Long-Term Forest Dynamic After Land Abandonment in a Fire Prone

Landscape Ecology (2005) 20: 101–112 Springer 2005 DOI 10.1007/s10980-004-1297-5 -1 Research article

Long-term forest dynamic after land abandonment in a ﬁre prone Mediterranean landscape (central Corsica, France)

Florent Mouillot1,*, Jean-Pierre Ratte1, Richard Joffre1, David Mouillot2 and Serge Rambal1 1IRD-UR 060, DREAM CEFE/CNRS, 34293 Montpellier Cedex 05, France; 2UMR CNRS-UM2 5119, 34095 Montpellier Cedex 05, France; *Author for correspondence (e-mail: [email protected])

Received 30 December 2003; accepted in revised form 12 July 2004

Key words: Land cover change, Landscape patterns, Mediterranean-type ecosystem, Transition matrix

Abstract

Two hundred years of landscape changes were studied on a 3,760 ha area of central Corsica (France) representing a typical Mediterranean environment. Different historical sources, including an accurate land-cover map from 1774 and statistics on land cover from 1848 and 1913, were used. Three additional maps (1960, 1975 and 1990) were drawn, and a complete ﬁre history from 1957 to 1997 was created. Forests expanded slowly by a border effect. Forest expansion was more rapid in unburnt sites (0.59% per year) than in burnt sites (0.23% per year), mostly because the initial amount of forests was greater. Because of the border effect, the combination of past landscape pattern and short distance colonization abilities of forest species may have allowed the shrublands to persist in some places after land abandonment. This persistence may explain the pattern of ﬁre in the landscape, since shrubland burn more readily than forests.

Introduction In recent times, most Mediterranean landscapes of southern Europe have undergone dramatic land Plant growth rates and life history traits have been abandonment (Debussche et al. 1987; Etienne et al. shown to have strong effects on local processes of 1998; Le Houe´rou 1992; Lepart and Debussche succession in disturbed habitats (Grime 1977; Noble 1992; Moreira et al. 2001; Pausas 2004). Although and Slatyer 1980). However, dispersal ability is a landscape changes in the Mediterranean basin have major spatial process driving landscape dynamics, and been recently studied, most of those studies focussed recent studies have increased understanding of how on quantifying the rate of land abandonment (Deb- the spatial pattern of seed sources and colonizable ussche et al. 1999; Debussche et al. 1987; Garcia- patches, can influence the spatial and temporal course Ruiz et al. 1996), forest expansion (Debussche and of colonization (Gustafson and Gardner 1996; Holt Lepart 1992; Moreira et al. 2001), or fire effects et al. 1995; Lavorel et al. 1995; Turner et al. 1997; (Trabaud and Galtie´ 1996; Vazquez and Moreno With and King 1999). Indeed, in the case of land 2001). The processes driving these dynamics have abandonment, the initial landscape pattern may drive been identified to be controlled by species dispersal the course of succession by determining the compo- abilities (Ne’eman and Izhaki 1996), and locally sition and the spatial distribution of both the seed modified by soil properties (Roche et al. 1998; source and host community (Smith et al. 1993). This Tatoni et al. 1994). More recently, spatially explicit pattern may also influence the occurrence and the studies modeled landscape dynamics based on initial spread of disturbances (Turner et al. 1989). landscape pattern and few biologically relevant 102 variables (Carmel and Kadmon 1999; Carmel et al. 2001; Kadmon and Harari-Kremer 1999; Pausas 2003). However, analysing and understanding the interaction of historical spatial modifications of these landscapes with recurrent fires and environment has rarely been considered because few data are avail- able on both vegetation changes and fire history. Understanding these interactions would clarify the remaining question on the inter-related effects of land abandonment and fire on long-term Mediterra- nean vegetation dynamics. Particularly, our goal is to identify the potential feedbacks of spatial changes in landscape composition on the fire regime itself. In this study, we intend to answer this question i) by reconstructing both the land-cover dynamics and fire history over a period sufficiently long to reflect both land use changes and intrinsic successional processes, and ii) by analysing and simulating the rates and patterns of change in landscape composition after land abandonment, in relation to topography and fire.

Study site

The study area is located in the Venaco area (4312¢ N, 912¢ E), in the central part of Corsica (a French Mediterranean island covering 8752 km2 Figure 1. (a) Location of Corsica and the Venaco area in the west (Figure 1a). For centuries, the region has been covered Mediterranean basin. (b) The study area (black line) is also reported on the topographical map of the Venaco region (dotted line). by mixed areas of managed forest and agriculture (Diodore of Sicily, ND), to ‘provide food for livestock … on a rude, uneven and difficult terrain’ (Rousseau 1765). The collapse of the agricultural activity at the composed of sandy brown and slightly acid soils (pH beginning of the 20th century, due to the geographical = 6.5) (Mesleárd and Lepart 1991). isolation of the island and new agricultural policies, The climate is Mediterranean (cool moist winters allowed only cow, sheep and goat grazing as major and hot dry summers). Annual rainfalls recorded at activity (Lenclud 1980). The study area covers 3,760 Venaco for the 1960–1982 period range from 668 mm ha in the Parc Naturel Regional, with an elevation to 1709 mm with a mean of 1088 mm. Eighty percent ranging from 200 m to 1100 cm. The area consists of of precipitation occurs between October and April. foothills, with a Mediterranean-type vegetation Mean annual temperature is 13.5 C; it varies between growing on acidic soils. Patches of Quercus ilex L., Q. 6.2 C in February and 22 C in August. pubescens Willd., Q. suber L. or Pinus pinaster Ait. woodlands co-occur with patches of shrublands (called maquis): High shrublands (1-2m height) are Material and methods dominated by Arbutus unedo L. and Erica arborea L. Low shrublands (height < 1m) are dominated by Historical land cover reconstruction Cistus monspeliensis L. (Mesleárd and Lepart 1991). The maquis area is approximately delimited on its The creation of a map (Plan Terrier) was ordered in west side by a 1000m altitude contour line, and on the 1774 by King Louis the fifteenth to inventory the southern and eastern boundary by the Vecchio- natural resources of the island after its acquisition Tavignano river corridor (Figure 1). The substrate is (Caratini 1995). A precise map at 1:10,000 scale was 103 consequently published in 1793 (Albitreccia 1942). tween 75% and 100% of the polygon, ‘open forest’ Land-cover patches are represented by polygons on a between 25% and 75%. The remaining 5 non-forested schematic topographic background. A copy of this types (trees covering 0–25%) were then defined map was scanned and georeferenced using IDRISI according to the percentage cover of high shrubs (Eastman 1987). Confluence and ridge lines were used (>1m, <2m), low shrubs (<1m) and herbaceous, as as base points for georeferencing the map with Lam- described in Table 1. The vegetation types were bert 4 coordinates; twenty-two base points were used aggregated into the 4 ‘generic’ land cover types for on the whole landscape. The polygons were redrawn in some analysis (Table 1). vector format by tracing the lines of the scanned and The three maps (1960, 1975, 1990) were digitized georeferenced image. Each polygon was assigned to and converted to 20m resolution raster format in a GIS one of eight land cover types (Figure 2a). Since species database (Figure 2b, 2c, 2d). The aspects map was are not mentioned in the initial legend, we considered derived from a 20m Digital Elevation Model (DEM). that ‘Chestnut forests’ were dense forests containing not only Castanea sativa but also Quercus ilex and Fire maps Quercus pubescens, and that ‘woodlands’ were exclusively Quercus ilex. Pinus pinaster forests are not Major fires have been mapped yearly on the study mentioned in the legend, but they may have been area since 1970 based on police and fire reports, and sporadically present in the shrublands. We aggregated registered on a GIS database in vector format (source olive trees, vineyards and crop cultivations into a ODARC-Bastia: Agricultural Office of Corsica). This ‘cultivated areas’ generic land cover type. The map database was validated and updated using local was converted to raster format with 20 m resolution. reports (Joffre 1981) and aerial photograph surveys Two additional surveys were carried out on the from 1975 and 1990. Additional fire boundaries territory by the French fiscal services (Source: Ar- (Barry and Maniere 1975; Joffre et al. 1982) were chives departementales de Haute-Corse). These sur- added on a topographical map at 1:25,000 scale, veys provide landscape composition for the years digitized and converted to 20 m resolution raster 1848 and 1913, but in a non-spatial format. We used format. This work led to a complete database for the them as intermediate updates of landscape composi- 1957–1997 period (Mouillot et al. 1998) (Figure 2e). tion in Figure 3. They consider the four ‘generic’ land cover typesÀforested, shrublands-grasslands, culti- Land cover change analysis vated and urban areas – that we used as the common classification for long-term translation across time. Temporal analyses were performed in 1960, 1975 and 1990, using transition matrices. We limited this approach to these three dates because those maps have Present land-cover maps and additional data a common legend, and the time interval between two consecutive maps is long enough for changes in Joffre (1981) and Amandier et al. (1984) established vegetation types to be detected. This interval was not precise land cover maps for 1960 and 1975 respec- so long as to hide trends due to changes in land use or tively. Each map was built from 1:17,000 aerial disturbance regime. Transition matrices were obtained photograph interpretations (panchromatic for 1960, by cross-tabulating the maps, pixel by pixel. Cells infra-red, false colour for 1975). We used the same above the diagonal represent areas which have approach to draw a vegetation map for 1990 on reached a more advanced successional stage. The cells topographical maps at 1:25,000 scale, based on color below the diagonal represent regression to earlier aerial photographs. successional stages. On these maps, cultivated areas, urban areas and riparian forests are precisely located. We then Analysis and modeling of forest expansion assigned vegetation types to the remaining polygons. Seven vegetation types were defined according to a Forest expansion in relation to topography and fire typology based on vegetation layers developed by was examined by spatial map analysis and modeling Godron (1968) and Trabaud (1973) for the Mediter- within a GIS. From the continuous map of aspect ranean vegetation. The vegetation type ‘dense forest’ values (0–360) and the fire map (burnt at least once was assigned when trees (height > 2m) covered be- vs not burnt between 1957 and 1997), the landscape 104

Figure 2. Land-cover maps of the Venaco county in (a) 1774, (b) 1960, (c) 1975 and (d) 1990 with their corresponding keys, and (e) cumulated ﬁre frequency map on the Venaco study site for the 1960–1990 period. 105

To determine whether forests are evenly distributed across the landscape, we tested for correlation between the forest surface area in each burn x aspect category (in ha) and the total surface area of these categories (in ha). We tested the slope different from 0 by a t-test. The differences in forest expansion between burnt and unburnt sites were tested using a matched pairs Student T-test. Both tests used a 5% critical value.

Results

Land cover changes Figure 3. Percentage cover of forests, shrublands-grasslands, cultivated and urban areas in the Venaco county (3,760 ha) from 1774 From 1774 to 1913, cultivated areas dominated to 1990 according to the initial map ‘Plan Terrier’ (1774), the two (Figure 3). Throughout this period, forests had a fiscal surveys (1848,1913) and the 3 land-cover maps (1960,1975 and 1990). rather stable cover of about 20%. The cover of shrubland-grasslands was inversely correlated with agricultural pressure. Shrublands-grasslands were reduced to less than 10% in the middle of the 19th was reclassified into 16 environmental categories, (8 century, when agricultural activity was at its maxi- aspect classes (45 interval, i.e., [337.5 À 22.5], [22.5 mum. From 1913 onwards, agricultural activity À 45], · 2 burn categories). The percentage area of strongly declined. It reached its minimum in the each environmental category actually covered by 1970’s, during which time small gardens of chest- forest was calculated for the years 1774, 1960 and nuts, olive trees and orchards around urban areas 1990, as was the expansion between 1960 and 1990 covered less than 3% of the landscape. When the (Table 3). land abandonment and rural exodus processes To test for forest expansion by isotropic diffusion reached their maximum (1913–1960), shrubs rapidly around the initial forest, we used the DISTANCE invaded up to 68% of the landscape, a nearly six procedure of the IDRISI software. From the initial fold increase, whereas forests expanded only from 1774 map, the distance to the nearest forest edge was 20% to 38%. calculated for each non-forested pixel. The pixels Transition matrices for 1960–1975 and 1975– were grouped according to 50 m-wide intervals. We 1990 (Table 2) show that forests have mainly in- then calculated the fraction of each distance class creased at the expense of shrublands (9.3% and actually covered by forest in 1960, 1975 and 1990. 5.6%) rather than cultivated areas (0.8 + 0.1% and This fraction describes the overall probability of a site 0.3 + 0.2%), and few forested areas disappeared (2.8 to be colonized by forest according to its distance and 1.1%). Dynamics in the shrublands and grass- from the initial forest edge (see Figure 5). land is less clear, as we observe in the same pro- We fixed three thresholds on the continuous dis- portion, either a progressive dynamic towards a tance map to build three hypothetical forest cover more developed vegetation (9.4% and 11.7%), or a maps, which have the same surface area as observed regressive dynamic towards a lower biomass vege- in the actual maps of 1960, 1975 and 1990. The actual tation (8.2 and 6.6%), leading to a dynamic equi- and simulated maps differ only in the spatial distri- librium between herbaceous and shrubland areas. bution of the forested area. The three hypothetical Bare soil is mostly the result of recent fires, as for maps represent the forest patterns which would have example the 8.5% of the landscape covered by bare occurred with the hypothesis of isotropic forest dif- soil in 1975 was rapidly transformed into a shrubby fusion, or, theoretically, the forest patterns which vegetation within 15 years (7.0% out of the 8.5% of would have occurred neither fires nor other abiotic bare soil of 1975 are back to a shrubby vegetation parameter influenced on forest expansion. in 1990 in Table 2). 106

Table 1. Land cover types (vegetation types 0–7 and land use types 8–9) as deﬁned by anthropogenic inﬂuence and percentage cover of the four vegetation layers ‘tree’, ‘high shrub’, ‘low shrub’ and ‘herbaceous’. [0; 2] types refer to the generic type ‘forested’ areas, [3; 6] to ‘shrublands and grasslands’.

Vegetation layer Tree layer >2m High shrub layer 1m< >2m Low shrub layer <1m Herbaceous layer

0 Riparian forest 1 Dense forest 75–100 0–100 0–100 0–100 2 Open forest 25–75 0–100 0–100 0–100 3 High maquis shrubland 0–25 25–100 0–25 0–25 4 Low shrubland 0–25 0–25 25–100 0–25 5 Mixed shrub-grassland 0–25 25–100 25–100 25–100 6 grassland 0–25 0–25 0–25 25–100 7 Bare soil 0–25 0–25 0–25 0–25 8 Cultivated 9 Urban

Biotic and abiotic controls on landscape dynamic given source (Greene and Johnson 1996). We used this information to simulate the pattern of forest Forest cover in 1774 was not significantly correlated expansion which would have occurred if fire had no to total cover across aspect categories (R2 = 0.42, t = influence by making expansion proportional to the 2.1 < t 0.025,6 = 2.447, ns). N, NW and NE aspects distance to forest edge. It appears that, under this were proportionately more forested (more than 27%), constraint, predicted forest area increase would have whereas S aspects were only 4% covered by forests been greater than observed within the burnt area (Figure 4). Throughout the study period, forest (8.3% compared to 7.0% in Table 3), but significantly remained most important on aspects where it was lower than on the unburnt sites (8.3% vs 15%) (mat- initially most important. However, the correlation ched pairs student test t = 4.09, p = 0.0024). between the total cover and the observed forest cover across aspects became stronger (t1960 = 4.57, t1975 = 6.44, t1990 = 7.06), particularly on the unburnt S, SE, SW and E aspects where forest expanded on 20.3% to Discussion 26.8% of their aspect, compared to 11.2% to 15.3% elsewhere (Table 3). Total differences between Past land cover unburnt and burnt sites (17.8% vs 7.0%) in forest expansion between 1960 and 1990 were significant Our study illustrates the long history of land use in the (matched pairs student test t = 4.41, p = 0.002) Mediterranean basin and the dramatic shift in the 20th (Table 3). The unburnt sites experienced greater century which has influenced landscape structure and absolute forest increase on the southern aspects, with composition (Barbero et al. 1990; Moreira et al. up to 26.8% of the area becoming forest in 30 years 2001). In 1774, chestnut forests covered the largest (Table 3). Forest expansion was however more uni- fraction of wooded areas. The development of agri- formly distributed within the burnt area, with a slight culture (Bourcet 1996), and the extensive use of preference for NW aspects (10.9%). Rates of expan- chestnut forests (Pitte 1986), had caused replacement sion (as the ratio between expansion and the initial of the initial Quercus ilex forests, which palynoeco- cover) are, however, not significantly different logical analyses reveal to have been dominant on the between burnt areas (1.52%.yÀ1) and unburnt areas island for thousands of years (Carcaillet et al. 1997; (1.42%.yÀ1) (matched pairs student test t = 0.2, p Reille 1992). Chestnut forests were mainly distributed 0.1), indicating the fast dynamic, even in unburnt on the North-facing slopes, mostly because they could sites, relatively to the initial cover. not tolerate high summer water stress (Bacilieri et al. Our results also indicate that forests colonized less 1994; Pigott and Pigott 1993). The other human area with increasing distance to the 1774 initial forest activities (including orchards, vineyards, and wheat edge (Figure 5). This shape follows the usual seed cultivation) typically took place on south facing dispersal curve observed for most species, as a geo- slopes and in valley bottoms, in order to avoid low metric consequence of the dispersal process from a winter temperatures (Lepart and Debussche 1992). 107

Table 2. Transition matrices between 1960–1975 and 1975–1990 performed on the 10 land cover types. Totals of each year and for each type are in % of the landscape surface (3760 ha). We indicated the no-change diagonal line as well as the three generic types (Forest, Shrublands- grasslands, urban-cultivated areas). Vertical and underlined values correspond to the sum of transition rates within the delimited transition sub- matrix it belongs to. The dark lined region represent the progressive transition of shrublands to forests, the double lined region represent the regressive transition from forests to shrublands/grasslands and the grey background represent the internal progressive and regressive dynamics within shrublands. 108

Table 3. Surface area (ha) covered by each environmental category (burn x aspect), followed by the percentage of these areas actually covered by forests in 1774, 1960 and 1990 (observed). The forest percentage cover simulated for 1960 and 1990, with the assumption of an isotropic forest expansion in absence of ﬁre, is also shown. The forest expansions are represented for the observed and simulated maps, as the difference between the percentage covers of 1990 and 1960. Yearly rates of forest expansion are the ratios between forest expansion and forest cover in 1960 divided by the 30 years of the study period.

Burnt Unburnt

N NE E SE S SW W NW total N NE E SE S SW W NW total

area (ha) 181 276 319 342 320 196 58 83 1775 240 452 512 346 143 106 94 105 1996 area (%) 4.8 7.3 8.5 9.1 8.5 5.2 1.5 2.2 47.1 6.4 12.0 13.6 9.2 3.8 2.8 2.5 2.8 52.9 1774 observed (%) 7.7 8.2 8.6 2.5 1.6 2.1 3.1 18.0 7.6 43.8 39.2 22.8 10.8 9.3 19.2 22.4 42.7 26.8 1960 observed (%) 27.3 29.2 20.5 13.9 7.3 3.0 3.0 13.0 15.2 51.2 53.4 41.6 34.6 29.0 23.4 24.2 43.3 41.6 simulated (%) 19.3 19.1 16.3 7.0 6.5 6.3 8.5 29.0 14.2 64.4 59.1 38.4 25.6 23.0 38.3 41.7 61.1 44.2 1990 observed (%) 34.1 34.9 26.3 18.4 14.7 11.0 7.8 23.9 22.2 66.5 64.6 62.8 56.8 55.8 43.7 35.7 55.2 59.4 simulated (%) 31.8 31.8 24.7 16.3 15.5 14.5 14.7 34.4 19.7 77.0 73.6 53.0 43.8 38.3 51.4 60.2 72.9 59.2 forest expansion observed (%) 6.8 5.7 5.9 4.5 7.3 8.0 4.8 10.9 7.0 15.3 11.2 21.1 22.2 26.8 20.3 11.5 11.9 17.8 rate (% yÀ1) 0.8 0.7 1.0 1.1 3.3 8.8 5.3 2.8 1.5 1.0 0.7 1.7 2.1 3.1 2.9 1.6 0.9 1.4 1960–1990 simulated (%) 12.4 12.7 8.5 9.3 9.0 8.2 6.2 5.4 8.3 12.6 14.5 14.6 18.2 15.3 13.1 18.4 11.7 15.0 rate (% yÀ1) 1.5 1.5 1.4 2.2 4.1 9.0 6.8 1.4 1.8 0.8 0.9 1.2 1.8 1.8 1.9 2.5 0.9 1.2

(note: for both the ‘observed’ and the ‘simulated’ datasets, the burns vs unburnt categories refer to the same ﬁre maps, issues from the actual 1957–1997 database). 109

result of internal dynamics. This is a common out- come within most Mediterranean ecosystems, e.g., Chilean matorral (Fuentes et al. 1984) or Californian chaparral (Franklin 1998; Hilbert and Larigauderie 1990), where edaphic limitations on forest development and recurrent ﬁres are important in maintaining this vegetation type (Callaway and Davis 1993).

Forest dynamics Figure 4. Forest percentage cover on each of the 8 aspects (45 interval) for the years 1774 (white), 1960 (light gray), 1975 (dark Between 1960 and 1990, a 13% increase in forest gray) and 1990 (black). cover is observed (0.42% y–1). We focused on ana- lyzing total expansion rather than the rate of expansion as forest development is a spatial process, and burnt sites dynamic is as much influenced by the status of neighboring burnt sites than neighboring unburnt sites. It makes less sense to analyze the ratio between total forest increase to the surface of the initial forest on a given environmental category as all the neighboring categories can influence one site’s dynamic. Our detailed study identifies a slower forest expansion (in % of the landscape covered by new forest) on the burnt sites, with an average increase of 7% (=0.23% yÀ1), compared to 17.8% (=0.59% yÀ1) for the unburnt sites. Fires reset the vegetation to the early dynamics stages and forest species have been Figure 5. Forest percentage cover in 1960 (black dot), 1975 (gray dot) and 1990 (open dot) represented as a function of the distance shown to be weak invaders (Ne’eman et al. 1999), for (in m) from the edge of the initial forest (1774). Lines represent the some species (like Quercus sp. dominating our study fits to the datasets (power function y=axb+c). site) due to a shorter distance dispersal compared to anemochoric seeders (Debussche et al. 2001; Gustaf- Today, the only apparent remaining evidence of past son and Gardner 1996; Lepart and Debussche 1993), forest management and agricultural activity is the and a high seedling mortality due to summer drought gardens and orchards located around urban areas, and in open areas (Garcia-Fayos and Verdu 1998). some grassland plots in the most accessible parts of The average forest expansion is around 1.06% yÀ1 the valley where the slope is not steep and the soil is in southern France in the absence of disturbance deeper (Poesen et al. 1998). (Debussche et al. 1987), and varies between 0.69% yÀ1 and 1.39% yÀ1 in the Near East (Carmel and Shrubland dynamics Kadmon 1999; Kadmon and Harari-Kremer 1999). This average expansion of about 1% yÀ1 seems to be Shrubland expansion, following land abandonment, a common value in many ecosystems (Bragg and has been rapid (Figure 3). An efficient dispersal Hulbert 1976; Callaway and Davis 1993; Mast et al. strategy, greater germination ability and better seed- 1997). However, this expansion can significantly de- ling survival have been identified as the major traits crease for drier conditions (Callaway and Davis 1993; favoring the large-scale invasion of shrubs rather than Mast et al. 1997) (Kadmon and Harari-Kremer 1999), trees (DeSimone and Zedler 2001; Ferrandis et al. or in fire prone areas (Bragg and Hulbert 1976). These 1999; Lookingbill and Zavala 2000; Verdu and Gar- lower expansion are more in accordance with our cia-Fayos 1996). The pattern of forest expansion results for the fire-prone Mediterranean type ecosys- illustrates that shrubland persistence might be largely tem observed in Corsica, in absence of any plantation. dependent upon lack of tree invasion, rather than the More particularly, our results highlight a slower than 110 usually observed forest expansion in unburnt sites Acknowledgements (a maximum of 0.89% yÀ1 compared to an average value of 1% yÀ1 generally observed). This result This study was supported by the European contract contrasts with the rather efficient forest recovery we LUCIFER ENV4-CT96-0320 and a personal PhD observe here on burnt sites (more than 0.15% yÀ1), grant to FM from Collectivite´ Territoriale de Corse considering the high fire recurrence. and Office de l’Environnement de la Corse. Between the burnt categories, environmental factors differentially influence forest dynamics. 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