Historical (1860) forest structure in ponderosa pine forests of the northern Front Range, Colorado

Abstract

Management of many dry conifer forests in western North America is focused on promoting resilience to future wildfires, climate change, and land use impacts through restoration of historical patterns of forest structure and disturbance processes. Historical structural data provide models for past resilient conditions that inform the design of silvicultural treatments and help to assess the success of treatments at achieving desired conditions. We used dendrochronological data to reconstruct nonspatial and spatial forest structure at 1860 in fourteen 0.5 ha plots in lower elevation (∼1900–2100 m) ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) forests across two study areas in northern Colorado. Fires recorded by trees in two or more plots from 1667 to 1859 occurred, on average, every 8–15 years depending on scale of analysis. The last fire recorded in two or more plots occurred in 1859. Reconstructed 1860 stand structures were very diverse, with tree densities ranging from 0 to 320 trees·ha−1, basal areas ranging from 0.0 to 17.1 m2·ha−1, and quadratic mean diameters ranging from 0.0 to 57.5 cm. All trees in 1860 were ponderosa pine. Trees were significantly aggregated in 62% of plots in which spatial patterns could be estimated, with 10% to 90% of trees mainly occurring in groups of two to eight (maximum, 26). Current stands based on living trees with a diameter at breast height of ≥4 cm are more dense (range, 175–1010 trees·ha−1) with generally increased basal areas (4.4 to 23.1 m2·ha−1) and smaller trees (quadratic mean diameters ranging from 15.7 to 28.2 cm) and contain greater proportions of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and Rocky Mountain juniper (Juniperus scopulorum Sarg.). This is the first study to provide detailed quantitative metrics to guide restoration prescription development, implementation, and evaluation in these and similar ponderosa pine forests in northern Colorado.

Mixed-Severity Fire Fosters Heterogeneous Spatial Patterns of Conifer Regeneration in a Dry Conifer Forest

“…Furthermore, the clustered nature of the residual forest suggests that trees survived in groups. These results indicate that fire-caused mortality is at least partly responsible for creating the historical spatial pattern reported in a number of studies [7,8,64]. Many of these studies have speculated that the spatial clustering was a result of fire, but this is one of the only studies that have documented this pattern immediately post-fire.…”

“…The re-introduction of mixed-severity fire after a century of fire exclusion resulted in a clustered spatial pattern of residual trees, with densities that were still higher than those reported for historical forests elsewhere in the region [8]. However, residual tree densities were lower than those reported for undisturbed forests in the region [31,37,63], as well as for recently restored forests [31,63].…”

“…We described non-spatial aspects of forest structure using tree density (trees per hectare; TPH), diameter at breast height (DBH), tree height, basal area (BA), and quadratic mean diameter (QMD) [42,43]. To estimate canopy cover of the residual forest, we used a crown radius of 3 m for all overstory trees [8]. We measured the nearest neighbor distance for each tree and the distance to the nearest tree for each 1 m pixel within the plot using the R package spatstat [44,45].…”

“…Points are more likely to be found near other points in a clustered pattern (CEI < 1), and the average distance from any arbitrary point to its nearest-neighbor is less than expected in a completely random pattern. We also evaluated positioning with Ripley's K [7,8,47,48] using the R package spatstat [44,45]. The K function estimates spatial dependence between points of the same type (e.g., residual trees) across spatial scales by determining the expected number of points within a distance (r) from any randomly sampled point.…”

“…These fires also acted as a control on tree recruitment [4,5], thereby resulting in a variety of tree densities and tree size and age distributions [5,6]. The clustered spatial pattern commonly associated with historical dry conifer stands-where well-defined groups of trees and individual trees were interspersed in a matrix of treeless openings is further attributed to the relatively frequent historical fire regime [7][8][9]. This heterogeneity in historical forest structure is thought to be more resilient to drought and subsequent wildfires [1], yet little is known regarding the process responsible for this spatial structure.…”