The Importance of Aspen
Sierra aspen are hot spots for biodiversity, provide critical habitat for native birds and are known for their beautiful fall color and the fluttering of their leaves on warm summer days. As SYRCL continues to work with the Tahoe National Forest on developing comprehensive meadow restoration plans, we focus on hands-on restoration activities directed at the enhancement of Sierra aspen stands by removing encroaching conifers. With altered fire regimes and hydrology, and a changing climate, conifers have begun to intrude on aspen stands and are outcompeting aspen for sunlight and water. Unfortunately, more than 96% of historic aspen stands have been lost due to fire suppression, conifer encroachment and other factors. SYRCL has completed several aspen regeneration projects and continues this work in the meadows of the Yuba watershed.
- Aspen typically live about 150 years.
- Aspen reproduce primarily from asexual root sprouting.
- Each colony is its own clone, and all trees in the clone have identical genetics, characteristics and share a single root structure.
- Aspen stands support more species than surrounding conifer vegetation types.
- It may take 3-4 years before aspen will sprout in a stand where conifers have been removed.
Aspens are deciduous trees, which means that they loose their leaves in the winter. They are also hardwoods, unlike the conifers (firs and pines) in the rest of the landscape.
Western Sierra aspen are unique because all of the stands are seral. This means that the stands are not stable and will be lost if there is no disturbance event, such as a fire, in the stand to remove conifers. Aspen as shade intolerant species, so they need ample sunlight to grow. When this sunlight is not available, aspen will not produce suckers. Western Sierra aspen are in what is called the Upper Montane Zone. Aspen have a specific soil requirement for growth. The soil must retain moisture for a good part of the year, which is why we see aspen on the fringes of meadows, or in riparian areas. Sometimes you can see aspen stands high up on ridges, particularly on the northern side, and these aspen stands are able to survive due to the snow that can stay there for longer.
Another way aspen are unique is how they regenerate. Aspen primarily regenerate clonally through a shared root system. The younger ‘suckers’ (ramets) will grow from roots of the adult aspen, and will use the older aspen for resources until they can produce enough food for themselves. Aspen can reproduce sexually, but these tend to be rarer events. Aspen are also dioicous, meaning that the aspen are single-sexed, somewhat unique among plants. These seeding events are thought to be triggered by fire or particulate matter in the air (as from smoke). Aspen are triggered into producing suckers by the lack of auxin. So, adult aspen will produce auxin, which inhibits the growth of the ramets from the roots. When the adult aspen dies or is in distress, the auxin is no longer produce, and the ramets can grow as there is not auxin to prevent the growth of suckers. This is why aspen need disturbance events to grow, with their sunlight demands and the need for young suckers to replace the older trees, aspen are in this disturbance cycle for the stand to continue. Without these disturbance events, aspen stands will move away from the successional phase they are in and progress into a conifer forest. This is also why it can be difficult to age an aspen stand; aspen stands continually replace their trees, and therefore even if a scientist ages the largest tree present, the stand could be much older, some scientists think even 1000 years old. This is a bit like the thought experiment of the ship of Theseus, is a ship still the same ship if it is slowly replaced and traces of the original are no longer present?
Aspen are thought to have first come to North America at the end of the last Ice Age 8,000 years ago as the glaciers receded. The Receding glaciers left large deposits of sediment, pushed down the mountains by the glaciers. This soil is nutrient rich (check) and has a higher water retention rate. The soil from the moraines is also flatter, due to the glacial carving in the mountainous landscape. As a primary succession species, aspen were most likely the first colonizers to grow post Ice Age.
After the Ice Age, aspen stands were impacted by the Native American forestry management strategies. The Native Americans practiced promoting and starting small fires to remove the undergrowth in forests to create a more open landscape. We are not sure how much of this occurred at this elevation, but fires would have occurred higher in the Sierra through lightening strikes.
Beginning with the Gold Rush, the Sierra had serious impacts from European-American settlers within the Yuba Watershed. Aspen were impacted by the clearing of forests, high levels of erosion due to hydraulic mining, and stream/water diversions. Surprisingly, these events are disturbance events, and most likely promoted the growth of aspen stands due to the removal of vegetation. However, erosion and stream diversion would have negatively impacted aspens due to the changing of their water sources, flooding of stands/roots, and increased sediment loads burying root systems. Later in the 19th century the railway construction led to increased timber harvesting, and post-harvest burning. Sheep herding was very popular in the high Sierra, and the increased grazing by sheep would have been bad for the aspen stands, but the small burns the herders conducted at the end of the grazing season would have been helpful.
Later in the 20th century, the United States Forest Service took over management of much of the forested land in the Sierra. For many reasons, the USFS instigated a fire suppression policy, and this led to crowded conifer stands, but also removed the primary disturbance factor that allows aspen to regenerate within the Western Sierra landscape. Coupled with the increased infrastructure in the mountains and continued livestock grazing, aspen stands have been in decline. Roadways and towns like to be near waterways and on flat land, as do aspen. If you drive through Truckee you can see how the aspens there are inundated with the town itself.
In the present day, the USFS is trying to undo much of the damage caused by the 100 years of fire suppression, along with a warming climate. The aspen now face problems of conifer encroachment, livestock grazing, and damage from human recreation.
To reduce the impact from ungulates (hooved animals), you can either fence the aspen stands or not allow grazing in the meadows. There are variety of fencing types which can be used. Cattle fencing will not deter deer from entering the stand, so the deer will still benefit from the aspen stands, but will then still browse on the young aspen. Fencing must also be placed in the spring and removed in the fall so that the snow does not damage the fences. For natural fencing, trees can be what is called jackstrawed, which means they are split and placed as a natural barrier around the stand. This reduces the number of ungulates who can access the stand, but does leave fuels.
Prescribed burns are ideal for promoting aspen growth. They will increase the soil temperature, and expose the soil for aspen to grow. This will clear the vegetation within and around the stand, reducing competition.
The removal of conifers both within and around the stand is necessary for aspen stand longevity. The conifers slowly encroach within the aspen as they are more shade tolerant. So the idea here is to remove conifers which provide seeding opportunities within or around the stand, and to also remove young conifer saplings or seedlings within the sand until the aspen suckers are at recruitment height. Conifers can also be girdled, which is the wrapping of a wire around an tree to kill it slowly, and then leave a snag on the landscape, which provides habitat for many different animals, especially birds.
Coppice is a more extreme form of aspen restoration treatment. This is the clear cutting of conifers and aspen, with the idea that the aspen will grow back faster than the conifers, and are stimulated to produce suckers due to the cutting of the mature trees
There are also other indirect restoration efforts. Meadow restoration in general is good for aspen as this removes the deep water channels, and conifers that are on the fringes of the aspen stands. Riparian restoration will also help aspen, as well as fuel reduction management, as this is another form of conifer removal.
SYRCL is currently in the planning portion of the aspen restoration project in order to get it to be “shovel ready”.
Volunteers & Aspen
Since 2011, SYRCL has partnered with the Tahoe National Forest to work with volunteers and remove encroaching conifers from aspen tree patches that were struggling for light. These efforts are ongoing and projects to monitor success and continue to remove conifers are active at Rucker Lake, Pierce Wetlands, Loney Meadows, and Butcher Ranch. Check out this video produced by Tony Loro highlighting this work.
In the last few years volunteers have primarily been working to put up and take down cattle fencing around aspen stands.
Funding for our aspen regeneration work has been provided by the Wildlife Conservation Board, the Nevada County RAC, Sierra Nevada Conservancy, Earthwatch, and the California Department of Water Resources through the Cosumnes American Bear Yuba Integrated Regional Water Management Group.