The Construction of Mediterranean Scrub in Biogeography and Ecology, C.M. Rodrigue, AAG 2005

The Construction of Mediterranean Scrub in Biogeography and Ecology

Association of American Geographers, Denver, 5-9 April 2005

Christine M. Rodrigue
Department of Geography
California State University
Long Beach, CA 90840-1101
https://home.csulb.edu/~rodrigue/
rodrigue@csulb.edu

Abstract

There is a marked difference in the representation of Mediterranean scrub vegetation (e.g., chaparral, maquis) in North American and European literature in biogeography and ecology. Authors discussing this vegetation in the California context accept that it is a natural response to the summer drought climate and steep terrain and is adapted to and may exploit its late summer and fall fires. Debate here focusses on the extent to which humans have modified or, indeed, can modify "natural" fire regimes. European authors frame this vegetation instead as a secondary successional formation in a landscape that "should" be dominated by oak woodland and forest. The widespread presence of Mediterranean scrub is seen as an artifact of human disturbance over thousands of years, mediated through overgrazing, deforestation, accelerated erosion, and anthropogenic fire. This paper will present a content analysis of the Mediterranean scrub literature, in order to engage both traditions in the construction of a unified framework for these pyrogenic formations.

Introduction

Mediterranean summer drought climates are found in only a few places on Earth [ PPT 2 map]. Woody vegetation in these regions displays a number of typical xerophytic adaptations to the summer drought: reduced leaf size, coriaceous or sclerophyllous leaves, oily or waxy leaves, dense and thick bark, deep root systems, and the capacity to stump sprout after fire. The vegetation is capable of tolerating fires, which often take place in late summer or early fall with the advent of adiabatic winds coupled with anthropogenic or lightning set fires. Many species rely on stump sprouting to recover from fire, but fire-dependent germination is utilized by many species to ensure optimal survival conditions for seedlings.

One of the Mediterranean vegetation formations is the Mediterranean scrub, familiar to Californians as chaparral and to Europeans as maquis [ PPT 3 photographs ]. This is a shrub-dominated formation, typically ranging from 0.5-4.0 m in height and generally forming a dense cover with nearly 100% ground cover and leaf area indices around 2. While Mediterranean scrub species can generally grow in a variety of soils, it competes best on steeper slopes with coarse, unstable, and poorly profiled soils. As such, it can commonly be found on steep slopes from 400-2,000 m.

The similarities among the various Mediterranean scrub formations has not led to similarity in interpreting them among the biogeographers, ecologists, botanists, and foresters dealing with them in the different Mediterranean regions. [ PPT 4 ] The purpose of this paper is to establish the differences in interpretation between the American and the European traditions in these fields, with an eye towards the differences in fire hazard management they imply.

The Literature of the Mediterranean Scrub

The two literatures agree on the basic characteristics of the Mediterranean scrub, as described above. They disagree in interpreting the functions of such scrub in the seral stages of succession. There are also differences in the debates in each literature about the rtle of wildfire in Mediterranean landscapes. It is possible at least partially to reconcile these differences, and it is the underlying goal of this paper to initiate this reconciliation by calling attention to these differences.

Succession in Mediterranean Landscapes in the European Tradition

In European writings, Mediterranean scrub is commonly described as degraded secondary successional formations that express the negative impact on the landscape of thousands of years of human activities: deforestation for agricultural clearing or for timber, overgrazing or overbrowsing by livestock (notably goats), and by wanton use of fire for clearance and convenience. The regional "climax" vegetation is characterized as oak-dominated forests and woodlands, which hang on as relict stands in a few scattered areas. Some examples follow (with links to expanded quotations)

A popular British textbook in physical geography puts it this way:

In all Mediterranean regions ...The native woodland has therefore been replaced by dense scrub...

Walter's classic text argues that

If cultivation or grazing is stopped then successions tending towards the true zonal vegetation take over

Woodward, an American author, takes a European stance towards "Mediterranean scrub" in her 1996 biogeography course web page:

Much of the formation is considered a subclimax developed on degraded and eroded soils and maintained in part by fire and goats.

LaBianca, et al., write that:

... the outlines of the story of how the prehistoric Mediterranean woodland forest was destroyed has begun to come to light. ... other findings ... point to the existence of a Mediterranean Woodland Forest here during Early Holocene and Neolithic times.

A suspicion towards this canonical view is raised by Trabaud and Galtié:

Has the frequency of fire produced the presence of the shrublands or has the existence of the shrublands allowed the occurrence of repeated wildfires?

A British historical ecologist, Rackham, expresses the strongest skepticism towards this common perception:

Most Mediterranean countries regard themselves as ruined landscapes, "degraded" through thousands of years of misuse of the land, which might be "restored" to the forests supposed to have existed in an idealized past.

Chaparral in the American Tradition

A different impression is conveyed in American literature. In this tradition, chaparral is described as one of the "climax" formations that develops in Mediterranean climate rigimes under slope, edaphic, and fire conditions that give it the competitive advantage over oak park and woodland, mixed woodland, California prairie, or coastal sage/soft chaparral. This is not to overlook the anthropogenic impacts in Mediterranean ecosystems, but the premise is that the formation itself is a natural element of the landscape.

For example, in a common American introductory physical geography textbook, the flavor of this perspective comes through:

Thus the vegetation is a mosaic related to site characteristics and microclimate.

In an American biogeography textbook, MacDonald writes:

The distribution of woodland, shrubland, and grassland can reflect a number of factors, including regional rainfall differences, slope aspect, substrate, and disturbance. ... In addition, it is thought that some of the extensive shrublands around the Mediterranean Sea are the result of overgrazing by goats and other livestock, followed by erosion of the topsoil.

Applied landscape management literature expresses a similar view, as seen in Radtke:

The mixtures of plant species within these areas are determined by such factors as aspect and steepness of slope, soils, elevation, fire frequency, and local climate.

Minnich emphasizes the particularity of chaparral and other vegetation associations in terms of zonal and elevational factors, and slope and aspect:

Grassland, coastal sage scrub, and chaparral, in which nearly all the burning detectable by Landsat imagery occurs, are divergent in terms of physical appearance, rooting structure, phenology, drought stress, fuel, and fire response.

The European view of Mediterranean scrub did affect early perceptions of California chaparral until regional ecology and biogeography matured with fieldwork in the area. Hanes noted some of these views in 1971:

One (misconception) is that primeval forests were open and park-like, and brushy areas were small and insignificant until white men (sic) settled California.
The successional status of chaparral has been debated for years, some botanists even questioning whether chaparral is a climax association.

A Shifting European Perception of the Human Rôle in Mediterranean Scrublands

As seen in the Trabaud and Galtié and Rackham excerpts, there is a new skepticism within the European tradition towards the rigidity of interpretation of the Mediterranean scrub. One factor driving this re-examination is the increase in large fires coonciding with the abandonment of Mediterranean agropastoralism with the globalization of economic competition and the increase in home construction in fire-prone amenity areas.

For example, Rackham puts it:

There is now much more to burn than when the countryside was densely populated and used. Abandoned farmland or neglected pasture turn into forest or shrubland. ...Foresters promote fire in another way, when they exclude browsing animals and thus allow native vegetation to grow up and accumulate fuel.

In a similar vein, Quézel, et al. write:

In countries on the northern Mediterranean basin, the collapse of the age-old agrosilvipastoral system is leading to deep changes in the structure and architecture of forest and pre-forest communities of plants and animals.

Mazzoleni, et al. drew out the same concerns about agricultural abandonment:

Records show that grazing of semi-natural Mediterranean vegetation by sheep and goats has virtually ceased in several European countries where marginal land has been abandoned. ... This in turn then affects the frequency and intensity of fires.

The American Debate over Chaparral Fire Dependency Heats up

The European literature is not alone in re-assessing its assumptions. In the American literature, however, the focus is not on the naturalness of chaparral but on the rtle fire plays in vegetation succession. The oldest writings on California chaparral followed European disparagement of the scrublands, giving way to a more empirically-grounded view as early as the 1920s. Much attention began to focus on adjusting the climax vegetation concept to accommodate the kind of seral mosaic landscape produced by this vegetation that adapts to and exploits fire disturbances associated with the summer drought climate.

An entire school of thought grew up around the reconceptualization of chaparral as pyrogenic, not just adapted to fire but actually dependent on it and capable of producing the conditions that enable fire. Examples of writings in this vein include these.

Hanes, for example, concludes:

Chaparral fires are both natural and inevitable. A fire-exclusion policy does not prevent fire, it only forestalls fire.

Minnich made perhaps the most forceful statement of the fire-dependency of chaparral:

The present regime of large, intense conflagrations in southern California chaparral appears to be an artifact of fire suppression..... Since 1910, small fires have been replaced by ever-larger ones, with numerous conflagrations since the 1950s despite increased suppression investment.

In my own 1993 article, I expressed this line of thought as:

The longer the period since a fire, the greater both the probability and the magnitude of the next fire.

This view has recently been challenged. A number of studies have directly questioned whether fire hazard in chaparral is dependent on the age of the stands and whether large fires have, in fact, actually increased in frequency in the wake of fire suppression. They argue, further, that too-frequent fires lead to type-conversion from chaparral to grasslands dominated by exotic species and susceptible to invasive species. Examples in this critical vein include:

Keeley and Fotheringham, who stated:

As is the case today, the natural fire regime was likely characterized by many small fires and a few large fires that consumed the bulk of the landscape. ...Today, fire suppression is required just to maintain some semblance of the natural fire regime.

Moritz argues that age-dependency is almost completely trumped by meteorological conditions in accounting for the spatial distribution of fires in chaparral:

I hypothesize that fires during extreme weather conditions have been capable of burning through all age classes of the vegetation mosaic.... Findings contradict the assertion that, in the absence of fire suppression, large fires would be constrained by more complex age-patch mosaics on the landscape.

No matter the vehemence of this debate over the relative importance of fuel accumulation and weather conditions in setting the stage for the kind of massive fires seen in California in 2003, both camps do converge on a common implication for wildfire hazard prevention and mitigation. That is, whether wildfire magnitude and probability increase through time since a previous fire or whether massive landscape-clearing fires are the outcome of a random convergence of extremely dry conditions coupled with anthropogenic or lightning-induced fires, continued residential development of the wildfire- urban interface is hazardous.

It also entails a massive social subsidy: The firefighting taxes and insurance assigned risk pools represent the externalization and socialization of private household-level risk assumption. As I put it in 1993:

...the benefits of an amenity view are privatized, while the private hazard costs to the household are reduced by the socialization of fire hazard mitigations.

The irony of this socialization of vulnerability to chaparral wildfire hazard is that home survival is quite possible even in the most fire-susceptible landscapes. As Cohen and Saveland put it:

...structure survival is the essence of the W-UI fire problem, but structure ignition is the critical element for survival. ... experiments and model results indicate that flames are an ignition threat only at close distances to a structure. ... Vegetation management beyond the structure's immediate vicinity has little effect on structure ignitions.

Toward Reconciliation

There are conceptually a few lines of potential reconciliation between the European and the American traditions in their representations of the Mediterranean scrublands and the rtle of fire in them.

First, the European tradition could resituate Mediterranean scrub as a natural formation as validly present in the Mediterranean borderlands as woodland and forest but adapted to dominate different specific situations: areas of steep slope, unstable soil, and recurrent fire.

Second, the anthropogenic rtle in altering the landscapes of the Mediterranean is not diminished by recognizing that it works through altering the relative areas suitable for dominance by the oak-dominated woodlands/forests and by the various scrub formations. Deforestation and overgrazing resulted in accelerated erosion and that increased the areas of steep and unstable slope suitable for expansion of the natural scrub elements.

Third, European scholars and many others need to reframe what exactly is "natural." How far back does one have to go to get to a "pristine" nature, one not "sullied" by human activities, as a goal for landscape restoration? In the Mediterranean borderlands, agriculture and animal husbandry and their transformations of the landscape go back 10,000 to 11,000 years ago. The millenia just before that comprised the colder Younger Dryas (~13,000 - 11,500 BP) and before that the warm Late Glacial Interstadial of ~15,000 - 13,000 BP. It is hopeless romanticism to hearken back to a pristine nature in the Mediterranean borderlands and possibly in California as well, given human settlement here sometime after 15,000 BP.

Fourth, agriculture and animal husbandry may well have produced the kind of mosaicking in the Mediterranean landscape that really could prevent or reduce massive conflagration by reducing the ground cover, particularly the highly flammable ground covers of maquis, garrigue, and grassland. The rural depopulation and abandonment of agropastoralism in the European Mediterranean has ironically been accompanied by an increase in landscape-consuming fires. The goats might not have been so bad, after all! This counterintuitive effect might be explicable in terms of Cohen's findings about fire behavior and ignition potential. The low cover of crop plants and fodder deny wildfire ladders into the crown through prevention of brand-formation and amplified temperatures.

Fifth, California researchers need to settle the acrimonious debate over the balance among fuel accumulation, stand age, meteorologic effects, and ignition sources to understand wildfire dynamics in California chaparral, coastal sage, and grassland and their susceptibility to invasive exotic species.

Sixth, no matter how the California "Fire Wars" work out, the message for urban planning in the Wildfire-Urban Interface is clear: Discourage residential development in the Wildfire-Urban Interface and reprivatize chaparral fire vulnerability back to the household level to encourage firewise home modification and thereby promote structural survival in the worst California firestorms.

Acknowledgments

I wish to acknowledge the field projects in Malibu, the Santa Monica Mountains, and the Orange County South Coast Wilderness conducted during the Geoscience Diversity Enhancement Project at CSULB (NSF Grant # GEO- 0119891) for inspiring my interest in this literature.

And to the GDEP summer interns:

Terry Lumati (Long Beach City College)
Simeon Haynes (Cabrillo High School)
Carlos Takashima (El Camino College)
Barbara Talalemotu (El Camino College)
Sally Lwin (Lakewood High School)
Luz Mendez (Cerritos College)

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