The Sierra Nevada Tree Mortality Event: Historical Context · Ponderosa Pine Forest Source:...

The Sierra Nevada Tree Mortality Event: Historical

Context Marc Meyer

Southern Sierra Province Ecologist USDA Forest Service, Region 5 Ecology Program

Sierra, Sequoia, and Inyo National Forests

Tree Mortality Drivers

• Loss of forest ecosystem integrity • Bark beetles • Hotter droughts

Bass Lake Recreation Area, Sierra NF

Loss of Forest Ecosystem Integrity • Removal of fire as a key ecological process • Increased forest densification • Forest structural homogenization

– Due to past logging & fire exclusion

Ililouette Basin, Yosemite NP

Altered Fire Frequency

Source: Safford and Van de Water et al. (2014)

Altered Fire Frequency

Too little fire (~84%)

Source: Safford and Van de Water et al. (2014)

Increased Densification Ponderosa Pine Forest

Source: Stephens et al. (2015) *Based on 1911 historical dataset based on 378 transects covering a large geographic extent (~10,000 ha) and over 300 ha in transect area.

8 times increase in density

Increased Densification Mixed Conifer Forest

Source: Stephens et al. (2015) *Based on 1911 historical dataset based on 378 transects covering a large geographic extent (~10,000 ha) and over 300 ha in transect area.

2.5 times increase in density

Loss of Forest Heterogeneity • Historical ICO pattern

– Individual trees – Clumps of trees – Openings

• Loss of openings and single trees over time

Sierra and Sequoia National Forest Historic Photo Archives

Bark Beetle Activity: Historic

Source: Stephens et al. (2015)

Mountain Pine Beetle activity in the Western US

*Source: Hicke et al. (2006)

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Precipitation Trends: Yosemite NP

*Source: Western Regional Climate Center

Temperature Trends: Yosemite NP

*Source: Western Regional Climate Center

Hotter Droughts • Regional warming trends resulting in more extreme

droughts – increased tree moisture stress

Source: Dobrowski, Univ. of Montana Source: US Drought Monitor

Mountain Pine Beetle activity in the Western US

*Source: Hicke et al. (2006)

Summary • Reduced forest ecosystem integrity

– Removal of fire as key process – Forest densification & homogenization

• Warming temperatures • Increased beetle activity

nancy grulke ngrulke@fs.fed.us tree ecophysiologist and director, western wildlands environmental threats assessment center pnwrs

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MODERATE PHYSIOLOGICAL DROUGHT STRESS

% OF LONG TERM AVERAGE PRECIPITATION, 1883 – 2016 BIG BEAR DAM, SAN BERNARDINO NATIONAL FOREST, CA

Last 10 yrs averages 49%, and last 18 yrs averages 62% of LT average ppt

DROUGHT

grulke et al., 2009

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% of average ppt vs. and BB outbreaks

running 2 yrs running 3 yrs running 4 yrs

1. CLOSED FOREST CANOPY (DENUDED BY LATE 1800’S) 2. POST-WWII INDUSTRIALIZATION AND POPULATION EXPLOSION IN LA (UPWIND) >> AIR POLLUTION 3. 4 YRS OF MODERATE DROUGHT 4. SUBSEQUENT BARK BEETLE OUTBREAK 5. MANY SUBSEQUENT OUTBREAKS, THEN 100+ MILLION TREES IN 201

preisler et al., 2017; grulke et al., 2009

^ BB

Flat terrain- trees have access to upper soil horizons, plus

cracks in weathered bedrock >> reduce stand density: more water for each tree, lower

tree drought stress, less susceptibility to pests/paths, lower probably for outbreaks, and increase water output in watershed

>>in a severe drought, the whole stand will likely be susceptible

Sloped terrain- shallow upper soil horizons; trees may have access to springs which increases within-stand differences in tree drought stress

>> reduce stand density: may be more water availability for some, but not all trees; increase water output in watershed

>>in a severe drought, not all trees will be susceptible >> in an outbreak, most trees will still be attacked, with

differing capacity to resist attack differing capacities include ability to find soil moisture,

capacity to control plant water loss, resin production, and resin quality

sequoia nat’l park: grulke et al., 2003a; 2003b;

slow: drought stress reduces transpiration such that insufficient CO2 is absorbed by the leaf, there is less sugar production and lower storage of carbohydrates in bole and roots. eventually there are insufficient carbs to permit above-ground growth, and then root growth to continue to explore soil horizons for water (and nutrients). tree dies.

faster: some trees have slower stomatal closure in response to drought, or O3 induces sluggish stomatal closure, or O3 induces nighttime transpirational loss, such that transpiration losses exceed water transport rates from roots thru bole xylem to leaf. water column breaks (cavitates), and less and less conductive surface is available to transport water to transpiring leaves. Less carbon is absorbed… (same process as above). This can happen within growing season (2002 in eastern SBNF in PIJE; recently on western and eastern slopes of SN)

R² = 0.6203

R² = 0.4117

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Log. (NL, dense)

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INFLECTION POINT

Data from thinned and dense stands in San Bernardino, Sequoia, Inyo, Tahoe, and

Lower needle elongation growth Lower canopy primary branch excision Growing point dieback Whole tree desiccation Within whorl needle excision Photosynthetic pigment oxidation

FIELD ID OF TREE DROUGHT STRESS:

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Christopher J. Fettig

Pacific Southwest Research Station USDA Forest Service

Drought & Tree Mortality in the Sierra Nevada – Considerations of the Past and Expectations for the Future

July 2017

Which environmental conditions and actions contributed to tree mortality

and associated resource effects?

• Environmental conditions – Low precipitation – High temperatures – Forest densification – “Aggressive” bark beetle fauna

• Actions – CO2 emissions – Reduction in harvests/thinning;

highly effective fire suppression

– Other past management actions

Precipitation

The state of California ended five years of consecutive drought in 2017. Water Year 2015 (ended 30 September 2015) was the hottest and driest on record. 2014 was the third driest and second warmest. Cause―a persistent ridge of high pressure over the northern Pacific that blocked winter storms from reaching California.

Warming is estimated to account for 8–27% of the observed anomaly in 2012–2013 and 5–18% in 2014.

Warming

When soil moisture is limited trees close their stomata to avoid excessive water loss, which inherently leads to reduced productivity as stomatal closure also prohibits uptake of carbon dioxide and therefore Ps.

Carbon starvation occurs when drought duration is long enough to curtail Ps longer than the equivalent storage of carbon reserves for maintenance of metabolism. Hydraulic failure occurs if drought intensity is sufficient to push a plant past its threshold for irreversible desiccation before carbon starvation occurs.

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Bark beetles

Bark beetles

• 550 species in North America.

• Relatively few are economically important.

• Regulate certain aspects of primary production, nutrient cycling, ecological succession, and the size, distribution and abundance of forest trees.

Deceased mountain pine beetle adult in parental gallery.

Bark beetles and forest densification - Growing space

• Individual trees utilize growth factors (i.e., sunlight, water, nutrients, temperature, oxygen, carbon dioxide, etc.) until one or more factors become limiting.

• A forest contains a certain amount of intangible growing space, which varies spatially and temporally.

• As growing space diminishes, a tree’s photosynthates are allocated to different uses in an order of priorities.

Ps allocations

• maintenance respiration (metabolism) • production of fine roots • reproduction • primary (height) growth • xylem (diameter) growth • insect and disease resistance mechanisms

adapted from Oliver and Larson (1996), Fettig et al. (2007)

Mass attack • A critical minimum number of

beetles is required to overcome defenses (Wood 1972, Hodges et al. 1979, 1985; Raffa et al. 1993), which varies with changes in host vigor.

• Several bark beetle species (e.g., Ips spp.) preferentially attack logs, slash, or dead and dying trees. In these cases, little or no host resistance is encountered.

• All other things equal: - Less competition among trees =

higher individual tree vigor = more beetles required to kill the tree

Western pine beetle females fighting (above) and encapsulated (below) in resin produced by vigorous trees.

Hayes et al. 2009. J. Econ. Entomol. 102:2170 − 2182.

Stand density and host abundance are consistently linked with the occurrence and severity of bark beetle infestations.

Meta-analysis of stand density index threshold

relationship with Dendroctonus spp. attack in yellow pine forests across the western U.S.

Drought exerts a downward force on these relationships

Thinning guidelines (e.g., 230 SDI; Oliver 1995 GTR-RM-267) derived under a different climate signal and bark beetle pressure index are insufficient today.

Christopher J. Fettig cfettig@fs.fed.us

530-759-1708

• Bentz, B.J., J. Régnière, C.J. Fettig et al.. 2010. Climate change and bark beetles of the western United States and Canada: Direct and indirect effects. Bioscience 60:602−613.

• Fettig, C.J. 2016. Native bark beetles and wood borers in Mediterranean forests of California. In Lieutier, F. and T.D. Paine, eds. Insects and Diseases of Mediterranean Forest Systems. Springer International Publishing, Switzerland, p. 499–528.

• Fettig, C.J., M.L. Reid, B.J. Bentz et al. 2013. Changing climates, changing forests: A western North American perspective. Journal of Forestry 111:214–228

• Kolb, T.E., C.J. Fettig, M.P. Ayres et al. 2016. Observed and anticipated impacts of drought on forests insects and diseases in the United States. Forest Ecology and Management 380:321–334.

California’s Tree Mortality: Lessons from the Intermountain West Dr. Jodi N. Axelson

Cooperative Extension Specialist

Context

McDowell, N., D. Beerling, D. Breshears, R. Fisher, K. Raffa, M. Stitt. 2011. The Interdependence of Mechanisms Underlying Climate-Driven Vegetation Mortality. Trends in Ecology and Evolution, 26: 523–532

Hicke, J, A. Meddens, C. Kolden. 2016. Recent Tree Morality in the Western United States by Bark Beetles and Forest Fires. Forest Science, 62: 141-153

Life After Beetle

Regeneration of pine Regeneration of shade tolerant species

However…

Shrubs, shrubs, shrubs → different outcomes

In California….

Shrubs, shrubs, shrubs → different outcomes

The Future

Questions/Unknowns

• In California’s Mediterranean climate will shrubs usually win?

• Will ponderosa pine be lost at lower elevations?

• Will pine species that died naturally regenerate?

Climate change, changes to disturbances regimes, species invasions, (…) all pose significant challenges for maintaining resilient & healthy forests

Expect the Unexpected

A Solution…

• Science → Management (aka Action) → Policy:

– Relationships & Trust – Iterative – Multi-disciplinary