Present and past old-growth forests of the Lake Tahoe Basin, Sierra Nevada, US

Abstract

We described 38 relictual old-growth stands -with data on the mortality, regeneration, floristic richness, fuel load and disease incidence in our study area in the Tahoe Basin of California and Nevada. The stands are within the lower and upper montane zones (1900-2400 m a.s.l.) and they are rare, occupying < 2% of the land in the Basin's watershed. Correlation matrices and ANOVAs of forest types and conifer species with environmental gradients revealed significant relationships with elevation, distance east of the Sierran crest, slope aspect, annual precipitation, date of complete snow melt, litter depth and degree of soil profile development. Pathogens, parasites and wood-boring insects were present on 23% of living trees; 16% of all trees were dead. We compared these stands to a reconstruction of pre-contact Basin forests and to ecologically analogous old-growth forests of Baja California that have never experienced fire suppression management. Currently, overstorey trees (> 180 yr old) in the Basin stands have ca. 33% cover, 54 m 2 .ha -1 basal area and 107 individuals.ha -1 , values very similar to reconstructions of pre-contact Basin forests and to modern Baja California forests. Understorey trees (60-180 yr old), however, are several times more dense than historic levels and species composition is strongly dominated by A. concolor, regardless of the overstorey composition. The ratio of Pinus : Abies has increased -and the age structure of extant stands predicts that it will continue to increase -from approximately 1:1 in pre-contact time to 1:7 within the next century. Disease incidence and mortality in Baja forests were lower. Although we quantitatively defined current Basin oldgrowth forests -in terms of stand structure -we realize that our definition will differ from that of both past and future oldgrowth forests unless management protocols are changed.

Fuel loads, snag abundance, and snag recruitment in an unmanaged Jeffrey pine–mixed conifer forest in Northwestern Mexico

“…Jeffrey pine-mixed conifer forests in this study had similar snag densities as those reported in Ganey (1999) and are the lower range of Barbour et al (2002) recommendation, but in 1998 there were no snags in 35% of plots, and after a severe drought that significantly increased snag abundance, there were still zero snags on 28% of plots. Average snag density of 5/ha is similar to that found in other pine-dominated forests before harvesting or fire suppression (Table 4) but this density was produced by approximately 60% of the inventoried area in this study.…”

“…There are very few small, dead trees in the SSPM (only 7% with DBH < 10 cm). This is in contrast to ponderosa pine and mixed conifer forests in Arizona and California, where current snag populations are dominated by small snags (Ganey, 1999;Barbour et al, 2002). Fire suppression has increased tree density and canopy cover in many western US forests and the resulting competition has probably contributed to the high number of small dead tress.…”

“…When fire is reintroduced into these forests, the high horizontal fuel continuity results in the majority of the area burning within a fire perimeter, even under modest fire weather. Many snags are old and are in the latter stages of decay in western US forests (Ganey, 1999;Barbour et al, 2002) and this increases their susceptibility to spot-fire ignitions. When fire is reintroduced in these conditions it will consume the majority of old, decayed snags but will create new snags from direct fire effects and the interaction with bark beetles (Dendroctonus and Ips spp.).…”

Tree mortality from fire and bark beetles following early and late season prescribed fires in a Sierra Nevada mixed-conifer forest

“…The introduction of grazing, decimation of the Native American population, and more recent fire suppression policies however, resulted in fire exclusion and increased tree density, fuel accumulation, and fuel continuity on the forest floor (Skinner and Chang, 1996;McKelvey and Busse, 1996;Bradley and Tueller, 2001). Fire exclusion has promoted uniform tree age and structural distributions, and has altered stand composition by allowing shade tolerant species such as firs (Abies) to crowd out shade intolerant species such as pines (Pinus) (Parsons and DeBennedetti, 1979;Barbour et al, 2002). The unnatural accumulation of fuels and changes to stand structure are feared to have detrimental effects on the ecosystem while also greatly increasing the risk of high intensity crown fires.…”

Growth of Norway spruce (Picea abies L.) saplings in subalpine forests in Switzerland: Does spring climate matter?

“…It also protects young trees from wind or ice abrasion (Marr, 1977;Hadley and Smith, 1989;Engelmark et al, 1998) and needle damage due to spring frost (Wardle, 1968;Hänninen, 1996). Deep and persistent snow cover in spring increases water availability in early summer, which in combination with high summer temperatures was found to increase the elevation of treeline in some subalpine forests (Hessl and Baker, 1997) or to favor tree growth in others (Barbour et al, 2002). However, late-laying snow in spring shortens the growing season (Walsh et al, 1994;Alftine and Malanson, 2004), increases the risk of black snow mold (Herpotrichia nigra R. Hartig) infection (Simms, 1967), and may lead to the drowning of seedlings by snow (Wardle, 1968).…”