Case study No 2: Reliability-based service life assessment of a damped bridge stay

Preguntas específicas

Las preguntas que guiaron este capítulo son:

 ¿Cómo cambia la regulación de los caudales y la capacidad de almacenamiento de agua durante el desarrollo del bosque a través de la sucesión forestal?

 ¿Existen diferencias en la función de protección de suelos de los bosques en distintos estados de la sucesión forestal?

 ¿Cómo influyen las características morfométricas de las cuencas, vegetación y propiedades físicas de los suelos en la regulación del flujo hidrológico y protección de suelos de la erosión en las diferentes etapas de la sucesión?

Para establecer la relación entre sucesión forestal y funciones ecosistémicas se evaluó los atributos del suelo, vegetación y morfometría de cuencas en distintos estados sucesionales de un bosque templado, en microcuencas ubicadas al interior de la cuenca del estero Llancahue, con el fin de relacionarlas con el ciclo hidrológico y el transporte de sedimentos.

Objetivos específicos

 Evaluar el rendimiento hídrico (relación precipitación/escorrentía) y capacidad de almacenamiento de agua en los suelos de microcuencas con distintos estados sucesionales de bosque, para estimar su capacidad de regulación de caudales

 Evaluar el transporte de sedimentos hacia los cursos de agua a escala de microcuencas, en distintos estados sucesionales de bosque, para evaluar la función de protección de suelos  Analizar las características estructurales y composicionales de la vegetación, las

características físicas de los suelos en distintos estados sucesionales del bosque y la morfometría de cuencas, para determinar su participación en las funciones ecosistémicas de regulación hídrica y protección de suelos

Cristián Frêne, José Dörner, Felipe Zúñiga, Jaime G. Cuevas, Fernando D. Alfaro & Juan J. Armesto

ABSTRACT

Ecosystem functions in forests can vary significantly during the course of succession, depending on vegetation structure and soil biophysical characteristics. We examined streamflow regulation, water storage, and soil protection functions for a chronosequence of small catchments covered by temperate rain forests over volcanic ash soils in southern South America. Our aim was to understand the changes in ecosystem functioning during successional dynamics, assessing patterns and mechanisms of storage, regulation and protection functions from early shrubby stages to the old-growth forest condition.

Streamflow for the entire study period in old-growth forests catchments (OG) averaged 2102 mm (±110), followed by secondary forests (SF) catchments with 1770 mm (±284), and finally shrub catchments (CH) with 1684 mm (±267). Evapotranspiration showed higher values in SF catchments (643 mm), followed by CH catchments (612 mm), and OG catchments (346 mm). OG and CH catchments had the lowest sediment concentrations in streamflow, without statistical difference between them, while SF catchments had the highest concentrations of sediment in all seasons of the year. Principal component analysis showed that soil physical properties, vegetation and catchment morphometry were all relevant variables to explain the

streamflow regulation and water storage capacity of forest soils are enhanced as forest succession progresses, with maximum soil protection function in early and late stages of forest succession.

INTRODUCTION

Streamflow regulation, water storage capacity and soil protective from erosion are forests ecosystem functions that can vary significantly during the course of succession, depending on vegetation structure and soil biophysical characteristics (Gorham et al. 1979, Likens 2004, Levia et al. 2011, Dörner et al., 2015). Understanding successional changes is highly relevant for enhancing the regulation and distribution of water supplies in complex, human-managed landscapes, where hydrologic cycles are being altered by land-cover and climate change, as well as by the growing societal demands for water use. Understanding how vegetation dynamics is likely to influence soil water storage capacity, their supply to humans, and their capacity to support natural processes has become a highly relevant question in this context. Successional changes in species composition, structure and function of ecosystems have ecological, hydrologic and biogeochemical implications (Margalef 1963, Odum 1969, Veblen

et al. 1981, Vitousek & Reiners 1975, Likens & Borman 1995). Patterns and mechanisms of change in ecosystem structure-function relationships are primarily understood from the point of view of the forest succession paradigm (Lienard et al. 2015). However, patterns of

America, as well as Northern and Central Europe. Accordingly, much of our current understanding of eco-hydrologic processes in changing temperate forests is based on studies conducted in industrially polluted and/or young successional forests, that have regenerated from large-scale logging and forest clearing by humans in the past 100 years across the Northern Hemisphere.

Because the composition of species, disturbance regimes, geology and soils of temperate forests differ significantly between hemispheres (Gilliam 2016), it is questionable the direct application to the southern hemisphere of models developed in the northern hemisphere. This biogeographical and historical bias severely constrains our views and understanding of successional processes in general (Bosch & Hewlett 1982, Swank & Crossley 1988, Hedin et al. 1995, Perakis & Hedin 2002, Levia et al. 2011). In these terms, studies of forest hydrology and biogeochemistry in southern hemisphere forests, although more recent and less numerous, often focus on remnant old-growth forests, in relatively unpolluted environments, and areas with a limited history of large-scale human intervention (Godoy et al. 2014). In southern South America there has been less preoccupation for understanding ecosystem dynamics during the different stages of forest succession following anthropogenic disturbance.

Enhancing the capacity of future forests to capture carbon and store water under a changing climate will depend on our ability to understand the linkages between hydrologic processes and successional dynamics. This paper will examine the change in streamflow regulation, water storage, and soil protection functions in a series of small catchments differing in their

second-growth native temperate rain forests in southern South America (40° S).

A model for understanding hydrologic changes during forest succession

In a forest ecosystem plant cover is first impacted by rainfall, thus reducing the rate at which water impacts the soil via canopy interception (Martínez & Navarro 1995). Rainwater that contact forest soil surface (throughfall) can either runoff or infiltrate the soils, depending on soil physical properties (Laio et al. 2001, Dörner et al. 2015). This water fills in the pores of the upper soil horizons, eventually reaching deeper layers where it can recharge the water table (Dörner et al. 2015) or move horizontally, giving rise to the subsurface runoff or lateral flow (Martínez & Navarro 1995). The functional aspects of the soil porous system are closely related to soil hydraulic properties (Dörner & Horn 2006), which are essential components of water flow regulation.

Soil hydrologic connectivity regulates runoff and export of solutes downstream (Pringle 2003), while soil hydraulic conductivity and the characteristics of pore space influence water dynamics (Martínez & Navarro 1995, Dörner et al. 2015, Dec et al. 2017). In a water saturated soil, hydraulic conductivity depends on the quantity, size, morphology, continuity and orientation of pores. Because this variable depends to a great extent on the shape and continuity of the porous system, it varies strongly from site to site and differs also in the different soil horizons (Ellies et al. 1997).

sediment transport (Geißler et al. 2013). Disturbances, such as logging or fire, may increase surface runoff especially during storm events, thus reducing water infiltration into the soil (Martinez & Navarro 1995), altering soil properties, affecting soil particle transport, and watershed outputs for specific periods of the year.

On a global scale, estimated natural erosion rates range from 0.001-2 t/ha/year on flat lands with established herbaceous and tree cover, up to 1-5 t/ha/year in mountainous areas with limited plant cover (Pimentel 2006). Transport rate of soil particles downslope is determined by topography and vegetation cover (Tsukamoto 1989, Miura et al. 2002, Geißler et al. 2013), which changes during succession (Miura et al. 2002).

To study changes in water flows and sediment dynamics during forest succession, we propose to use a watershed approach (Likens 2001). This approach consists of observational or experimental studies of comparable watersheds differing in vegetation cover and composition to gather quantitative information on post-disturbance hydrological and sediment dynamics in structurally complex landscapes, including forest mosaics (Hedin & Campos 1991, Likens 2001). Small catchments are suitable and sensible units to assess ecosystem responses to compositional and structural changes following natural or human-induced disturbance (Hedin & Campos 1991). Studies at the watershed scale allow for a complete account of inputs and outputs of matter and energy in the ecosystem and facilitate monitoring changes in state variables (Likens 2001).

Accordingly, we will use the watershed approach in a region of southern South America to address our question about the relationship between forest successional stages and the amounts of hydrologic and sediment fluxes. Specifically, we investigated the eco-hydrological functions in nine forested catchments, representing three different successional stages in a temperate rainforest from southern Chile. Previous information about vegetation changes in this system (Donoso et al. 2014), allowed the distinction of (1) an early successional phase, 20 years following logging and cattle, with a cover of shrubs dominated by indigenous bamboo

Chusquea quila, followed by (2) secondary forest developed for 100-110 years following anthropogenic fire, which are dominated by fairly even-aged Nothofagus dombeyi trees and, finally (3) an old-growth stage with presence of a heterogeneous composition of trees 350-400 years old, dominated by shade-tolerant tree species in the upper canopy.

Our aim was to understand the changes in ecosystem functioning during succession, specifically assessing the patterns and mechanisms of water storage and regulation, and soil protection function from early shrubby stages to the old-growth forest condition. Considering observed changes in composition and structure of vegetation among these catchments (Table 1), we addressed the following specific questions: 1) How do physical soil properties change through succession? 2) How is water flow regulated and storage capacity modified through the different successional stages? 3) Are there differences in soil protective function among the three compared forest successional stages? 4) What physical and biotic factors influence ecosystem functions related to water flow and soil protection during the different seasons of

Existing hypotheses derive from early work by Odum (1969), Vitousek & Reiners (1975), and Gorham et al. (1979), propose that hydrologic flow regulation and soil protection functions will be maximized in watersheds with an old-growth forest cover, where soil and vegetation are interconnected, in contrast to younger and less complex stages, where soils are less developed (Ohte & Tokuchi 2011, Likens 2012). Therefore, we hypothesize that forest attributes (vegetation structure and soil properties) that regulate streamflow and thereby enhance water storage will become more prominent as forest succession progresses, reaching maximum values in the old-growth stage. This is because water consumption by vegetation is greater in early stages of succession, when forest presents exponential growth, also considering that after many years of forest growth, in a late succession, soil physical properties will improve the infiltration rates and water storage capacity in the soil profile.

Regarding soil protection, we hypothesize that this ecosystem function will be more effective in early (scrub) and late stages (old-growth forest) of forest succession, reaching higher rates of erosion in the intermediate stages (secondary forests) of succession. This would be explained by the lower height and dense canopy cover, in the case of pioneer bamboo cover, and the multiple vegetational strata in the vertical profile of old-growth forest, in addition to the dense layer of litter present in both succesional stages. Secondary forest would have less soil protection due to canopy height, poor vertical structure and lower litter content in soil. Hence, vegetation structure and soil physical properties for different successional stages should play key roles in modulating stream and underground water flow, water storage and