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\chapter{Study area and species} % Chapter title
\label{ch:studyArea} % For referencing the chapter elsewhere, use \autoref{ch:examples} 

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\color{gray} {\slshape Keep close to Nature’s heart… and break clear away, once in awhile, and climb a mountain or spend a week in the woods. Wash your spirit clean.}
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John Muir
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In the Belgian Ardennes, the mixture of sessile oak (\textit{Quercus petraea} (Matt.) Liebl.) and European beech (\textit{Fagus sylvatica} L.) is of considerable importance as it covers most broadleaf forests. While increasing attention is nowadays being paid to the maintainance of species diversity in forests, oak populations are decreasing in this region. Oak regeneration diminishes, whereas beech regeneration invades the understory. 

\section{Ecological conditions}

The Belgian Ardennes (\ang{50}N, \ang{5}E; \autoref{fig:21map}), is an ecoregion that is characterized by rather homogeneous soil and climate conditions and is fairly representative of medio-European acidophilous beech forests (CORINE classification 41.111). For the period of 1971 to 2000, mean annual rainfall varied between \SI{933}{\milli\metre\per\year} and \SI{1357}{\milli\metre\per\year} and precipitation was well distributed throughout the year. Mean annual temperature ranges between \SI{7.4}{\degreeCelsius} and \SI{9.0}{\degreeCelsius}\footnote{These values were computed by spatial interpolation of climatic data (Tanguy Dejaegere, 2014, Pers. Comm.).}. Dominant soils are well drained brown acidic soils (WRB soil classification) of variable depth, which have developed on Hercynian oligotrophic schist and sandstone substrates. 

\begin{figure}[tb]
\includegraphics[width=\textwidth]{fig21map}
\caption{Locations of the 27 sites in the study area.}
\label{fig:21map}
\end{figure}

In the Belgian Ardennes, sessile oak (\textit{Quercus petraea} (Matt.) Liebl.) and pedunculate oak (\textit{Quercus robur} L.) are both indigenous species that are frequently mixed with European beech (\textit{Fagus sylvatica} L.). According to the regional forest inventory that does not distinguish pedunculate oak and sessile oak \citep{Lecomte2003}, pure and mixed stands with beech and oaks cover almost three-quarters (\SI{71}{\percent}) of the broadleaf forests. These stands have varied structures, ranging from secondary oak forests to late-successional beech forest.
 
In the Belgian Ardennes, oaks and beech have similar requirements regarding soil conditions and most sites are suitable for both species, with few exceptions. They both thrive in acidic soil conditions with good water supplies. Nevertheless, oak thrives better at elevations lower than \SI{500}{\metre} above sea level. Sessile oak also thrives better than beech on dry south-facing slopes and pedunculate oak thrives better than beech in valley bottoms (wetter and richer soil conditions). The data that is collected by the regional forest inventory, highlights well that oaks and beech are found in similar soil conditions (\autoref{fig:22soil}) but the abundance of oaks decreases as elevation increases and oaks are more abundant than beech on well-exposed slopes and valley bottoms.

For the purposes of this study, 27 sites were selected in public forests where European beech and sessile oak grow well (\autoref{fig:21map}). Sites at high elevation, valley bottoms, and steep-sloping sites were avoided. The elevation of the sites ranged between \SI{244}{\metre} and \SI{514}{\metre} above sea level. The average slope was about \SI{9}{\percent} with a maximum of \SI{36}{\percent}. The soil depth ranged between \SI{15}{\centi\metre} and \SI{110}{\centi\metre}, with an average of \SI{47}{\centi\metre}. No pedunculate oak was found in the sites. Because of the homogeneity of the substrates and topographic positions, water and nutrient supplies were assumed to be similar among sites. The ground flora was indeed homogeneous among sites. The dominant herbaceous species were \textit{Luzula luzuloides} (Lam.) Dandy \& Willm, \textit{Vaccinium myrtillus} L., \textit{Deschampsia flexuosa} (L.) Trin, \textit{Dryopteris carthusiana} (Villar) H.P. Fuchs, and \textit{Pteridium aquilinum} (L.) Kuhn. 

\begin{figure}[htb]
\includegraphics[width=\textwidth]{fig22soil}
\caption[Soil conditions in which oaks and beech are found.]{Soil conditions in which oaks (sessile oak and pedunculate oak) and beech are found in the Belgian Ardennes. Soil conditions were inferred from Ecoflore indexes \citep{Bartoli2000} computed from the floristic surveys gathered by the regional forest inventory of Wallonia (\SI{2596}{plots}).}
\label{fig:22soil}
\end{figure}

\section{Reduction of oak population}
The area that is covered by oak is decreasing. According to data that were compiled by the regional forest inventory, there is a shortage of oak trees with diameters less than \SI{60}{\centi\metre}, which clearly indicated that oak regeneration has been deficient (\autoref{fig:23structure}). Several factors contribute to the decrease in oak abundance. The evolution of social and economic conditions, the contrasted shade tolerances of the two species, and the pressure exerted by wild game are the main contributing factors.

Until the end of the 19\textsuperscript{th} century, oak was preferred over beech when the strength, hardness, resistance and tannins of oak wood were needed for the construction of ships, timber-framed buildings, furniture, weapons, plows, and for leather tanning. However, the evolution of social and economic conditions led foresters to stop investing in oak regeneration. The demand for oak wood has decreased and coppicing of oak was abandoned and replaced by the planting of fast-growing species. Harvests in broadleaf stands were limited and oak stands gradually evolved from coppices with standard to well-stocked high forests \citep{Lemaire2001}.

In the absence of natural and human perturbation, oak regeneration is rapidly suppressed by beech regeneration. Under closed canopies, beech is indeed a strong competitor and a very shade-tolerant species. Beech seedlings usually establish before other less shade-tolerant species and before canopy opening. Once light availability increases, less shade-tolerant species start to develop but beech rapidly suppresses them. This common observation has been reported many times since the beginning of the 20\textsuperscript{th} century \citep{Poskin1934}. Oak, the less shade-tolerant species, has persisted in mixtures with beech mainly because of its longevity and through human intervention. Yet nowadays, the natural succession of mixed stands that are composed of beech and oak toward pure beech stands is clearly observed.
 
Finally, red deer (\textit{Cervus elaphus} L.) and roe deer (\textit{Capreolus capreolus} L.) preferentially browse oak saplings over beech saplings. The browsing pressure exerted by wild game depends mainly upon the density of game populations and environment carrying capacity. In the Belgian Ardennes, carrying capacity is limited due to the acidic soils, but game populations have increased remarkably over the last decades. Wildlife pressure on regeneration has become dramatically important in some places, ruining the efforts of forest managers to renew their stands and obliging them to fence large areas in the hope of promoting oak regeneration \citep{Licoppe2005}. 

\begin{figure}[htb]
\includegraphics[width=\textwidth]{fig23structure.pdf}
\caption[Frequency histogram of oaks and beech.]{The data of the regional forest inventory of Wallonia indicates a shortage of oak (Sessile oak and pedunculate oak) trees with girth of less than \SI{60}{\centi\metre} which underlines the deficiency of oak regeneration.}
\label{fig:23structure}
\end{figure}

\section{Why maintain oak?}
\subsection{Biodiversity}
As a general rule, the coexistence of several tree species promotes forest biodiversity since different sets of organisms are associated with different tree species. Moreover, oak is known to have a large set of associated organisms \citep{Branquart2010}, with some ``charismatic'' species such as woodpeckers (Picidae). In contrast, beech tends to suppress the admixed tree species and, consequently, reduce ecosystem diversity.

\subsection{Resilience}
Mixed stands are more resilient and recover faster than pure stands because of interspecific differences in the sensitivity of individual species to biotic and abiotic stressors. The risk of major degradation is therefore weaker in mixed stands than in pure stands. Additionally, trees seem to be less affected by pest damage in mixed stands than in pure stands \citep{Jactel2005}. As a case in point, beech stands are known to be very sensitive to wind storms and drought. Indeed, beech has a superficial root system that is very sensitive to wind storms. Beech stands were among the most affected stands after the wind storm of 1999 in France \citep{Bock2005}. Furthermore, the striking decay of beech trees, that is partly attributable to an outbreak of ambrosia beetles (\textit{Trypodendron domesticum} L. and \textit{Trypodendron signatum} Fabricius, Scolytidae), in the beginning of the current century in the Belgian Ardennes \citep{Henin2003} has highlighted the sensitivity of beech stands to pests and pathogens.

\subsection{Productivity} 
Biomass productivity can be greater in mixed stands than in pure stands because of niche complementarity and positive interspecific interactions \citep{Hector1999}. According to \citet{Pretzsch2013}, mixtures of oak and beech have an average biomass production that is \SI{30}{\percent} greater than that of pure stands of oak or beech. Oak and beech have contrasting strategies for aboveground and belowground resource foraging. Due to the complementarity of their strategies, the proportion of captured resources can be higher in mixed stands than in pure stands. Moreover, several facilitation interactions between oak and beech have been reported, such as greater levels of atmospheric deposition \citep{Andre2008}, higher nutrient contents of leaf litter, faster litter decomposition \citep{Jonard2008}, and greater mycorrhizal diversity \citep{Tyler1992}. Recent studies have even shown that beech trees are less sensitive to drought if they grow in mixed stands with oak \citep{Pretzsch2013b, Molder2014}.

\subsection{Wood quality} 
Oak trees in the high forests of the Belgian Ardennes produce valuable wood for furniture \citep{Plak1987, Gruselle2002}. Although the quality of oak wood in the Belgian Ardennes has a poor reputation, this problem mostly has originated from past management practices in oak stands (oak coppicing). With the exception of sites at high elevations, the ecological conditions in the Belgian Ardennes and, presumably, the continuous-cover forestry management of mixtures of oak and beech, are suitable for producing wood of high quality. 

\subsection{Climate change}
According to the last report of the IPCC \citep{IPCC2013}, climate change is unequivocal, human intervention is clear, and continued greenhouse gas emissions are likely to cause further climate change. By the end of the 21\textsuperscript{st} century in the Belgian Ardennes, according to \citet{Laurent2009} and based on IPCC’s scenario A1B \citep{IPCC2007}, mean annual temperature is predicted to increase by about \SI{3}{\degreeCelsius}. Precipitation is supposed to increase during winter ($+$\SI{18}{\percent}) and decrease during summer (\SI{-15}{\percent}), with more severe drought episodes. Even if the extent of climate change is questionable, scientists agree on the temperature and precipitation trends for this century.

Such climate changes will likely affect forest ecosystems in various ways. Even though most of them are still unpredictable (e.g., species adaptations, community dynamics, pest outbreaks), scientists have agreed that summer drought will probably be one of the major drivers of ecosystem change in Western Europe. The increase in temperatures implies an increase in evapotranspiration, with an increasing demand for water resources. Competition for water resources is likely to increase and trees will suffer from periodic water shortages, depending upon site conditions and species ecology.

Summer drought is already a key factor that contributes to the annual variability of tree growth, as has been highlighted in numerous dendrochronological studies \citep{Scharnweber2011}. From that perspective, oak seems better adapted than beech. The ecological niche of oak with respect to mean annual temperatures (\SIrange{7.3}{13}{\degreeCelsius}) extends into warmer regions more than that of beech (\SIrange{5}{12}{\degreeCelsius}) \citep{Piedallu2009}. Oak has a deeper and more efficient root system, a lower vulnerability to cavitation, and is less drought-sensitive than beech \citep{Breda1993, Backes2008, Garcia2000, Cavin2013, Mette2013}.
 
Furthermore, recent studies have shown that oak competitiveness might increase with climate change. In France, the height growth of beech has recently declined, whereas the height growth of oak has substantially increased \citep{Bontemps2012}. A similar trend has been observed in Central Europe \citep{Mette2013}. In the latter study, researchers even estimated climatic conditions under which oak could become more competitive than beech. They estimated that rank reversal would occur with mean annual temperatures of \SIrange{11}{12}{\degreeCelsius} and annual precipitation of \SIrange{500}{530}{\milli\metre} (with \SI{230}{\milli\metre} during the growing period).
 
Therefore, there is serious evidence that beech will suffer from climate change to a greater degree than oak. However, the decline of beech in mixed stands will probably be mitigated by positive interspecific interactions. Beech decline in the Belgian Ardennes will likely be observed later than in surrounding areas because the Belgian Ardennes has a relatively cooler and wetter climate than the surrounding areas. Nevertheless, the forecasted \SI{3}{\degreeCelsius} increase in mean annual temperature endangers beech trees by the end of the 21\textsuperscript{st} century, especially in the most xeric conditions, as has already observed on south-facing slopes.

\section{Forest management}
With the gradual degradation of the market for small oak timber during the 20\textsuperscript{th} century, broadleaf forests of the Belgian Ardennes have been managed with continuous-cover forestry systems to convert oak coppices and oak coppices with standards progressively to high forests. Forest managers have usually maintained high forest stocking of adult trees \citep{Lemaire2001}, thereby promoting beech regeneration. Nevertheless, during the last decade, the outbreak of ambrosia beetles and the induced beech decay \citep{Henin2003} has opened the canopy of some of these forests providing opportunities for the regeneration of less shade-tolerant species.

Currently, the management of these forests aims to maintain irregular and mixed stands that rely upon the establishment of natural regeneration under a partially closed canopy. Despite the difficulties of maintaining the coexistence of beech and oak, continuous-cover and close-to-nature silviculture is considered to better fulfill the needs of today’s society than the other silvicultural systems \citep{Touzet1996} because it is considered to preserve ecosystem values and services (multiple-use management), to offer greater management flexibility \citep{deTurckheim2006}, and to reduce biotic and abiotic hazards in the context of climate change \citep{Pretzsch2013}. 


