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A gap-based approach for regenerating pine species and reducing surface fuels in multi-aged mixed conifer stands in the Sierra Nevada, California

Silvicultural systems that promote multi-aged stands through periodic diffuse harvests are a viable option under modern landscape management regimes attempting to emulate natural disturbances (Long, 2009). Within a framework of large-scale ecosystem management, such systems can be a means for achieving heterogeneous forest structures at the stand level (North et al., 2009; Puettmann et al., 2009). Smaller landowners may gain several additional benefits from multi-aged systems. For example, artificial regeneration costs are lower or do not exist, canopies remain relatively intact throughout time to provide continuous soil protection and productive capacity and income from harvests can be spread out to provide revenue at relatively short intervals (Mathews, 1992). For some landowners, multi-aged systems may actually be the only regeneration option that is permissible because of legal agreements restricting even-aged methods (i.e. easements) or because of local forest practice regulations governing small landowners. Finally, there is potential for multi-aged stands created by harvesting individual or small groups of trees to have long-term productivity rates comparable to even-aged stands (O’Hara and Nagel, 2006; Laiho et al., 2011). While the benefits listed above for some forests may be rooted in biophysical processes or management efficiencies, there also exists a general social perception that multi-aged stands are ‘more natural’ and therefore inherently preferable (O’Hara et al., 2007). As O’Hara et al. (2007) has noted, however, A gap-based approach for regenerating pine species and reducing surface fuels in multi-aged mixed conifer stands in the Sierra Nevada, California


Introduction
Silvicultural systems that promote multi-aged stands through periodic diffuse harvests are a viable option under modern landscape management regimes attempting to emulate natural disturbances (Long, 2009). Within a framework of large-scale ecosystem management, such systems can be a means for achieving heterogeneous forest structures at the stand level (North et al., 2009;Puettmann et al., 2009). Smaller landowners may gain several additional benefits from multi-aged systems. For example, artificial regeneration costs are lower or do not exist, canopies remain relatively intact throughout time to provide continuous soil protection and productive capacity and income from harvests can be spread out to provide revenue at relatively short intervals (Mathews, 1992). For some landowners, multi-aged systems may actually be the only regeneration option that is permissible because of legal agreements restricting even-aged methods (i.e. easements) or because of local forest practice regulations governing small landowners. Finally, there is potential for multi-aged stands created by harvesting individual or small groups of trees to have long-term productivity rates comparable to even-aged stands (O'Hara and Nagel, 2006;Laiho et al., 2011). While the benefits listed above for some forests may be rooted in biophysical processes or management efficiencies, there also exists a general social perception that multi-aged stands are 'more natural' and therefore inherently preferable (O'Hara et al., 2007). As O'Hara et al. (2007) has noted, however, A gap-based approach for regenerating pine species and reducing surface fuels in multi-aged mixed conifer stands in the Sierra Nevada, California the degree to which objectives are met should be measured in order to ultimately assess a multi-aged system's value.
Opposing the benefits of multi-aged systems are several risks (Guldin, 1996). A concern often mentioned by forest managers is the tendency for poor regeneration of pioneer tree species (i.e. shade intolerants) if too much canopy cover is retained or if seed dispersal from desired species does not happen to coincide with harvests. Another is that an unprepared seedbed (i.e. decomposing and fresh litter on forest floors) will restrict germination of some species, especially pioneer tree species that germinate on heavily disturbed forest floors. This lack of control over species composition is a substantial risk, especially when compared with the high degree of species control in even-aged systems. In forests where wildfires are common, there is an additional and profound risk of catastrophic loss, especially if intermediatesized trees (i.e. 'ladder fuels') and surface fuels increase as a result of selective harvests (Agee and Skinner, 2005).
In mixed conifer forests of Western North America, the effects of multi-aged systems on species composition and surface fuels are of particular importance. Ponderosa pine (Pinus ponderosa var. ponderosa Dougl. ex. Laws) regeneration requires disturbances that substantially reduce competition and prepare a disturbed seedbed for germination (Oliver and Ryker, 1990). Sugar pine (Pinus lambertiana Dougl.), while not as tightly linked with disturbance as ponderosa pine, also regenerates more readily following disturbances (Kinloch and Scheuner, 1990). Sugar pine can also be an especially high-priority species to regenerate in order to assist population recovery from impacts of the exotic pathogen, white pine blister rust (Cronartium ribicola Fischer, van Mantgem et al., 2004). Shade tolerant species such as white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.) and incense-cedar (Calocedrus decurrens Torr.), meanwhile, have been increasing in stands with low management intensities and where fire-exclusion policies prevail (Ansley and Battles, 1998;Pierce and Taylor, 2010).
Fire severity in mixed conifer forests of Western North America has been increasing (Westerling et al., 2006;Miller et al., 2009) and proactive management to reduce fire hazard is being urged (Collins et al., 2010). Traditional methods of applying multi-aged systems, however, may fall short in achieving either desired species compositions or in reducing fire hazard. Following harvests, site preparation activities that dispose of activity fuel (i.e. logging slash) and prepare seedbeds to encourage germination typically do not occur, leading to potential increases in surface fuel loads that are especially important in driving fire behaviour (Agee and Skinner, 2005). If multi-aged systems increase surface fuel loads and encourage regeneration of only tolerant species, they may be even more diminished in meeting health and diversity objectives than forests where fire suppression is the only management activity (Stephens and Moghaddas, 2005b).
Novel approaches are clearly needed to meet modern objectives while using multi-aged systems. One approach is to harvest in a dispersed manner throughout stands, as would traditionally occur, but also manage specific areas within stands for both fuel reduction and seedbed preparation for pine regeneration. By creating small canopy gaps during a harvest, then piling and burning debris within gaps, a high-quality seedbed of ash and/or bare soil could be created while simultaneously reducing surface fuel. Such an approach could potentially maintain the essence of multiaged systems (stand-level heterogeneity and multiple age classes), while also addressing concerns of low pine regeneration and surface fuel build-ups. Given that pine species in pre-settlement Sierra Nevada forests likely regenerated in distinct canopy gaps created by flare-ups in mixed-severity fires (Keeley and Zedler, 2000), it follows that higher germination rates in canopy gaps with ash substrates could be an adaptive trait. There is limited information from sowing studies to verify this, however (but see Zald et al., 2008). While there have been studies finding correlations between different substrate types and the presence of established conifer seedlings (e.g. Gray et al., 2005), these studies have not measured substrate type at the time of germination, which may have occurred several years earlier. This discrepancy is important because the availability of suitable germination sites precisely coincident with seed dispersal is a dominant influence on mixed conifer regeneration in the Sierra Nevada van Mantgem et al., 2006). Information about substrate effects on germination of different species is therefore critical for designing multiaged systems that rely on natural regeneration following regeneration harvests.
This study has three objectives. One is to present a case study on the feasibility and effectiveness of a pile-and-burn fuel reduction treatment in terms of reducing surface fuel and modifying predicted fire behaviour within multi-aged stands. Fuel treatments were concentrated in small canopy gaps dispersed throughout two multi-aged stands in a central Sierra Nevada mixed conifer forest. The second objective is to measure the influence of ash versus soil substrates on germination of ponderosa pine and sugar pine from a controlled seed-sowing experiment within canopy gaps created with multi-aged regeneration harvests. The third objective is to evaluate the influence of introducing canopy gaps on light heterogeneity and availability in the context of encouraging pine regeneration. We focus on informing management by evaluating the effectiveness of applying treatments in multi-aged stands to coincidentally reduce surface fuels and encourage regeneration of pine species.

Study area and treatments
The study was located at the University of California, Berkeley's Blodgett Forest Research Station (BFRS), in the central Sierra Nevada Mountains (1300 m elevation; 38° 52′ N, 120° 40′ W). The climate is Mediterranean, with annual precipitation (average = 166 cm) occurring mainly during the winter and spring months and a prolonged drought during the summer. Soils are well developed, originating from granodiorite parent material. Heights of codominant anopy trees typically reach 27-34 m in 50-60 years. Vegetation at BFRS is dominated by a mixed conifer forest type (Fites-Kaufman et al., 2007), composed of variable proportions of white fir, incense-cedar, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii), ugar pine, ponderosa pine and California black oak (Quercus kelloggii Newb.).
The treatment was conducted within two stands at BFRS (stands 230 and 110) ~1.5 km apart. Both stands have slight slopes, generally less than 15 per cent. Each stand has had several harvest entries on a roughly 10-year interval since the early 1970s. Harvests have removed individual trees of all size classes dispersed throughout the stands. As a result of the multiple harvest entries, stand structures are highly heterogeneous with multiple age and size classes present. Past harvest operations have all used conventional manual felling with chainsaws and ground-based yarding of sawlogs (minimum top diameter of 15 cm). Tops and limbs have traditionally been left on site.
In general, harvests occur in these stands when basal area densities surpass a predetermined upper threshold (~50 m 2 ha 21 ). Harvests reduce basal area density by ~40 per cent. Density therefore fluctuates over time within this density management zone of 30-50 m 2 ha 21 . This target has corresponded to a harvest about every 10 years in both stands used for this study. Forests with lower productivities or different desired harvest frequencies could have lower and/or wider density management zones. The harvests for this study were designed to remove trees of all size classes, in proportion to the growing space occupied by each size class. In addition to the harvest of individual trees throughout stands, ~10 per cent of each stand area was converted to 0.04-ha canopy gaps, distributed throughout the stands. Within these 0.04 ha areas, all merchantable trees were removed.
Stand 230 is 17 hectares, which given the 10 per cent gap conversion rate described above, corresponded to ~42, ~0.04-ha gaps created during the harvest. Of these, 30 were used for the study. Stand 110 is 18 hectares, corresponding to 45 gaps, of which, 35 were used in the study (the biggest and smallest gaps were excluded in both stands). The harvest in stand 230 took place in the summer of 2004. The harvest in stand 110 took place in the summer of 2006. The same experimental design was used in each stand. Only the timing of the treatment was different (along with differences in climate between the two periods). Because of the difference in treatment timing between stands, they were considered two different trials of the same experiment rather than replications of a single treatment.
Following the harvest of merchantable trees, nonmerchantable trees (<25 cm diameter growth at breast height (d.b.h.)) within gaps were cut and placed into a pile at gap centre. Surface fuels within gaps were also collected and placed into piles. A tracked excavator with a grapple head was used for the non-merchantable tree removal and piling in each gap. Debris piles were burned during the fall after each harvest. Burning followed ~10 cm of cumulative precipitation after the summer drought period. Piles were lit during the morning and then allowed to burn all day before throwing any unburned pieces on pile edges into pile centres by hand to increase consumption. The same operator for machine piling and the same hand crews for burning were used in both stands. The piling and burning activity within these gaps was similar to a typical site preparation operation following a regeneration harvest in an even-aged stand but on a much smaller spatial scale.
The seed-sowing experiment was designed to isolate substrate quality as a factor of germination frequency. Seeds were sown in the fall within gaps after the burned areas had cooled completely. Seeds were collected from several parent trees throughout BFRS in previous years during good cone crops and stored in a seed storage facility. Prior to sowing, seeds were stratified by soaking them in running water for 48 h, similar to the protocol used by nurseries prior to sowing. This served to emulate the soaking that the seeds would have received had they been dispersed from parent trees surrounding the gaps in late summer. Seeds were sown within gaps on three different substrates: ash, ash/soil edge and bare soil ( Figure 1). For each of the three substrates, two sowing spots were located within 1 m of each other, one for ponderosa pine and one for sugar pine. Five seeds were sown at each sowing spot. Seeds were embedded into the sowing substrate by hand so that all or most of the seed were buried within 1 cm of the surface. All five seeds were sown within a 10 × 10 cm area. Pine seeds are relatively large and many, if not most, dispersed seeds are collected and cached by small mammals (Vander Wall, 1994). To prevent seed predation and relocation, 'seed caps' made from metal mesh were secured over the sown seeds. Wire stake flags were used to mark sowing locations. Four hundred and fifty seeds of each pine species were sown in stand 230 (30 gaps × 3 sowing spots × 5 seeds per spot), while 525 seeds of each species were sown in stand 110 (35 gaps).
There is a potential confounding effect in this seed-sowing design that could be caused by a centre-to-edge gradient in microclimate along the line of substrate treatments from ash to soil ( Figure 1). If edge environments are shadier, then germination might be different at ash substrate locations because of a direct influence of radiation on germination. To assess this potential, we measured the per cent of total transmitted radiation at north edge and central locations of 14 randomly selected gaps in stand 230 with hemisperhical photography (see methods for light measurements below) to see if such a gradient existed. The mean per cent of total transmitted radiation reaching gap centres was 39 per cent, while radiation at north edges of gaps was 42 per cent. These means were statistically indistinguishable (two-tailed t test P = 0.44). The actual difference in light between ash and soil substrate sowing locations is even less than this 3 per cent non-significant difference since the substrate treatments were oriented along an east-west line and not a south-north line where the light gradient in gaps is steepest (Canham et al., 1990;York et al., 2003). Furthermore, seeds were not sown at gap edges but well within the perimeters of gap boundaries as projected by the drip-lines of the surrounding canopy. Within-gap differences between sowing locations other than those caused by the substrate treatment are therefore not considered to be confounding.

Measurements and analysis
The influence of within-gap fuel treatments on stand-level surface fuels was measured with line-intersect transects (Brown, 1974). Sampling points were distributed throughout the stands on a 120-m square grid and measured prior to harvests, then again after the harvests and fuel treatments described above. There were 13 sampling points in stand 230 and 14 points in stand 110. Sampling points were used whether or not they overlapped with a gap (i.e. they measure net change in stand-level fuels). At each sampling point, two transects were measured. The first was oriented in a random direction and the second oriented positive 60 degrees from the first. Because surface fuels are often variable within stands (e.g. Reiner et al., 2009), the two transects were averaged to increase precision at the level of the sampling point. One-hour (0-0.64 cm) and 10-h (0.64-2.54 cm) fuels were intercept-sampled from 0 to 2 m, 100 h (2.54-7.62 cm) fuels from 0 to 3 m and 1000 h (>7.62 cm) and larger fuels from 0 to 11.3 m along transects. Duff and litter depth were measured at 0.3 and 0.9 m on each transect. Fuel depth was measured at three points along each transect. Fuel loads were calculated using equations developed for local species (van Wagtendonk et al., 1996(van Wagtendonk et al., , 1998. The objective of the surface fuel analysis was to quantify the effect of treating surface fuels using this gap-treatment approach in multi-aged stands. The fuel treatment was not an experimental treatment, per se (with different levels of fuels treatment intensity, for example). To determine the net effect of the fuel treatment on surface fuel load at the stand level, we compared the means of total fuel loading before and after the harvest. Comparisons were done separately for the two stands. The 95 per cent confidence interval of the difference between means and the magnitude of change in total fuel load following treatments was the basis for inference.
We followed the approach used by Stephens and Moghaddas (2005a) to evaluate changes in fire behaviour associated with harvest treatments. Since our goal was to evaluate a worst-case scenario with respect to fire hazard, we modelled fire behaviour responses under extreme fire weather with Fuels Management Analyst (FMA) Plus, Version 3 (Carlton, 2005). We used the fire weather parameters calculated for Blodgett Forest that represent extreme (97.5th percentile) conditions observed during the last 41 years at a nearby weather station (values reported in Table 1 in Stephens and Moghaddas (2005a)). FMA Plus relies on sitespecific information on fuel loading and forest structure to estimate stand-level fire behaviour. Surface fuels (e.g. leaf litter, forest duff and coarse woody debris) provide the substrate that carries fire and determine fire intensity and spread rate. Canopy attributes (e.g. crown bulk density, crown base height and vertical continuity) influence the probability of a crown fire and subsequent tree mortality. Forest structural attributes such as canopy cover and tree height modulate wind speeds and fuel moisture that in turn influence fire behaviour (Pierce et al., 2009). FMA Plus incorporates field-based measurements of these key factors and uses published methodologies to compute fire behaviour and effects for given fire weather conditions (Stephens and Moghaddas, 2005a). We developed the necessary inputs for FMA Plus from the pretreatment and posttreatment field measurements of surface fuels and trees. Similar approaches have been used throughout the dry forests of the western US to evaluate the efficacy of fuel treatments on wildfire effects (Hartsough et al., 2008). Sown seeds were monitored for germination starting in the spring after the fall sowing. Surveys started immediately following snow melt and, in both stands, timing of the start of survey was ideal for capturing the first germination events because snow had just recently melted. Surveys began on 6 May 2005 in stand 230 and on 23 April 2007 in stand 110. In stand 230, surveys occurred every 2 weeks until no new germinants were found (June 22nd) and then, a final survey was done 1 month later to ensure capture of any late germination. In stand 110, surveys were done every month until no new germinants were found, followed 1 month later by a final survey. Following the experience of surveying in stand 230, 1-month intervals were found to be adequate for the purpose of this study because germinating seeds were protected from predation by the mesh covers and germinating seeds did not decompose or otherwise disappear within any 1-month period. A seed was considered germinated if the hypocotyl had breached the seed coat and made contact with the soil. During surveys, the number of germinants at each spot was recorded. Following the final survey, the cumulative number of germinants occurring at each spot was summed. Zero to five germinants were possible at each spot.
The objective of the germination analysis was to describe the relationship, if any, between substrate type and germination frequency. Our primary interest was in finding any effect, positive or negative, of the ash substrate on germination compared with the bare soil substrate. When no seeds germinated in a given sowing location, the germination frequency was zero. These zero values were particularly important in this case to keep as zero values (as opposed to adding one and doing a data transformation) since zero germination was an ecologically distinct event and not simply a very small number along a continuous range of numbers. Germination frequency was therefore modelled as a polytomous response variable (i.e. an ordinal variable), with six levels of germination frequency possible (either 0, 0.2, 0.4, 0.6, 0.8 or 1.0, corresponding to the number of germinants found at each sowing spot). Substrate (ash, soil or edge) and species (ponderosa or sugar pine) nested within the two stands were the predictor variables of interest. Gap (65 levels) was included as a variable to account for variability among gaps. Logistic regression was used to detect significant effects of the predictor variables (Trexler and Travis, 1993). Likelihood ratio tests comparing the full model with all variables to the model without a given variable were used to verify significance. The critical α-value to compare with the χ 2 -statistic was 0.05. Post hoc tests were done for significant effects from the saturated model with logistic regression and likelihood ratio tests. Analyses were done with JMP software (SAS Institute Inc., Cary, NC).
In addition to the light measurements within gaps in stand 230 described above, we measured stand-wide light availability from a grid both before and immediately after the harvest in order to characterize the change in the light environment across the stand. This was done to assess the effectiveness of the treatment in producing stand-level heterogeneity in light conditions and to find out if the created gaps had more light available within them compared with the stand in general (i.e. to find if they were ecologically distinct areas with respect to increased light availability for encouraging pine regeneration). Photos were collected from the same 120-m sampling grid (n = 13) as the fuel measurements using a Nikon 35-mm camera and a Nikkor fish-eye lens (8 mm f/2.8) placed 1 m aboveground. To provide a reference for how light conditions within the harvested stands compare with unharvested reserve stands, we also collected photos from a similarly spaced grid (n = 35) in an adjacent stand that has not been harvested for 100+ years. These measurements provide a context for what light conditions would be like in these stands if no harvests had ever occurred. Photos were taken near dawn or dusk to minimize direct lighting effects. Colour slides were converted to digital images (900 d.p.i.) that were analysed with GLA software (Frazer et al., 2000) to compute the per cent of total transmitted photosynthetically active radiation (per cent Total Transmitted Radiation). This index of irradiance calculated from film-based hemispherical photographs has proven to be one of the most reliable measures of light influence on seedling growth (Kobe and Hogarth, 2007). We obtained a precision estimate of ±2 per cent from remeasurement of a random subset (7 per cent) of photos.
Randomization tests were performed to test if the harvest created an overall change in the light environment at the stand level. Means and standard deviations (SDs) from grid measurements prior to and following the harvest were calculated for each of 1000 iterations (random sampling with replacement). Differences (pre-versus post-harvest) in the mean and SD were considered significant when P < 0.05. The use of randomization tests provided a common approach to statistically evaluate the impact of the treatments on both the magnitude (mean) and heterogeneity (SD) of the light environment while also accounting for any non-normality in the observed data. The other comparison of interest for statistical testing was between within-gap locations and the stand in general (including both gap and non-gap areas). Light in the unharvested reserve stand is not analysed with statistical tests but the stand's light data distribution is given to provide an example of the light environment for a stand not having any recent harvest or canopy disturbance.

Results
Prior to harvest, both stands had similar structures with mean forest basal area equal to 55 m 2 ha 21 and mean tree d.b.h. = 40 cm. However, the surface fuel loading in stand 230 was considerably heavier pre-harvest compared with stand 110 (202 tons ha 21 versus 69.4 tons ha 21 , Table 1). Following the harvest and the within-gap fuel treatment, stand-level fuel load decreased in stand 230 compared with the pre-harvest amount by 33 per cent to 135.1 tons ha 21 (Table 1), a moderate decrease (33 per cent) that was statistically significant (P < 0.05). In contrast, the treatments had no influence on surface fuel in stand 110. Following   the harvest, fuel loading was 74.5 tons ha 21 , a small increase (7.3 per cent) not detectible with a 95 per cent confidence interval of the difference. More logging slash (tops and limbs) was created during the harvest in 110 because more large trees were harvested, explaining the primary reason for the difference in surface fuel change between the two locations. More logging slash was added to stand 110, where the gap piling had the effect of simply negating the addition of the slash to the surface fuel load. In 230, where the harvest resulted in less slash, the gap piling was able to negate the addition of logging slash plus reduce it compared with the pre-harvest surface fuel load.
Harvests had only minor impacts on fire behaviour and fire effects. Even under the most extreme fire weather conditions, these managed stands were at low risk of experiencing destructive crown fires. In both compartments, regardless of treatment, surface fires dominated ( Table 2). The flame lengths of the modelled fires were all <1 m, well short of the base of the live crown in these stands (none lower than 5.8 m). The predicted plot-level tree mortality rates associated with these low intensity, slow moving fires ranged from a high of 42.6 per cent (standard error (SE) = 3.7 per cent) in Compartment 110 after treatment to a low of 36.2 per cent (SE = 2.4 per cent) in Compartment 230 after treatment. In no case did the harvest treatments cause a significant increase in fire-related tree mortality (t test, P-values > 0. 37).
Seed germination was different between stands (i.e. years), species and gaps. Within stands, substrate type (P = 0.001) and species (P < 0.001) were significant predictors of germination frequency in the model, with species having the majority of the influence (Table 3). In stand 230, there was little germination of ponderosa pine at all. Only 20 of the 90 sowing spots had any germinants. Despite the low overall germination in stand 230, however, there was a clear trend towards more germination occurring in ash substrates compared with soil substrates (P = 0.009; Figure 2). In stand 110, ponderosa pine germination was much more common, with 88 of 105 sowing spots having at least one germinant and no clear trend in germination frequency between substrates (P = 0.09).
For sugar pine, there was also a distinct difference in overall germination between the two stands. Interestingly, however, it was the opposite of ponderosa pine. Germination was much more frequent in stand 230 (64 of 90 spots had at least one germinant), while it was considerably lower in stand 110 (32 of 105 spots). There was no detectable Prior to the harvest in stand 230, the average light reaching 1 m above the forest floor across the stand was 12 per cent of total transmitted radiation, compared with 6 per cent in the unharvested reserve. Light heterogeneity in both the reserve and the treated stand prior to harvest was relatively low (Figure 3). The harvest increased stand-level light to 26 per cent (P = 0.002) and increased SD in light from 4 to 13 per cent; the range of light availability levels was dramatically increased (Figure 3). Mean light within gaps (40 per cent) was well above the stand mean of 26 per cent (P = 0.002). A difference in SD was not detected (P = 0.56) between gaps (9 per cent) and the stand in general (13 per cent). The effect of the harvest was, in general, to increase the amount and heterogeneity of light across the stand. The creation of gaps created distinct pockets of increased light availability nested within the stand.

Effectiveness of fuel treatment
Although the fuel treatment following the harvest was restricted to small areas dispersed throughout the stand and covered only 10 per cent of the total area, the treatment was effective in avoiding a net increase in stand-level surface fuel following conventional harvesting methods that left all tops and limbs of harvested trees on-site. Where surface fuel load was particularly high prior to harvest, the treatment actually resulted in a net decrease in surface fuel following the harvest.
In general, we found that these stands to be exposed to only modest fire hazard following the harvest and fuel treatments. The multi-aged prescription maintains a heterogeneous canopy structure with much of the productivity concentrated in large diameter trees. Clearly, surface fuels are a concern given their important role in determining fire behaviour (Agee and Skinner, 2005). However, despite the potential to accumulate high surface fuel loads, particularly in the 10-and 100-h size classes (Table 1), there were limited ladder fuels to carry modelled fires into the canopy. While the treatments resulted in increased modelled fire line intensity and rate of spread (Table 2) relative to pretreatment conditions, the modelled fire never got off the surface and never caused widespread mortality. Stephens and Moghaddas (2005a) reached a similar conclusion regarding the efficacy of mechanical thinning treatments at modifying fire behaviour in these forests.
In situations where there is an especially large amount of surface fuel (similar to stand 230 in this study), the modest decrease in surface fuels that were achieved in this case might not be enough for managers wanting to reduce fire hazards to very low levels. Options for further reducing surface fuels in multi-aged stands include more intense treatments (either prior to or following harvests) that cover more than 10 per cent of the stand area or using a different yarding method to reduce activity fuel. Whole-tree Unharvested reserve Selection stand before harvest Selection stand after harvest Figure 3. The relative distribution of the per cent of total transmitted radiation above the canopy reaching the forest floor in an unharvested reserve stand and in a stand prior to and following a multi-aged selective harvest at Blodgett Forest Research Station, California. Stand-level measurements are from samples distributed across the stand, regardless of proximity to canopy gaps. FORESTRY 8 harvesting, in particular, could transport tops and limbs to landings rather than leave them in the stand as surface fuel (Schmidt et al., 2008).
The effectiveness of fuel treatments to a large degree depends on the details of how treatments are carried out. During the treatments for this study, only 10 per cent of the stand area was treated but more than 10 per cent of the slash present following the harvest was actually piled. The debris piles were made within 0.04-ha gaps, where high concentrations of slash resulted from the harvest of a clump of adjacent trees. This resulted in higher amounts of slash within gaps compared with the stand in general. During the treatments for this study, the operator did not distinguish between fresh logging slash and slash that had been created by previous operations or debris that had fallen on its own. More than just recent activity fuel created by the harvest, therefore, was piled and burned.

Substrate effects on pine germination
When ponderosa pine germination was in general more frequent, it made no difference whether the substrate was ash or soil. However, ponderosa pine germinated more frequently in the ash substrate when germination was in general low. This tendency towards slightly higher ponderosa pine germination in ash substrates was noted long ago in a nursery experiment (Fisher, 1935). More recently, an increase in germination on ash substrates was noted for Jeffrey pine, a species very similar to ponderosa pine (Zald et al., 2008). Our interpretation is that ash substrates are not likely to provide substantially better conditions for germination than bare soil substrates during years when conditions for germination are favourable. What little positive effect the ash substrate has is likely to be swamped by climatic factors that influence seedling establishment and mortality. However, our results suggest that silvicultural treatments that create ash substrates could increase ponderosa pine germination during years when conditions for seedling establishment are marginal. Marginal conditions for ponderosa pine regeneration occur frequently simply because seed production is highly variable and often very low (McDonald, 1992;Krannitz and Duralia, 2004;Moghaddas et al., 2008). Seed predation by small mammals can greatly reduce the amount of seed that is available to germinate (Vander Wall, 1994), especially in low seed production years. Higher germination rates, even if only slightly higher, in ash substrates may be an adaptive trait that ponderosa pine evolved under a mixed-severity fire regime. It is plausible that such a trait would be advantageous for increasing regeneration success given the tenuous nature of seed production and the limiting effects of predation and given that ponderosa pine regeneration was often confined to canopy gaps created by locally intense fires prior to the current era of fire suppression (Keeley and Zedler, 2000). The importance of substrate quality as a general influence on regeneration is well established (e.g. Simard et al., 1998;Gray et al., 2005;van Mantgem et al., 2006;Moghaddas et al., 2008). Regeneration is a process made up of several phases. Dispersal, germination and establishment must occur as a prerequisite to recruitment, and each may be influenced by different factors. In this study, we isolated only one of these phases. Substrate conditions for germination may be very good for a particular species, yet little recruitment may occur if there is no dispersal or if conditions for establishment are poor. Our results therefore do not broadly imply that burning to create ash substrates will consistently improve regeneration of ponderosa pine following harvests since influences on other phases of regeneration are also important during the period following a harvest.
There was no evidence of sugar pine germination being influenced by ash substrates. Other studies that have not measured sugar pine germination but have measured establishment found sugar pine to be relatively insensitive to low-intensity burn treatments (McDonald, 1992;van Mantgem et al., 2006;Moghaddas et al., 2008). When trying to manage for sugar pine natural regeneration in multi-aged stands, burning does not appear to benefit germination more than having bare soil seedbeds. Seed production and conditions that facilitate establishment and growth are likely to be more important. As with ponderosa pine, sugar pine cone production is highly variable between years, stands and individuals (Yeaton, 1984). An especially important factor in sugar pine may be seed predation. Sugar pine seeds are much larger than the other seeds in the mixed conifer forest (Kinloch and Scheuner, 1990). Such large seeds tend to be preferred as a source of food for small mammals (Vander Wall, 1994). In one case, small mammals moved as many as 99 per cent of dispersed Jeffrey pine seeds (Vander Wall, 1992), which are considerably smaller than sugar pine seeds. While it is uncertain how many of these seeds are consumed versus left behind in caches, seed predation during low-production years in sugar pine could be an important factor that limits regeneration.
Harvests designed to initiate a new cohort of seedlings cannot feasibly be timed to coincide with favourable seedling germination/establishment conditions that are controlled by future weather patterns. Some practices, however, could be implemented to hedge bets against the risk of poor regeneration conditions following a regeneration harvest. If the harvest occurs in late summer, the cone crop should be visible in canopies and dispersal amounts could be estimated (McDonald, 1992), so that harvests could be timed to coincide with 'bumper' cone crops. Such timing seems more feasible for sugar pine given their conspicuous cones that take 2 years to mature. Planting seedlings is also a viable option that may be a particularly effective way of establishing cohorts of pioneer species within small canopy gaps (York et al., 2011).
Because detailed microclimate data were not collected, there is no clear explanation for the difference in germination rates between the two stands. Some possible reasons can be eliminated, however. Gap sizes between the two stands were similar, as were soil types and topography. Considering the wide range in environments that are optimal for germination between species, even within the Pinus genus (Minore, 1979), the most probable reason is differences in weather prior to and during germination.
A weather station within 2 km of both stands recorded weather data during the germination periods. In the spring of 2005, ponderosa pine germination was low and sugar pine germination was high. This period of high sugar pine germination was relatively wet and cold. Precipitation from January to June of 2005 was much higher (1260 mm) and soil temperature somewhat colder (5.6°C) than in the spring of 2007, which was relatively dry (653 mm) and warm (7.4°C). During the dry and warm year, ponderosa pine germination was high and sugar pine germination was low. This pattern in sugar pine is consistent with nursery studies of sugar pine germination requirements, where long cold stratification periods have been noted to greatly increase germination (Heit, 1968). It is unclear why ponderosa pine had greater germination frequency during the dryer and hotter year, considering moisture stress has been observed to reduce germination in P. ponderosa var. arizonica (Schubert, 1974). Site-specific information on phenology of soil moisture and temperature are needed to explain these differences. Staging harvests over multiple years or planting seedlings are both viable options that mitigate temporal variability in germination environments.

Influence on light environment
Prior to the gap-based approach most recently used in these stands, past selective harvests of individual trees from many size classes had achieved several of the traditional goals of multi-aged management: multiple age classes were initiated, productivity in terms of timber yield was relatively high, size structure was well balanced between size classes and periodic income was attained while avoiding costs of intermediate treatments. Yet this traditional approach was failing to achieve two objectives that have more recently become higher priorities throughout western US forests. Surface fuels had increased and pine regeneration, especially ponderosa pine, had greatly declined to the point where it was almost absent. Although the amount of light reaching the forest floor on average (12 per cent) prior to harvest was twice the average than in a nearby reserve stand, it was still not nearly enough to regenerate the local shade-intolerant species. This was in spite of significant reductions in stand-wide basal area (~40 per cent) during previous harvests. Past regeneration surveys following prior harvests indicate very little initiation of sugar pine regeneration and virtually no ponderosa pine regeneration. We therefore expect that adequate resource availability for pine regeneration will be restricted primarily to within the gaps created during these harvests. Even many of these gaps, which had an average of 40 per cent light availability within them, may be marginal for ponderosa pine recruitment. Inadequate light availability could be associated with gaps that are either too small or that have surrounding canopies that are too dense. Planting would likely improve recruitment through bypassing the germination and establishment phase of regeneration and allowing early seedling growth into higher light environments of the upper canopy. Survival of planted pine seedlings in this forest type was consistent across a gap-size gradient from 0.1 to 1.0 ha (York et al., 2007). However, survival in smaller gaps created in this study (~0.04 ha) could be considerably lower. Again, planting may be a viable option for addressing survival in small gaps. The cost-effectiveness of planting in multi-aged stands warrants further study to further evaluate the effectiveness of the gap-based approach to multi-aged management. Control of competing vegetation within gaps would also likely improve recruitment, as would the placement of gaps adjacent to particularly good specimen parent trees that are likely to produce more seed for natural regeneration.

Synthesis and conclusions
Many of the negative outcomes traditionally associated with multi-aged stands (Guldin, 1996) can be moderated or resolved by designing harvests and post-harvest treatments to specifically meet modern objectives. The creation of small gaps followed by burning within gaps in this study effectively addressed two of the major concerns of multi-aged stands in western North American forests -surface fuel loading and regeneration of shade-intolerant pine species. While this type of micro-management of small regeneration areas nested within multi-aged stands may seem counter to the concept of these systems being less intensive or 'more natural', they nonetheless are treatment alternatives for achieving what would otherwise be unachievable objectives using more traditional multi-aged approaches. Further, the 10-year re-entry cycle is similar to the known fire return interval in these forests (Stephens and Collins, 2004) and therefore to an extent emulates disturbance frequency of the natural disturbance regime (sensu, Long, 2009).
The addition of fire after gap creation could be viewed as an additional step towards the emulation of a natural disturbance regime (Long, 2009). While the highly controlled nature of fires confined to piles within gaps does not represent the spatial heterogeneity that can be achieved with prescribed fires that cover larger areas, the gap-based approach also avoids the blunt and often unpredictable effects of stand-level prescribed fires (Hartsough et al., 2008). Where risk of loss of commercial timber value or the risk of fire escape is unacceptable (e.g. stands designated for timber production or areas adjacent to neighbouring landowners), this gap-based approach to harvesting and burning represents a viable alternative that retains the benefit of fire while avoiding many of its risks.

Funding
California Agricultural Experiment Station to J.J.B.