Climate Change and Western Forests

Temperature Influences

Latitude, orographic effects, ocean proximity

West to East precip observations

Increase in proportion of summer precip

North to south precip observations

Decline in annual precip totals; decline in winter precip and increase in summer monsoon rainfall

North American Monsoon

Summer: descending air from the Hadley cell migrates northward, low pressure systems form over the SW where intense radiation draws air inward and locally reverses Hadley cell circulation

Low Level Jets

lower troposphere winds carry moisture from Gulf of Mexico and E Pacific Ocean to SW US

Winter Climate: NW

Frequent extra tropical cyclones from polar jet stream

Winter Climate: SW

Less organized circulation patterns characterized by high pressure over Great Basin and ENSO state

Summer Climate: NW

Cooler, drier air masses

Summer Climate: SW

Warmer wetter air masses cause thunderstorms across central rocky mountains, NAM starts in July/August

Use of SWE

Snow depth used in the 1900s, started to be used in 1970s to account for texture and packing

SNOTEL Stations

Transmit wind, temp, depth of snow, relative humidity, and solar radiation

Ablation

Direct evaporation of snow

Forest increasing effects on snow

Provide shade, reduce winds and therefore ablation

Forest decreasing effects on snow

intercept snow, canopies can heat up causing ablation,

Wildfire frequency is positively related too

Increasing temperatures, early snowmelt, length of the fire season

Effects of early snowmelt

Lack of high river flow when water demand is high, drought stress, plants start growing earlier leading to soil water depletion, positive feedbacks from reduced local evaporative cooling and reduced cloud formation, annual grasses complete life cycles sooner and dry out, woody fuels dry out earlier, dust promotes summer melt of high elevation snow/glaciers

Vapor Pressure Deficit

Difference between saturated and actual vapor pressure, “atmospheric water demand”

VPD is a product of:

Mostly air temperature but also relative humidity

How trees die

hydraulic failure, carbon starvation, pests/pathogens

Indirect drought effects on tree mortality

Amplified life cycles of pests/pathogens, more frequent/larger/more severe wildfires

Arrhenius equation approach

predicting fire via growing season max. temp, precip, and partial pressure of O2 changing w/ elevation

1800s fire management goals

Outlawing of cultural burning practices

1910s-1950s fire management goals

1911 Weeks act in response to large amounts of burning lead to complete suppression

1960s-1980s fire management goals

Multiple uses of federal land

1990s-2000s fire management goals

Restoration era and the Healthy Forests Initiative (conservative response to 60s and 80s)

2010-2024

Climate change era, socio-economic resilience

Restoration/resilience forest mgmt

Reduce forest densities, perscribed fire, could possible use levels of fire that reduce drought stress instead of 1800s forest structures