Historical and current landscape‐scale ponderosa pine and mixed conifer forest structure in the Southern Sierra Nevada

Abstract

Abstract. Many managers today are tasked with restoring forests to mitigate the potential for uncharacteristically severe fire. One challenge to this mandate is the lack of large-scale reference information on forest structure prior to impacts from Euro-American settlement. We used a robust 1911 historical dataset that covers a large geographic extent (.10,000 ha) and has unbiased sampling locations to compare past and current forest conditions for ponderosa pine and mixed conifer forests in the southern Sierra Nevada. The 1911 dataset contained records from 18,052 trees in 378 sampled transects, totaling just over 300 ha in transect area. Forest structure was highly variable in 1911 and shrubs were found in 54% of transects. Total tree basal area ranged from 1 to 60 m 2 ha À1 and tree density from 2 to 170 ha À1 (based on trees .30 cm dbh). K-means cluster analysis divided transects into four groups: mixed conifer-high basal area (MC High BA), mixed conifer-average basal area (MC Ave BA), mixed conifer-average basal area-high shrubs (MC Ave BA Shrubs), and ponderosa pine (Pond Pine). The percentage of this 1911 landscape that experienced high severity fire was low and varied from 1-3% in mixed conifer forests and 4-6% in ponderosa pine forests. Comparing forest inventory data from 1911 to the present indicates that current forests have changed drastically, particularly in tree density, canopy cover, the density of large trees, dominance of white fir in mixed conifer forests, and the similarity of tree basal area in contemporary ponderosa pine and mixed conifer forests. Average forest canopy cover increased from 25-49% in mixed conifer forests, and from 12-49% in ponderosa pine forests from 1911 to the present; canopy cover in current forest types is similar but in 1911 mixed conifer forests had twice the canopy cover as ponderosa pine forests. Current forest restoration goals in the southern Sierra Nevada are often skewed toward the higher range of these historical values, which will limit the effectiveness of these treatments if the objective is to produce resilient forest ecosystems into the future.

Tamm Review: Management of mixed-severity fire regime forests in Oregon, Washington, and Northern California

“…While these data are useful for general descriptions and tabulations of historical vegetation conditions, they are unsuited to making spatially accurate inferences as to local historical vegetation conditions, or for inferring disturbance regimes from size distributions of trees (Fulé et al, 2014;Stevens et al, in press). While needs vary both regionally and locally, we strongly disagree with the contention that ecological restoration is unnecessary in MSForests of the Inland Pacific West, as do a host of authors throughout the Inland West: Barth et al (2015), Collins et al (2011aCollins et al ( , 2015, Gaines et al (2010aGaines et al ( , 2010b, Spies et al (2010), Hessburg and Agee (2003), Hessburg et al (1999aHessburg et al ( , 1999bHessburg et al ( , 2000aHessburg et al ( , 2005Hessburg et al ( , 2013Hessburg et al ( , 2015, Taylor (2004), Stephens et al (2009Stephens et al ( , 2010Stephens et al ( , 2015, Moghaddas et al (2010), Scholl and Taylor (2010), Hagmann et al (2013Hagmann et al ( , 2014, Merschel et al (2014), Perry et al (2011), Harris and Taylor (2015), and Franklin and Johnson (2012). However, we recognize the importance of stand-replacing fire in appropriate forest types, and at appropriate spatial and temporal scales.…”

Analysing fragmentation in vulnerable biodiversity hotspots in Tanzania from 1975 to 2012 using remote sensing and fragstats

“…Game-Howell test is ideal for unequal sample sizes charaterised by heterogeneity and has been widely used in vegetation mapping that include taxonomic profiles in the Atlantic and Caatinga biomes of northestern Brazil (Pacchioni et al 2014), forest transformation in Uluguru mountains (Ojoyi et al 2015), the effect of fire on Penderosa pine forest density, canopy cover, tree size and basal area (Stephens et al 2015) and shrub density in Zegros forest, southwest Iran (Askari et al 2013). Games Howell test results showed significant patterns of fragmentation between 1975 and 1995 in all habitats (p≤0.05).…”

Restoring fire-prone Inland Pacific landscapes: seven core principles

“…For example, in the Blue Mountains and Northern Cascades provinces, as much as a third of the total area that was capable of producing dry, mesic, or cold forests was in either pre-forest or early seral condition (Hessburg et al 1999a(Hessburg et al , b, 2015. The resulting patchwork of successional and environmental conditions resisted abrupt and widespread changes at local and regional landscape scales by reducing fuel contagion and the likelihood of large and severe fires (Collins et al 2009, Hessburg et al 1999bKeane et al 2009;Malamud et al 1998;Moritz et al 2013;Peterson 2002;Stephens et al 2015). Large wildfires and insect outbreaks (e.g., Miller and Keen 1960) occurred when extreme climate and weather conditions overrode the spatial controls that successional and fuel patterns, and topography provided (Littell et al 2009;McKenzie et al 2004;Westerling et al 2006), but even large fires resulted in patchy successional landscapes (Cansler and McKenzie 2014).…”