The experimental design CPI-M1
The principal objective of this experimental design is to estimate the efficiency of the irregular shelterwood method (CPI*) for the conservation of mixed stands characteristics and composition while being economically interesting. The conservation of old-growth characteristics is an other aim of this protocol.
The site is divided between 20 plots of 70 x 70 m equally distributed between five treatments : irregular shelterwood system with continuous cover (CPI-CP) ; irregular shelterwood system extended (CPI-RL) ; shelterwood system (CPR*) ; clearcut (CPRS*) and control plots were no treatments have been made. Some of these forestry processes occur in several steps spread over twenty years (CPI-RL and CPR). At the moment this study has been realized, we only have observed the effects of the establishment cuts for two of these treatments and not those of the final cut.
The experimental design SSAM-II
This protocol studies the effect of selection cuts made on small tree groups on the dynamic and conservation of old-growth attributes in yellow birch – conifer stands. Another aim is the study of red spruce regeneration, a declining species for which the conservation in managed forest is complex (Dumais & Prévost, 2007).
The site is divided between 20 plots of 80 x 80 m where 4 treatments (5 plots per treatment) are equally applied. Three of them consist in a selection cut by groups. The size of these gaps varies and their distribution is not homogeneous. They differ by the harvest intensity, defined by the residual basal area (strong : 13 m²/ha ; medium : 16 m²/ha ; low : 19 m²/ha). The last scenario is used as reference.
Literature review on naturalness evaluation
In order to realize a methodology for the evaluation of old-growth characteristics, we studied 6 different protocols of naturalness evaluation : Schnitzler & Borlea, 1998 ; Bus de Warnaffe & Devillez, 2002 ; Haye, 2006 ; Winter & al., 2010 ; St Hilaire, 2011 ; Rossi & Vallauri, 2013.
These studies show a wide variety of clues (annex III). A selection had to be done to retain only the most relevant for our study. This choice has been made following three main axes directions :
– Simplicity : we try to obtain an ergonomic methodology, which requires a small amount of data and is easily reusable. Moreover, we are also constrained by our study limits.
– Old-growth characteristics : As point 1.4.2. shows, the concept of naturalness goes further than old-growth aspects. If some clues can be pertinent for the evaluation of the first, they may not apply to the former.
– Québec context : the main part of naturalness literature comes from Europe,in a context where forests are mostly recent, fragmented and where exploitation has been strong and regular. But the context is different in Québec and some parameters hold no relevance here.
Comparison of CPI-M1 and SSAM-II before the harvest
We tried to establish if some significant differences existed at the initial state between the experimental designs CPI-M1 and SSAM-II and on which parameters. The objective was to estimate the relevance of a comparison between these two stands.
In order to do so, we used for each quantitative variable a non-paired Student test to verify if the data respected normality and equality of the variance, or, if not, a Wilcoxon test.
Comparison of CPI-M1 and SSAM-II
The stands studied in the experimental designs CPI-M1 and SSAM-II are very different. Every parameter, except the diversity, show significant differences or tendency between the two sites (table 2).
CPI-M1 experimental design presents a stand with an high basal area and high density (30.49 m²/ha and 1115.6 stems/ha) but with a lack of mature elements. However, 73% of the stems recorded (dbh ≥ 9 cm) have a dbh under 30 cm and these of 50 cm or more are quite inexistent. Same pattern applies for the snags. The density in CPI-M1 is superior compared to SSAM-II (256 snags/ha in the first and 107.2 in the second) but again, they are mainly composed of snags with a dbh under 30 cm.
SSAM-II plots show significantly higher diameter stems for living trees as well as for snags. Only the living stems with a diameter superior or equal to 60 cm show a tendency (p-value = 0.05513). The reason is probably the natural low density of these elements which limits the test efficiency. The results of the mean and maximum diameters confirm this image of SSAM-II : a richer in mature elements experimental design.
Even if the specific richness is the same, the repartition of the different species is different. The stable state (dominated by the yellow birch – red spruce – balsam fir group) is, on average, the most commonly found on the two sites. Nevertheless, the strong proportion of red maple in CPI-M1 causes a domination of the intermediate group characterized by red maple, white birch and balsam fir in 6 plots. This group is absent of SSAM-II. This explains the significant difference at the Khi² test (p-value = 0.02068).
As a summary of these observations, it appears the stands studied on experimental designs CPI-M1 and SSAM-II are strongly different. The old-growth characteristics are significantly more present in the second one. Due to the absence of coarse woody debris data for CPI-M1 before the harvest, we cannot display results on this aspect.
Initial variability between the treatments
For all parameters, there is no difference between plots grouped by their future treatment in CPI-M1 (Annex V). The comparison between treatments after the harvest will not be influenced by a pre-existing variability. The experimental design SSAM-II has different results for two of its parameters: snags with a dbh ≥ 30 cm (p-value = 0.021) and coarse woody debris with a diameter ≥ 30 cm (p-value = 0.0634).
For the snags, this difference occurred between the control and the STR13 and STR19 treatments. The control plots contain twice more snags with a dbh ≥ 30 cm compared to these two treatments. However, there is no significant difference with the snags with a dbh ≥ 50 cm. This disparity seems to occur only for snags with a dbh between 30 and 49 cm. For the coarse woody debris, we see a significant difference between the control and the STR16 treatment. The values are strongly inferior for the second. The absence of any coarse woody debris with a diameter ≥ 30 cm explain this result.
These differences always occurred over the control treatment. Thus, it will be easy to consider them in the post-harvest analysis.
Old-growth forest in the yellow birch-conifer domain
Old-growth forests can appear under different aspects in the yellow birch – conifer domain, according to the dominant species in the stand. If yellow birch is the main specie, we stay close of old-growth characteristics usually expected in temperate broadleaved forests. The structure is complex and high diameter living or dead trees are common. If balsam fir is the dominant specie then we are not in this representation of old-growth forest anymore. Its composition is strongly variable on many parameters (density, snags…) and mature elements lack. The complex structure is the only certainty about these stands. This natural behaviour is a potential explanation for the prevalence of stands of the MJ ecological class in the yellow birch – conifer domain in the studied EFE : the MS stands are not consistent with the usual representation of old-growth forest and are therefore rarely selected.
Yet, the lack of knowledge about old-growth stands in the yellow birch conifer domain (Doyon & Varady-Szabo, 2013) have made our work more difficult. The experimental designs SSAM-II and CPI-M1 are insufficient to provide an accurate picture of old-growth forests. Furthermore, the ecological differences between these sites and those studied in the literature also limit the comparison. More important studies must be made in the yellow birch – conifer domain to provide more precise clues for the ecosystem management.
Impact of the harvest on old-growth elements
Among the different management processes applied in CPI-M1, one result is obvious : the strong impact of the CPRS. As a clearcut, its intensity compared to the other treatments was predictable. Such harvests can even change, or at least severely slow down, the natural dynamic of the stand (Arcahmbault & al., 1997, Tremblay, 2009). Thus, a decrease of these practices seems necessary. The other treatments are not much different from one another, control excepted. The low density of trees with natural cavities seems to be the only common effect between the four harvest methods. Again, the lack of previous data limits the interpretation for this parameter.
Two points may explain these results. First, the initial density in mature and large elements was low in the majority of plots. Then, these densities were close to the minimal conservation standard for old-growth elements used during the hammer finish (Déry & Leblanc, 2005). The harvest of these elements was then strongly limited for each treatment, except for the CPRS. For the stems with a dbh ≥ 50 cm, the comparison with clearcut does not show any result because of their initial low density.
It is also necessary to take into account the next harvests. For two treatments, CPI-RL and CPR, it was only an opening of the canopy for the development of the regeneration. In the next decades, the remaining will also be harvested. We can therefore expect an impact close to the CPRS for the old-growth elements. However, for the CPI-CP treatment, the exact same harvest will be repeated around 20 years. The aim of this scenario is to always keep at least 40% of the canopy. This treatment is expected to enable a great conservation of old-growth elements (Raymond & al., 2009 ; Doyon & Varady-Szabo, 2011).
Table of contents :
1.1 The Direction of Forest Research
1.2 The context of the ecosystem management
1.3 The yellow birch – conifer domain
1.4 Old-growth and naturalness
1.4.1 Old-growth stands, an important ecosystem
1.4.2 The evaluation of naturalness, an inspiration
1.5 Aim of the study
2. Material and methods
2.1 CPI-M1 et SSAM-II sites
2.1.1 General characteristics
2.1.2 The experimental design CPI-M1
2.1.3 The experimental design SSAM-II
2.1.4 Collected data
2.2 EFE data
2.3 Old-growth clues selection
2.3.1 Literature review on naturalness evaluation
2.3.2 Final parameters selection
2.4 Statistical analysis
2.4.1 Comparison of CPI-M1 and SSAM-II before the harvest
2.4.2 Effect of the different treatments
3.1 Initial state
3.1.1 Comparison of CPI-M1 and SSAM-II
3.1.2 Initial variability between the treatments
3.1.3 Comparison of our results with the values of other old-growth forests
3.2 Results after harvest
3.2.1 Results in CPI-M1
3.2.2 Results in SSAM-II
4.1 An overview of old-growth yellow birch – conifer stands
4.1.1 Stands dominated by balsam fir
4.1.2 Stands dominated by yellow birch
4.1.3 Old-growth forest in the yellow birch-conifer domain
4.2 Impact of the harvest on old-growth elements
4.2.1 Experimental design CPI-M1
4.2.2 Experimental design SSAM-II
4.2.3 Comparison of these results with old-growth standards
4.3 Critic of our methodology