Fire Ecology

Post-Cedar Fire Mixed Conifer-Hardwood Monitoring and Mixed Coniferous Forest Restoration at Cuyamaca Rancho State Park, San Diego County, California

Sponsored by California State Parks, Colorado Desert District, 2004-2009 (overseen by Kim Marsden, Assoc. Resource Ecologist)

Post-Cedar Fire Monitoring and Mixed Coniferous Forest Restoration (2008-2009)

We examined establishment patterns of pines following a large, severe wildfire in the Peninsular Ranges of southern California, USA. The October 2003 Cedar Fire caused 98% pine mortality. In this study, we asked (i) where did seedlings establish and survive in formerly forested areas of the Cuyamaca Mountains 5 years following the high severity fire and (ii) what factors were associated with the spatial pattern of seedling establishment? Factors analyzed were pre-fire vegetation type, fire severity, post-fire vegetation characteristics, topography (slope, aspect, and elevation), and mapped soil type. We used a unique belt-transect survey method following the existing trail network that resulted in a representative sample of post-fire environments. Almost 1300 100 m × 20 m quadrats were searched in 2008–2009, one third of which supported juvenile pines. Regeneration primarily consisted of Coulter pine (Pinus coulteri D. Don), a weakly serotinous pine that was establishing at densities of 5–2320/ha on half of the quadrats where it had occurred pre-fire. Pinus coulteri regenerated in areas burned at high severity where pre-fire pine cover was high and its abundance was positively associated with higher elevation and cover of bare soil. In contrast, minimal regeneration of nonserotinous pines occurred patchily in areas that were not severely burned.

Post-Cedar Fire Mixed Conifer-Hardwood Monitoring (2007) -- Executive Summary – Findings and Recommendations

The 2003 Cedar Fire burned extensive forested areas of the Cuyamaca Mountains in the Peninsula Ranges of San Diego County, CA, USA. This large fire severely affected these forests. The purpose of this study was to examine the effect of the heterogeneity of both the landscape and disturbance on patterns of post-fire vegetation dynamics. An earlier project (Agreement C0443021) reported that conifer mortality in Cuyamaca Rancho State Park (CRSP) was extremely high and positively related to fire severity, and that early post-fire vegetation dynamics during the first two growing seasons were dominated by the establishment of abundant and diverse native herbs (including fire-obligates) as well as shrub seedlings, and resprouting by shrubs and oak trees. This study reports on the analysis of data from 38 vegetation monitoring plots in West and East Mesas resurveyed in May-June 2007, the fourth post-fire growing season. Vegetation changes in the first four years following a large, severe crown fire in Coulter pine and mixed conifer forests are described.

  • Areas with high cover of Ceanothus shrubs

Dense shrub cover, primarily Ceanothus palmeri, has established on about 40% of the area surveyed, especially in stands with higher former forest cover and fire severity. About half of these stands had lower shrub cover, averaging 32%, and half had high cover, averaging 70%. On a landscape scale I recommend no vegetation management of C. palmeri dominated stands. Ceanothus is a nitrogen fixing genus of California shrubs that serves an important ecosystem function, especially following fire on low-nutrient soils in California’s montane forests. These shrub stands will naturally thin over time, resulting in lower, patchier cover, and allowing establishment of conifers.

Site-specific removal of shrubs over small areas may be required for tree planting projects, but the disadvantage of Ceanothus removal, again, is that it may affect the availability of nutrients for successful tree establishment. Ceanothus is also known to be an intermediate host for mycorrhyzal fungi associated with conifer species and necessary for their survival. In the stands examined that fell at lower elevations in the forested zone (1300-1400 m), shrub cover averaged around 60% and chaparral species dominated vegetation recovery, especially Ceanothus leucodermis. Again, Ceanothus plays an important nutrient cycling role post-fire in California ecosystems. These sites appear to be following a normal trajectory of succession for chaparral-dominated sites. There is no indication that vegetation management is required.

  • Areas with high cover of invasive, exotic annual grasses and forbs

By 2007, 4 years after the Cedar Fire, exotic annual brome grasses (Bromus spp.) and a mustard, Sisymbrium altissimum, were the most abundant herbaceous species in the areas surveyed. Forested areas of CRSP are interspersed with dry meadows (grasslands) with a history of grazing, and the most abundant brome species have been established there for at least a 

Although it is disheartening to see plant diversity dominated by exotic species, I think it is unlikely for these exotic Eurasian grasses to lead to altered fire regimes or type conversion, as has been found elsewhere in sagebrush ecosystems. These grasses are dependent on dispersal in order to establish in forested areas following disturbance because they do not have persistent seed banks. They did not reach potential establishment sites in abundance during the first post-disturbance year, when most resources are available, owing to dispersal limitation. Because of this, they had little impact on the post-fire annuals and bulbs that are a significant component of biodiversity in this forest community. They are shade-intolerant, and therefore I predict that they will not persist in such great abundance in these sites after 5-10 years as the woody canopy closes. Given their temporal and spatial patterns of establishment, they have the greatest potential impact native perennials and opportunistic annuals. They are also likely to re-invade, but only during the same time window (2-10 years after fire), given the proximity of propagule pools, the dry meadows found in the mountains where non-native grasses have long been established. If
Sisymbrium altissimum forms a persistent seed bank it could have a greater impact on native plant communities.century. Bromus tectorum, in particular, has been observed to invade following fire in low elevation pine forest. Therefore, it is not surprising that these species were found in abundance following a stand-replacing crown fire. Concerns that are raised, however, are: what is the impact of high exotic herbaceous cover on 1) native herbs, and 2) conifer regeneration? In other words, is there a negative impact on native biodiversity and natural patterns of succession?

Post-Cedar Fire Mixed Conifer-Hardwood Monitoring (2004-2006) -- Executive Summary – Findings and Recommendations

In late October 2003 the Cedar Fire, the largest wildfire in southern California in over 100 years, burned almost all of Cuyamaca Rancho State Park (CRSP; 10,000 ha) at high severity. CRSP, situated in the Peninsular Ranges, harbors one of southern California’s unique sky islands of montane conifer habitat surrounded y a sea of foothills shrublands (chaparral). The landscape looked like scorched earth to anyone who drove through CRSP after the fire. However, many ecosystems in California are fire adapted – their plants have traits that allow them to survive or regenerate after fire, or even that require fire for regeneration.

The purpose of this study was to measure thee effect of the Cedar Fire on the forested lands of Cuyamaca, and to monitor the early recovery of the plant community. We selected the West Mesa area of the Park (about 1100 ha) for study because a previous forest survey was conducted there 11 years before the Cedar Fire, and because it appeared to be less completely burned than other areas (Middle Peak). This selection was ironic for two reasons. First, the previous survey did not locate all sample plots precisely enough that they could be relocated exactly. Therefore, before and after comparisons of forest structure can only be made at an aggregated level. Secondly, we determined that in fact fire severity for all but five of the 37 plots revisited was very high. While this in itself is useful information, it limited our ability to determine the effect of fire severity on forest recovery.

However, we found the following in the first post-fire year (2004):

  • Almost all pines and other conifer trees were killed by the fire. The greater the fire severity in a location the higher the mortality. Most trees measured in our study area were Coulter Pine but there were also Jeffrey Pine, Incense Cedarand White Fir. Coulter Pine is partially serotinous meaning that some cones on some trees are sealed with resin that is melted by fire, releasing seeds. Only five pine seedlings were found in our plots, or less than 20 seedlings per hectares
  • Most oak trees were killed above-ground but were resprouting from the base – the individual survived the fire but the mighty oak was replaced by a few stems. Resprouting was unrelated to characteristics of the environment or the fire.
  • Shrub species found in chaparral patches and under forest were regenerating vigorously by resprouting and/or establishing thousands of seedlings.
  • Over 100 species of native wildflowers (annual and perennial) flowered prolifically in 2004, in spite of low rainfall, producing a typical flush of post-fire biodiversity. Non-native plant species were not very abundant or diverse.

The 2003-04 rain year was extremely dry, followed by the extremely wet year of 2004-05. Continued monitoring in 2005 revealed the following:

  • Even fewer new pine seedlings established and we estimate that survival of those establishing in 2004 to 2005 was low (<20%). Pines are not reestablishing.
  • In a single stand where several Incense Cedar trees survived the fire we found thousands of seedlings, highlighting the importance of small unburned patches to forest regeneration.
  • Cover of resprouting oaks, shrubs and of shrub seedlings increased.
  • A group of “fire-following” herbs, predictably, disappeared in the second year, and unseen to us remain as seeds in the soil waiting for the next fire.
  • Cover of other herbs increased, presumably in response to the high rainfall.
  • Unfortunately, a large portion of that cover was non-native (exotic) grasses that are known to be invasive (displace native plants in California ecosystems) and alter fire cycles.
  • We added new survey plots in an area of East Mesa, part of which had been burned by Park officials in a fuel-reducing prescribed burn one year before the Cedar Fire. East Mesa had much lower fire severity and lower conifer mortality than West Mesa (50% versus 95% mortality). It is not known if this is due to previous fire history, specific behavior of the Cedar Fire, or stand conditions, but it is a dramatic difference. East Mesa is drier, with more open forest stands dominated by Jeffrey Pine.

Predictions and Recommendations

  • We predict that severely burned forest in CRSP will be dominated by shrubs and oaks for decades to centuries. The expanding cover of these species may prevent pine seedling survival even if pines can disperse seed to these areas naturally.
  • A review of ecological restoration and plantation forestry projects would provide an estimate of the cost versus effectiveness of promoting pine forest reestablishment by large-scale seeding or planting
  • More worrisome is the expansion of exotic grasses of forbs following the rains of 2005. This should be closely monitored. These species are known to be detrimental to native plant communities.
  • Other exotic plant removal programs should be reviewed to determine the feasibility of a removal program. Specifically, there may be strategic places on the landscape where, if done, this would be most effective.

Finally, to paraphrase Professor Paul Zedler, is may be fruitless to debate the natural fire cycle in this region when climate and human use of fire have varied at the time scale of the fire regime itself – hundreds to 10,000 years. Instead, as Wheeler in his book on fire ecology remarked, resource managers must determine what elements of biodiversity, ecosystem function or resource production they want to manage with fire, and then determine how, when and where to use fire (prescription, suppression, let-burn) to achieve their resource management goals. 

Chaparral succession following the Coyote Fire at Sky Oaks Field Station

This project formed the Masters thesis research of Heather Karnes Schmalbach who completed her degree in December 2005. The main findings are summarized below.

Main Findings

Effects of pre-fire stand age, fire severity, and hillslope position on postfire vegetation recovery during the first two postfire seasons were studied in two contrasting stands of mixed chaparral at San Diego State University’s Sky Oaks Field Station (San Diego County, CA). The old stand was approximately 60-year old and the young stand was 12-years old when both burned in the July 2003 Coyote Fire.
The two stands showed contrasting patterns of postfire community composition among different plant groups based on their modes of postfire regeneration. Fire severity was significantly higher in the older stand. Increased fire severity was positively correlated with the establishment of Ceanothus greggii var. perplexans (an obligate seeder) seedlings, and negatively correlated with Adenostoma fasciculatum (a facultative seeder) seedling abundance. Hillslope position was also important in determining patterns of abundance, suggesting that soil erosion and deposition following fire may have a significant effect on postfire community recovery in these steep sites. Alternatively, prefire differences in the chaparral community that were correlated with hillslope position may account for these differences.
The postfire herbaceous community in the first year was dominated by Phacelia brachyloba (a fire annual). Contrary to expectations, this species was found in greater abundance in the old stand than in the young. Fire annuals were largely absent from the community in the second year, and were replaced in abundance by a variety of opportunistic native and exotic annuals.

Publications (also see Publications)

Franklin, J. and Bergman, E., 2011. Patterns of pine regeneration following a large, severe wildfire in the mountains of southern California. Canadian Journal of Forest Research 41(4):810-821 10.1139/x11-024.

Franklin, J., 2010, Vegetation dynamics and exotic plant invasion following high severity crown fire in a southern California conifer forest, Plant Ecology 207:281-295. DOI 10.1007/s11258-009-9672-6.

Bergman, Erin, 2009. Pine regeneration in Cuyamaca Rancho State Park, San Diego County, five years after a high severity crown fire, Thesis (M.S.)-- San Diego State University (Biology)

Schmalbach, H., J. Franklin and J. F. O’Leary, 2007, Patterns of post-fire regeneration in a southern California mixed chaparral community, Madroño 54(1): 1-12

Franklin, J., Spears-Lebrun, L., D. Deutschman, and K. Marsden, 2006, Impact of a high-intensity fire on mixed evergreen and mixed conifer forests in the Peninsular Ranges of southern California, USA, Forest Ecology and Management 235: 18-29.

Spears, Linnea Anne, 2005, Tree mortality and forest recovery in Cuyamaca Rancho State Park, San Diego County, California following the 2003 Cedar Fire, Thesis (M.S.)-- San Diego State University (Biology), 45 p.8.

Schmalbach, Heather, 2005, Patterns of post-fire regeneration in the chaparral community at Sky Oaks Field Station, Thesis (M.S.)-- San Diego State University (Biology)