BRACKEN FERN INVASION IN SOUTHERN YUCATAN: A CASE FOR LAND …sypr.asu.edu/pubs/Environmental Science/geographical_review_schn… · land use suggests a more complex process involving

BRACKEN FERN INVASION IN SOUTHERN YUCATAN:A CASE FOR LAND-CHANGE SCIENCE*

LAURA C. SCHNEIDER

Land-change science seeks to understand land dynamics and their various conse-quences through an examination of coupled human-environment systems. This syn-thetic approach is used to address the recent, fourfold spread of bracken fern (Pteridiumaquilinum (L.) Kuhn) throughout the cultivated landscapes of the southern YucatinPeninsula, in Mexico. By joining biophysical, socioeconomic, and remote sensing-geographic information system (GIs) evidence, some of the one-dimensional expla-nations for the advance and persistence of the invasive species are rejected in favor ofthose focused on the synergy between human and environmental dimensions of theproblem.

Global environmental change and related studies increasingly recognize the use-fulness of integrative approaches that address the dynamics of coupled human-environment systems (Kates and others 20O1; Steffen and others 2003; Turner andothers 2003; Rindfuss and others 2004). Such approaches combine the human, eco-logical, and remote sensing and GIS sciences (Liverman and others 1998) and havebeen labeled "integrated land-change science" (Klepeis and Turner 2oOl; Turner2003; Moran, Skole, and Turner 2004; Rindfuss and others 2004). Since 1997 theSouthern Yucatdn Peninsular Region (SYPR) project has exemplified this kind ofscience in its efforts to understand and model the interactions and consequences oftropical deforestation and agricultural expansion around and within the CalakmulBiosphere Reserve and Mesoamerican Biological Corridor in the states of Campecheand Quintana Roo in Mexico (Turner and others 20oi; Turner, Geoghegan, andFoster 2004). The SYPR project early on identified an invasive species, bracken fern,as an important element of land-use dynamics in the region. That role, which isexamined in this note, illustrates the kind of understanding that results from usinga broadly integrated approach.

Research on invasive species is centered within the ecological community andon how invasion affects ecosystem structure, environmental productivity, biotic di-versity, and other factors related to ecosystem function in the face of land-coverchange (Elton 1958; Lodge 1993; Vitousek 1994; Vitousek and others 1997; Hobbs2ooo; Mooney and Hobbs 2ooo; Sutherst 2000). Although this research recognizesthe human-environment relationships embedded in invasions, most attention todate has focused on the ecological consequences of the invasion, as well as on the

SThis research was supported by the Land-Use and Land-Cover Change Program of the National Aeronautic andSpace Administration through Southern YucatOn Peninsular Region grants NAG 564o6 and NAG 511134 and by thesustained support of the Center for Integrated Studies on the Human Dimensions of Global Environmental Change,Carnegie Mellon University (National Science Foundation NSF-SBR 95-21914). My thanks to an anonymous re-viewer, to Billie L. Turner, and to Mazen Labban for their comments on the manuscript.

*Vo DR. SCHNEIDER is an assistant professor of geography at Rutgers University, Piscataway, New

Jersey o8854-8045.

The Geographical Review 94 (2): 229-241, April 2004Copyright © 2005 by the American Geographical Society of New York

THE GEOGRAPHICAL REVIEW

resulting land-cover changes (Hobbs 2000). This is attested by the Global InvasiveSpecies Programme and its concern for the effects of biological invasions on ecosys-tems services (for example, productivity and nutrient cycling; D'Antonio and Kark2002). Much less attention has been given to assessing the human-environment in-teractions in which the biological nature of plant invasions is explicitly linked tosocial, economic, and cultural causes of land transformation.

The Southern Yucatdtn Peninsular Region provides an ideal setting for the studyof human-environment relationships and an increasing plant invasion that threat-ens forest ecosystems (function and structure), landscapes (homogenization), andhuman well-being (agricultural production). The region's designation as a hot spotof biodiversity at great risk of deforestation is due in part to the significant growthof its agrarian population since the 196os (Achard and others 1998). At the sametime, much of the region's forest has been set aside as the Calakmul BiosphereReserve and is now connected to El Mundo Maya, an archaeoecotourism scheme,and to the World Bank and Global Environment Fund's Mesoamerican BiologicalCorridor (Primack and others 1998; Kaiser 2ooi; Turner and others 2oOl; Turner,Geoghegan, and Foster 2004). Plant invasions, which affect ecosystem recovery andhousehold economics, are an important part of land-use change in the region andare closely related to ecosystem function and landscapes. Bracken fern has increasedfourfold in the area since 1985, impeding regular succession of the vegetation andincreasing the areas under forest opened for cultivation (Schneider 2004). The gen-eral assumption in the literature is that this invasion is linked to land degradation(Suazo 1998), but the spatial distribution of bracken fern and its relationship toland use suggests a more complex process involving land-use strategies, land degra-dation, and fire regimes. In this field note I provide an overview and description ofthe "integrated" methodology used to understand such dynamics in the region.

AN INTEGRATED APPROACH TO UNDERSTANDING PLANT INVASIONS

Integrated land-change science calls for an approach in which human, ecological,remote sensing, and GIS sciences are combined in problem solving (Turner 2003;Moran, Skole, and Turner 2004; Rindfuss and others 2004). Since 1997 the SYPR hasexplored this kind of science in its efforts to understand and model the interactionsand consequences of tropical deforestation and agricultural expansion in the con-text of the needs of local land managers and the Calakmul Biosphere Reserve, ElMundo Maya, and the Mesoamerican Biological Corridor (Turner, Geohegan, andFoster 2004). The project examines the ways in which human activities and bio-physical processes interact to create the landscapes of the region. Bracken fern inva-sions appear to be a critical component of the landscapes resulting from theseprocesses (Turner, Klepeis, and Schneider 2003; Schneider 2004).

In order to understand the dynamics of bracken fern invasion, its links to vari-ous components of the human-environment system must be documented and ana-lyzed, particularly: socioeconomic conditions, based on studies of land-use historyand current land-management practices shown in household surveys; spatial land-

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GEOGRAPHICAL FIELD NOTES

FIG. i-The YucatAn Peninsula of Mexico, showing the Southern Yucatan Peninsular Region and theCalakmul Biosphere Reserve. (Cartography by the author)

scape conditions, based on remote sensing and GIs analysis of past and currentdistributions of land covers; and environmental conditions, based on ecologicaltransects and landscape ecology metrics. Once the pieces are understood they arelinked in land-change models to explain current bracken fern invasion and predictscenarios of its spread in the region. What is critical in this approach is the need toassess the coupled human-environment system as a whole rather than as an assem-blage of isolated major components.

THE SOUTHERN YUCATAN PENINSULAR REGION

The SYPR occupies about 22,ooo square kilometers of southwestern Quintana Rooand southeastern Campeche, north of the Mexico-Guatemala border (Figure i). Itcontains the largest and most rapidly disappearing contiguous tract of seasonaltropical forest in Mexico (P6rez-Salicrup 2004). The vegetation in the region is amosaic of forest types with different structural appearances (Turner and others 2oo1;Vester 2002; P6rez-Salicrup 2004) that reflect the variation in soils and other bio-physical conditions. The region's karst topography is characterized by undulatinguplands with elevations ranging from ioo to 350 meters above mean sea level; themain soil types are redzinas (mollisols) (Turner and others 2001). A significantdifference in total annual rainfall is observed from the southeast to the northwest,ranging from approximately 1,400 millimeters to approximately 900 millimeters

Mexico's Yucatan Peninsula

D Southern Yucatan Peninsular RegionMCalakmul Biosphere Reserve

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FIG. 2-A farm being invaded by bracken fern in the northern region of Calakmul Municipality,Yucatdn Peninsula, Mexico. Bracken fern (Pteridium aquilinum (L.) Kuhn) is creeping uphill from theforeground and mixing with Sabal mexicana Mart., a fire-resistant species known locally as "guano."(Photograph by J. Ronald Eastman, January 2003).

(Turner and others 2001). Given high evapotranspiration, few permanent sourcesof surface water, and groundwater sources at depths in excess of 150-200 meters,acute shortages of surface water are common toward the end of the dry season. Twomain types of seasonal deciduous forest-upland forest (selva mediana) and wet-land forest (selva baja inundable, or bajo)--cover the SYPR.

The region was the northern part of the central lowlands of the Classic Mayacivilization, and experienced one long wave in the rise and decline of occupationand use associated with that civilization until about A.D. 1000 (Turner 1983). Theforest returned, though altered in species composition, and remained largely unpop-ulated until recently. The region was significantly logged of its hardwoods (Spanishcedar and mahogany) in the mid-l9oos and then extensively colonized from vari-ous parts of Mexico. With an increase of the population from 2,500 to approxi-mately 35,000 since 196o, the SYPR is struggling to find a balance between theagricultural needs of the new settlements and the broader aims of preservingthe region as a biotic reserve and biological corridor and of supporting El MundoMaya (Foster and Turner 2004; Klepeis 2004). Currently, the dominant land-tenuresystem in the region is the ejido (communally managed land granted by the Mexi-can government to farmers, usually with usufruct tenure). The main activity of

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farmers is subsistence milpa (swidden) agriculture, combined with commercial chilicultivation and other strategies.

Most ejido agriculture is based on slash-and-burn techniques with continualrotation through forest fallow (Klepeis and others 2004). The system, referred tolocally by the Maya word "milpa," is dominated by maize but is often intercroppedwith squash and beans. In recent years, a growing number of farmers have intro-duced commercial crops, most notably chili, and pasture into their land-use port-folios. Although the incorporation of such uses has increased the complexity andvariation of swidden practices, earlier research in the region (Turner 1983) suggeststhat farmers employed a plot-fallow rotation predicated on three years of cultiva-tion and nine to twelve years of fallow. Except for the impact of some chili-culti-vation practices, this range still applies (Keys 2004; Klepeis and others 2004).

Remote sensing analysis shows that approximately 3.8 percent of forest (mainlyupland growth) was lost from 1987 to 1997 (Roy Chowdhury and Schneider 2004),

but with significant subregional variations in amount and rates of loss. The eastcentral and west central edges of the region have been substantially deforested forthe longest period, but new deforestation is focused along the southeastern edge ofthe Calakmul Biosphere Reserve. Overall, cultivated land appears to be shifting tosuccessional forests, indicating a shortening of fallow cycles (Roy Chowdhury andSchneider 2004). Significantly, much of this shift is linked to the emergence of mar-ket jalapefio (chili) production, as it is woven into milpa or undertaken throughmore permanent kinds of cultivation (Keys 2004). These and other land pressurespose various problems with respect to the needs of the conservation and archaeo-touristic programs for maintenance of mature forest in the region and for lesseneddemands on the remaining forest on ejido lands (Primack and others 1998; Turnerand others 2004).

BRACKEN FERN

Bracken fern is considered one of the most successful invasive plant species (Taylor199o). Except in desert regions, it occurs around the world. It is found in varioustypes of habitats, from woodlands to grasslands, and becomes a difficult weed toeradicate because of its persistent underground rhizome (Fletcher and Kirkwood1979). It establishes itself in areas dominated by fires, deforestation, and agriculturalactivities (Page 1986), causing severe problems to both farmers and conservation-ists (Pakeman and Marrs 1992). The increase of bracken fern in homogeneous areasis correlated with human activities (Rymer 1976). The high success of bracken ferninvasions in tropical regions is due in part to its dispersion mechanisms: Spores aredispersed all through the year, and once individuals are established, deep rhizomesallow their local persistence. Bracken fern's success in establishing itself is also dueto its high resistance to diseases and pests (Cooper-Driver 1990), to the presence ofallelopathic substances (chemicals that inhibit the growth of nearby plants) (Gliess-man 1976), to its prolific vegetative reproduction (Page 1986), to the high density of

the frond canopy and litter suppressing the ground flora, and to tolerance of a broadrange of climatic and edaphic conditions (Gliessman 1976; Page 1986). Another fac-

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tor that contributes to bracken fern invasion is the resistance of the rhizome to fireand adverse weather conditions, allowing the colony to spread vegetatively (Fletcherand Kirkwood 1979).

Bracken fern is reported as an agricultural weed in Europe, Canada, the UnitedStates, Australia, and New Zealand; however, the dynamics of invasion in tropicallandscapes has been minimally addressed (Suazo 1998). Research on this invasive inMexico is modest, although its disruption of land uses is well known. In Chiapas,for example, bracken fern covers as much as 36o square kilometers, disrupting bothagricultural and cattle-ranching activities (Suazo 1998). It is also a successful in-vader in the Southern Yucatdn Peninsular Region (Figure 2). Characteristics such asresistance to fire, tolerance to long dry periods, low susceptibility to diseases, carci-nogenic effects on potential predators (for example, cattle), and high rates of dis-persion make bracken fern very difficult to eradicate (Suazo 1998). Bracken fernplots tend to be quite large and homogeneous, due mainly to a frequent fire regime(two or three years) that eliminates other vegetation. Bracken fern rhizomes rundeeply into the soil and are the main reason for colony expansion. Areas covered bythe fern are structurally distinct from forest and other disturbed land covers in theSYPR region, which allows them to be detected and differentiated in remote sensinganalysis.

DATA AND ANALYSIS

Current data collection and analysis includes satellite data, household surveys, andecological transects. The remotely sensed data for 1985, 1994, and 20ol are used tocreate maps of the current extent and increase of bracken fern. Using principalcomponents analysis and texture analysis, it was possible to separate clearly the spec-tral signature of bracken fern from other land covers (Roy Chowdhury and Schneider2004; Schneider 2004). The analysis was undertaken with Landsat Thematic Map-per (TM) from 1985 to 1996 and with Landsat Enhanced Thematic Mapper 7 (ETM+)

from 1999 to 2001. Due to the radiometric and geometric improvements in ETM+,

principal components analysis and texture analysis were performed only on TM

imagery. Maps containing seven classes for each time period were created: uplandforest, wetland forest, secondary vegetation (four to fifteen years), agriculture,bracken fern, semi-inundated savannas, and water.

Using information on fern density from the satellite data, three villages wererandomly selected for detailed study, representing low, medium, and high densityin fern invasion. A stratified random sample of sixty farmers in the three villageswas selected for interview; each interview took approximately one day. All of theinterviews were conducted in Spanish and translated into English by the authorwithout any additional interpreters. The household heads were asked to respond toa standardized questionnaire to explore their socioeconomic and land-use history.The questions dealt with the household's immigration history, off-farm employ-ment, and demographic composition, as well as farm production, inputs, and fal-low cycle, with detailed questions about bracken fern for farmers who were coping

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with invasion. In addition, each interview involved a guided tour of the respondent'sagricultural plots. Using a global positioning system, a georeferenced sketch mapdetailing the configuration of the land and its uses was created, permitting linkagesto the remotely sensed data.

FIG. 3-The invasion of bracken fern (Pteridium aquilinum (L.) Kuhn)in the northern region of Calakmul Municipality, Yucatan Peninsula,Mexico, makes farmers' access to some areas a challenge, for individualplants usually reach a height of 2 meters or more. (Photograph by theauthor, May 2002)

Ecological data were collected from parcels selected from approximately half ofthe interviewed households (Figure 3). The parcels were grouped according to thesize of their fern patches (0-5, 5-10, and more than io hectares) and the time sincelast burn (less than 1, 2-5, and more than 5 years). Transects and soil sampling were

undertaken for each combination of parcel size and burn year, resulting in thirty

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TABLE 1-LAND CHANGE IN THE SOUTHERN YUCATAN PENINSULAR REGION

(in kM 2)

IMAGE MOSAICS

LAND COVER OR USE 1984-1988 1994-1997

Wetland forest 3,286 3,264Upland forest 12,502 12,064Secondary growth (7-15 years) 784 1,060Agriculture and pasture 524 599Bracken fern 39 149Semi-inundated savannas 48 48

Source: Adapted from Roy Chowdhury and Schneider 2004.

TABLE II-BRACKEN FERN INVASION AND LAND-MANAGEMENT CHARACTERISTICS OF Two EJIDOS

IN THE SOUTHERN YUCATAN PENINSULAR REGION

CHARACTERISTIC EJIDO NICOLAS BRAVO EJIDO LA GUADALUPE

Year of establishment 1941 1985Total area 885 km 2 50 km 2

Parcel size per farmer 100 ha 40 haTotal area under bracken fern in 2001 5,679 ha 88 haPercentage of all parcels with bracken fern present 49 20Average size of bracken fern patches 19 ha 2 haAverage age of bracken fern patches 25 years 10 yearsAverage size of plots being farmed 3.8 ha 4.7 haArea of pasture and mechanized agriculture 14 ha 6 ha

Source: Adapted from Schneider 2004.

transects and sixty soil samples. The transects consisted of six i-square-meter sampleplots separated by 15 meters in smaller parcels and by 30 meters in larger parcels, inwhich vegetation was measured (height and diameter at breast height), weighed (bothlive and dead biomass), and classified (common name or species). This evidence wasused to generate measures of fern invasion, composition, and structure. The soilsanalysis included main macronutrients and micronutrients (for example, nitrogen,phosphorous, manganese) and other variables, such as organic matter and pH.

Preliminary results for the areas analyzed using remote sensing, as well as therelationship between management practices and bracken fern invasion, are sum-marized in Table I. The woody land-cover distribution includes upland forest (ap-proximately 12,000 square kilometers), wetland forest (approximately 3,200 squarekilometers), and secondary growth (approximately i,ooo square kilometers), lo-cated mostly next to areas currently being farmed. Less-disturbed forest occurs inthe west central part of the region, where the Calakmul Biosphere Reserve is lo-cated. Agriculture predominates in the three other areas. Of these, the eastern areacontains most of the older ejidos and shows high-density forest disturbance but

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decreasing cultivated and pasture lands and increasing bracken fern invasion. Thesetrends could be explained by the fact that the older ejidos in the east are expandingtheir household incomes beyond agriculture and limiting cultivation to those par-cels or amounts covered by government subsidies (Klepeis and Vance 2003).

Bracken Fern Spread on the Ejido Nicol6s Bravo, Mexico

4

A ForestAgriculture

5 k 1985 2001 Bracken fern

FIG. 4-Comparison of satellite images reveals the extent to which Pteridium aquilinum (L.) Kuhn,

or bracken fern, spread on the Ejido Nicol]s Bravo, in Mexico's Yucatdn Peninsula, between 1985 and

2001. (Cartography by the author)

Although bracken fern has increased in area across the region (Table I), theeastern part of the SYPR remains the epicenter of its importance. Analysis of remotesensing imagery shows a rate of increase of 3 percent from 1985 to 2001, comparedwith o.8 percent for the western part. Bracken fern appears to be denser on private

lands than on ejido lands. Most private lands in the region are small ranches whosemanagers do not expend the labor and other costs needed to control the invasion.All of the bracken fern in the Calakmul Biosphere Reserve is in the buffer zone ofagricultural ejidos. Virtually no fern exists in the core zone of the reserve or in theseveral forest extension ejidos, forest lands given to agricultural ejidos for variousextractive uses.

A more detailed characterization of bracken fern is under way for the threecase-study ejidos. Initial analysis of survey data in two of these ejidos (Table II,Figure 4) suggests that bracken fern invasion is negatively correlated with land avail-ability. Bracken fern density is low in land-sparse ejidos characterized by high land

pressures (4o hectares per household) and intensive cultivation (especially com-mercial chili cultivation). In contrast, fern density is high in land-surplus ejidoscharacterized by low land pressures (8o hectares per household) and less-intensivecultivation. This pattern, which appears to be consistent with observations of otherejidos in the region, is counterintuitive to the perception that longer-fallow cultiva-tion is more environmentally benign than are shorter-fallow systems and to the

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SYPR project speculation that bracken fern prevalence is associated with longer-term land use and repeated burning of the landscape.

Combining biophysical, socioeconomic, and spatial data relevant to brackenfern prevalence provides a different understanding. Analysis of topographic char-acteristics indicates that steep slopes and areas facing the sun from a northeasterlydirection are more susceptible to the establishment of bracken fern than are otherareas in the region. Repeated burning of fern-dominated areas also favors retentionof the species. These factors are the subject of ongoing research. Socioeconomic andspatial data also indicate, however, that farmers' willingness to combat bracken ferninvasion is related to the land, labor, and capital conditions of the farm household.In land-surplus conditions, as among the eastern ejidos, the high labor and othercosts involved in controlling bracken fern lead to a common response: Leave theinvaded land, and cultivate the noninvaded land. This option is not viable in land-sparse conditions, as among the southern ejidos, where the fern is controlled themoment it begins to invade parcels. Land-use intensity, therefore, appears to play afundamental role in decisions about bracken fern management, consistent withvarious induced intensification theses (for example, Turner and Shajaat Ali 1996).

These tentative findings are supported by the land-use history of the Ejido NicolisBravo, where remote sensing analysis (TM and ETM+ imagery) revealed that a largeexpanse of bracken fern had developed between 1985 and 2001 (Figure 4). For ex-ample, ground surveys revealed a parcel to the northwest of the ejido to be a formercattle-ranch project sponsored by the Banco Nacional de Cr6dito Rural to assistejidatarios (farmers). Starting in the early 198os, farmers cleared approximately 200

hectares used for maize mixed with pasture. After two years the land was fully cov-ered with pasture grasses. The project persisted until 1987, when it ended due mainlyto internal conflicts among its members; the cattle were sold, and the land was aban-doned. Opened but unmanaged land provides ideal conditions for bracken fern:minimal competition for light and easily desiccated vegetation receptive to inci-dental burning from swidden fires generated on land surrounding the abandonedranch. In this case, the invaded lands were under no specific farmer's control andwere of no value to any individual household. Therefore, no one attempted to com-bat the fern. Firmly established, its fire ecology ensures its persistence in the nearterm. Some bracken fern stands in the region have existed for more than twenty-five years.

This example was used because of the size of the invaded area and its visibilityin the imagery (Figure 4). The base conditions involved, however, are similar tothose throughout the larger, eastern ejidos, where the fern is prevalent. Individualfarmers have sufficient land (up to loo hectares), such that several hectares of swid-den land lost to the invasion can be offset by shifting to other parcels. The land-stressed farmers in the south do not have this luxury, so the moment the fern appearsin cultivated or fallowed parcels, weeding begins. In this labor-intensive way, otherplant species are given an advantage: Once they reach sufficient height, their shadeinhibits bracken fern growth.

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HUMAN-ENVIRONMENT RELATIONS DRIVE BRACKEN FERN INVASION

Bracken fern has become an important component of the coupled human-envi-ronment system of the Southern Yucatdn Peninsular Region. Its nearly fourfoldincrease between 1985 and 2001 was associated directly with human disturbance,primarily agricultural activities. Once established, bracken fern's persistence is sup-ported by fire, mostly incidental burns from the large number of swidden fires setevery year to clear farmland and pastureland. Its impacts include impediment offorest succession and farmland fallow, reduction in biotic diversity, and high laborcosts to combat its spread. Although other factors are involved, bracken fern ap-pears to become dominant in those disturbed parcels in which, for whatever rea-

son, direct management has ceased. In many cases, the decision as to whether tomanage the invasive species is tied to household and ejido land pressures. This un-

derstanding of the invasion dynamics is facilitated by linking the three arms of inte-grated land-change science: biophysical, socioeconomic, and remote sensing / GIs

analysis.

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TITLE: Bracken Fern Invasion in Southern Yucat%an: A Casefor Land-Change Science

SOURCE: Geogr Rev 94 no2 Ap 2004WN: 0409403133008

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