John F. O'Leary

Simulating the effects of different fire regimes on plant functional groups in Southern California

A spatially explicit landscape model of disturbance and vegetation succession, LANDIS, was used to examine the effect of fire regime on landscape patterns of functional group dominance in the shrublands and forests of the southern California foothills and mountains. Three model treatments, frequent (35 year), moderate (70 year), and infrequent (1050 year) fire cycles, were applied to the landscape for 500 year. The model was calibrated and tested using a dataset representing an initial random distribution of six plant functional groups on an even-aged landscape. Calibration of the three fire regime treatments resulted in simulation of fire cycles within 7% of these intended values when fire cycles were averaged across ten replicated model runs per treatment. Within individual 500-year model runs, the error in the simulated fire cycle (average area burned per decade) reached 11% for the moderate and frequent fire cycle treatments and 53% for infrequent. The infrequent fire regime resulted in an old landscape dominated by the three most shade tolerant and long-lived functional groups, while shorter-lived and less shade tolerant seeders and resprouters disappeared from the landscape. The moderate fire regime, similar to what is considered the current fire regime in the southern California foothills, resulted in a younger landscape where the facultative resprouter persisted along with the long-lived shade tolerant functional groups, but the obligate seeder with low fire tolerance disappeared, despite its moderate shade tolerance. The frequent fire regime resulted in the persistence of all functional groups on the landscape with more even cover, but the same rank order as under the moderate regime. The model, originally developed for northern temperate forests, appears to be useful for simulating the disturbance regime in this fire-prone Mediterranean-type ecosystem.

Variations in a regional fire regime related to vegetation type in San Diego County, California (USA)

This study considers variations in a regional fire regime that are related to vegetation structure. Using a Geographic Information System, the vegetation of San Diego County, Southern coastal California USA is divided into six generalized classes based on dominant plant form and include: herbaceous, sage scrub, chaparral, hardwood forest, conifer forest and desert. Mapped fire occurrences for the 20th century are then overlain to produce records of stand age, fire frequency and transitional stability for each of the vegetation classes. A ‘Manhattan’ similarity index is used to compare and group transition matrices for the six classes of vegetation. This analysis groups herbaceous, hardwood and conifer forests in one group, sage scrub and chaparral in a second, and desert in a third. In general, sage scrub and chaparral have burned more frequently than other vegetation types during the course of the 20th century. Temporal trends suggest that the rate of burning in shrub-dominated vegetation is either stable (chaparral) or increasing (sage scrub), while the rate of burning in both hardwood and conifer forest is declining. This is consistent with a pattern of increased fire ignitions along the relatively low elevation urban-wildland interface, and an increase in the efficiency of fire suppression in high elevation forests.

Influence of short-interval fire occurrence on post-fire recovery of fire-prone shrublands in California, USA

In recent decades, fire frequency has increased with population growth at the wildland-urban interface in southern California, USA. Short intervals (,5 years) between successive fires can be detrimental to plant species that require longer intervals between fires to reach reproductive maturity. The resilience of vegetation to changes in fire regimes is of particular interest for shrublands in Mediterranean climates, which in general are susceptible to high frequenciesof wildfire. Research on how fire hasshaped the life historiesof shrubs inMediterranean climates is central to understanding how to protect sensitive habitat while allowing for the natural occurrence of wildfire in these regions. FrequentlyburnedchamisechaparralshrublandsinSanDiegoCounty,California,weremappedinthefieldwiththeaidof satellite imagery and analysed to investigate changes in vegetation condition and composition associated with short intervals between fires. Fire history, terrain and land-cover characteristics of mapped stands were tested to determine thefactorsassociatedwithdisturbedandconvertedvegetation.Resultsindicatethatnumberofburnsandmeanfireinterval are contributing factors in post-fire change in chaparral stands. Chamise chaparral is vulnerable to alteration and type conversion when fire return intervals are 4-5 years or less.

Post-fire diversity patterns in two subassociations of Californian coastal sage scrub

. Coastal sage scrub exhibits floristically dissimilar subassociations on opposing aspects in the Santa Monica Mountains of southern California. Southward-facing (Venturan I) and northward-facing (Ventaran II) subassociations were sampled at two scales (1 m2 and 625 m2) each spring for five years following a fire. Mature (10- and 28 yr-old) sites of these subassociation types were also sampled in the study area. The subshrub and herbaceous components that were most prevalent on northerly slopes following the first post-burn year contributed the most cover for all ages examined. Aspect-related differences in species richness were most apparent at the smaller sampling scale where they were equal the first post-burn year and varied inversely thereafter. At this scale, richness values were two times higher on older north-facing sites, largely due to the herbaceous component. South-facing sites displayed progressive decreases in equitability through time. This pattern reflects gradual reduction in overall species richness combined with progressive site domination by fewer species, as manifested by increasing dominance concentration values. By contrast equitability on north-facing sites fluctuated little through time, though dominance concentration tended to decrease. Higher richness, equitability, and overall cover with time after burning on north-facing sites is associated with the relatively mesic habitats upon which this subassociation develops. Post-burn patterns of species richness and stratal differentiation on south-facing slopes are consistent with those reported for other mediterranean-type shrublands, i.e. peak richness was attained the first two years following a fire with subsequent decrease to a constant level.

Monitoring Detailed Land‐Cover Changes in Shrubland Habitat Reserves Using Multi‐temporal IKONOS Data

Abstract The objective of this study was to explore the utility of multi‐temporal, multi‐spectral image data acquired by the IKONOS satellite system for monitoring detailed land cover changes within shrubland habitat reserves. Sub‐pixel accuracy in date‐to‐date registration was achieved, in spite of the irregular relief of the study area and the high spatial resolution of the imagery. Change vector classification enabled features ranging in size from tens of square meters to several hectares to be detected and six general land cover change classes to be identified. Interpretation of the change vector classification product in conjunction with visual inspection of the multi‐temporal imagery enabled identification of specific change types such as: vegetation disturbance and associated increase in soil exposure, shrub removal, urban edge vegetation clearing and fire maintenance, increase in vegetation cover, spread of invasive plant species, fire scars and subsequent recovery, erosional scouring, trail and road development, and expansion of bicycle disturbances.

Chaparral growth-ring analysis as an indicator of stand biomass development

Chaparral wildfires typically create even-aged stands of vegetation that grow quickly in the first 2 decades following a fire. Patterns of this growth are important for understanding ecosystem productivity and re-establishment success, but are logistically challenging to measure over long time periods. We tested the utility of a novel method of using shrub growth rings to estimate stand-level biomass accumulation at an annual time scale in southern California chaparral. We examined how temporal variation in precipitation and spatial variation in solar irradiation influence that accumulation. Using field measurements and a relationship between stem basal area and aboveground biomass, we estimated current biomass levels in an 11-year-old chaparral stand, and used growth-ring diameters to estimate growth in each year from age 4 to 11 years. We found that annual growth as measured by shrub growth rings tracked closely with patterns of annual precipitation, but not with time since fire. Solar irradiation was not found to be a significant covariate with total biomass by plot, possibly due to sampling area limitations. The close relationship of annual biomass accumulation with annual precipitation indicates that shrub growth-ring measurements can provide a useful metric of stand-level recovery.

PATTERNS OF POSTFIRE REGENERATION IN A SOUTHERN CALIFORNIA MIXED CHAPARRAL COMMUNITY

ABSTRACT Effects of prefire stand age, fire intensity, 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 Universitys Sky Oaks Field Station (San Diego County, CA). One stand was approximately 60-years old and the other 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 intensity was significantly higher in the older stand. Increased fire intensity was positively correlated with establishment of seedlings of Ceanothus greggii var. perplexans (an obligate seeder), and negatively correlated with seedling abundance of Adenostoma fasciculatum (a facultative seeder). 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 on 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.

Towards greater uniformity of species diversity studies in Mediterranean-type ecosystems

Abstract Understanding of inter- and intra-continental patterns of plant species diversity in a variety of ecosystems has been hindered by several conceptual and methodological shortcomings. In this paper two major areas of concern are identified and general guidelines for improving species diversity studies in Mediterranean-type ecosystems are suggested. A primary suggestion would be to standardize sampling techniques whenever possible. An example would be to include progressive nesting of quadrats at various scales within a 0.1 ha sample plot. Plots should be adequately stratified to represent the full range of environmental and disturbance gradients that occur within the overall study area. In addition, each habitat and/or disturbance type should be adequately replicated to ensure statistical validity of any comparisons undertaken. Sampling should take place during spring months when flowering and phenological activity is maximized. Secondly, greater uniformity in the analysis and presentation of diversity data would facilitate interstudy comparisons. Use of the nested quadrat approach permits analysis of species richness trends at scales ranging from 1 m 2 to 1000 m 2 or greater. Preference should be given to those diversity models and indices recognized as best depicting proportional abundances of species. In addition, identifying components of structural diversity contributed by various life- or growth-forms as well as native versus non-native elements would lead to fuller understanding of the data.