Posts Tagged: Sierra Nevada
This winter, a generous amount of rain and snow has fallen on California, but it can't erase the brown swaths of dead and dying trees in the Sierra Nevada caused by five years of drought and decades of forest mismanagement.
Fire suppression and the harvest of the largest and most resilient trees in the forest led to a large population of weak trees. The prolonged drought further weakened the trees' defenses against native insects. Aerial detection surveys show that more than 102 million trees have died since 2010; more than 62 million died in 2016 alone. Public and private landowners are now struggling to recover from this natural disaster.
UC Cooperative Extension forester and natural resources advisor Susie Kocher recommends dead trees be removed and the landscape reevaluated.
“The dead trees will eventually fall,” Kocher said. “Removing trees around homes and other buildings is especially important for safety. Also, when they fall on the ground they become large fuels on the forest floor, leading to more intense fires.”
The cost of removing the trees can be substantial. The State of California is funneling disaster relief funds through California counties, utilities are felling trees that pose a threat to power lines, and local jurisdictions are removing trees that could fall on roads and other public infrastructure. However, most tree removal is the responsibility of private landowners.
When the dead trees are gone, before considering replanting, Kocher suggests Sierra residents carefully assess what has survived.
“There is often a lot of live vegetation remaining,” Kocher said. “Make a map and mark where you find living trees and shrubs and identify them by species and size. If you have a significant number of trees left, you may not need to replant.”
Kocher suggests nurturing the remaining young trees.
“You may want to thin trees out so that available sun and soil moisture are focused on the healthiest individuals. Some watering in the summer may help counter stress caused by increased solar radiation,” she said.
If removing the dead trees leaves the landscape too bare, replanting native conifers is a good strategy. Conifers include pine, cedar and fir trees, but in California's dense forests, firs and cedars – which do well in shady conditions – are beginning to dominate. Replanting may be a time to give native pines – such as Jeffrey, ponderosa and sugar pines – a chance to recover ground.
“The fact that many pines have died does not necessarily mean they are no longer adapted to your location, even with our warming climate,” Kocher said. “There may be a few locations that are less suitable for trees that have grown there in the past, but for most areas, local growing conditions should support native conifers in the near future.”
Native plants and shrubs that died during the drought or were damaged during tree removal will likely come back on their own without replanting. Shrubs and oaks can re-sprout and native herbaceous plants generally store seed in the soil that will grow under native rainfall conditions.
Replanting of trees also gives landowners the chance to shape the landscape for best effect. Kocher offers the following recommendations on replanting trees in natural landscapes:
- Space trees at least 10 feet apart.
- Trees and flammable vegetation should be kept at least 10 feet away from the home, planted sparsely within 30 feet of the home and spaced widely enough in the 30 to 100-food zone so the crowns of the trees will not touch when they are mature. Beyond 100 feet, trees can fill into a more natural looking forest.
- Plant trees at least 10 feet from power lines.
- Do not plant trees within the road right of way to prevent interference with snow clearance, maintenance and construction projects.
- Plant pines where there is a lot of sun. Do not plant sugar pine on the driest sites.
- Avoid planting where the mature trees will block desired views.
The fourth winter in a row of disappointing precipitation has triggered a die off of trees in the Sierra Nevada, most of which is now in ‘exceptional drought' status. The US Forest Service conducted aerial monitoring surveys by airplane in April 2015 and observed a large increase in tree mortality in the Southern Sierra (from Sonora south). Surveyors flew over 4.1 million acres of public and private forest land and found that about 20 percent had tree mortality on it, totaling over 10 million dead trees.
The Forest Service found severe mortality in many pine species especially ponderosa pine. On private lands along the foothills of the Sierras, surveyors found extensive areas of dead pines. Large areas of blue and live oak mortality were also suspected though it was too early in the season to be sure.
The insects killing trees in the Sierra are all native insects that are multiplying because of drought conditions. Native insects are a necessary part of the forest ecosystem that speed decay of wood back into nutrients, prey on other insects, and provide food for wildlife. They are normally present at low levels and cause tree mortality only in localized areas.
However, drought weakens trees and reduces their ability to withstand insect attacks. Normally trees use pitch to expel beetles that attempt to burrow into the tree through the bark. Weakened trees cannot produce the pitch needed to repel these beetles which are able to enter under the bark and lay eggs. Larvae feed on a tree's inner bark cutting off the tree's ability to transport nutrients and eventually kill it. Attacking beetles release chemicals called pheromones that attract other beetles until a mass attack overcomes the tree. Many beetles also carry fungi that weaken the tree's defenses.
Western pine beetle is one of the main culprits killing pines in the Sierra during this drought. It is a bark beetle, one of a genus of beetles named Dendroctonus which literally means ‘tree-killer'. Adult beetles are dark brown and about a quarter-inch long. Adults bore into ponderosa pines, lay eggs which develop into larvae in the inner bark then complete development in the outer bark. When beetle populations are high, such as during drought periods, even healthy trees may not be able to produce enough pitch to ward off hundreds of beetle attacks.
Western pine beetle often attacks in conjunction with other insects. Other beetles causing tree mortality in Sierra forests include mountain pine beetle, red turpentine beetle, Jeffrey pine beetle, engraver beetles (Ips) and fir engravers. Forests with a higher diversity of tree species are typically less affected because beetles often have a preference for specific tree species. Some species may attack only one tree type. For example Jeffrey pine beetles attack only Jeffrey pine.
The best defense against bark beetles is to keep trees healthy so they are able fight off insects themselves. Widely spaced trees are typically less susceptible to successful attack by bark beetles since they face less competition for moisture, light, and nutrients compared to densely growing and overcrowded trees. Forest health can be promoted by thinning to reduce overcrowding (so each tree has access to more resources) and removing high risk trees during thinning (such as those that are suppressed or unhealthy).
For landscape trees of high value close to a home, watering may be one option to increase tree vigor against bark beetle attacks. Apply about 10 gallons of water for each inch of tree diameter (measured at chest height) around the dripline of the tree once or several times a month during dry weather.
There are some insecticides registered for bark beetle control, but all are preventative only. Carbaryl may prevent attack for up to two years, while pyrethroids can deter attack for up to a year. Spraying can be tricky because the chemical must be applied up to 50 feet up the trunk of the tree usually while standing on the ground. Since misapplication may have toxic consequences, any insecticide must be administered by a licensed pesticide applicator. All applications must follow the label. Though some systemic treatments applied to the soil or inserted into the tree may work in some cases, there is not a lot of documented evidence that they are effective against western pine beetle. No insecticide can prevent tree death once a tree has been successfully attacked.
Author: Susie Kocher, UC Agriculture and Natural Resources Cooperative Extension advisor
Since 2006, a team of University of California Agriculture and Natural Resources scientists has been studying the effects of vegetation management in the Sierra Nevada forest on fire behavior, forest health, water quality and quantity, the Pacific fisher (a small mammal in the weasel family) and the California spotted owl. The researchers are writing up their final reports and seeking public feedback on their recommendations and next steps in the process.
On Wednesday, May 27, community members are invited to discuss the recommendations with the Sierra Nevada Adaptive Management Project (SNAMP) team at an all-day meeting in the Sacramento area.
“Although adaptive management as a theory of practice in resource management has been in the literature for decades, few studies have been done to truly apply theory to actual practice,” said Susie Kocher, a UC ANR Cooperative Extension forestry and natural resources advisor for the Central Sierra area.
The US Forest Service's 2004 Sierra Nevada Forest Plan Amendment calls for managing the 11 national forests in the Sierra Nevada using the best information available to protect forests and homes. SNAMP is designed to provide resource managers with research-based information for making forest management decisions.
The SNAMP meeting will be held 9 a.m. to 4 p.m. on May 27 at the Wildland Fire Training Center, 3237 Peacekeeper Way in McClellan (near McClellan Airfield outside Sacramento).
To attend, please register at http://ucanr.edu/snamp2015annualmeeting by Sunday, May 24. Registration is free.
For more information about the project, visit http://snamp.cnr.berkeley.edu. The final SNAMP report will be available for download at http://snamp.cnr.berkeley.edu/snamp-final-report. Comments will be accepted online at http://ucanr.edu/snampreportcomments until July 15.
Besides their beauty, aspen groves are valued for providing myriad ecological services. The groves add species and landscape diversity to the Sierra Nevada. They offer higher water retention than the adjoining conifer forest, plus wildlife habitat and forage for livestock and wildlife.
Aspens are native to the Sierra Nevada and other areas with cool summer temperatures. Groves are typically clonal colonies that grew from a single seedling and spread when roots come to the surface and start a new tree. Each tree lives 40 to 150 years above the ground, but the root system can be much older; one colony in Utah is said to be 80,000 years old.
In the Sierra Nevada, aspen stands have been declining in vigor and sometimes died because of pressures from encroaching conifers, according to Malcolm North, research scientist with the U.S. Forest Service's Pacific Southwest Research Station in Davis.
Aspens thrive in bright sunlight. Without regular fires, conifers eventually shade them out. More than 90 percent of aspen stands in forested areas of California have some conifer shading. Removing conifers with mechanical equipment to increase sunlight in aspen groves is an effective technique to restore stands, but land managers have been reluctant to thin out conifers because of concern about impacts on nearby mountain streams.
A recent collaborative research project involving UC Cooperative Extension, UC Davis and the U.S. Forest Service confirmed that conifer removal to restore aspen stands can be conducted without degrading aquatic ecosystems.
“This project is an excellent example of the value of collaboration between the university, U.S. Forest Service, and stakeholders to integrate best management practices and best available science for the restoration of critical habitats in our national forests,” Tate said.
The scientists evaluated two aspen restoration projects next to mountain streams in the Lassen National Forest. Prior to conifer tree removal, they evaluated stream temperature, water quality, stream shade, overstory tree canopy cover, aquatic insects, soil compaction and soil moisture. Followup data collection took place two to seven years after the conifers were removed from the stands.
Water quality did not change following the timber harvest. Removing conifers reduced canopy cover, which resulted in an increase of sunlight on the stream, but there was no corresponding increase in water temperature. Soil moisture increased in stands where conifers were removed, compared to aspen stands that were untreated.
“It is a real win-win when we can use contemporary timber management strategies to restore aspen in the absence of a natural fire regime, and also safeguard the health of our aquatic ecosystems,” Tate said.
An aspen stand released from conifer encroachment. The fallen conifers are in the foreground.
“The fire interval is completely out of whack compared to pre-settlement conditions,” Kocher said.
In a historical, natural and healthy fire regime, nearly half of Sierra forests would experience fire every 12 years and three-quarters would burn every 20 years. However, only 0.2 percent of Sierra forest land has burned repeatedly at least every 20 years in modern times, while 74 percent has not had a single wildfire or prescribed burn in the last 103 years.
These conclusions are part of a report Kocher produced for the Sierra Nevada Conservancy, a state agency created by the California legislature in 2004 with the understanding that the environmental, economic and social well-being of the Sierra Nevada and local communities would benefit from an organization providing strategic direction.
SNC commissioned reports about Sierra Nevada indicators, such as Demographics and the Economy, Land Conserved and Habitat, and Water and Air Quality and Climate. Kocher’s report is titled Forest Health and Carbon Storage. Future system indicator reports will cover Fire Threat and Agricultural Lands and Ranches.
Forest health is a particularly difficult indicator to frame and quantify as there is no consensus on exactly what defines forest health, Kocher writes in the introduction.
“Forest health is a concept that is commonly used but not well defined,” Kocher said. “You’d think you’d be able to answer how healthy a forest is in a straight forward way. But you have to look at stressors, the natural processes that are happening in a forest. What does the landscape look like?”
Because forest health cannot be characterized by any single, simple measure, Kocher selected three data sets to provide an indication of the health of forests in the region: fire return interval, wildfire threat to ecosystems and forest pest impact and threat. She found that fire return interval told the most provocative story.
Fire return interval tracks the frequency with which wildfire revisits the same land over and over. Kocher compared the frequency of fire return between the pre-settlement “natural” state and modern times.
The pre-settlement or “reference” fire return interval is an estimate of how often, on average, a given forest type likely burned in the three or four centuries prior to Euro-American settlement in the middle of the 19th century. As such, the reference state includes fires deliberately set by Native Americans to manage forest vegetation and wildlife.
Researchers determined fire frequency by analyzing fire scars in tree rings of live and dead trees. Results show that forested areas previously burned every 11 years on average in the warmer and drier lower elevation forests such as ponderosa pine. On the other end of the spectrum, fire occurred only every 133 years for sub-alpine forests where it takes much longer for fuels to accumulate and dry.
Fire suppression in the last 100 years has left much of the Sierra forest land vulnerable to high severity wildfire, Kocher wrote in her report. High severity wildfire across wide areas poses a threat to forest plant and animal life, to forest communities and watershed function, particularly in the northern half of the Sierra Nevada Conservancy Region.
Kocher’s report also reviewed carbon storage in Sierra Nevada forests. Since carbon in the atmosphere is leading to global climate change, there is increased interest in storing carbon in forests.
“It could be said that by suppressing fire, we have sequestered more carbon,” Kocher said. “In an overstocked forest, there are more standing trees and more fuel on the ground. Storing more carbon may help in the short run, but in the long term, your stand has a higher risk of burning up. That puts a lot of carbon into the atmosphere.”
Forest thinning improves the stability of carbon storage in the long-term. Whether it reduces the amount of carbon stored depends on what factors are taken into consideration, Kocher suggests. Part of the complexity of the equation stems from whether or not forest products removed from the forest are factored in. Most studies that account for the carbon sequestered by lumber and other products removed during thinning find that thinning both increases the quantity and stability of carbon sequestered by forests over time.