M. Joao Martins

University of Lisbon | UL · DCEB, CEF

Assessing a plant’s nutritional status and fertilizer rates and types that can optimize fruit quality and yield are critical in intensive apple orchards. The aim of this work was to identify correlations between nutrients in the different organs that allow the early diagnosis of the nutritional status and to assess the impact on the optimal nutrien...

Climate change is a challenge for forests in the coming decades, with a major impact on species adaptation and distribution. The Mediterranean Basin is one of the most vulnerable hotspots for biodiversity conservation under climate change in the world. This research aimed at studying a Mediterranean species well adapted to the region: the Arbutus u...

Riverine areas are considered large carbon reserves because they support long-term woody communities, which in their natural state are dense and diverse. In this study, we analysed the carbon storage alterations across 20.5 km of riverine Sampling Units (SUs) downstream Touvedo (a run-of-river dam) and Fronhas (a storage reservoir), by adopting a L...

River regulation may filter out riparian plants often resulting in reduced functional diversity, i.e., in the range of functions that organisms have in communities and ecosystems. There is, however, little empirical evidence about the magnitude of such reductions in different regions. We investigated the functional diversity patterns of riparian wo...

This study aims to estimate the total biomass aboveground and soil carbon stocks in a Mediterranean riparian forest and identify the contribution of the different species and ecosystem compartments to the overall riparian carbon reservoir. We used a combined field and object-based image analysis (OBIA) approach, based on unmanned aerial vehicle (UA...

Large rivers on the Iberian Peninsula reflect a long-history of human interventions and natural disturbances in the fluvial corridors and on the surrounding valleys. In this study we aim to characterize the river morphology in pre-regulation times and understand the morphological changes of the large river Tagus (Central Portugal), across space and...

Dammed rivers have unnatural stream flows, disrupted sediment dynamics, and rearranged geomorphologic settings. Consequently, fluvial biota experiences disturbed functioning in the novel ecosystems. The case study is the large irrigation reservoir Alqueva in Guadiana River, Southern Iberia. The study area was divided into three zones: upstream and...

Riparian ecosystems have a great potential for long-term carbon storage and sequestration. However, there is a growing demand on accurate estimations for the Mediterranean region and particularly using remote sensing approaches. In this study, we estimated the Above Ground Biomass (AGB) of a Mediterranean riparian forest using high-resolution multi...

Developing spatially explicit models of Ecosystem Services (ES) distribution and diversity across the territory has been increasingly attracting the interest of researchers and policy-makers due to its potential to operacionalize and mainstream the ES concept into existing planning and policy tools. In this paper we explore the use of social media...

The aim of this study was to analyse the effect of relative humidity in suppressing Sitophilus zeamais, in paddy rice stored under hermetic conditions, during four and seven months, at different average temperatures, as well as the impact on rice quality. Hermetic bags, GrainPro® SuperGrainbag® Farm™, were used to store two rice varieties under thr...

1. The first attempts to describe species ecological niches were simple geometric procedures that depict the niche boundaries directly from environmental data. The convex hull was one of such procedures, popular for its simplicity, clear ecological rational and precise definition of the niche. However, it lacked the ability to differentiate areas o...

The main objective of this study is to characterize the hydromorphological evolution of Tagus and Minho rivers, using a planform image analysis, and assess the contribution of human disturbance to global change. We performed a temporal analysis by choosing dates representing a pre-regulation period (19th century) and a post-regulation period (21th...

Rivers and humans compete for territory, water and ecological resources. Hydrological alterations and land-use and land cover (LULC) are known to alter riparian ecosystem functions and processes. We studied ecological and hydromorphological alterations by regulation and LULC in three hydropower rivers of north and centre Portugal and in a very larg...

Many riparian ecosystems in Mediterranean Europe are affected by land use and flow alteration by dams. We focused on understanding how these stressors and their components affect riparian forests in the region. We asked: (i) are there well-defined, responsive riparian guilds?; (ii) do dam-induced stream flows determine abundance and distribution of...

Long-term changes of fluvial landscapes: evolutionary trajectories of vegetation patterns

In: R. Guarino, G. Bazan, G. Barbera, The 60th IAVS Annual Symposium “vegetation patterns in natural and cultural landscapes” Abstract. Palermo University Press, Palermo, Italy: 47-48.

Dams strongly impair the fluvial environment by altering downstream flows. We analysed riverscapes downstream of three dams and hypothesized that different dam types in rivers with diverse history of land-use and land cover (LULC) change have significant riparian cover differences at diverse biogeomorphic units (banks, riverbanks, islands). We perf...

We present a bioclimatological diagnosis of mainland Portugal, namely the thermotype and ombrotype maps following Rivas-Martínez's worldwide bioclimatic classification system. In order to obtain this diagnosis, we produced maps of bioclimatological indices using, as base data, geostatistical interpolations of air temperature and precipitation. We p...

Resampling methodologies, like the generalised jackknife and the bootstrap are important tools for a reliable semi-parametric estimation of parameters of extreme or even rare events. Among these parameters we mention the extreme value index, denoted ξ, the primary parameter in statistics of extremes, and the extremal index, denoted θ, a measure of...

Resampling methodologies, like the generalised jackknife and the bootstrap are important tools for a reliable semi-parametric estimation of parameters of extreme or even rare events. Among these parameters we mention the extreme value index, denoted ξ, the primary parameter in statistics of extremes, and the extremal index, denoted θ, a measure of...

Dams are undoubtedly one of the major driving forces of change in fluvial systems. They alter the aquatic and riparian ecology by affecting river hydrology in quality, quantity and timing of downstream flows. On the other hand, riparian landscapes are usually constrained by land-use. The main goal of this study is to quantify and understand the cha...

Over the past decades, we have assisted to an increasing understanding of the effects of regulated-flow conditions on aquatic ecosystems. However, the responses to altered flows of the ecotonal ecosystems remain a long-standing chal-lenge in the Mediterranean regions. This is probably due to the multiple pressures that in-fluence directly and indir...

Background: Stream flow is a fundamental driver of riparian plant communities. The increasing need for water resources to manifold purposes (hydropower energy, agriculture, industry, urban use) resulted in a widespread disturbance of flow regimes, with regulation by dams having a major impact in fluvial and riparian landscapes. There is increasing...

Background & Aim: Alteration of flow regimes by dams causes shifts in the composition and diversity as well as the trait syndromes of streamside plant communities [1]. There is still a limited knowledge of riparian strategies to streamflow regulation and moreover to different dam operations. Such an understanding would enable us to more strategical...

Hutchinson's pioneering work on the niche concept, dating from 1957, inspired the development of many ecological models. The first proposals, BIOCLIM and HABITAT, were simple geometric approximations to the shape of the niche. Despite their simplicity, they combine two features that make them adequate for the purpose of exploring the niche: they fi...

Given a set $X$ of $k$ points and a point $z$ in the $n$ -dimensional euclidean space, the Tukey depth of $z$ with respect to $X$ , is defined as $m/k$ , where $m$ is the minimum integer such that $z$ is not in the convex hull of some set of $k-m$ points of $X$ . If $z$ belongs to the closed region $B$ delimited by an elli...

What are the species and functional trait composition of riparian forests in near-natural Southwest European rivers? Are functional trait and species assemblages constrained by environment? Near-natural riparian habitats throughout mainland Portugal, Southwest Europe. We collected data on riparian woody abundances and environmental variables from 1...

In a top-down design, vegetation-types mapping can be approached with an environmental-based model, usually relying on two main premises: i) presence of different vegetation-types is mutually exclusive in the landscape, although ecotones might exist between them; ii) there is an ecological causation for the vegetation-types distribution in the land...

As florestas ripárias são componentes fundamentais da paisagem mediterrânica. Para além do elevado contributo para o incremento da biodiversidade regional, estas comunidades de écotono desempenham várias funções físicas, ecológicas, sociais e económicas. Os sistemas fluviais estão sujeitos a severas alterações no regime hidrológico, devido à regula...

Riparian or streamside forests are key-components of river systems generally adapted to natural flow dynamics and disturbance regimes. Changes in the natural flows and fragmentation of riparian corridors by river regulation interfere in the ecological processes of riparian zones and may induce compositional shifts in species and functional traits o...

We present a framework for the mapping of vegetation-types, based on environmental data, using a simple and interpretable geometric approach, adopting the convex hull notion. Two main premises are acknowledged: i) presence of different vegetation-types is mutually exclusive in the landscape, although ecotones might exist between them; ii) there is...

In this article, we revisit the importance of the generalized jackknife in the construction of reliable semi-parametric estimates of some parameters of extreme or even rare events. The generalized jackknife statistic is applied to a minimum-variance reduced-bias estimator of a positive extreme value index—a primary parameter in statistics of extrem...

Separability of clusters is an issue that arises in many different areas, and is often used in a rather vague and subjective manner. We introduce a combinatorial notion of interiority to derive a global view on separability of a set of entities. We develop this approach further to evaluate the overall separability of a partition in the context of c...

Over the last four decades the world has been losing biodiversity at an alarming rate despite the increasing number of protected areas (PAs). Certified forest management may complement the role of PAs in protecting biodiversity. Forest certification aims to promote sustainable forest management and to maintain or enhance the conservation value of c...

In this article, we deal with the importance of the generalized jackknife methodology in the construction of a reliable semi-parametric estimate of any parameter of extreme or even rare events. In order to illustrate such a kind of methodology, we shall apply it to corrected-bias estimators of a positive extreme value index, the primary parameter i...

Gap analysis is a protocol for assessing the extent to which valued biodiversity attributes are represented within protected areas. Such analysis involves overlaying the distribution of biodiversity features (e.g. species) with protected areas, but the protocol entails arbitrary assumptions that affect the outcome of the assessments. In particular,...

Alternative kernel estimators of a positive extreme value index are proposed. Associated Generalized Jackknife estimators are also considered.

Classical extreme value index estimators are known to be quite sensitive to the num-ber k of top order statistics used in the estimation. The recently developed second order reduced-bias estimators show much less sensitivity to changes in k. Here, we are interested in the improvement of the performance of reduced-bias extreme value index estimators...

In many environmental situations, it is of key importance to focus on the extreme values of a sample, on extremely high quantiles or on small tail probabilities. Heavy tailed-models are quite useful in many environmental fields, like biology, hydrology and meteorology, and the problem of estimating the extreme value index, γ, for heavy-tailed distr...

Classical tail index estimators are known to be quite sensitive to the number k of top order statistics used in the estimation. The recently developed second order reduced bias' estimators show much less sensitivity to changes in k. Here, we are interested in reduced bias' tail index estimation, based on a "classical" exponential second order regre...

A set of second order reduced bias tail index estimators are compared under a third order framework

For heavy-tailed models F, and in a sub-class of Hall's class of models, minimum-variance second order reduced bias tail index estimators will be discussed under a third order framework.

Generalizations of Hill's estimator (1975) have recently been proposed, and widely used in extreme value theory, namely in tail index estimation of distribution function with positive tail index. The kernel estimators, proposed by Csörgo et al. (1985) are a large class of tail index estimators, including Hill's estimator and averages of Hill's esti...

In this paper, and in a context of regularly varying tails, we analyse particular but interesting cases of the maximum likelihood and least squares estimators proposed by Feuerverger and Hall (Ann. Statist. 27 (1999) 760). All these estimators are alternatives to a well-known estimator of the tail index, the Hill estimator (Ann. Statist. 3 (1997) 1...

Averaging Hill's estimators leads to a reduction in the volatility of Hill's plot. We deal with a generalization of the procedure proposed by Resnick and Stărică (1997), and, propose alternatives, assymptotically equivalent at the respective optimal levels, but with more interesting sample paths. Asymptotic normality is derived for intermediate lev...

The problem of estimating the tail index in heavy tailed-distributions is very important in many applications. One of the most popular semi-parametric estimators, Hill’s estimator, presents some drawbacks such as high dependence on the number of upper order statistics. Generalizations of the Hill estimator have been recently proposed by several aut...

Generalizações do estimador de Hill têm sido propostas recentemente e largamente usadas em teoria de valores extremos na estimação do índice de cauda associado a uma função de distribuição com índice de cauda positivo. Os estimadores de núcleo constituem uma vasta classe de estimadores do índice de cauda na qual se incluem o estimador de Hill e méd...

Bias reduction in the tail index estimation is considered.

New estimators of the tail index, denoted by Mean Bootstrap Estimators are obtained and studied. A bootstrap estimator of the Mean Squared Error is also considered.

In this paper we shall deal with the asymptotic and finite sample properties of "asymptotically unbiased" estimators of the tail index γ, based on "external" adequate estimators of the second order parameter ρ. The behaviour of the ρ-estimator considered has indeed a high impact on the distributional properties of the final estimator of γ, and must...

In this paper we shall deal with the asymptotic and finite sample properties of “asymptotically unbiased” estimators of the tail index γ, based on “external” adequate estimators of the second order parameter ρ. The behavior of the ρ-estimator considered has indeed a high impact on the distributional properties of the final estimator of γ, and must...

In this paper, and in a context of regularly varying tails, we analyse particular but interesting cases of the Maximum Likelihood (M L) and Least Squares (LS) estimators proposed by Feuerverger and Hall (1999). These estimators are alternatives to a well-known estimator of the tail index, the Hill estimator (Hill, 1975), and jointly with the Genera...

In this paper we shall deal with the asymptotic and finite sample properties of "asymptotically unbiased" Generalized Jackknife estimators of the tail index γ, based on adequate estimators of the second order parameter ρ. As a by-product of the final study we first present finite sample properties of the ρ-estimators to be considered in the General...

In this paper we shall consider a natural Generalized Jackknife estimator of the index of regular variation γ of a heavy right-tail 1 − F (x), as x → +∞, associated to any adequate semi-parametric estimator of γ. Such an estimator is merely a linear combination of the original estimator at two different levels. We study such a general linear combin...

The main goal of this paper is to present generalized Hill estimators parametrized in a positive real α (and equal to the Hill estimator when α=1), which are asymptotically more efficient than the Hill estimator for a large region of values of α for any point of the (γ,ρ)-plane, where γ>0 is the tail index, related to the heaviness of the tail 1−F...

Em Teoria de Valores Extremos um dos problemas de grande interesse é o da estimação do índice de cauda de uma distribuição. Os estimadores que têm vindo a ser propostos em contexto semi-paramétrico, baseiam-se num número adequado de estatísticas ordinais de topo, que depende não só do estimador considerado como de parâmetros ‘perturbadores’. A meto...

In this paper, and in a context of regularly varying tails, we propose different alternatives to a well-known estimator of the tail index—the Hill estimator (Hill, 1975). These alternatives have essentially in mind a reduction in bias, preferably without increasing Mean Square Error, by the use of suitable Generalized Jackknife methodologies (Gray...

The main goal of this paper is to present Generalized Hill estimators parametrized in a positive real α (and equal to the Hill estimator when α = 1), which are asymptotically more efficient than the Hill estimator for a large region of values of α for any point of the (γ, ρ)-plane, where γ > 0 is the tail index, related to the heaviness of the tail...

An averaging technique is proposed to reduce the variance of Hill's estimator.

We propose a modification of Resnick and Starica (1997) proposal to reduce the volatility of the Hill-plot, or to smooth the plot, in which the averaging technique is realized over a fixed number of Hill’s estimators

Asymptotically efficient alternatives to the Hill estimator are introduced and studied.

Book with extended abstracts accepted for presentation at the Conference on Extreme Values and Additive Laws, Estoril, October 2-6, 1999.

The maximum of a large number of random variables, suitably normalized, follows, under certain general conditions, the Generalized Extreme Value (GEV) distribution . Hill’s estimator provides an estimation for γ > 0 from a finite sample, requiring the largest m observations, out of n. For different underlying distributions with heavy tails, we prov...

In this paper, and in a context of regularly varying tails, we propose dif- ferent alternatives to a well-known estimator of the tail index — the Hill estimator (Hill, 1975). These alternatives have essentially in mind a reduction in bias, preferably without increasing Mean Square Error, by the use of suitable Generalized Jackknife methodologies (G...

The maxima’s sequence of a sample of n i.i.d. random variables converges, under some conditions, to a random variable with the Generalized Extreme value Distribution. The question of estimating the extreme value index from a finite sample is a very important one, when we need to predict extremal behaviour. We here consider convex mixtures of the Hi...