Forest structure in unroaded old-growth: understanding the influence of soils on variability of long-term vegetation dynamics and fire history
Forests are often perceived by the public to be static landscape features and disturbances that cause changes to forest ecosystems (e.g., fire, pine or spruce beetle outbreaks) are perceived as deleterious instead of natural components of a dynamic landscape. In order to encourage a public understanding of the dynamic nature of ecosystems and to enhance resiliency, diversity, and productivity of forest ecosystems, it is essential to encourage collaborative, science-based restoration of forest landscapes. The USDA’s Collaborative Forest Landscape Restoration Program provides guidelines for community stakeholders and private advocacy groups to engage with federal, state, and local agency stewards in the development of monitoring and assessment goals for forest restoration projects. Through coordination with the Uncompahgre Plateau Collaborative Restoration Project, my research will focus on unique unroaded old-growth stands with three primary objectives. Objective 1) map forest types and develop associations between soils and vegetation; Objective 2) develop a detailed fire history; and Objective 3) reconstruct historic stand structure based on extrapolation from the fire history and intensive aging of living trees, snags, and logs. Soil characteristics (particularly rock cover) strongly influence the development of forest canopies which influence fire regimes and tree survival. I anticipate the oldest trees will occur on shallow, rocky soils where either fire return intervals were longer or fire severity was lower. In contrast, I expect deeper, less rocky soils to host vegetation conducive to fuller canopies with greater risk of stand-replacing fire behavior. These sorts of interactions between soils, vegetation, and fire may be important in weaving the tapestry of the overall landscape-scale ecology of these forests. These insights will assist landscape scale treatments designed to restore ecosystem structure, composition, and function while reintegrating and managing wildfire as a natural landscape scale component to reduce the risk of unnaturally severe crown fires.
The unroaded areas of the Uncompahgre Plateau (USFS stand numbers 7703010135, 7702020137, and 7703010140) offer an opportunity for in-depth analysis of changes in stand composition over time and in relation to differences in soils. Species composition and tree ages likely differ between shallower soils (with veteran ponderosa pines and Douglas-fir legacy trees) and deeper soils (with more spruce, fuller canopy, and greater risk of stand-replacing fire). This investigation into long-term forest patterns includes three foundational objectives.
Objective 1) Mapping forest types and developing associations between soils and vegetation. This objective encompasses two separate questions regarding soil and vegetation association. First, is there an association between soils and vegetation type? Second, where are particular soils and vegetation located in the study areas? The primary emphasis of this objective is the tree/soil association with deep vs. shallow soils.
*Revision: Soil “rockiness” sampling shifted to “Motley” and “Goodtimes” Mesas for summer 2011 and excluded Monitor Mesa (“Free” Mesa) due to different topography/hydrology on “Free” mesa.
A regular 50m x 50m grid was overlaid on both “Goodtimes” and “Motley” mesa. Forty randomly selected points on each mesa were selected and mapped for sampling. Each random point was assigned an ID number with UTM Northing/Easting coordinates and after using a GPS to navigate to each point we sampled under the following protocol:
1) Measure soil depth (both litter layer / O-horizon) and depth to rock at 5m from the plot center point in the four Cardinal directions (N, S, E, W). Soil depth to rock is a continuous variable, except when depth exceeded the length of the screwdriver shaft; then recorded as “> 30cm”.
2) At each randomly selected point, use a 20 BAF prism to measure stand basal area. Record the species and DBH of all “in” trees.
3) In order to establish an age cap, use increment borers to core the two largest trees in each plot. Since DBH does not directly correlate to age of the tree, cores were taken from comparably sized trees of different species when DBH was close. Additionally, a visual search for any significantly larger trees in proximity to each plot was conducted. If noticeably larger/older trees were found, an additional core was taken and recorded.
4) Plot demographics (average soil depth, basal area, species composition, age) will be compared to existing aerial photos for comparison/classification.
Objective 2) Detailed reconstruction of fire history. We know there were large fires on the Plateau in 1842 and 1879. What other low-intensity fire events occurred and at what frequency? There seems to be an association between soil, vegetation type, and fire frequency and we expect that soils influence fuel structure (particularly crown bulk density) and this difference in structure leads to differences in fires.
Heritage tree identification/mapping.
1) Map and characterize each heritage/veteran tree we could locate.
-Record species and DBH
-Characterize soil rockiness at 1m and 5m from each tree in the N, S, E, & W directions. Provide additional notes (e.g., how many visible fire scars) for future sampling. We currently do not have an increment borer large enough to sample these heritage trees though one is on order
- Living trees: ensure sampling does not undermine the longevity of the sampled tree. Increment borer for single scars, partial cross-section for trees with multiple scars. If a tree with multiple fire-scars would be seriously injured in collecting a sample, a diligent alternate search for fallen snags will be conducted for a cross-section. Fallen snags often record the same apparent sequence of fires. Pay particular attention to fallen snags & logs… often fire-scars are not apparent until looking at the underside of the of the log but these sometimes record even older scars than the living trees. Intent is to sample enough fire-scars to determine the fire-regime pattern, but will forego sampling of trees/snags that are of particular aesthetic or wildlife value.
- Map all fire scar locations. This type of fire-scar map might be useful as a sort of index of past surface-fire frequency. Even if we don’t know the dates of all of the fires, we are hypothesizing that surface fires were more frequent in the open stands, which would mean that fire-scars should be more numerous in the open stands.
Objective 3) Reconstruction of historic stand structure based on intensive aging of living trees, snags, and logs. Recognizing large fires on the Plateau in 1842 and 1879, historic stand structure can be extrapolated based on fire-history results and the size/composition of live trees, snags, and logs.
Use data gathered/analyzed from the above methods to reconstruct historic stand structure.