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\chapter{General introduction} % Chapter title
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\color{gray} {\slshape Come forth into the light of things. Let nature be your teacher.}
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William Wordsworth
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Like the management of other environmental resources, forest management has been questioned and more sustainable practices of forest management are being sought. The management of even-aged pure stands with clearcutting has been particularly debated because clearcuts can incur long-lasting environmental damage such as soil mass movement, stream siltation, windthrow, and biodiversity loss \citep{Keenan1993}. The goal of new forest management practices is to be adaptive and close-to-nature, while sustaining biodiversity and ecosystem services. Forest managers must consider the constantly changing and growing needs of society, take into account climate change, and evaluate of the effects of past management strategies. Attempts are being made to reproduce the structure of old-growth forests in which trees of different sizes and species coexist in the hope of preserving ecosystem values and functions \citep{Touzet1996}. Natural processes, such as natural regeneration, are being favored over human interventions with the aim of preserving ecosystem services and reducing management costs. More particularly, continuous-cover forestry is being favored. Such silvicultural techniques have the goal of relying on natural regeneration, while maintaining irregular stand structure and tree species mixture \citep{Pommerening2004, Bruciamacchie2005, Schutz2012}. 

Successfully managing irregular and mixed forests by relying upon natural processes requires comprehensive knowledge of the ecology of natural regeneration. Continuous-cover forestry systems mimic the dynamics of forests without anthropogenic perturbation and where large-scale disturbances are rare. The dynamics of such forests are dominated by small canopy openings, which originate following the death of single or multiple canopy trees. These small-scale disturbances create a mosaic of patches in which vegetation continuously changes through time and space. Regeneration of different tree species can establish within canopy openings, depending upon the size of the openings. Regeneration of late-successional species can colonize small gaps that result, for example, from the death of a single tree. Regeneration of mid-successional species can colonize gaps of moderate size that originate following the death of a small cluster of trees. Regeneration of early-successional species can establish in large gaps that originate, for example, from windthrow. Once the canopy of early- or mid-successional species closes, understory conditions evolve in favor of mid- or late-successional species, respectively \citep{Finegan1984, Rameau1999}. Environmental conditions in partially shaded understories contrast with those that have been observed in clearcut areas. Tree retention creates microclimates with limited light availability, reduced air movement, and modified water and nutrient cycling \citep{Messier1999}. Because species respond differently to these particular conditions, management of the forest canopy can be used, in theory, to manipulate the microclimate and, hence, control understory development and composition \citep{Lieffers1999}. Understanding regeneration ecology is therefore a requirement for predicting ecosystem dynamics and manipulating them.
 
The amount of radiation transmitted to the understory is a critical factor determining regeneration dynamics wherever nutrient and water availability satisfies regeneration requirements. Solar radiation drives plant photosynthesis and the exchange of mass and energy between soil, vegetation and the atmosphere. As a consequence, the quantity and the spectrum of light that is intercepted by the regeneration greatly affects its survival and growth \citep{Lieffers1999}, as well as stem, branch, leaf and root morphology \citep{Balandier2006b, Galen2007, Niinemets2010}. In temperate and boreal forests that are managed with a continuous-cover forestry system, due to radiation interception by overstory trees, light availability in the understory is often the predominant limiting resource for regeneration and, consequently, drives regeneration dynamics. The amount of transmitted radiation determines, at least in part, regeneration composition because of interspecific differences in growth and survival under shade, i.e., interspecific differences in shade tolerance. Shade-intolerant species are capable of invading open areas, while shade-tolerant species establish under a closed canopy.
 
Our quantitative understanding of understory light environments in uneven-aged and mixed stands remains incomplete. Due to the complexity of multilayered and mixed canopies, understory light conditions vary greatly in time and space \citep{Pukkala1991}. Measuring and predicting light is complicated under complex canopies \citep{Lieffers1999}. Assessing understory light requires repeated measurements and specific equipment. In contrast to the situation in even-aged stands, understory light in uneven-aged and mixed stands is poorly predicted by stand density \citep{Lochhead2012}. Understory light also depends upon the vertical and horizontal distribution of gaps and biomass elements. Consequently, new tools need to be developed and validated to predict understory light availability and improve our understanding of the dynamics of heterogeneous forests.

A better quantitative understanding of understory light is required to provide guidelines for the management of understory light regimes in uneven-aged and mixed stands. Forest managers indeed lack quantitative instructions for controlling understory light levels with partial cutting and, thereby control regeneration growth and survival. Using continuous-cover forestry systems, partial cutting reduces the density and modifies the structure of the vegetation that absorbs incoming light. By maintaining a closed canopy or by opening gaps, forest managers can theoretically create distinct regeneration niches \citep{Grubb1977} that promote shade-tolerant species or less shade-tolerant species, respectively. Even though these key principles have long been described, important questions remain regarding their practical implementation. How a canopy that is favorable to the regeneration of shade-tolerant species can be distinguished from a canopy that is favorable to the regeneration of less shade-tolerant species? What are the stand characteristics that should be considered apart from stand density to predict understory light? Close-to-nature silviculture can create microclimates that are favorable for the coexistence of species with different shade tolerances, but this requires further scientific investigation.

As a matter of fact, while forest managers maintain complex stand structure with continuous-cover silviculture, they often face difficulties or even fail to maintain some desired species mixtures \citep{Schutz1999}. Problematic situations occur mostly with mixtures of species with contrasting shade tolerances (\autoref{fig:examples}). Such difficulties have been reported worldwide, for example, with the regeneration of valuable and less shade-tolerant species in tree-fall gaps in African rain forests \citep{Hall2008, Doucet2009}, the regeneration of yellow birch (\textit{Betula alleghaniensis} Britton) or sugar maple (\textit{Acer saccharum} Marsh.) after partial cutting in stands with, respectively, balsam fir (\textit{Abies balsamea} (L.) Mill.) or American beech (\textit{Fagus grandifolia} Ehrh.) in the province of Quebec (Canada) \citep{Delagrange2004, Prevost2010}, or the regeneration of admixed less shade-tolerant species in European beech (\textit{Fagus sylvatica} L.) forests \citep{VonLupke1999, Petritan2007, VanCouwenberghe2013, Petritan2014}.

\begin{figure}[bt]
\myfloatalign
\subfloat[oak-beech in the Belgian Ardennes]
{\includegraphics[width=.45\linewidth]{gfx/chenehetre2}} \quad
\subfloat[oak-pine in Orlean's forest]
{\label{fig:example-b}
\includegraphics[width=.45\linewidth]{gfx/chenePinLorris}} \\
\subfloat[maple-beech in Quebec]
{\includegraphics[width=.45\linewidth]{gfx/sthilaire}} \quad
\caption[Examples of mixed forests.]{Examples of mixed forests with species of contrasting shade tolerances.}\label{fig:examples}
\end{figure}
 
The purpose of this thesis is to determine how close-to-nature forest management can maintain mixtures of species with contrasting shade tolerances. I consider ecological conditions in which light is presumably the major factor driving regeneration composition and, hence, I study the ecology of mixed regeneration with respect to light availability, the interception of light by irregular and mixed canopies, and how forest management affects understory light and regeneration dynamics.

I examine the mixture of sessile oak (\textit{Quercus petraea} (Matt.) Liebl.) and European beech (\textit{Fagus sylvatica} L.) in the Belgian Ardennes (\autoref{ch:studyArea}). The mixture of oak and beech is of considerable economic importance in European temperate forests and makes an interesting ecological model for studying the coexistence of species with contrasting shade tolerances. Indeed, the two species are often mixed together and they have contrasting shade tolerances. Beech is a late-successional species that is very shade-tolerant. In contrast, oak is a mid-successional species that is less shade-tolerant. Consequently, the mixture of oak and beech naturally evolves towards pure beech forest, i.e., the climax forest.
  
First, I study the ecology of advance regeneration of the two species. I study competition between these two species along a gradient of light availability (\autoref{ch:rege}). Because both species have contrasting shade tolerances, I expect that the shade-tolerant species dominates regeneration in low-light environments, while the opposite situation occurs in high-light environments. In low-light environments, the growth of the less shade-tolerant species is low and the mortality risk is high \citep{Ellenberg1974, Pedersen1998}, whereas in high-light environments the less shade-tolerant species can outgrow the shade-tolerant species \citep{Huston1979, Smith1989, Kobe1995}. According to this theory, adequately managing understory light allows regeneration composition to be controlled. However, it hardly explains why forest managers have difficulties in maintaining the mixture of the two species. In modeling competition between the two species, I test whether there is a rank reversal between the height growth rates of both species in high-light environments or in low-light environments and I seek after the understory light conditions that are favorable for the coexistence of both species.

Second, I attempt to better understand and model light interception by irregular and mixed forest canopies (\autoref{ch:lightModel}). Measuring the amount of radiation that is intercepted by the diverse components of a forest canopy is very tedious and time-consuming, especially in heterogeneous forests \citep{Lieffers1999}. Therefore, I investigate how a model of light interception can advantageously replace light measurements. Since several modeling approaches have already been proposed, I synthesize their advantages and drawbacks, and I identify which approaches are the most appropriate to which application. Then, I implement one of these approaches in a simulation model with the goal of exploring understory dynamics regarding the density, structure and composition of heterogeneous canopies. Once implemented, I validate this approach with field measurements of understory light availability.
 
Third, the implemented model of light interception provides an excellent opportunity for exploring how canopies can be manipulated to maintain mixtures of species with different shade tolerances (\autoref{ch:simulation}). I use the model to explore how modifications to stand density, structure and composition can affect understory light availability. With continuous-cover forestry systems, sylvicultural operations mimic small-scale perturbations with group- and single-tree selection cuttings. Cutting aggregated groups of trees opens canopy gaps, substantially increases understory light \citep{Coates2003, Beaudet2011}, and is often recommended for promoting the regeneration of less shade-tolerant species \citep{VonLupke1998, Bruciamacchie2005}. Yet other strategies that harvest scattered trees can also create microsites with sufficient light availability for less shade-tolerant species. I quantify the understory light conditions following the application of 5 selection cutting systems and I identify how these systems can be manipulated to optimize the understory area favorable for either shade-tolerant species or less shade-tolerant species. This work yields new insights into the interception of light by heterogeneous canopies and provides quantitative guidelines for maintaining mixtures of species with contrasting shade tolerances.

