Regeneration monitoring

The dominant herbaceous species were Luzula luzuloides (Lam.) Dandy & Willm, Vaccinium myrtillus L., Deschampsia flexuosa (L.) Trin, Dryopteris carthusiana (Villar) H.P. Fuchs and Pteridium aquilinum (L.) Kuhn, typical of acid soils. Stands were selected in public forests managed with a continuous-cover forestry system. Studied stands had complex vertical and horizontal structures and included a wide range of diameter at breast height and total height classes. Stand composition was also diverse, varying from monospecific beech stands to stands almost completely dominated by oak. Other species (mainly Carpinus betulus L., Betula pendula Roth and Acer pseudoplatanus L.) accounted for up to 20% of stand composition.

In spring 2007, 27 sites were selected with a well-established regeneration (10 to 300 cm high) in order to span a wide range of stand structure and composition. Regeneration areas (100 to 6500 m2) were fenced off to protect them from browsing by deer. Inside each fence, the saplings within 5 to 31 square plots of 4 m² (total of 242 plots) were sampled. The plots were laid out every 4 m following a square grid. The saplings in these plots were measured. At the beginning of the monitoring, all saplings were measured whereas only the tallest saplings were measured at the end of the monitoring. Inside the fences and within 20 m outside the fences, every tree with a diameter at breast height greater than 6.4 cm was measured and mapped. 

Hemispherical photographs and light sensor were used to quantify the amount of light available to saplings. The percentage of above canopy light measured in the understorey ranged between 1% and 61% for plots dominated by oak, and between 2% and 43% for plots dominated by beech. This encompassed a wide gradient ranging from a close canopy to a canopy with a gap size varying up to approximately 1200 m² (area without overtopping crown). 

Soil and humus description as well as floristic surveys (Braun-Blanquet method) were carried out in order to compute the levels of nutrients and water supply at every site.

These stands originated from plantations of Norway spruce (Picea abies (L.) Karst), Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and, in two stands, silver fir (Abies alba Mill.) and Japanese larch (Larix Kaempferi (Lamb) Carr). The stands are additionally composed of Sorbus aucuparia L, Fagus sylvatica L., Western hemlock (Tsuga heterophylla (Ref.) Sarg.), Betula sp. and Pinus sylvestris L. that naturally regenerated or were preserved by past forest management. All these stands are in public forests managed with continuous cover forestry. Single tree selective thinnings or partial harvests are realized periodically (about every 6-12 years). Red deers (Cervus elaphus L.) and Roe deers (Capreolus capreolus L.) were present in the area but in limited abundance and causing relatively few browsing damages.

In each stand, the trees with diameters greater than 10 cm were mapped and their diameter is measured. Moreover, 12 circular subplots of 3 m radius were laid out systematically to monitor regeneration (density, height structure) and light conditions. These measures are repeated every two years.

In addition, in 2016, tree total height, the height of the base of the crown and crown radii were measured. In March-April 2018 and in October 2018, the three tallest saplings of each species were measured in each subplot. Their height, the length of the terminal shoots of years 2016, 2017 and 2018 were also measured, as well as the longest lateral shoots that was selected among the lateral shoots growing at the last whorl of the corresponding year. 

Hemispherical photographs were taken in the centre of subplots, above regeneration, at dawn or dusk, in spring 2018 (Nikon D90, Sigma 4.5 mm fisheye lens). 

Because the growth of saplings could have been affected by different pest and pathogens associated with defoliation symptoms, the foliage retention index was estimated. Considering the phytosanitary problems affecting Douglas fir in the study area, additional observations and analyses were performed to assess in more details the damages of pest and pathogens of Douglas fir regeneration. 

In summer 2021, the floristic diversity, the abundance of tree microhabitats and deadwood were assessed in each stand.

The enclosure-exclosure network is thus not representative of all the forest area of the study zone, but of the conditions considered by experts as the best conditions encountered for the forest regeneration. 

The state forest managers measured yearly the seedling height, seedling number and cover of herbaceous and woody strata as well as the cover of frequent herbaceous species. The number of tree seedlings were counted for all species present, but only the heights of the tallest were measured. Up to 5 seedlings for one or two species considered as the local target species, and 10 more seedling from the other species.