Response of plant functional types to changes in the fire regime in Mediterranean ecosystems: A simulation approach

Abstract. In the Mediterranean basin, the climate is predicted to be warmer and effectively drier, leading to changes in fuel conditions and fire regime. Land abandonment in the Mediterranean basin is also changing the fire regime through the increase in fuel loads. In the present study, two simulation models of vegetation dynamics were tested in order to predict changes in plant functional types due to changes in fire recurrence in eastern Spain. The two modelling approaches are the FATE-model (based on vital attributes) and the gap model BROLLA (based on the gap-phase theory). The models were arranged to simulate four functional types, based mainly on their regenerative strategies after disturbance: Quercus (resprouter), Pinus (non-resprouter with serotinous cones), Erica (resprouter), and Cistus (non-resprouter with germination stimulated by fire). The simulation results suggested a decrease in Quercus abundance, an increase in Cistus and Erica, and a maximum of Pinus at intermediate recurrence scenarios. Despite their different approaches, both models predicted a similar response to increased fire recurrence, and the results were consistent with field observations.     

The effect of landscape pattern on Mediterranean vegetation dynamics: A modelling approach using functional types

Abstract. In the framework of land use changes in the Mediterranean area, I asked to what extent different landscape structures might determine long‐term dynamics in Mediterranean ecosystems. To answer this question, a spatially explicit model was developed (the Melca model), incorporating two functional types of woody species dominant in Mediterranean ecosystems: a resprouter (R) and a non‐resprouter fire‐recruiter (seeders, S). The model was used as a tool for generating hypotheses on the possible consequences of different landscape scenarios. Thus, five different hierarchically structured random landscapes were generated, all having the same cover for the two functional types but different landscape structure (ranging from highly heterogeneous to homogeneous landscapes). After a 100‐yr simulation, plant cover and spatial pattern had changed and the changes were different for the different initial spatial configurations, suggesting that long‐term vegetation dynamics is spatially dependent (the resultant dynamics are sensitive to the initial spatial structure). In the landscapes where R‐type species had a low number of large patches and S‐species had a large number of small patches, the number of R‐patches increased and their size decreased, while the number of S‐patches decreased. In these cases, the final cover of the two types changed little from the initial cover. Landscapes with a large number of small R‐patches interspersed with S‐patches had a decrease in the number of R‐patches, an increase in the number of S‐patches and a decrease in the size of S‐patches. In these landscapes, final cover was significantly changed, increasing in R‐type and decreasing in S‐type species. These results suggest that low spatial autocorrelation (low aggregation) favours R‐type species. Implications for land management are also discussed.     

Plant functional types and disturbance dynamics – Introduction

Abstract. Plant functional traits and types are useful concepts in relation to disturbance responses of natural and managed ecosystems. To explore their applicability in greater depth, a set of 12 papers presents a broad range of issues from methodologies to the results of particular trait studies in the field, and modelling approaches. So far, empirical studies have only allowed us to identify a few functional traits that are consistently associated with disturbance. To determine the trait variations associated with climate, disturbance history and current disturbance regime as well as the interactions between these factors, global-scale comparisons of numerous individual studies are required. Significant advances toward this ambitious goal are presented in these papers, and include: (1) the articulation of experimental and analytical methodologies for individual studies that could usefully contribute to a global comparison; (2) the identification of core traits that can be used in the further search for disturbance-related traits common to a range of environments; (3) further information on vegetation response to disturbance in terms of trait representation, and the identification of attribute syndromes; (4) the identification of issues for modelling disturbance dynamics using functional types.     

Post-fire bryophyte dynamics in Mediterranean vegetation

Abstract. Bryophyte dynamics after fire in the Mediterranean macchia of Southern Italy was studied both by diachronic and synchronic approaches. Changes of bryophyte cover and species composition were found in relation to both age and fire intensity. During the first 2 yr after fire, bryophytes dominated the plots which had experienced the highest fire intensity while herbs were dominant in plots affected by lighter fires. Pioneer species, such as Funaria hygrometrica, Barbula convoluta and Bryum dunense, characterized recent intense fires, whereas Bryum torquescens, B. radiculosum and B. ruderale were dominant after less intense burning. Pleurochaete squarrosa, Tortula ruraliformis and Tortella flavovirens dominated intermediate successional stages. Pleurocarpous mosses were dominant only in the older closed stands. Different patterns of regeneration strategies were described: spores dominated early stages of intense fire, while vegetative propagules characterized later successional stages and less severely burned areas. Although bryophytes usually have a low abundance in Mediterranean vegetation, their role in post-fire vegetation dynamics may be locally enhanced according to burning conditions.     

Landscape-scale regeneration dynamics of disturbed Mediterranean maquis

Abstract. Long-term regeneration dynamics of Mediterranean maquis was investigated by analysing historical aerial photographs of Mt. Carmel, one of the largest protected areas in the Mediterranean region of Israel. Two sets of aerial photographs were processed, one from 1960 (before the area was protected), and the second from 1992 (21 yr after the area was declared a nature reserve). The photographs of each year were classified into three vegetation states based on the percentage cover of trees: open maquis with tree cover < 33.3 %, moderately developed maquis (tree cover 33.3–66.6 %), and closed maquis (tree cover > 66.6 %). Grid maps constructed from the classified images were used to determine probabilities of transition between vegetation states. Closed maquis showed zero probability of transition to either open or moderately developed maquis. Probabilities of ‘forward’ transitions (transitions from low-cover to high-cover classes) were higher on north-facing than on south-facing slopes. On north-facing slopes, the area of open maquis decreased from 87 % to 46 % during the period studied, while that of closed maquis increased from 3 % to 29 %. On south-facing slopes open maquis decreased from 87 % to 69 % while closed maquis increased from 1 % to 8 %. Within a particular aspect, tree cover in 1960 was a reliable predictor of tree cover in 1992. This indicates that micro-scale patterns of tree distribution in 1960 were important in determining the structure of the maquis 32 yr later. Simulations based on the empirically derived transition probabilities suggest that, under current climatic conditions, the process of maquis regeneration on Mt. Carmel may take 500–1000 yr.     

High mortality and enhanced recovery: modelling the countervailing effects of disturbance on population dynamics

Disturbances cause high mortality in populations while simultaneously enhancing population growth by improving habitats. These countervailing effects make it difficult to predict population dynamics following disturbance events. To address this challenge, we derived a novel form of the logistic growth equation that permits time‐varying carrying capacity and growth rate. We combined this equation with concepts drawn from disturbance ecology to create a general model for population dynamics in disturbance‐prone systems. A river flooding example using three insect species (a fast life‐cycle mayfly, a slow life‐cycle dragonfly and an ostracod) found optimal tradeoffs between disturbance frequency vs. magnitude and a close fit to empirical data in 62% of cases. A savanna fire analysis identified fire frequencies of 3–4 years that maximised population size of a perennial grass. The model shows promise for predicting population dynamics after multiple disturbance events and for management of river flows and fire regimes.     

Gap ecology in Florida scrub: Species occurrence,diversity and gap properties

Questions: Studies of gap effects have been conducted mainly in forests. We studied gap ecology in a pyrogenic Ceratiola ericoides (Florida rosemary) dominated shrubland and asked: How do gap size and the frequency of large gaps change across the fire chronosequence? Do larger gaps differ from smaller gaps in vegetation structure or species diversity? Are effects of gaps independent of, or dependent upon, time‐since‐fire? Are larger gaps refugia for herbs and subshrubs? Location: Archbold Biological Station, Lake Wales Ridge, south‐central Florida, USA. Methods: We investigated plant species occurrence and diversity in 805 gaps (areas free of shrubs taller than 50 cm) in 28 fire‐dependent Florida rosemary scrub sites. We collected quantitative cover data in a subset of seven sites. Results: Gap area distribution was lognormal. The largest gaps occurred throughout all but the longest time‐since‐fire intervals. Gaps were smallest in the longest unburned site but otherwise did not show strong patterns across the fire chronosequence. Species diversity measures increased with increasing gap area, with herbaceous diversity increasing with both gap area and bare sand. Herb diversity (H') decreased with time‐since‐fire. Larger gaps are refugia for some species. Of 14 species occurring in 25–75% of gaps, 13 had increased occupancy with increasing gap area, and gap area was the strongest predictor of occupancy for seven species of herbs and shrubs. Time‐since‐fire was the strongest predictor of occupancy for five species, including four ground lichens that increased with time‐since‐fire. Conclusions: Community structure within Florida scrub gaps is influenced by gap size, which in turn is affected by fire, the dominant ecological disturbance. We present a conceptual model that considers both gap size and time‐since‐fire as drivers of community structure and herbaceous plant diversity in Florida scrub. Because gap properties (independently of fire) have strong influences on species assemblages in Florida rosemary scrub gaps, fire management should consider the number and size of gaps across the landscape.     

Spatial patterns and dynamics of woody vegetation in an arid savanna

The spatial distribution of woody plants was studied in an arid savanna in Botswana. The study included stands of mixed species and sizes as well as monospecific even-sized stands of different size classes of the tree Acacia erioloba and the shrub Acacia mellifera. In the case of A. mellifera both dense stands on overgrazed land and more open stands were included. The analysis used all plant-to-plant distances, and individuals were represented with a realistic canopy extension. The mixed stands showed aggregated distribution of individuals, mainly caused by strong clumping of small shrubs. In A. erioloba saplings were aggregated, small trees were randomly or regularly distributed and large trees were randomly spaced. In open stands of A. mellifera aggregation increased with size of the shrubs, while in dense stands with overgrazing aggregation decreased with increasing size. The different patterns are discussed in relation to the relative importance of inter- and intraspecific competition for water and of disturbance by fire as regulatory mechanisms for total amount and spatial distribution of woody plants in this savanna.     

A hierarchical deductive approach for functional types in disturbed ecosystems

Abstract. We propose a hierarchical approach for plant functional classification in disturbed ecosystems to be used for vegetation modelling and global plant trait comparisons. Our framework is based on the persistence of plants at different levels of organization. We assume that the main parameters to determine persistence in chronically disturbed ecosystems are those related to: I ndividual‐persistence capacity, P ropagule‐persistence capacity (persistence at the population level), C ompetitive capacity (persistence at the community level) and D ispersal capacity (persistence at the landscape level). The IPCD approach is illustrated for fire‐prone and grazed ecosystems from the Mediterranean region and Australia and by assuming a binary classification of the four traits determining persistence which give a total 16 possible functional types. The IPCD framework provides a simple structured and synthetic view from which more elaborated schemes can be developed.     

Responses of Mediterranean Plant Species to different fire frequencies in Garraf Natural Park (Catalonia,Spain): field observations and modelling predictions.

Dynamics of the coexisting Mediterranean species Pinus halepensis, Quercus coccifera, Erica multiflora, Rosmarinus officinalis, Cistus albidus, C. salviifolius and Ampelodesmos mauritanica, with contrasted life history traits have been studied under different fire scenarios, following two approaches: a) field survey in areas with three different fire histories (unburned for the last 31 years, once burned in 1982, and twice burned in 1982 and 1994), and b) simulations with different fire recurrence using the FATE vegetation model. We compared observed abundance in the field survey to simulation outputs obtained from fire scenarios that mimicked field fire histories. Substantial mismatching did not occur between field survey and simulations. Higher fire recurrences were associated with an increase in the resprouting Ampelodesmos grass, together with a decrease in Pinus abundance. Resprouting shrubs did not show contrasting changes, but trends of increase in Quercus and decrease in Erica were observed. The seeders Rosmarinus and Cistus achieved maximum abundance at intermediate fire recurrence. We also performed ten 200 year simulations of increasing fire recurrence with average times between fires of 100, 40, 20, 10, and 5 years. A scenario without fire was also simulated. High fire recurrence produces an increase in Ampelodesmos, a grass which is becoming dominant in the area, and a small increase in Erica, but Quercus abundance decreases and Pinus disappears. Rosmarinus and Cistus abundance peaks at intermediate fire frequencies. When comparing these simulations to those in which Ampelodesmos was excluded, we found that the absence of the grass only increased Cistus occurrence in the community, this effect being more important at frequent fire recurrence. The study suggests that simple models based on life history traits may be useful in interpreting plant community dynamics in Mediterranean ecosystems that are greatly influenced by differences in the fire regime.     

Spatial modelling of succession-disturbance dynamics in forest ecosystems: Concepts and examples

Over the last few decades it has become increasingly obvious that disturbance, whether natural or anthropogenic in origin, is ubiquitous in ecosystems. Disturbance-related processes are now considered to be important determinants of the composition, structure and function of ecological systems. However, because disturbance and succession processes occur across a wide range of spatio-temporal scales their empirical investigation is difficult. To counter these difficulties much use has been made of spatial modelling to explore the response of ecological systems to disturbance(s) occurring at spatial scales from the individual to the landscape and above, and temporal scales from minutes to centuries. Here we consider such models by contrasting two alternative motivations for their development and use: prediction and exploration, with a focus on forested ecosystems. We consider the two approaches to be complementary rather than competing. Predictive modelling aims to combine knowledge (understanding and data) with the goal of predicting system dynamics; conversely, exploratory models focus on developing understanding in systems where uncertainty is high. Examples of exploratory modelling include model-based explorations of generic issues of criticality in ecological systems, whereas predictive models tend to be more heavily data-driven (e.g. species distribution models). By considering predictive and exploratory modelling alongside each other, we aim to illustrate the range of methods used to model succession and disturbance dynamics and the challenges involved in the model-building and evaluation processes in this arena.     

Rainfall seasonality determines annual/perennial grass balance in vegetation of Mediterranean Iberian

Much recent attention has been focused on the invasion and dominance of annual grass species in areas thought to have been historically dominated by perennial life forms. Explanations of this phenomenon in the literature have focused on two mechanisms favoring annuals: ruderal strategy associated with disturbance, and stress escaping associated with dry sites or deserts. Here I present evidence from vegetation surveys at 50 sites across a 1,200 km band of the Iberian Peninsula—a source region for many invasive annuals—showing that relative annual versus perennial grass composition is not well correlated with degree of disturbance or average annual precipitation. However, annual dominance is strongly and significantly linked to the seasonality of precipitation, in particular the relative intensity of summer drought. Disturbance was significantly associated with annual grass dominance in Iberia, but with much less explanatory power than summer drought intensity. Slope, aspect, and soil parent material were not significantly correlated with annual versus perennial grass dominance. These results suggest that subtle differences in rainfall seasonality largely drive grass composition in herbaceous Mediterranean vegetation. Furthermore, the patterns of annual grass invasion observed in the world’s other Mediterranean climate regions may be associated with similar climatic drivers.     

Synergistic influences of introduced herbivores and fire on vegetation change in northern Patagonia,Argentina

Question: We investigated how cattle and European hares, the two most widespread exotic herbivores in Patagonia, affect species composition, life‐form composition and community structure during the first 6 years of vegetation recovery following severe burning of fire‐resistant subalpine forests and fire‐prone tall shrublands. We asked how the effects of introduced herbivores on post‐fire plant community attributes affect flammability of the vegetation. Location: Nahuel Huapi National Park, northwest Patagonia, Argentina Methods: We installed fenced plots to exclude livestock and European hares from severely burned subalpine forests of Nothofagus pumilio and adjacent tall shrublands of N. antarctica. The former is an obligate seed reproducer, whereas the latter and all other woody dominants of the shrubland vigorously resprout after burning. Results: Repeated measures ANOVA of annual measurements over the 2001‐2006 period indicate that cattle and hare exclusion had significant but complex effects on the cover of graminoids, forbs, climber species and woody species in the two burned community types. Significant interactions between the effects of cattle and hares varied by plant life forms between the two communities, which implies that their synergistic effects are community dependent. Conclusions: Following severe fires, the combined effects of cattle and hares inhibit forest recovery and favour transition to shrublands dominated by resprouting woody species. This herbivore‐induced trend in vegetation structure is consistent with the hypothesis that the effects of exotic herbivores at recently burned sites contribute to an increase in the overall flammability of the Patagonian landscape.     

Ecological and evolutionary differences between Mediterranean seeders and resprouters

Abstract. Differences in allocation patterns between seeders and resprouters in several Mediterranean plant communities (Australia, California and South Africa) have led to the prediction that seedlings of seeders grow faster than those of resprouters. In the Mediterranean Basin, it has also been hypothesized that regeneration strategy of plants after fire is associated with several other life history traits. This paper tests both hypotheses for the dominant plants in the Mediterranean Basin from literature data. Results show that seeders from the Mediterranean Basin grow significantly faster and allocate more biomass to leaf plus paracotyledons than resprouters. Seeders are mainly non‐sclerophyllous, anemochorous, dry‐fruited, small‐seeded species that evolved in the Quaternary (post‐Pliocene) and are associated with earlier successional stages. Resprouters are mainly sclero‐phyllous, vertebrate‐dispersed, fleshy‐fruited, large‐seeded species that evolved in the Tertiary (pre‐Pliocene) and are associated with late successional stages.     

Fire-scars and polymodal age-structure provide evidence of fire-events in an Aleppo pine population in southern France

Aleppo pine (Pinus halepensis Mill.) is adapted to fire, the most important disturbance in Mediterranean ecosystems. This species is known to be sensitive to fire and is usually killed when burned, but has a powerful regenerative capacity following burning. Sometimes, however, pines survive within burned areas, suggesting a certain potential for recording fire events in terms of age-structure and eventually scars. In the present study, fire scars on Aleppo pine trunks were used to construct a 150-year fire record in a burned forest in southern France. Four fires were detected with a mean fire return interval of 27 years: minimum 13 years and maximum 45 years. The age structure of the Aleppo pine population was analyzed on the basis of dated fire-scars in order to test the role of fire in creating the polymodal structure of the pine population. Results show the presence of multiple “cohorts” (subpopulations) of Aleppo pine. There appear to have been several pulses of tree recruitment, with the two largest cohorts corresponding to regeneration after the 1919 and 2000 fires. Other fires were recognized based on fire scars but these were not associated with obvious post-fire pine recruitment. Regeneration success is likely to depend on fire severity and on the length of fire-free intervals. Fire history and associated age-structure can thus be reconstructed using Aleppo pine stands.     

A functional classification for predicting the dynamics of landscapes

Abstract. Functional classifications have been derived for various purposes using subjective, objective and deductive approaches. Most of the classifications were derived to describe a static state of a region or landscape rather than to predict the dynamics of the system. Here, we suggest a simple, but comprehensive functional classification based on life history parameters that can predict the dynamics of plant communities subject to recurrent disturbances. The predicted dynamics are described in terms of survival and local extinction of the functional groups. The groups derived from the classification are probably largely independent of functional groupings that may be derived for other aspects of community composition (e.g. structure, phenology) and community interactions (roughness, albedo etc.). We emphasize that functional classification is context-dependent and we should not expect to find a useful, universal classification into functional groups. Software has been developed to help classify the species into functional groups, to derive successional sequences and to predict community composition under different disturbance regimes both in point and landscape models.