\chapter{General discussion} % Chapter title
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\color{gray} {\slshape Science goes from question to question; big questions, and little, tentative answers. The questions as they age grow ever broader, the answers are seen to be more limited.}
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George Wald
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\section[Maintaining species diversity]{Maintaining the mixture of species with contrasted shade tolerances}
The main purpose of this thesis was to determine how forest managers could maintain a mixture of species with contrasting shade tolerances in uneven-aged and mixed forests that are managed using continuous-cover forestry systems.
 
The results suggest that maintaining a mixture of species with contrasting shade tolerances requires fine control of understory light to promote regeneration of the different species. Such a control of understory light can be achieved by controlling the density and structure of the overstory with partial cutting. Moreover, regeneration cleaning may be essential to suppress regeneration of the shade-tolerant species and to promote regeneration of the less shade-tolerant species.

Understory light was found to be a key parameter in the dynamics of heterogeneous stands, as it affects regeneration growth and composition. There are interspecific differences in survival and growth response to the availability of light. Under the deep shade of a closed canopy (PACL $<$ \SI{5}{\percent}), only small seedlings of shade-tolerant species are able to survive \citep{Madsen1997, Emborg1998, LeDuc1998, Lieffers1999, VonLupke1999, Emborg2000, Collet2001, VonLupke2004, Collet2006, Petritan2007}. Consistent with other studies, as understory light increases (PACL $=$ \SIrange{10}{20}{\percent}), the seedlings of less shade-tolerant species are able to survive and grow, but the seedlings of shade-tolerant species grow faster and suppress the seedlings of less shade-tolerant species. After canopy release (PACL $=$ \SIrange{20}{40}{\percent}), the regeneration of both shade-tolerant and less shade-tolerant species develops well \citep{Dineur1951, Emborg1998, VonLupke1998, Stancioiu2006c}. Less shade-tolerant species require indeed greater amount of radiation than shade-tolerant species. Additionally, less shade-tolerant likely require sufficient amount of direct radiation that can be obtained after such canopy release \citep{Diaci2002, Diaci2007}. Following further canopy opening (PACL $>$ \SI{40}{\percent}), the growth of the regeneration saturates and becomes less sensitive to variations in light availability \citep{Stancioiu2006c, VanCouwenberghe2013}. Such large canopy openings alter the microclimate of the understory and regeneration can additionally suffer from the development of competitive herbaceous species \citep{Gaudio2008}.

The control of understory light is consequently an important issue, although it is also a difficult task. I explored different strategies that forest managers could apply to increase the availability of light to the understory. This work provided indications of how partial cutting can be manipulated to change stand composition and structure and how such changes affect understory light. The control of understory light requires controlling harvest intensity jointly with the location, size and species of the harvested trees. Different silvicultural treatments can be utilized to control understory light. For example, forest managers can preferentially harvest small or large trees, trees of shade-tolerant species, or aggregated groups of trees. All of these silvicultural strategies could provide satisfactory results as long as harvest intensity is adapted to the chosen strategy (\autoref{fig:61lightsimulation}).

\begin{figure}[htb]
\includegraphics[width=\textwidth]{fig61lightsimulation}
\caption[Post-harvest proportion of microsites favorable to less shade-tolerant species.]{Post-harvest understory light conditions depend upon cutting intensity and cutting type. To maximize the understory surface that is favorable to regeneration of less shade-tolerant species, forest managers need to choose an adequate combination of cutting type and cutting intensity.}
\label{fig:61lightsimulation}
\end{figure}

The results of this thesis also underscore the problem that even under good light conditions (PACL $>$ \SI{20}{\percent}), regeneration of less shade-tolerant species might not overcome the regeneration of shade-tolerant species. This finding goes against the theory of shade tolerance as proposed by \citet{Kobe1995} and the results of greenhouse experiments \citep{Dreyer2005}. Indeed, according to \citet{Kobe1995}, there is a tradeoff between high-light growth and low-light survival. In low-light environments, shade-tolerant species survive better than less shade-tolerant species, whereas in high-light environments, shade-tolerant species grow slower than less shade-tolerant species. Nevertheless, the results of this thesis are in accordance with the observations of forest managers within the study area and with the recently published results of \citet{VanCouwenberghe2013} and \citet{Petritan2014}, who studied the same two species in France and in Romania, respectively. Additionally, the suppression of the regeneration of less shade-tolerant species by regeneration of shade-tolerant species in high-light environments has been reported in other mixed forests: in France with mixtures of \textit{Quercus pubescens}-\textit{Fagus sylvatica} \citep{Kunstler2005}, in central Europe with mixtures of \textit{Acer pseudoplatanus}-\textit{Fraxinus excelsior}-\textit{Fagus sylvatica} \citep{Petritan2009}, or in Quebec with mixtures of \textit{Acer saccharum}–\textit{Fagus grandifolia} \citep{Delagrange2010}.

Consequently, maintaining less shade-tolerant species in stands with shade-tolerant species might require silvicultural interventions jointly in the overstory and regeneration layers. Establishing saplings of less shade-tolerant species may require competition from the saplings of shade-tolerant species to be decreased manually. The dominant saplings of less shade-tolerant regeneration would have a greater chance of survival if they are taller than any neighboring saplings of shade-tolerant species.

\section[Silvicultural recommandations]{Silvicultural recommendations for maintaining mixtures of oak and beech} 
Beech saplings naturally outgrow oak saplings under a partially closed canopy. In addition, beech saplings usually establish before oak saplings and require half as much light to reach an optimum growth than do oak saplings. In these conditions, oaks are rapidly suppressed and mixed stands of oak and beech evolve naturally toward pure beech stands. The two species mainly coexist because beech naturally regenerates under well-established oak stands where oak has previously been favored by selective thinning, coppicing and plantations \citep{Claessens2010}. The reverse situation is unlikely to occur naturally without frequent disturbances \citep{Messier1999} and (or) disturbances that damage the beech regeneration \citep{Reyes2010}.
 
As a result, foresters applying continuous-cover silviculture need to proceed in three phases:
 
First, in waiting for the emergence of oak seedlings, they must maintain low-light levels in the understory (PACL $<$ \SI{5}{\percent}) to reduce the development of beech seedlings that continuously emerge under the canopy. Indeed, germination does not depend upon light conditions \citep{Turbang1954, Welander1998, Chaar1999, Nicolini2000}.

Second, reducing overstory density is essential to promote oak regeneration. This can be achieved with partial cutting that increases the availability of radiation to oak regeneration to about \SIrange{20}{40}{PACL}. The target basal area after partial cutting ranges between \SI{10}{\square\metre\per\hectare} and \SI{25}{\square\metre\per\hectare}, depending upon residual stand structure. The creation of gaps of about \SI{500}{\square\metre} provides locally large amounts of radiation while minimizing the reduction of stand stocking. Preferential cutting of small trees and individuals of shade-tolerant species provides sufficient amounts of light with a target basal area of about \SIrange{15}{20}{\square\metre\per\hectare}. Cutting without consideration of tree species, size and location requires a lower target basal area, i.e., about \SIrange{10}{15}{\square\metre\per\hectare}. Preferentially cutting large trees maintains large areas in the understory with low-light availability, which favors shade-tolerant species. However, as saplings become taller, foresters need to gradually open the gaps and reduce residual stand density to satisfy the increasing light requirements of older saplings \citep{Messier1999}.
 
Third, given the ecological conditions of the Belgian Ardennes, oak could not be promoted over beech by only managing the overstory. Foresters need to frequently remove or break manually the dominant and co-dominant beech competitors that will systematically overtop oak saplings \citep{VonLupke1998}, especially if beech seedlings were established before oak seedlings. At the end of this treatment, the dominant oak saplings must indeed dominate all remaining beech saplings.

\section{Study limitations and perspectives}
This study provides us with a better understanding of the regeneration dynamics of uneven-aged stands with a mixture of species with contrasting shade tolerances. I was not able to study all of the processes that drive the dynamics of such a complex ecosystem. In the following discussion, I acknowledge the limitations of my work and, consequently, indicate remaining knowledge gaps and interesting research perspectives.

I assumed that the coexistence between tree species depends mainly upon interspecific differences in the development of established regeneration, as only the dominant saplings survive and reach the upper canopy layers. Even though the evolution of stand composition does not depend only upon competition between saplings of different species, the suppression of all saplings of a given species from the regeneration layer certainly affects future stand composition because new regeneration establishment usually does not occur where there is abundant advance regeneration \citep{Reyes2010}. The composition of tree recruitment could additionally depend upon interspecific competition between poles and trees, interspecific differences in tree survival, seed production, seed dispersal, and seed germination.
 
I assumed that sapling survival depends upon the capacity of saplings to grow taller than their neighbors and, consequently, to capture a larger amount of solar radiation. Consequently, the analysis of regeneration height growth (primary growth) was prioritized over the analysis of regeneration diameter growth (secondary growth) and regeneration survival. \citet{VanCouwenberghe2013} studied the growth of regeneration of sessile oak and European beech in France and found that diameter growth is closely related to height growth. In their study, diameter growth of oak saplings was less than the diameter growth of beech saplings under all investigated light levels. \citet{Baudry2013} had studied the survival of sessile oak and European beech in Belgium and demonstrated that sapling survival depends largely upon sapling capacity to dominate neighboring individuals. Moreover, a relationship can usually be found between sapling mortality and diameter growth \citep{Kobe1995, Pacala1996}. The probability of sapling mortality decreases as sapling radial growth increases. Sustained growth indicates good photosynthetic activity and reduces the period in which saplings are exposed to competition with neighboring vegetation or other agents that could cause damage such as wild game (e.g., deer) or disease (e.g., oak powdery mildew).

I assumed that the development of advance regeneration mostly depends upon light quantity, as expressed by the percentage of above canopy light. The findings of this thesis corroborate the paramount importance of the amount of light that is required for regeneration development and composition. Nevertheless, other factors certainly drive competition between species with contrasting shade tolerances because some of the results of this thesis are in contradiction with the results of other studies. Particularly, in a greenhouse experiments \citep{Dreyer2005}, saplings of sessile oak outgrew beech saplings in high-light conditions. While, in the field experiments that was performed for this thesis project, beech saplings dominated oak saplings under all of the study conditions, even though regeneration was measured over a wide gradient of light availability. In stands that are managed with continuous-cover forestry systems, the availability of light does not necessarily allow conditions to be identified in which the saplings of less shade-tolerant species outgrow saplings of shade-tolerant species. I hypothesize that other factors drive competition between species with contrasting shade tolerances, and subsequently explain why the saplings of less shade-tolerant species do not outgrow saplings of shade-tolerant species in bright understories. Different results could therefore be obtained in other areas or with other tree species. In particular, a rank reversal between the growth of species with contrasting shade tolerances along a light gradient has been observed and modeled in other mixed forests \citep{Pacala1994}. Site conditions might also affect species performance ranks \citep{Kobe2006, Valladares2008}. Consequently, the promotion of less shade-tolerant species over shade-tolerant species does not automatically require regeneration tending. Among the factors that have already been identified as modifying the performance rank of species of differing shade tolerance, the availability of water likely plays a key role. The other factors that should be considered are tree ontogeny, nutrient availability, and sunfleck duration and frequency \citep{Valladares2008}.

Regeneration of less shade-tolerant species likely requires direct radiation whereas diffuse radiation can be enough for regeneration of shade-tolerant species \citep{Diaci2002}. Further investigations are required to better identify the requirements of regenerations to direct radiation and how forest manager can control the amount of transmitted direct radiation. I hypothesize that (\textit{i}) an increase in the availability of direct radiation promotes the regeneration of less shade-tolerant species, (\textit{ii}) post-harvest availability of direct radiation depends upon cutting type, and (\textit{iii}) gap creation is the cutting type that induce the greatest increase in the availability of direct radiation. 
   
The availability of understory light was investigated under particular conditions. The availability of understory light depends upon complex interactions between latitude, canopy height, topography, and foliage orientation and aggregation \citep{Canham1990, Kuuluvainen1992, Prevost2012}. In the northern hemisphere, maximum solar elevation decreases with latitude and maximum light penetration through canopy gaps is offset northwards from the gap center (\autoref{fig:62light}). At high latitudes, the area receiving the largest gains in understory light is located closer to the northern gap edge or even under closed canopy. Consequently, the area receiving the largest gain in understory light does not coincide with the area of increased nutrient and water availability \citep{Lieffers1999}. Moreover, south-facing surfaces (sloping ground and leaves) receive greater amounts of energy from direct radiation than north-facing surfaces because direct radiation strikes the surface at an angle closer to the perpendicular. In hilly areas, some of the radiation entering at low elevation angles is also intercepted by the horizon. For example, west-facing slopes do not receive direct radiation at the beginning of sunrise. The study forests were around \ang{50}N latitude, had a canopy height of about \SI{30}{\metre}, and were present on gentle slopes. In different forest types, at different latitudes, and (or) on hilly terrain, different results could be obtained regarding the effects of partial cutting on understory light. For example, I hypothesize that (\textit{i}) smaller gaps (and higher stand densities) are required on south-facing slopes or at lower latitudes in order to promote the regeneration of less shade-tolerant species, and (\textit{ii}) cutting from below induces a lower increase in understory light at low latitude than at high latitude.

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\includegraphics[width=\linewidth]{fig62light}
\caption[Availability of direct radiation at varying slopes and canopy heights.]{The transmission of direct radiation through canopy gaps depends upon the interaction between site latitude, forest type (e.g., canopy height), slope and aspect. For each combination of these factors, the figure schematically depicts the spatial distribution of direct radiation along a North-South transect that is centered on a canopy gap. The angular distribution of diffuse radiation can be considered as relatively uniform. Consequently, microsites in gap center receive the greatest amount of diffuse radiation.}
\label{fig:62light}
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} 

Furthermore, the availability of light for regenerating stems evolves through time due to canopy closure and growth of regeneration. In irregular forest stands that are managed with continuous-cover forestry systems, saplings, poles and trees react to canopy release. The crowns of poles and trees expand laterally and vertically. As saplings become taller, the availability of light does not necessarily increase because of the proximity of the crowns of poles and trees \citep{Ban1998}. Regeneration growing under the canopy or at the edge of canopy gaps can therefore face more rapid and more severe competition from the remaining trees and poles than does regeneration growing in the center of a canopy gap. When analyzing the effects of partial cutting, I considered understory light conditions immediately after the harvest, whereas different findings could have been obtained if I had considered understory light conditions several years after the harvest. Further work is required to tackle the modeling of canopy closure and investigate the evolution of understory light conditions with canopy closure. As gap centers, northern gap edges, and southern gap edges are microsites with different light regimes \citep{Diaci2002}(\autoref{fig:63microsites}), some interesting hypotheses that could be tested are that (\textit{i}) the availability of light to regenerating stems at gap edges decreases as regenerating stems become taller because of the competition with dominated trees and canopy closure, and (\textit{ii}) there is a rank reversal over time in the availability of light between the gap center and northern gap edge.   
   
The studied regeneration was protected from game browsing by fences, as browsing by wildlife is known to affect regeneration composition \citep{Hidding2013, McGarvey2013}. Indeed, palatable species are preferentially browsed by deer and less palatable species are therefore promoted. This is a severe limitation to the results of this thesis, especially if game populations are abundant. Depending upon the shade tolerance of the palatable species, game browsing can either promote shade-tolerant species (e.g., European beech versus sessile oak in the study area) or less shade-tolerant species (e.g., spruce versus fir in the Vosges mountains \citep{Heuze2005} or on Anticosti Island \citep{Casabon2007, Hidding2013}). Within the studied area, I hypothesize that (\textit{i}) abundant deer populations accelerate the transition of secondary forests to late-successional forests, (\textit{ii}) density of deer populations is a more important factor than understory light availability to the diversity of understory regeneration, and (\textit{iii}) maintaining the coexistence of beech and oak requires control of the density of wild game populations.
  
Competition between species is likely affected by changes in environmental conditions. Climate change displaces the ecological niches of species and ecosystem boundaries. Depending upon site conditions, species autecology and interspecific interactions, species will either benefit or suffer from climate change, which in the long run can modify stand composition \citep{Bontemps2012, Mette2013}. However, the effects of climate change on the dynamics of uneven-aged and mixed stands require further investigation. Interactions between the admixed species can attenuate the sensitivity of species to climate change \citep{Pretzsch2013b}. Therefore, it is unclear whether the findings about interspecific competition in the understory are likely hold under a future climate. Further studies should identify whether interspecific competition in the understory of uneven-aged forests, when managed with a continuous-cover forestry system, is sensitive to climate change.
 
The reaction of regeneration to a sudden increase in solar radiation due to canopy disturbances or silvicultural treatments was not investigated. Modification of stem allometry is expected in the years following canopy release \citep{Collet2001}, but further work is required to quantify the reaction of saplings to canopy release and assess the efficiency of regeneration cleaning. In particular, the growth of released saplings should be further studied in order to provide more precise silvicultural recommendations, such as the appropriate timing for regeneration tending.

In order to further generalize the findings of this thesis, the simulation approach that was outlined in this study should be replicated for other forest ecosystems with different species compositions, stand structures, topography, and at different latitudes. With this in mind, developing models of stand structure and crown dimensions would be particularly helpful. Such models would enable us to simulate the dynamics of stands with controlled density, composition and structure with limited field work. Rigorous experimental planning could hence be elaborated to identify clearly the importance of the various factors of understory light regime.

\begin{figure}[tb]
\includegraphics[width=\textwidth]{fig63microsites}
\caption[Light microsites in a canopy gap.]{Northern gap edge (a), gap center (b) and southern gap edge (c) are microsites with different light regimes. The availability of light to saplings hypothetically decreases as saplings grow because of competition with dominated trees at gap edges; a rank reversal over time of the availability of light between the gap center and northern gap edge can be expected.}
\label{fig:63microsites}
\end{figure}

\section{Conclusion}
The increasing awareness of decision makers and forests managers with respect to the necessity of preserving biodiversity and ecosystem services with sustainable management practices have provided the impetus for promoting a silviculture of mixed and uneven-aged stands that mimics natural processes. However, a lack of knowledge of the natural dynamics of such stands has hindered our ability to manage for some desired mixture of tree species.
 
In order to fill this gap, I modeled some key processes of the dynamics of mixed and uneven-aged stands. I modeled the development of mixed regeneration under partially closed canopies and the interception of light by heterogeneous canopies. I implemented these models in a simulation program to explore how these factors affect stand dynamics and how various silvicultural treatments can be manipulated to provide or maintain favorable understory light conditions for natural regeneration. The results provide guidelines for forest managers who want to maintain the coexistence of species with contrasting shade tolerances.
 
Finally, maintaining mixtures of species with different shade tolerances offers many advantages, but at the same time runs contrary to natural forest succession. Mixtures preserve forest biodiversity, forest productivity and ecosystem services, and enhance forest resilience. In the context of pursuing sustainable forest management practices combined with global change, maintaining species mixtures is a good strategy. However, continuous-cover silviculture of mixed stands with species of different shade tolerances requires many efforts to fight against the natural transition of secondary forests towards late-successional forests. Despite the numerous silvicultural advantages of mixed stands, maintaining such species mixtures is questionable, since they do not mimic natural forest dynamics, might prove to be labor intensive and, therefore, partly contradict the essence of close-to-nature forestry, the goal of which is to rely upon natural processes over human interventions. At the end of this thesis, I therefore wonder whether the management of mixed stands should accommodate natural forest succession rather than maintaining, by any and all means, unstable ecological communities. 




