A.
1USDA
2
Charlet
, Robert
D.
1
Westfall ,
and Diane L.
Forest Service, PSW Research Station, Albany, CA
2College of Southern Nevada, Dept. of Biological Sciences, Henderson, NV
1
Delany
BACKGROUND
HYPOTHESIS
The Great Basin, USA, contains hundreds of mountain ranges, 319 in
Nevada alone (Charlet, in review; Fig. 1). Many reach alpine elevations,
and are separated from each other by low-elevation basins currently
inhospitable to conifer growth. Montane and subalpine conifer species
occur across many ranges. These have affinities to the Sierra Nevada or
Rocky Mountains (Charlet 2007), from which conifers migrated during
cool periods of the Pleistocene. Under Holocene climates, the Great
Basin geography became a terrestrial island-archipelago, wherein
conifer populations are isolated among ranges, and inter-range
migration is highly restricted. During warm intervals of the Holocene,
conifers would be expected to have migrated upslope toward and off
mountain summits, and extirpation would be assumed to result during
continued warming. Lack of inter-mountain corridors would prevent
re-colonization even when climates ameliorated, and gradual loss of
upland conifers by the late Holocene would result. The widespread
distribution of extant conifers across many Great Basin ranges,
however, suggests a different history.
Independent patterns, repeating across multiple species’ distributions, suggest that
low-elevation environments served as climatic refugia for upland conifers during warm
periods of the Holocene, from which re-colonization within ranges was possible following
return to cool climates. We hypothesize that narrow, north-aspect ravines, which
during cool climates support persistent or seasonal streams and riparian
communities, become available as conifer habitat when warming climates desiccate
creeks and deplete riparian vegetation. We further speculate that cold-air drainage,
reduced solar insolation, lower wind exposure, and greater soil moisture in these
topographic positions enable populations of montane and subalpine conifers to
persist even during warm climate intervals when high elevations are unfavorable
for conifer persistence. On return to cool climates, low elevation refugia become
sources for recolonizing higher slopes, and/or continue to persist as relictual
populations.
We present three lines of evidence supporting this hypothesis, and speculate that
low-elevation, riparian environments might also act as climate refugia for Great
Basin conifers in the future.
1. Disjunct Distributions of Montane and Subalpine Conifers within Nevada Mountain Ranges
Patterns of within-species distributions across 23 conifers and 229 mountain ranges implicate low-elevations—and especially riparian contexts—as cool,
refugial sites (Table 1). 677 modern records document conifers occupying low elevation sites relative to their altitudinal distribution within mountain
ranges (Fig. 2); 23% of these are species that have occurrences only in low elevation contexts within a range. In the 463 cases where the low elevation of a
species in a range is known, 36% are in a riparian context. These patterns provide evidence that conifers currently find low ravines to be suitable habitat;
the presence of low-only sites further suggests that these locations might be relictual (and hence refugial) from widespread distributions at higher
elevations in former warmer times.
Table 1. Conifers of Nevada with low and high elevation
records, elevation ranges, and environmental contexts
based on low-elevation records within a range.
Fig. 1. Mountain ranges of the Great
Basin physiographic region (brown);
dots show conifer records used to assess
low-elevation occurrences within
Nevada.
A.
B.
Fig. 2. Limber pine elevation ranges in the Great Basin.
A. Montane distribution (Toiyabe Mtns). B. Low-elevation, riparian context (2097 m; Wassuk Range).
2. Temperature Patterns; Low vs. Montane Populations of Limber Pine (Pinus flexilis), Great Basin
In a focused study on limber pine (a subalpine species), temperatures measured by thermochrons (iButtons)
document warmer temperatures at low elevation riparian sites relative to uplands. However, temperatures at
low elevations are colder than expected: Differences among elevation zones in summer temperatures, especially
minimum temperatures, are much less than expected based on elevation difference (Table 2). Lapse rates
between low-elevation and upland populations corroborate this, being much smaller than regional standard
values (-6.5° C/km), and in some cases positive (Table 2). The pattern of summer hourly temperatures shows
that low-elevation sites can be as cool as uplands in the mornings, and that relatively hot temperatures at low
elevations are limited to short intervals in afternoons (Fig. 3).
Fig. 3. Representative mean summer hourly
temperatures at low-elevation and upland,
riparian limber pine locations (based on
thermochrons). Owens Gorge, 2045 m; Cell
Phone, 2985 m; Schulman Grove, 3100 m.
3. Late-Holocene Distribution Implies Recovery from Refugia: Limber Pine, Wassuk Range, NV
Abundant relict wood, scattered across slopes of the high Wassuk Range, contrasts with the present sparse
distribution of live trees, limited primarily to NE slopes and ravines (Fig 4). Tree-ring analysis indicates that
these trees grew throughout intervals of the late-Holocene (neoglacial), at altitudinal ranges similar to present
(Fig. 5). No samples were found that dated from the warm mid-Holocene, i.e., prior to 3600 years ago, despite
that sample depth was high starting in the earliest intervals. Lack of relict wood in the abundant alpine zone
implies that limber pine did not migrate upslope in the warm periods, rather that stands were extirpated across
the primary upland range. Although old wood dating prior to 3600 yrs has yet to be found in low riparian
areas (where wood preserves poorly), limber pine in those contexts could have been mid-Holocene refugia,
and served as source s for rapid re-colonization when climates ameliorated at the start of the neoglacial.
Table 2. Temperature and lapse-rate differences for low-elevation
riparian sites versus high-elevation montane sites, limber pine.
A. PRISM-based values (1970-2000 norms, Daly et al. 1994).
B. Thermochron-based values.
A.
B.
Fig. 4. Limber pine on Mt Grant, Wassuk Range.
A. Modern sparse distribution on NE slopes. B. Historic distribution (relict wood) on diverse aspects.
REFERENCES
Charlet, D. A. In review. Atlas of Nevada conifers: A
phytogeographic reference. 2nd Edition.
Charlet, D. A. 2007. Distribution patterns of Great Basin conifers:
Implications of extinction and immigration. Aliso 24:31-61.
Fig. 5. Limber pine tree-ring series, showing historic distribution across diverse
aspects of Mt Grant, Wassuk Range.
Daly, C., R.P. Neilson, & D.L. Phillips. 1994. A statisticaltopographic model for mapping climatological precipitation over
mountainous terrain. Journal of Applied Meteorology 33:140-158.
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