Complex Responses of Subalpine Forests to Climate Change Connie Millar, Robert Westfall, and Diane Delany USDA Forest Service Sierra Nevada Research Center Albany & Lee Vining, CA Two Lines of Active Research 1. Species & Communities Move Uphill in Response to Warming Standard lapse-rates (e.g., -6.5°C/km) incorporated into regional modeling & conservation assessments Hayhoe et al. 2004 PNAS 101: 12422-12427 California Climate Change Center. 2006. Our Changing Climate; Assessing the Risks to California Projected losses of 70-95% ASF A Classic Interpretation Also in Paleoecology 1967 Thompson 1988 Western North America Vegetation History K Cole Dots = LGM Lines = Current 2. Complex Mountain Climatology Cold-air pooling & inversions are common, and define widely varying lapse rates from positive to negative July Atmospheric conditions for these conditions are projected to increase in the future and may overwhelm changes due to warming Cascade Watersheds, Oregon Daly 2008 Cold-Air Pooling Studies, Lundquist 2008 Loch Vale Watershed, CO Studies on temperature inversions in the White Mtns, Chris van de Ven, Stu Weiss, 2008 Uphill migration (shift in treeline) is not the only or typical response to warming climates… Species in the alpine and subalpine zone respond to climate complexly and individualistically I. FOREST DENSIFICATION (without change in treeline elevation) A. Treeline Zone Infilling Whitebark pine Kuna Crest • Deadwood scattered within live zone but not higher • Abundance changes over time; currently recruits are increasing within upper deadwood zone • Krummholz individuals persist by vegetative layering King & Graumlich 1999, Rogers, Millar, Westfall, 1999 I. FOREST DENSIFICATION (without change in treeline elevation) B. General Subalpine Forest Infilling 1907 Gaylor Pk -- Tioga Pass Vale & Vale 1984 1984 Vale & Vale 1994 I. FOREST DENSIFICATION (without change in treeline elevation) C. Colonization of Formerly Persistent Snowfields Trees Colonizing Snowfields Primary response is to temperature Mean & SD T min Millar et al. 2004 I. FOREST DENSIFICATION (without change in treeline elevation) D. Colonization of Subalpine Meadows Trees in Mdws Mean & SD Correlation with Negative PDO and Low PDSI PDO/ PDSI Tmin Precip/ Streamflow Complex interactions between temperature, precipitation, & interannual variability Millar et al. 2004 Warren Bench, 2005 Meadow colonization is readily reversible Horse Mdws, 2006 Warren Bench, 2006 II. CHANGE IN FORM & GROWTH (without change in treeline elevation) krummholz flags & skirts Whitebark pine, a sublimely plastic subalpine species upright trees Flags Mean & SD PDO/ PDSI Development of Flags Responds to Temperature and PDO As with meadow colonization, flag response is reversible Tmin Precip/ Streamflow Millar et al. 2004 Stem Growth in Krummholz Whitebark Pine at Treeline Doubled over the 20th Century 1907 Stem Growth Mean & SD 1984 Parker Pass, Kuna Pk, YNP Vale & Vale. 1994 Millar et al. 2004, Rogers, Millar, Westfall 1999 III. CHANGE IN PATTERNS OF MORTALITY (without change in treeline elevation) A. Change in Drought and Insect & Disease Effects Mortality event in subalpine limber pine Mt Warren, Mono Basin 1988-1994 drought Death Dates 3 Sites Tmin/ Tmax + elevated temperatures + mountain pine beetle + mistletoe infection + blue-stain fungus = “Global-Warming Style Drought” Mortality Event Precip Millar et al. 2007 III. CHANGE IN PATTERNS OF MORTALITY (without change in treeline elevation) B. Change in Genetic Diversity & Adaptation Millar et al. 2007 Westfall & Millar 2006 Live trees remain Typical sparse old-growth stand Whitebark Pine Mortality Event 2008 …differential growth response as in limber pine? Owens River Headwaters Rock Creek 2008 field season III. CHANGE IN PATTERNS OF MORTALITY (without change in treeline elevation) C. Change in Subalpine Fire Relationships Cascade Crest Complex, OR, 2006 High elevation pine & fir forests of western NA IV. CHANGE IN ASPECT (without change in treeline elevation) Limber pine, Great Basin Sparse live stands now only N/NE aspects Mt Grant, Wassuk Range Deadwood on all aspects Ages of individual limber pine tree-ring series, Wassuk Range, NV NE N Millar, Westfall, King, unpub Forest Presence & Absence Correlated to Wet vs Dry Periods; Upper & Lower Forest-Border Elevations Didn’t Change Significantly Over ~3600 yrs Mean Elevation of Forest Borders Live Forest: Upper 3222m Lower 2721m Deadwood: Upper 3215m Lower 2715m V. CHANGE IN ELEVATION A. Differential Shifts in Elevation – Individualistic Response Example 1: Whitewing Mtn and San Joaquin Mtn Deadwood Stem Dates 821-1350 CE Millar et al. 2006 Deadwood Species 42 Whitebark Pine krumm 13 W White Pine ↓250 m 7 Lodgepole Pine ↓250 m 5 Jeffrey Pine ↓500 m 6 Mountain Hemlock ↓250 m 5 Sugar Pine ↓600 m & W SN Differences Modeled between Whitewing Mtn Paleo- and Current Climates Medieval: Ann ppt -24 mm Ann min T +3.2°C Ann max T +2.3°C V. CHANGE IN ELEVATION A. Differential Shifts in Elevation Example 2: Leapfrog Recruitment of Limber Pine vs Bristlecone Pine in the White Mtns, CA 3570 m * 300 m above current limber-pine treeline * Dolomitic soils * No or few live bristlecone pine * Area of historic bristlecone pine Millar et al., in prep Highest Elevation Sites, High Densities Limber Pine, Episodic Recruitment Correlate with: -- Annual Tmin, Tmax -- May & June Tmin -- Sept Precipitation -- Atlantic Multidecadal Oscillation Pulse ~ 1970-1991 V. CHANGE IN ELEVATION B. Shifts Down in Elevation with Warming and Drying Limber Pine is recruiting below its lower forest border in north ravines & riparian conditions High-Elevation Forests Respond Complexly to Climate 1. Increases in Forest Density 2. Changes in Growth & Form 3. Changes in Mortality & Genetic Diversity 4. Shifts in Aspect 5. Differential Shifts Up & Down in Elevation Simple movements uphill are inadequate interpretations of forest response to warming Conservation & Adaptation Lodgepole pine – really at risk? Implications Hannah 2006 * Future ranges – Moving or not? Which direction? * Refugia -- Where? How abundant? * Individualistic responses – Novel community compositions? * Assisted migration? * Germplasm diversity choices? * Triage decisions? * Corridors? Millar & Brubaker 2006 Millar, Stephenson, & Stephens 2007 Science 2008