GENETIC CONSEQUENCES OF
FOREST POPULATION DYNAMICS
INFLUENCED BY CLIMATIC VARIABLITY IN THE WESTERN USA
R.D. Westfall, C.I Millar and D.L. Delany
USDA Forest Service, Pacific Southwest Research Station,
Albany, California, USA
J.C. King, Lone Pine Research, Bozeman, Montana, USA
Outline of Presentation
• List known climatic cycles from the multimillennial to the
interannual.
• Give examples of climatic effects on vegetation
dynamics from the Great Basin and Sierra Nevada
• List effects of climatically-driven vegetation dynamics on:
Genetic structures
Use of genetic statistics
Scales of Climatic Variability
100,000 Year Patterns
Milankovitch
Ruddiman & Raymo, 1988
A--Adam, 1988;
B--Heusser, 1995
Century-Millennial Patterns
Bond Cycles
Bond et al., 2001
Upper forest border changes in P. albicaulis
High
Medium
Low
Interannual-Decadal Patterns
ENSO/PDO
Mantua, 2001
20th Century Responses:
Meadow Invasion
Response surface analysis of yearly recruitment into meadow vs annual PDO indices,
precipitation, and mean minimum temperature. Contour intervals are numbers of
trees. Dots indicate temperature/precipitation values for the period of record,
1900-2000 CE; gray dots indicate values positive PDO indices, black dots indicate
those at negative index values.
“This is déjà vu all over again”
- Yogi Berra
•
Western species dynamically track climate at multiple scales
•
Responses include oscillatory patterns of major range shifts, contractions,
fragmentations, expansions, changes in abundance
•
Western species do not shift dramatically in latitude, but accommodate
climate changes through shifts in population size, local-regional habitats, and
elevation
Anderson & Smith, 1994
Consequences - genetic structure
• Non-equilibrium conditions within and among populations
• The current niche spaces of species neither predict those
of past populations nor of future ones (Jackson and
Overpeck 2000; Pease et al. 1989)
• Seed dispersal is an important factor in gene flow
• Lack of isolation by distance at regional scale for neutral
markers
• Apparent lack of geographic structure at single locus basis
• May be important factor in the maintenance of quantitative
variation
“We are faced with insurmountable
opportunities”
- Walt Kelly (in Pogo)
Consequences - genetic structure
Adaptation of populations to environmental gradients
• Local populations may not be optimal (Namkoong 1969).
• Pease, Lande, & Bull (1989) predict that adaptation lags
climatic change.
• Gene flow from neighboring populations along
environmental gradients moves the local ones away from
optimal adaptation (García-Ramos and Kirkpatrick 1997).
Under changing environments, biotic communities may be
continually shifting mosaics (Pease et al. 1989)
Example: Populations are away from their optima
In common garden provenance tests, trait maxima are often in
populations lower in latitude or in elevations than that of the test
locations (reviewed in Wright 1976)
Rehfeldt, et al. (1999) examined 20-yr data from over 120
populations of Pinus contorta in 60 test plantations
At environments when a population is at it’s optimum, populations
from warmer environments were maximal
Reaction norms for height for populations growing along a temperature
gradient. Solid arrow indicates a population whose growth maximum is at about
1o C; dotted arrow indicates the population with the growth maximum for
populations growing at at that temperature, but with a growth maximum at a
warmer temperature. Adapted from Rehfeldt et al. 2001
Example: Populations are shifting mosaics
Genetic differences between krummholz and upright forms of
Pinus albicualis (Rogers et al. 1999)
• Krummholz (prostrate growth form) – at tree line,
longevity of patches (especially layered ones) in
excess of 500 years and up to 1700 years (King and
Graumlich 1998)
• Upright clumps – occupy lower elevations, those below
krummholz are young (<100 years)
• 0.052 >Fst < 0.062 between krummholz and upright
forms (Fst is ratio of the variation between populations
vs the total variation)
Conclusion: The two forms are likely to have originated from
different source populations
Upright Clumps
Krummholz Patch
Implications - on the use of genetic statistics
Not all statistics are created equal - Choice of
statistic depends upon assumptions about
population history
Examples:
Gst vs Weir’s θ
Reynold’s distance vs Nei’s distance
Conclusions
• Populations are in non-equilibrium states
• Seed dispersal is likely to be an important, sometimes
dominant, component of gene flow
• Like the Red Queen, populations of tree species are
constantly chasing optima
• Effects of climatic variability on populations affect the choice
of genetic statistics and affect their interpretation
“…For the uncertain future has yet to
come, with every possible variety of
fortune...”
- From Plutarch's “Solon”
Some Key Literature
Davis, M. B., and R. G. Shaw. 2001. Range shifts and adaptive
responses to Quaternary climate change. Science 292:673-679.
García-Ramos, G., and M. Kirkpatrick. 1997.
Genetic models of adaptation and gene flow in peripheral populations.
Evolution 51:21-28.
Jackson, S. T., and J. T. Overpeck. 2000.
Responses of plant populations and communities to environmental
changes of the late Quaternary.
Paleobiology 25:194-220.
Pease, C. M., R. Lande, and J. J. Bull. 1989.
A model of population growth, dispersal and evolution in a changing
environment.
Ecology 70:1657-1664.