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Post-fire Regeneration of the Ponderosa Pine

June 8, 2012

The exquisite pollen cones of the ponderosa pine.

Understanding at multiple spatial scales the complexity of factors controlling tree regeneration and distribution constitutes one of the fundamental goals of forest ecology (Bonnet, 2005).  With this being said, let’s explore some of these factors as they relate to the post-fire regeneration of the ponderosa pine.

There are many factors that are involved, such as: fire behavior, fire intensity and severity, degree of fire-related damage, season of burn (dormant versus growing season), and weather conditions (Keyser, 2010) that complicate the ability of the ponderosa pine to successfully reproduce.  Some of these factors, such as wildfire are considered large-scale disturbances, while events like lightning, tree falls, and diseases are considered small-scale disturbances (Bonnet, 2005). Thinning and prescribed-burning treatments can also influence reproductive output in ponderosa pine (yet the extent to which the application of prescribed fire, after thinning, affects the ponderosa pine reproductive output is not known) (Peters, 2008), and because of its propensity to benefit from mineral-soil seedbeds and open habitats, the ponderosa pine has long been considered a species well adapted to fire (Bonnet, 2005).

Experimental use of prescribed fire in masticated fuel beds has also resulted in fires with long residence times and substantial soil heating, which may reduce the soil seed bank density relative to other methods of disposal such as incorporation into the soil through tilling (Kane, 2010).  Periods of fire exclusion allows long-lived seeds of these species to build up in the soil and have resulted in prolific germination and establishment after reintroducing fire (Kane, 2010).

Regeneration of ponderosa pine after a fire depends on patterns of seed availability and the environmental conditions that define safe sites for seedling establishment (Bonnet, 2005).  Ponderosa pine seeds disperse within ~1-1.5 times the parent tree height (Lentile, 2005).  Ponderosa pine seedlings need intermediate shade conditions for good establishment (Bonnet, 2005).  Safe sites are characterized, in part, by the presence of scorched needle litter on blackened mineral soil and low vegetation cover (Bonnet, 2005).

Ponderosa Pine branch with immature pine cones.

In addition to the environmental condition defined as a safe site, for successful reproductive output, the ponderosa pine seeds need moisture within the soil (Bonnet, 2005).  The presence of scorched needles on the soil surface increases soil moisture retention (Bonnet, 2005).  Litter consisting of recently fallen needles, leaves and masticated woody debris (Kane, 2010), and scorched needles on the surface of the burned soil may also provide mechanical protection for seeds by restricting secondary movement (Bonnet, 2005).  Burned areas provide a general increase in soil nutrients such as mineralizable nitrogen and provide opened habitats with less herbaceous competition (Bonnet, 2005).

Landscape patch structure, typically assessed with aerial photography or satellite imagery, has been used to reconstruct fire extent and severity, particularly where stand-replacing fires leave distinct boundaries (Lentile, 2005).  The effects of wildfire on landscapes vary depending on fire regime (type of fire, intensity, season, frequency, duration) and on the community burned (Bonnet, 2005).  In landscapes with less distinct spatial gradients, historical fire size and the proportions of severity within a fire are particularly difficult to reconstruct (Lentile, 2005), but the patterns or “patches” of the burn areas are clearly apparent as the regeneration begins.

Burn severity has been described by the degree of tree mortality, soil heating, organic biomass consumption, change in soil physical properties, a combination of these fire effects, or broadly defined as the degree of ecosystem change induced by fire (Lentile, 2005).  In areas of low burn severity, the density of surviving trees will likely be high and fire scars will probably be present on ~20% of surviving trees (Lentile, 2005).  Moderate-severity fire killed many small and some larger trees, and larger trees took longer to die (Lentile, 2005).  Seedling densities were not significantly different in low and moderately burned areas, but differed significantly from densities in high-severity areas (Lentile, 2005).

Heavy thinning, similar to what occurred in moderate-severity sites, of second-growth ponderosa pine stands has been shown to decrease competition for water and nutrients and increase tree growth (Keyser, 2010).

In high-severity areas, 55% of the burned area has an opening size that is small and most of the area is within the effective seeding distance of ponderosa pine (Lentile, 2005).  In areas larger than the effective seeding distance of ponderosa pine, 45% of the high-severity burn, tree regeneration has been rare, and it is likely that persistent shrub and grasslands may develop (Lentile, 2005).  No tree seedlings were found in high-severity areas >30 m from a patch edge (Lentile, 2005).

The ponderosa pine is characterized by a cyclic reproductive strategy where years of heavy cone crops (masting years) typically occur after several years of low to intermediate cone crops, effects of management practices on reproductive output may be apparent several years after the treatment application depending on the timing (Peters, 2008).  Trees in the burned treatments produced cones filled with more seeds than trees in unburned treatments (Peters, 2008).  Even the season of the fire has an effect on the quality of the seed; the seed mass, and percentage germination of seeds harvested in spring-burns were significantly lower than those examined from all other burn times (Peters, 2008).

The lower seed quality in spring-burn trees could possibly be related to increases in the relative proportion of male to female cones, which has been shown to increase self-fertilization and reduce seed quality (Peters, 2008).  In ponderosa pine, as well as in many other conifers, female cones tend to be concentrated at the top of the canopy, whereas male cones occur at lower canopy positions, a pattern thought to increase cross-pollination by wind (Peters, 2008).  Spring-burn trees had a lower proportion of female cones in upper canopy positions relative to other treatments (Peters, 2008).

While burning increased the number of filled seeds per cone, the specific season of burning had a stronger effect on seed quality and subsequent seedling growth: seeds from spring burning were smaller and produced smaller seedlings, whereas seeds from fall burning tended to produce larger seedlings (Peters, 2008).

Ponderosa pine limb with opened seed cones and pollen cones present.

Fall-burn seedlings had significantly larger diameters than spring-burn and thin-only seedlings and significantly greater total biomass relative to spring-burns (Peters, 2008).  Overall, seed quality and subsequent seedling growth tended to be lowest in seeds from trees in the thin + spring-burn treatment and highest in the thin + fall-burn treatment, with intermediate values in the control and thin-only treatments (Peters, 2008).

Consistent with past studies, greater seedling densities were found in the burned compared with the unburned areas (Bonnet, 2005).  The highest seedling densities were found within the burned patches near the edges of the unburned forest canopy and densities decreased with increasing distance away from the edge both into the unburned forest and into the burned patch (Bonnet, 2005).  The high number of seedlings close to the unburned canopy edge but inside the burned patches was explained by the coincidence of both high seed availability and environmental conditions conducive to seedling-establishment success (Bonnet, 2005).  Low seed availability and poor environmental conditions for seedling establishment were responsible for the absence of seedlings in interior regions of severely burned patches (Lentile, 2005).

Greater regeneration success was observed inside the burned patch near the unburned edge and at isolated locations within the burned patch than in unburned areas, which is different from what would be expected from seed availability alone (Bonnet, 2005).  As anticipated in response to the influence of seed sources (trees producing scorched needles may also have been an unpredicted source of seeds), seedling density exponentially decreased from the edge toward the center of burned patches (Bonnet, 2005).  Areas with high understory cover restricted the undisturbed forest and reduced seedling survival in the burned areas (Bonnet, 2005).

Inside burned patches, the spatial pattern of regeneration appears to reflect seed availability; however, upon closer inspection, it was shown that this pattern is a result of both the distributions of predictable seed availability and safe sites (Bonnet, 2005).  Within the conditions favorable for seedling occurrence, small patches with high vegetation covers were correlated with high seedling mortality (Bonnet, 2005).  Though floristic richness was usually low within burned patches, covers of ruderal species sporadically reached up to 75% in these patches (Bonnet, 2005).

Variables that favored the presence of ponderosa pine seedlings were strongly related to the abundance of scorched needles on top of burned mineral soil, characteristic of edge areas (Bonnet, 2005).  While the mean soil temperature was very similar with or without scorched needles, the variations in soil temperature were attenuated by scorched needles compared with the bare soil (Bonnet, 2005).  Scorched needles on the surface of burned mineral soil, which are conditions most favorable for seedling occurrence, reduced soil temperature variation during the day (Bonnet, 2005).  Scorched needles that have fallen onto blackened mineral soil, after fire, serve as a mulch to mitigate subsurface soil temperature extremes that affect seed and seedling viability (Bonnet, 2005).

The abundance of mosses, lichens, and trees with burned crowns was also highly correlated with seedling occurrence, whereas high herbaceous and total vegetation covers negatively affected seedling incidence (Bonnet, 2005).  The protection of seeds by lichen crusts is particularly effective in extreme systems such as xeric or high-elevation habitats (Bonnet, 2005).  These crusts are very sensitive to fire, and their survival in micro-patches contributes to the generation success of ponderosa pine (Bonnet, 2005).

Increases in understory diversity required both the opening of the canopy (provided by thinning) as well as bare mineral soil exposure (provided by prescribed burning) (Kane, 2010).  Increases in the amount of bare ground allow for greater recruitment and establishment of individual plant species by reducing competition and increasing the available growing space (Kane, 2010).  Widely spaced, large trees form an open canopy (Lentile, 2005).  Seedlings in this environment are abundant and likely to survive and grow into the overstory, forming a dense, multiaged forest (Lentile, 2005).

Future surface fires or prolonged drought may kill most of the surviving trees or seedlings that are recruited following fire (Lentile, 2005).

Resources:

Bonnet, V.H., A.W. Schoettle, and W.D. Shepperd. 2005. Postfire environmental conditions influence the spatial pattern of regeneration for Pinus ponderosa. Can. J. For. Res. 35: 37-47.

Kane, J.M., E.E. Knapp, R.F. Powers, and J.M. Varner. 2010. Understory vegetation response to mechanical mastication and other fuel treatments in a ponderosa pine forest. Appl. Veg. Sci. 13:207-220.

Keyser, T.L., F.W. Smith, and W.D Shepperd. 2010. Growth response of Pinus ponderosa following a mixed-severity wildfire in the Black Hills, South Dakota. West. J. Appl. For. 25:49-54.

Lentile, L.B., F. W. Smith, and W.D. Shepperd. 2005. Patch structure, fire-scar formation, and tree regeneration in a large mixed-severity fire in the South Dakota Black Hills, USA. Can. J. For. Res. 35:2875-2885.

Peters, G., and A. Sala. 2008. Reproductive output of ponderosa pine in response to thinning and prescribed burning in western Montana. Can. J. For. Res. 38:844-850.

Photo Resources:

Powell, Dave. wikimedia.org. 2009. Pinus ponderosa subsp. scopulorumpollen cones.

http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Pinus_ponderosa_scopulorum_pollen_cones.jpg/512px-Pinus_ponderosa_scopulorum_pollen_cones.jpg

Siegmund, Walter. wikimedia.org. 2011. Pinus ponderosa var. ponderosa.

http://upload.wikimedia.org/wikipedia/commons/thumb/5/50/Pinus_ponderosa_3813.JPG/400px-Pinus_ponderosa_3813.JPG

USDA-NRCS Plants Database. wikimedia.org. 1996. Pinus ponderosa branch cones.

http://upload.wikimedia.org/wikipedia/commons/thumb/8/89/Pinus_ponderosa_branch_cones.jpg/800px-Pinus_ponderosa_branch_cones.jpg

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