Giancarlo Avena

The flaming sandpile: self-organized criticality and wildfires

A large series of wildfire records of the Regional Forest Service of Liguria (northern Italy) from 1986 to 1993 was examined for agreement with power-law behavior between frequency of occurrence and size of the burned area. The statistical analysis shows that the idea of self-organized criticality (SOC) applies well to explain wildfire occurrence on a regional basis.

Computing β-diversity from species-area curves

Summary A measure of β-diversity is proposed for equal-sized presence-absence vegetation sample plots. The method begins by computing the minimum variance unbiased estimator of the expected species richness E[Sn] as a function of sample size n for a set of N plots. Using a semilogarithmic relation, β-diversity is then determined from the slope of the best fitting line of the E[Sn] vs. log n plot. β-diversity is thus interpretable as the linear relation between expected species richness and log sample size. The method is illustrated using 15 square plots of 100 m2 each from a Matorral community in the Arid Chaco of central Argentina. Further, some analogies between the proposed method and Whittakers multiplicative model of β-diversity are discussed. Ein Mas fur die β-Diversitat wird vorgeschlagen, das fur “presence-absence”-Vegetationsaufnahmen auf gleichdimensionierten Probeflachen gilt. Nach der Methode wird zuerst der “minimum variance unbiased estimator” des erwarteten Artenreichtums als eine Funktion der Probengrose n fur eine Anzahl von N Flachen berechnet. Unter Verwendung der semilogarithmischen Beziehung wird die β-Diversitat uber die “best-fit”-Steigung des Graphen E[Sn] vs. log n bestimmt. Die β-Diversitat kann damit als die lineare Abhangigkeit des Artenreichtums von der logarithmierten Probengrose interpretiert werden. Die Methode wird anhand von 15 Probequadraten von jeweils 100 m2 aus der Matorall-Lebensgemeinschaft des ariden Chaco Zentralargentiniens erlautert. Zudem werden einige Analogien zwischen der vorgeschlagenen Methode und Whittakers multiplikativen Modell der β-Diversitat diskutiert.

Mapping and monitoring net primary productivity with AVHRR NDVI time-series: statistical equivalence of cumulative vegetation indices

Abstract In the last two decades, numerous investigators have proposed cumulative vegetation indices (i.e., functions which encode the cumulative effect of NDVI maximum value composite time-series into a single variable) for net primary productivity (NPP) mapping and monitoring on a regional to continental basis. In this paper, we investigate the relationships among three of the most commonly used cumulative vegetation indices, expanding on the definition of equivalence of remotely sensed vegetation indices for decision making. We consider two cumulative vegetation indices as equivalent, if the value of one index is statistically predictable from the value of the other index. Using an annual time-series of broad-scale AVHRR NDVI monthly maximum value composites of the island of Corsica (France), we show that the pairwise linear association among the analysed cumulative vegetation indices shows coefficients of determination ( R 2 ) higher than 0.99. That is, knowing the value of one index is statistically equivalent to knowing the value of the other indices for application purposes.

Are potential natural vegetation maps a meaningful alternative to neutral landscape models

Abstract In this paper, we present a short overview of neutral landscape models traditionally adopted in the landscape ecological literature to differentiate landscape patterns that are the result of simple random processes from patterns that are generated from more complex ecological processes. Then, we present another family of models based on Tüxen’s definition of potential natural vegetation that play an important role, especially in Europe, for landscape planning and management. While neutral landscape models by their very nature do not take into account vegetation dynamics, nor abiotic constraints to vegetation distribution, the concept of potential natural vegetation includes the effects of vegetation dynamics in a spatially explicit manner. Therefore, we believe that distribution maps of potential natural vegetation may represent an ecological meaningful alternative to neutral landscape models for evaluating the effects of landscape structure on ecological processes. Abbreviations: NLM = Neutral landscape model; PNV = Potential natural vegetation; PSV = Potential site-adapted vegetation; RNV = Reconstructed natural vegetation.

On the relationship between Pielou’s evenness and landscape dominance within the context of Hill’s diversity profiles

Abstract Entropy-related biodiversity indices deriving their conceptual basis from Shannon’s information theory have a long history of use in ecology for quantifying community structure and diversity. In addition, in the last two decades, numerous information–theoretical indices, such as the landscape dominance index, have been extensively applied to characterize landscape diversity in space and time. In this contribution, we offer a simple analytical relation between Pielou’s evenness J and landscape dominance D within the broader context of Hill’s parametric diversity family. Within this context, we recommend the use of Hill’s diversity number evenness E1,0 to overcome the shortcomings both of Pielou’s evenness J and the landscape dominance index D.

Topological analysis of the spatial distribution of plant species richness across the city of Rome (Italy) with the echelon approach

It is generally agreed that urban vegetation significantly contributes to the well-being of individuals and society. Therefore, plant species richness in urban environments is a variable of considerable interest to landscape planners and conservation biologists. While all monitoring activities have a spatial context to a varying degree, monitoring of urban plant species richness distribution requires an objective method for defining the boundaries of areas that are species rich or poor compared to their surroundings. By aggregating the cells of tessellated numerical surface variables into hierarchically related topological entities, the echelon approach provides a new way to objectively characterize the structure of spatial data bases and is thus appropriate for monitoring environmental indices such as urban plant species richness. In this paper, we apply the echelon approach to the characterization of the broad-scale spatial distribution of plant species richness across the city of Rome (Italy).

Quantitative comparison of the diversity of landscapes with actual vs. potential natural vegetation

. In the past 20 years, several metrics have been developed to quantify various aspects of landscape structure and diversity in space and time, and most have been tested on grid-based thematic maps. Once landscape patterns have been quantified, their effects on ecological functions can be explained if the expected pattern in the absence of specific processes is known. This type of expected pattern has been termed a neutral landscape model. In the landscape-ecological literature, researchers traditionally adopt random and fractal computer-generated neutral landscape models to verify the expected relationship between a given ecological process and landscape spatial heterogeneity. Conversely, little attention has been devoted to distribution patterns of potential natural vegetation (PNV) as an ecological baseline for the evaluation of pattern-process interactions at the landscape scale. As an application for demonstration, we propose a neutral model based on PNV as a possible reference for a quantitative comparison with actual vegetation (AC V) distribution. Within this context, we introduce an evenness-like index termed ‘actual-to-potential entropy ratio’ (HA/P = HACV/HPNV, where H is Shannons entropy). Results show that, despite the hypothetical character of most PNV maps, the use of PNV distribution as a baseline for a quantitative comparison with ACV distribution may represent a first step towards a general model for the evaluation of the effects of disturbance on vegetation patterns and diversity.

The remote sensing approach in broad‐scale phenological studies

. Satellite imagery provides a unique tool for monitoring seasonal dynamics of the Earths vegetation on a global scale. The combination of the normalized difference vegetation index (NDVI) data derived from the Advanced Very High Resolution Radiometer (AVHRR) with a daily repeat cycle and 1 km spatial resolution makes weather satellites operated by the National Oceanic and Atmospheric Administration very well suited for deriving broad-scale phenological metrics from satellite images. In this paper, similarities and differences between remotely sensed phenological studies and traditional symphenological studies conducted by ground-based observations are summarized. Finally, major shortcomings in deriving phenological metrics from NDVI time series are discussed.

On the information-theoretical meaning of Hill's parametric evenness.

The degree to which abundances are divided equitably among community species or evenness is a basic property of any biological community. Several evenness indices have been proposed to summarize community structure. However, despite their potential applicability in ecological research, none seems to be generally preferred. In this paper we show that, unlike other evenness indices without any clear information-theoretical meaning, Hills parametric diversity measure Eα,0 has an immediate relation to Rényis generalized information. Therefore, Eα,0 might be adequate for summarizing community structure within the context of a general theoretical framework of diversity analysis based on information theory.

An Information-Theoretical Measure of Taxonomic Diversity

Traditional diversity indices are computed from the abundances of species present and are insensitive to taxonomic differences between species. However, a community in which most species belong to the same genus is intuitively less diverse than another community with a similar number of species distributed more evenly between genera. In this paper, we propose an information-theoretical measure of taxonomic diversity that reflects both the abundances and taxonomic distinctness of the species. Unlike previous measures of taxonomic diversity, such as Raos quadratic entropy, in this new measure the analyzed taxonomic properties are associated with the single species instead of species pairs.