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Gayana. Botánica

versión impresa ISSN 0016-5301versión On-line ISSN 0717-6643

Gayana Bot. v.62 n.2 Concepción  2005

http://dx.doi.org/10.4067/S0717-66432005000200007 

 

Gayana Bot. 62(2): 120-129, 2005 ISSN 0016-5301

ARTÍCULOS REGULARES

 

POSSIBILITIES FOR ECOHYDROLOGICAL MONITORING IN NATURAL AND MANAGED ECOSYSTEMS IN SOUTHERN CHILE

MONITOREO ECOHIDROLOGICO EN ECOSISTEMAS NATURALES Y MANEJADOS EN EL SUR DE CHILE

 

Jan Peters1, Vanessa Wieme1, Pascal Boeckx2, Roeland Samson3, Roberto Godoy4, Carlos Oyarzún5 & Niko Verhoest1

1Laboratory of Hydrology and Water Management, Ghent University, Coupure links 653, B9000 Ghent, Belgium. Jan.Peters@ugent.be
2Laboratory of Applied Physical Chemistry (ISOFYS), Ghent University, Coupure links 653, B-9000 Gent, Belgium
3Laboratory of Plant Ecology, Ghent University, Coupure links 653, B-9000 Gent, Belgium
4Instituto de Botánica, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile
5Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile


ABSTRACT

Ecohydrology investigates how hydrological processes affect plant growth and vice versa. As these mutual interactions are important in many ecosystems, ecohydrology has wide applicability. An important aspect of ecohydrological research is the assessment and prediction of the occurrence of plant species or vegetation types in relation to hydrological or hydrogeochemical habitat conditions. Several models have been developed (e.g. DEMNAT, ICHORS, ITORS, ITORS-VL). All of these models are data driven. Consequently a monitoring network has to be installed, possibly in addition to existing databases. These networks are ecosystem specific but the most widely monitored variables are: water table depth, electric conductivity of groundwater, soil moisture content and chemistry (pH, concentrations of major ions and nutrients) and vegetation composition. The ITORS-VL model proved to be successful for predicting probabilities of plant species and plant community occurrence in Flemish (Belgium) wetland ecosystems, using regressions based on environmental variables ranging from hydrological variables as mean water table depth to chemical variables characterizing soil water such as nitrate concentration and electrical conductivity. However, because of its empirical and ecosystem specific nature, ITORS-VL would not be readily applicable to Chilean ecosystems. The possibility for implementing a comparable monitoring and modelling approach in different ecosystems in southern Chile is investigated. Benefits are of many kinds: (i) a holistic understanding of ecosystem functioning; (ii) the ability to determine the relative importance of site characteristics in various ecosystems and (iii) the possibility to predict vegetation successional stages under changing environmental conditions. These benefits have potential roles in decision support, ecosystem restoration and risk or environmental impact assessment.

Keywords: Ecohydrology, modelling, vegetation, hydrology, decision support systems.

RESUMEN

La ecohidrología se enfoca sobre las vinculaciones entre las plantas y el ambiente abiótico a partir del ciclo hidrológico. Como estas interacciones mutuas son importantes en muchos ecosistemas, la ecohidrología tiene una amplia aplicación. Un aspecto importante de la investigación en ecohidrología es la evaluación y predicción de la presencia de especies vegetales o tipos vegetacionales en relación con la hidrología o condiciones hidrogeoquímicas del hábitat. Para ello, diversos modelos han sido publicados (por ejemplo, DEMNAT, ICHORS, ITORS, ITORS-VL). Todos estos modelos necesitan datos. Consecuentemente una red de monitoreo se tiene que instalar en adición a las bases de datos existentes. Estas redes incluyen el monitoreo de propiedades importantes de los ecosistemas tales como: profundidad de la zona saturada, conductividad eléctrica del agua subterránea, contenido del agua del suelo, características químicas del agua del suelo (pH, concentraciones iónicas y nutrientes) y composición de la vegetación. El modelo ITORS-VL ha sido probado satisfactoriamente para predecir la ocurrencia de comunidades de plantas en los ecosistemas pantanosos de Flanders monitoring and modelling: Peters, J. et al. (Bélgica), usando regresiones basadas en variables ambientales que van desde variables hidrológicas como profundidad de la zona saturada a variables químicas que caracterizan el agua del suelo vía concentración de nitratos y conductividad eléctrica. Sin embargo, debido a su naturaleza empírica para un ecosistema específico, este modelo podría no ser adecuado para los ecosistemas en Chile. El presente trabajo investiga la posibilidad de implementar un monitoreo y modelamiento en diferentes ecosistemas del sur de Chile. Los beneficios de esta actividad son variados: (i) una comprensión holística del funcionamiento del ecosistema, (ii) la habilidad para determinar la importancia relativa de las características del sitio en varios ecosistemas, y (iii) la posibilidad de predecir estados sucesionales de la vegetación bajo condiciones ambientales variables. Estos beneficios presentan potenciales roles en la capacidad de gestión, restauración de ecosistemas y evaluación del riesgo o impacto ambiental.

Palabras claves: Ecohidrología, modelamiento, vegetación, hidrología, sistemas de soporte de decisiones.


INTRODUCTION

Ecosystems are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between direct gradients (nutrients, moisture, temperature), their environmental determinants (climate, geology, topography) and potential natural vegetation, and the processes that mediate between the potential and actual vegetation cover (Wilby & Schimel 1999). Ecohydrology is the research at the interface between hydrological and ecological sciences. Hannah et al. (2004) examined the evolution of the definition of ecohydrology through time. The first clear definition appeared in Wassen & Grootjans (1996) and covered the unidirectional nature of hydrological factors determining the natural development. Problems associated with the unidirectional nature of Wassen and Grootjans' definition were recognized by Baird and Wilby (1999) who broadened the definition to include ecohydrological interactions of vegetation on hydrological factors; ecohydrology investigates how hydrological processes affect plant growth and vegetation dynamics, and vice versa (e.g. Rodriguez-Iturbe et al. 2001, Seabloom et al. 2001, Cramer & Hobbs 2002, Zalewski et al. 2003). These ecohydrological relations can be revealed at various scale levels, and processes important at one scale are not necessarily so at another scale. Information is often lost as spatial or temporal data are considered at coarse scales of resolution (Turner 1990). A general overview of hierarchical relations between abiotic and biotic characteristics and properties in ecosystems at stand level is given in Fig. 1. Nutrient availability, soil moisture and temperature regimes tend to be strongly related to potential natural

vegetation (Franklin 1995, Leuschner 1997, Seghieri et al. 1997, Rodriguez-Iturbe et al. 2001, Laio et al. 2001a, 2001b, Porporato et al. 2001). The potential natural vegetation is a hypothetical climax state which is affected by a variety of natural (e.g. fire, flooding) and anthropogenic (e.g. harvesting, water management, climate change) disturbances. The resulting actual vegetation, on its turn, is responsible for feedbacks to nutrient availability, moisture and temperature regimes.

Among many others, the broad scope of ecohydrological research coveres the following aspects:

-interpretation of present soil and vegetation patterns from a hydrological point of view (Kemmers et al. 1995);

-the relationship between vegetation, soil and water based on an understanding of the physiological properties of plants (Baird & Wilby 1999);

-the effect of hydrological regimes on vegetation succession (Somodi & Botta-Dukat 2004);

-development of realistic goals in nature conservation (Kemmers et al. 1995);

-decision support in ecosystem restoration (Kemmers et al. 1995);

-sustainable development of water resources (Zalewski et al. 2003);

-Socio-economic aspects may be incorporated in ecohydrological studies as they affect many ecohydrological processes.

This review assesses the feasibility and applicability of an ecohydrological monitoring and modelling strategy in southern Chile, based on ecohydrological studies carried out in Flanders (Belgium) and The Netherlands. In the first part a monitoring strategy is outlined, serving different types of ecohydrological models (focus on empirical models). In the second part, ecohydrological models are described. Finally in the third part a preliminary exploration of possible ecohydrological studies in pristine and anthropogenic disturbed ecosystems in southern Chile is presented.

MONITORING STRATEGIES

A monitoring system is defined as a spatial network of measurement points on which measurements take place in a way that changes in time and space can be detected (Jansen et al. 1983). Variables for monitoring are selected on both scientific and practical arguments. Scientific arguments include the goal of a certain monitoring strategy: will data be used for empirical (i.e. description of abiotic and biotic ecosystem features, without understanding functional processes), or for mechanistic ecosystem description (in-depth understanding of a whole range of processes involved), and which variables tend to explain studied ecosystem features or processes? Practical arguments range from financial issues, over measurement tool limitations to site accessibility. Consequently, monitoring schemes might differ from one ecohydrological study to another in: (i) monitored ecosystem variables; (ii) temporal resolution, and (iii) spatial resolution. Variables considered frequently in monitoring strategies for ecohydrological studies in Flanders and the Netherlands are reviewed.

MONITORING OF ABIOTIC VARIABLES IN THE SATURATED ZONE

(1) Water table depth [m]: water table depth is a driving variable in ecosystems as it forms a boundary condition for soil moisture content in the root zone. It exerts a major influence upon the distribution and performance of plant species and the composition of the vegetation. Changes in water table depth due to management practices have severe impacts on all ecosystem compartments. Observed responses of established plants to these changes depend upon the magnitude of change and the species specific tolerance. Results can be used to calculate the timing, amplitude, frequency and duration of water table fluctuations. Mean highest water table depth and mean lowest water table depth are less dynamic hydrological parameters which can be calculated during long term monitoring. They suggest average circumstances and are indicative for both natural situations and hydrologically managed systems. Water table depth is measured using piezometers.

(2) Electrical conductivity (EC [ìS cm-1]): this variable is of minor importance in infiltration areas where precipitation water (low EC) is dominant. In areas with seeping water, tidal influences or irrigation, however, the EC of ground and soil water is deterministic for plant growth. The osmotic potential of the soil solution (Øs [MPa]), a measure of the osmotic stress a plant might experience, is linearly related with electrical conductivity (White, 1997). Different types of measuring probes can be used to determine EC in field and laboratory measurements.

(3) Hydrochemical variables [mg l-1]: Cation (NH4+, Na+, K+, Ca2+, Mg2+, Fe2+) and anion (HCO3-, PO43-, NO3-, SO42-, Cl-) concentration are determined. Available ions and ion balances in groundwater have important influences on plant growth and vegetation at some sites (e.g. seepage areas). At other sites (e.g. infiltration areas) the groundwater chemistry is not related to the soil solution. Homogeneous groundwater samples can be taken from the piezometers. Metal concentrations can be determined by plasma-emission-spectroscopy (ICP), HCO32- by titration, and PO43-, NO3-, SO42-, Cl- and NH4+ on the basis of ion chromatography, among other techniques.

MONITORING OF ABIOTIC VARIABLES IN THE UNSATURATED ZONE

In this monitoring section the quantity and quality of soil moisture is assessed. Soil moisture is an important link between the abiotic environment and vegetation (Fig. 1.).


 

FIGURE 1. Conceptual model showing the relationship between direct gradients (nutrients, moisture, temperature), their environmental determinants (climate, geology, topography) and potential natural vegetation, and the processes that mediate between the potential and actual vegetation cover. Legend: arrows indicate relationships; broken arrows indicate vegetation feedbacks; rounded squares and bent arrows indicate exogenous disturbances. Adapted from Franklin (1995).

FIGURA 1. Modelo conceptual representando la relación entre los gradientes directos (nutrientes, humedad, temperatura), sus condicionantes medioambientales (clima, geología, topografía) y la vegetación natural potencial, así como los procesos que actúan entre la vegetación potencial y la actual. Leyenda: las flechas indican relaciones, las flechas discontinuas indican retroalimentaciones de la vegetación, los cuadrados redondeados y las flechas curvadas indican alteraciones externas. Adaptado de Franklin (1995).

(4) Volumetric soil moisture content (%): Many ecohydrological studies suggest that soil moisture content is the key variable in ecosystems (e.g. Rodriguez-Iturbe et al. 2001). Soil moisture content controls rainfall infiltration, deep percolation and runoff generation, the heat budget of both soil and near-surface atmosphere, leading to an important coupling between processes taking place at the soil surface (Porporato et al. 2002). Direct and indirect methods for volumetric soil moisture content measurement are developed, among others: gravimetric method, time-domain reflectometry and neutron scattering.

(5) Soil moisture chemistry [mg l-1]: There are 13 mineral elements without which green plants cannot grow normally, neither reproduce. On the basis of their concentration in plants, these essential elements are subdivided into: the macronutrients (N, P, S, Ca, Mg, K, Cl) which are found at concentrations greater than 1000 mg kg-1 in the plant and the micronutrients (Fe, Mn, Zn, Cu, B, Mo) which are generally found at concentrations lower than 100 mg kg-1 in the plant. An analysis of soil water for K+, Ca2+, Mg2+, Fe2+, NO3-, NH4+, PO43-, SO42-, Cl- and HCO3-, soil water pH and electrical conductivity are essential for the explanation of vegetation patterns (Huybrechts et al. 2000). At some sites the soil water chemistry is directly related with the groundwater chemistry (seepage, thin unsaturated soil layer), but this is not a generally rule (Huybrechts et al. 2000). Soil pore moisture can be extracted by ceramic cup lysimeters, permanently installed beneath the ground surface. Alternatives for measuring cation and anion concentrations in samples were discussed above.

SOIL MONITORING

(6) Organic matter: changes in hydrological regime affect oxidation of organic matter. Retention and exchange of cations and anions, soil acidity, water absorption, and the formation and stabilization of soil aggregates are only a few of the many soil properties that are determined by reactions at organic surfaces (White 1997).

(7) Soil temperature: temperature is an important factor in relation to physicochemical and biological reactions in the soil as for plant growth and other characteristics such as germination. Generally soil temperature is measured at different depths using temperature probes.

(8) Soil aeration: the lack of oxygen (O2) in wetland soils results from flooding. During a flooding event, O2 diffusion is severely inhibited (Shuwen et al. 2004). The oxygen remaining in the soil is consumed by aerobic processes of roots and soil organisms, resulting in a severely reduced O2 partial pressure which induces alterations in microbial (Sierra & Renault 1995) and plant activity (Visser et al. 2003).

VEGETATION

Vegetation considers all plant life in a particular area. Vegetation can be represented by different plant communities, i.e. a noticeable, relatively homogeneous part of the vegetation. Delineation of those plant communities, however, might be seen as a problem to define borders. Two approaches have been used: (i) the Clements discontinuous approach (Clements 1916) with easily definable communities and (ii) the Gleason continuum approach (Gleason 1926) where vegetation results from individual plant responses. It is clear that the vegetation bears the result of interaction among many species, and interactions between species and the environment, so that when studying vegetation the existence of dynamical assemblages of species subjected to changes must be considered. This is successional changes or changes due to climatic variation and to human impacts (Biondi et al. 2004).

(9) The Braun-Blanquet approach (presented in van der Maarel 1975) has given the possibility of developing an abstract hierarchical classification system of vegetation that can be supported by logical and numerical methods, irrespective of whether we consider the vegetation evolving in a continuous or discrete manner. Variables assessed with the Braun-Blanquet methodology on permanent quadrates are species abundance, dominance and diversity.

MANAGEMENT

(10) Management practices are mostly described by non-continuous variables (annual mowing regime, cyclic mowing regime, grazing, etc.). The importance of management is obvious both on the biotic as the abiotic ecosystem compartments. Indeed, management practices can be focussed directly on the vegetation (mowing, pruning, herbicides, fire, grazing, etc.) or indirectly, on the abiotic site conditions (fertilizing, drainage, etc.) Consequently, a detailed management description is desirable.

A suitable experimental design covering some of cited ecosystem variables is given in Fig. 2 based on a study at the nature reserve Bourgoyen-Ossemeersen, Gent, Belgium.


FIGURE 2. Experimental design for an ecohydrological study at Bourgoyen-Ossemeersen, Gent, Belgium. (a) schematic overview of the measurement tools in the upper site; (b) delineation of monitoring transect.

FIGURA 2. Diseño experimental de un estudio ecohidrológico en Bourgoyen-Ossemeersen, Gante, Bélgica. (a) representación esquemática de los instrumentos de medida en la zona superior; (b) representación del transecto monitorizado.

MODELLING

An ecohydrological model is one that, within the model structure, accounts for ecohydrological processes. Models can be used for many purposes (Baird 1999): (i) simplistic models to illustrate the basic features of a system; (ii) for prediction or (iii) as research tool. Three main model types are defined: (i) conceptual models; (ii) physical and analogue models and (iii) mathematical models. A conventional breakdown of mathematical models is given by Hillel (1977), who considers empirical models (concerned with prediction rather than explanation), mechanistic models (based on known mechanisms and processes that operate within the modelled system), deterministic models (exact relationships are postulated, and the output of the model is predicted with certainty by the input) and stochastic models (a random or stochastic element is introduced into the model so that for a given input there is no longer a single output but a range or distribution of output values). As stated above a monitoring strategy is selected on both scientific and practical arguments. As far as the scientific arguments are concerned, Kirkby et al. (1993) stated that the choice of an appropriate model, and related monitoring strategy, should be centred on a number of questions:

(i) What is the purpose of the model? Models can be used for many purposes. As an example, some are used for prediction (e.g. empirical models), while others are used as a scientific tool (e.g. mechanistic models). In the latter, the predictive value of the model may not be of particular interest.
(ii) Should the model be process-based? Many empirical models that do not take explicit account of processes give accurate predictions of system behaviour. Then there may be no need to construct a process-based model. If, on the other hand, the model is to be used as a theoretical tool for investigating and explaining ecosystem behaviour, it will need to be process-based.
(iii) What resolution in space and time is required? Ecohydrological processes exhibit variations across space and time. A broad distinction can be made between dynamic models (simulation of time-dependent processes) and steady-state models (in which processes operate at a steady rate). For modelling at the same scale level, dynamic modelling generally needs greater temporal resolution data.
(iv) Is the model capable of being tested? A model may be plausible, it may contain what is thought to be a reasonable representation of the real world but, it cannot be tested. In this case results must be treated with great caution.

In Flanders and The Netherlands, several ecohydrological models have been developed (e.g. DEMNAT (Witte et al. 1992), ICHORS (Barendregt & Wassen 1989), ITORS (Ertsen et al. 1995), ITORS-VL (Huybrechts et al. 2002)). These models were designed for assessment and prediction of the occurrence of plant species or vegetation types in relation to hydrological or hydrogeochemical habitat conditions. All of these models are data driven, so that data obtained from monitoring networks were used.

ITORS-VL (Influence Terrestrial site conditions On the Response of Species - Flanders) is an empirical model which has proved to be successful for predicting probabilities of plant species and plant community occurrence in Flemish wetland ecosystems (Huybrechts et al. 2000, 2002). The model is divided into two different steps. First, a data-matrix is composed with collected data on hydrology, soil, management and plant species from the monitoring network. Data should cover the entire ecological amplitude of plant species under investigation, i.e. for all species the occurrence probability curve should be made up, ranging from the lowest tolerable level for an environmental factor (i.e. hydrological, soil or management variables) till the highest tolerable level and consequently condition the data-matrix dimensions. Second, mathematical (not necessarily causal) relationships between occurrence of plant species and monitored environmental variables are calculated by empirical multiple logistic regression. The compilation of all regression equations makes up the model. The model output does not stipulate whether plant species are present in the actual vegetation, only the probability of occurrence for different species is stated. Nevertheless, several benefits of the model application were found (Huybrechts et al. 2002), such as:

- extension of botanical knowledge, and species specific environmental preferences;

- determination of important abiotic site charac-teristics in relation to vegetation;

- useful as a management tool because the model provides the possibility to define realistic targets, to plan natural structures, to predict ecosystem development to some extent and to tune water management and nature conservation.

Emperical regression models are only valid within the systems where they were developed. Consequently, generalization of the ITORS-VL model to Chilean ecosystems is impossible. However, the possibilities and potential benefits of a comparable monitoring and modelling strategy for ecosystems in southern Chile is discussed.

POTENTIAL APPLICATIONS IN CHILE

A comparable monitoring and modelling strategy as discussed above can be applied to several ecosystems in southern Chile. A preliminary exploration is presented.

(1) Temperate forests of southern South America are globally important because of their high level of endemism (Smith-Ramírez 2004). Therefore these forests were included among the 25 global hotspots for biological diversity (Myers et al. 2000). However, at present a vast majority of coastal areas have been reforested with exotic species, mainly Pinus radiata D.Don and Eucalyptus globulus Labill. (Smith-Ramírez 2004). The hydrological consequences of intensive forest operations on water yield and quality have received much attention. Large-scale forest operations can severely affect water, nutrient and sediment cycling within a catchment (Huber and Iroumé 2001). Long term ecohydrological studies might be carried out in such catchments, both in the afforested sites as in neighboring natural patches. Results may show changes in the natural patches, both in abiotic circumstances and in vegetation. Consequently, the ecohydrological impact of plantation forestry on adjacent natural ecosystems can be assessed.
(2) The Santuario de la Naturaleza e Investigación Científica "Carlos Anwandter" on the Río Cruces, Valdivia, is a designated Ramsar site since 27 July 1991. The area (4877 ha) includes beds of the río Cruces and its tributaries and other adjacent flooded areas. These areas are influenced by tidal amplitudes smaller than 1 m and seasonal fluctuations in water level through the annual variation in precipitation. Previous monitoring of abiotic data of the río Cruces river freshwater includes: water temperature, dissolved oxygen levels, electrolyte and nutrient concentrations, pH and conductivity. The area can be divided in three ecological zones: (i) submerged plant zones; (ii) flooded areas or swamps and (iii) emergent vegetation areas and marshlands. In all three zones an ecohydrological monitoring system could be installed to supplement previous monitoring work. Short term results would include: the assignment of important site characteristics in different ecological zones; typical plant species in relation with abiotic characteristics; within-seasonal vegetation responses in relation to abiotic circumstances, etc. Results of long term monitoring would possibly include a description of environmental changes due to exotic plant species invasion, or human and natural disturbances.

CONCLUSION

Ecohydrological monitoring schemes and their subsequent modelling in different ecosystems of southern Chile could be beneficial for the: (i) improvement of the understanding of ecosystem functioning; (ii) determination of important site characteristics in relation to vegetation, and (iii) prediction of vegetation successional stages under changing environmental conditions. As many anthropogenic activities result in hydrologic alterations (with consequences for ecosystem functioning) or in other direct ecological impacts, understanding mutual relationships between hydrology and ecology and the ability to describe them in ecohydrological models can become a valuable management tool towards sustainable management.

ACKNOWLEDGMENTS

The authors wish to thank the Bilateral Scientific and Technological Cooperation between Flanders and Chile (Project BIL 01/04), the Fonds voor Wetenschappelijk Onderzoek - Vlaanderen (FWO-Vlaanderen) and the special research fund (BOF) of Ghent University (Belgium).

 

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Received 08/03/05
Accepted 21/08/05

 

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