54, November/December 2003
Fire Ecology I
by María del Carmen COBO and José Antonio CARREIRA
"Our study provides solid results to interpret the most important processes involved in the mid- to long-term evolution of litter and soil nutrient contents and pools in Mediterranean shrubland ecosystems in relation to changes in attributes of the fire perturbation regime (frequency and time elapsed since the last fire), which underlie desertification processes in fire-prone, semiarid areas."
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We hypothesize, however, that when fire frequency is abnormally high, the ecosystem nutrient capital does not fully recover, and the long-term balance between fire-related nutrient outputs and succession-related nutrient inputs is upset. Long-lasting soil fertility depletion slows rates of vegetation recovery which, in turn, increases erosion rates, initiating a positive feedback loop that can lead to irreversible desertification (Boerner 1982; Schlesinger et al. 1990; Carreira, Niell and Lajtha 1994).
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The study sites consisted of a four-stand, soil and vegetation chronosequence (Dyck and Cole 1990) of recent fire history. The sampling plots were selected within each of these study sites (four adjacent, steep sub-basins 1 to 3 km apart), all in mid-hillside positions with similar elevation (300-400 m), exposure (SE) and slope (36-41º). Prior to an initial "baseline" fire (1975), the whole study area was covered by a uniform old-growth Pinus pinaster forest with dense garrigue understory.
After 1975, the four study sites chosen, each 1000 m2, differed in their fire-disturbance regime (fire-frequency and time-lag between fire events): Unit D had not burned during the 1975 baseline fire or indeed since at least the 1950s, and was used as the reference plot. The three other plots, Units A, B, and C, burned in 1975; of these, Units A and B burned a second time in 1981, and Unit A burned a third time in 1986 (that is, Unit D had 0 fires, Unit C had 1 fire, Unit B had 2 fires, and Unit A had 3 fires during the period 1975 to 2002). Current species composition is similar in all units, with different proportions of abundance. Vegetation on relatively well-developed soils consists of discontinuous stands of garrigue shrublands with Juniperus oxycedrus, Buxus balearicae and Pistacia lentiscus as dominant species, and open pine (Pinus pinaster) forests. Where frequent fires have occurred, garrigue formations are replaced by gorse (Ulex rivasgodayanus) and rockrose (Cistus clusii)-dominated dwarf-shrub vegetation that includes many dolomitophilous plant endemics (Carreira, Lajtha and Niell 1997).
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Within 48 h after collection, litter and soil sub-samples were taken apart to calculate field moisture. Next, the litter samples were cleaned and air-dried. The soil samples were air-dried and sieved to 2 mm. Litter mass, soil gravel content and bulk density were recorded. After that, general soil chemical properties and total and labile nutrient (C, N and P) contents were measured.
For both sampling periods, litter and soil nutrient contents and pool sizes were expressed as a percentage of their value at the reference sampling plot (Unit D), and represented as a function of the number of years passed since the last fire event.
During the 1989-1991 sampling period, we also measured soil erosion rates using Verlagh boxes (Carreira, 1992). Nutrient pool sizes in litter and soil from this period were expressed as a percentage of their value at the reference plot (Unit D), together with erosion fluxes expressed as number of times above values at the reference plot, and represented as a function of site annual fire probability over the last 45 years.
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Another interesting issue that arose from our data relates to an alternative meaning for the concept of stability: elasticity (Grimm and Wissel 1997; Brang 2001). The soil nutrient pool at Unit A, the sampling plot that experienced an abnormally short fire return interval (3 fires) during 1975-1986, showed very small or even no recovery during the interval from 1989/91 (3-5 yr-old vegetation at Unit A, surface soil total N pool size about 30% of that in the reference plot) to 2000/02 (14-16 yr-old vegetation at Unit A, soil N pool size 35% of that in the reference plot), even though no new fires had taken place. In contrast, the surface soil total N pool size at Unit C (burned once), when its vegetation was 14-16 yr-old (in the 1989-91 period), was 125% of that in the reference plot. Thus, in the long term, the size of the soil nutrient capital depends more on the fact that a given threshold of fire recurrence has been surpassed at some time in the past, than on the time elapsed since the last fire event took place. This means that the soil nutrient pool, despite its relatively high inertia against fire, shows very low elasticity (that is, there are long-lasting effects if fire recurrence is high). On the other hand, the litter N content in Unit A, that was just 45% of N content in the reference plot during the first sampling period (1989-91), increased to 70% of reference values during the period 2000-02. This outstanding recovery for the litter N contents in the absence of new disturbance events, in comparison to the slight increments observed for the soil N contents and pools in the same circumstances, demonstrates the higher elasticity, although lower inertia, of nutrients in the litter layer.
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We found that erosion fluxes showed an exponential-type response curve with respect to a linear-increasing annual fire probability at the study area. However, when nutrient pools were considered, a threshold-type response curve was found. Above a given fire-frequency threshold, differences among sampling plots were more strongly related to the number of past fires (fire frequency) than to the age of the vegetation stand (number of years since the last fire). Our study indicated that in sampling plots experiencing an abnormally short fire-return interval, even transitory, during the last five decades, the current post-fire recovery rate of the nutrient capital and its associated process-rates was very low or is even absent (sensu positive degradation feedback leading to desertification; Schlesinger et al. 1990). On the other hand, in the sampling plots not surpassing such a threshold, recovery of the nutrient capital or organic matter cycling process-rates was always achieved in the short to mid-term, or is even increased with respect to the reference plot. This, in turn, stresses that perturbation is necessary for ecosystem sustainability, provided that a given perturbation regime is maintained through time and perturbation thresholds are not surpassed (Holling 1986; Chapin, Torn and Tatano 1996).
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María del Carmen COBO (email@example.com) is a Ph.D. student in the Department of Zoology, Botany and Ecology, University of Jaén; she will soon defend her doctoral thesis "Small scale spatial heterogeneity and soil-plant relationships in semiarid ecosystems from SE Spain." She also collaborates on research in Dr. M.W. Palmer's lab in the Department of Botany at Oklahoma State University, and is a member of the Andalusian research group "Forest ecology and landscape dynamics" conducted by her advisor, Dr. J.A. Carreira.
José Antonio CARREIRA (firstname.lastname@example.org) is a Professor of Ecology in the Department of Zoology, Botany and Ecology, University of Jaén. At present, he is the main researcher of the research group RNM-296 (Andalusian government catalogue) "Forest ecology and landscape dynamics," operating at the University of Jaén.
Authors' correspondence address:
Departamento de Biología Animal, Biología Vegetal y Ecología
Facultad de Ciencias Experimentales
Universidad de Jaén
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University of Jaen, RNM-296: Forest ecology and landscape dynamics
This Spanish-language site provides more information on the research activities of the co-authors and their colleagues in this research group.
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