Surprisingly little empirical and quantitative documentation of successional patterns in YPMC forests in the assessment area has been published. Nonethe-less, early observers of YPMC forests in the assessment area were already well acquainted with the different ecological tolerances and successional tendencies of the major tree species. For example, Sudworth (1900), Leiberg (1902), and Greeley (1907) all referred to the strong potential within YPMC forest for dense seedling recruitment of the shade-tolerant/fire-intolerant species in the absence of fire (see chapter 1). The species differences referred to in tables 1 and 2, and figures 3 and 4 interact with the environment and ecological disturbances to drive successional processes in YPMC forests.
Leiberg (1902) stated that the relative proportion of tree species in assessment area YPMC forests was changing because of timber harvest and fire. In general, he noted that the relative proportions of sugar pine and yellow pine were decreasing, as recruitment of young trees was not keeping up with their removal from the over-story by logging (one exception was the northern part of his central survey area).
At the same time, he described “a uniform increase” in the proportions of incense cedar and white fir across the survey area. Overall, the YPMC forests that Leiberg surveyed had low densities of tree seedlings and saplings, owing to the effects of
frequent fire. However, he noted that stands of YPMC forest that had escaped fire for 12 to 15 years were often filled with stands of saplings “so dense that a man can with difficulty force his way through” (Leiberg 1902: 43). Sudworth (1900) also noted that, “The frequent open spaces in yellow pine forests are sooner or later covered with dense patches of young trees, but these thickets may in turn be swept off by fire.” Show and Kotok (1924) made the same point, namely that fire protec -tion in the pine belt in the Sierra Nevada had resulted in “an enormous number of young forest trees that have appeared as individuals and in groups, or, in the more open virgin stand, as a veritable blanket under the mature timber.”
The rate of forest infilling in the absence of fire varies along environmental gradients. For example, studies in assessment area YPMC forests have found that seedling recruitment, survival, and growth are inversely related to eleva -tion (Hunter and Van Doren 1982, van Mantgem et al. 2006), and topographic exposure and insolation are also major drivers of seedling survival and growth rates (Kolb and Robberecht 1996, Maguire 1955). Local soil conditions and
topographic- and vegetation-defined (e.g., nurse plants) microhabitats can also play a major role in seedling survival, young tree growth, and rates of forest succes -sion and densification in the absence of frequent disturbance (Gomez et al. 2002, Tappeiner and Helms 1971). In the assessment area, dense shrub cover can have a major effect on future forest composition as well, as shade-tolerant trees (e.g., white fir) are more likely to survive the decades it may take to overtop the shrub canopy (assuming that fire can be kept out of the stand, in which case succession will be reset) (Stark 1965). Another major driver of seedling density and forest infilling is temporal coincidence between large seed crops and years with favor -able climate (high precipitation, occurrence of summer thundershowers, low summer temperatures, etc.) (Burns and Honkala 1990).
Bonnicksen and Stone (1982) provided a summary of successional dynamics in moist mixed-conifer forest (including giant sequoia). Bonnicksen and Stone (1982) popularized the notion of the “shifting mosaic” of successional stages on the landscape, where neighboring sites of the same ecological “potential” could be in dissimilar vegetation states owing to different spatiotemporal processes and their rates. They stressed that the nature and rate of different successional pathways depended on abiotic and biotic conditions of the site in question. That said, a generally recognized truism is that white fir is the competitive dominant in most YPMC forests in the assessment area, and the long-term absence of fire will ultimately lead to white fir forests (or Douglas-fir at lower elevations in the north assessment area). Fires in the presettlement period were frequent and mostly of low severity, but some aggregations of mature trees would nonetheless be periodically killed by fire, while others were left untouched, and in yet others the
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understory vegetation and tree regeneration would be consumed by the passage of fire. Such patches of high-severity fire would often be succeeded by dense areas of fire-promoted shrubs, and return to forest in such areas might take many decades (Show and Kotok 1924). Although fire was frequent, there were always tree clumps that had escaped fire for several FRIs, and thickets of shade-tolerant species (white fir, incense cedar) would often develop in these places. Regeneration of species like giant sequoia and the yellow pine requires mineral soil and canopy gaps large enough to bring sunlight to the forest floor (Burns and Honkala 1990, Meyer and Safford 2011). Recruitment of these species thus required fire intense enough to kill clumps of canopy trees; sugar pine tends to favor smaller gaps (Burns and Honkala 1990, Oliver and Dolph 1992). Note that Bonnicksen and Stone’s (1982) ideas were applicable to patchiness at a very fine scale (they explicitly state that they are referring to patches between 0.0135 and 0.16 ha). As is noted in multiple locations throughout this document, coarse-grained patchiness on the order of thousands, hundreds, or even tens of hectares was uncommon in YPMC forests before Euro-American settlement.
Overall, the general picture is one of very high potential for forest recruitment, especially by shade-tolerant species, with frequent fire or soil conditions maintain -ing the dominance of the pine (and in some places, giant sequoia) overstory and a more open forest condition (Bonnicksen and Stone 1982; Kilgore and Taylor 1979;
Leiberg 1902; North et al. 2002, 2005; Show and Kotok 1924, Sudworth 1900).
Models of YPMC forest successional dynamics—
Kercher and Axelrod (1984) developed a Monte Carlo-based model of YPMC forest succession (SILVA) at the stand level to better understand the effects of fire on forest dynamics in the Sierra Nevada. The SILVA model is complex, and includes more than 30 subroutines that model such phenomena as species-specific demographic rates (recruitment, growth, death, injury, etc.), stand structures, fire, and brush and litter dynamics. Fire effects on trees were estimated as a function of scorch height and tree diameter, but weather inputs were mostly held constant, so the simulated fire regime was relatively crude (Agee 2000). Kercher and Axelrod (1984) used SILVA to compare forest succession after a simulated clearcut for 500 years at two different elevations, 1520 m (5,000 ft) and 1830 m (6,000 ft). The lower elevation site is at the upper reaches of YPMC forests historically dominated by ponderosa pine; the upper site is nearer the upper limits of YPMC forests in the Sierra Nevada and historically included a significant component of fir species.
The time-averaged results of Kercher and Axelrod’s (1984) lower elevation simulation are shown in figure 11. Figure 12 shows the successional progression of the lower elevation YPMC stand through the 500 years of the SILVA simulations.
Coarse-grained
Fire was modeled as a stochastic process with a mean return interval of 7 years.
Ponderosa pine and black oak dominate the stand immediately after the initial stand-replacing fire, but black oak becomes subordinate to incense cedar and then white fir by 70 to 100 years and almost completely drops out of the stand by 300 years (fig. 12). After 200 years, overall basal area varies between 50 and 54 m2/ha, and the relative dominance of species changes but the proportions of shade-intol -erant to shade-tol-erant species fluctuate around 70:30 until after 400 years, when the proportion of ponderosa drops. Simulations without fire supported much higher basal area of shade-tolerant/fire-intolerant species like white fir and incense cedar.
The higher elevation simulation is in the “fir zone” and supported much more white fir than ponderosa pine, even under frequent fire (Kercher and Axelrod 1984).
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0 Ponderosa
pine
Percent of total basal area
Tree species
White
fir Sugar
pine Incense
cedar Black
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Figure 11—Box plots of upper and lower quartiles for basal area of five tree species modeled by Kercher and Axelrod (1984) for a yellow pine–mixed-conifer forest at 1524 m elevation with a mean fire return interval of 7 years. Values are from temporal distributions of basal areas as predicted by the SILVA forest succession model, averaged over 10 runs of 500 years. Horizontal lines within quar -tile boxes represent the median; the “error bars” represent upper and lower ranges for each species.
Douglas-fir was also modeled but accounted for only about 1 percent of the basal area. Illustration adapted from figure 6 in Kercher and Axelrod (1984).
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GENERAL TECHNICAL REPORT PSW-GTR-256
Van Wagtendonk (1984) carried out simulation modeling of YPMC forest succession using an improved version of the FYRCYCL stand-level forest dynam -ics model. The model included subroutines on vegetation, fuels, fire, weather, and lightning ignitions. Historical fire weather was used to drive the fire regime, and fires could be of any intensity. Van Wagtendonk (1984) carried out 200-year simulations under three management conditions: no fire, natural lightning igni -tion regime, and fire suppression (which permitted fires to burn under certain extreme conditions). Starting conditions were a seedling patch of 40 percent (by basal area) ponderosa pine, 25 percent sugar pine, 20 percent white fir, and 15 percent incense cedar. Under the no-fire scenario, ponderosa pine increased to >55 percent of basal area by 90 years (taking advantage of originally open stand conditions), then began to drop as white fir accrued individuals and basal area; white fir dominated the stand after 150 years. Under the lightning-ignition scenario, fires occurred on average every 8.9 years (the first fire occurred at 34 years) and ponderosa pine comprised more than 90 percent of stand basal area by the end of the simulation. Sugar pine was the most important codominant species in this scenario, but by the end of the simulation it was less than 8 percent of the
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0
500 400
300 200
100
Basal area (m2 /ha)
Time from clearcut (yr)
0
PIPO QUKE
PILA ABCO
CADE
Figure 12— Composite of SILVA simulation results for five tree species in a yellow pine–mixed-conifer forest at 1524 m elevation, under a mean fire-return interval of 7 years. Douglas-fir was also simulated but accounted for very little of the basal area. Data are from Kercher and Axelrod (1984).
stand as reckoned by either basal area or density. Stand densities varied widely in the lightning-ignition scenario, depending on fire frequency and intensity (van Wagtendonk 1984). Under the fire-suppression scenario (basically a modern business-as-usual scenario), two moderate- to high-severity fires occurred, reduc -ing white fir density much more than basal area (as some white fir had reached sufficient size to survive intense fire), and resulting in a system dominated by a fluctuating balance of ponderosa pine and white fir (but with ponderosa pine always dominant in terms of basal area). Sugar pine and incense cedar were only minor players in all three scenarios.
Keane et al. (1990) developed the FIRESUM successional process model as an upgrade to SILVA and applied it to understanding successional dynamics in ponderosa pine/Douglas-fir forests in the inland Northwest under different fire regimes. Keane et al.’s (1990) study area includes the northern tip of the assessment area, and four of the forest types (“fire groups”) modeled by FIRE -SUM either occur in assessment-area YPMC forests or are similar (warm, dry ponderosa pine; grand fir [Abies grandis is ecologically similar to white fir];
warm, dry Douglas-fir; moist Douglas-fir). In Keane et al. (1990), FIRESUM was used to carry out a 200-year model of successional dynamics in a semiarid ponderosa pine-dominant stand beginning in 1900. The major findings were that Douglas-fir was able to establish in the stand only when FRIs reached 50 years, but ponderosa pine still dominated the site under these conditions, with about 50 percent of the basal area at year 200 (Larix occidentalis and Douglas-fir comprised the remainder). Under no fire, Douglas-fir comprised one-third of the total basal area by year 100, and dominated the stand by 130 years; at the end of the simulation, Douglas-fir was about 65 percent of the total basal area (65 m2/ha). Under the frequent fire scenarios (<50-year FRIs), most basal area was contributed by large trees, but at FRIs of 50 years and above, fuels accumulated and fires were intense, which resulted in stands of small to intermediate trees at high densities.
Miller and Urban (1999a, 1999b) and Urban et al. (2000) described an adapta -tion of the ZELIG forest gap model for forests along an eleva-tional gradient in Sequoia & Kings Canyon National Parks in the southern Sierra Nevada. In various publications, Miller and Urban and colleagues employed the modified ZELIG model to study climate change scenarios, carbon dynamics, the effects of fire on stand parameters, the importance of the physical habitat and moisture availability, and so on, but they did not publish results on the actual successional dynamics between species within their simulations. We refer to these studies in various other places in this assessment.
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All the simulation models referred to above make clear the initial advantage that the yellow pine species have in frequent fire scenarios. Seedlings and saplings of ponderosa pine and Jeffrey pine (and sugar pine) grow rapidly in high light environments (fig. 3), and as young trees they support thicker bark than their competitors (fig. 4). Both adaptations provide for higher survival under recurrent fire. Where fire is not frequent, or overstory cover is high, the yellow pines are ultimately outgrown by shade-tolerant species.
Future—It is unknown how future climates and conditions may affect basic succes -sional processes in assessment-area YPMC forests. If future environmental condi -tions differentially affect key species in YPMC forest, then successional rela-tionships among species may change. An example is the effect of white pine blister rust on the five-needle pines, which in YPMC forests are represented by sugar pine, and, to a lesser extent, western white pine. Aside from these sorts of effects, it appears likely that warming temperatures and increasing fire activity on some of the landscape, but continued fire exclusion on most of the landscape, will simply accelerate the sorts of successional changes we have already witnessed for the past half century.