Plant diversity in a managed temperate deciduous forest: understorey response to two silvicultural systems

Summary

This study compared two silvicultural systems, 'traditional' coppice-with-standards (CWS) versus 'close-to-nature' selective cutting (SC), to assess their positive and negative effects on plant understorey diversity in an ancient temperate deciduous forest in France. These two systems are widely applied throughout Western and Central Europe. They can be viewed as two complexes of anthropogenic disturbances that differ in frequency (higher in SC), severity (stronger in CWS) and spatial extent (larger in SC than in CWS). In particular, the hypothesis that SC has more beneficial effects on plant diversity than CWS, was tested.

Study area: The study was conducted in the ancient forest of Le Nouvion (180–220 m altitude), Aisne department, France. The climate is suboceanic with an average annual temperature of 9.1 °C and annual precipitation of 950 mm. Soils are leached brown earths with a moderate drainage.

Dominant trees in this temperate deciduous forest are oak Quercus robur, hornbeam Carpinus betulus, ash Fraxinus excelsior and sycamore Acer pseudoplatanus. The forest was managed as hornbeam coppice with oak standards (CWS) from the mid-17th to the late 20th century. This system led to reduction of shade tree species (e.g. beech Fagus sylvatica), a promotion of woody species able to regenerate from stumps (e.g. Carpinus betulus and hazel Corylus avellana) and adverse effects on soil (e.g. strong compaction, raised watertable). CWS was considered unsustainable as some phases of the natural cycle were missing and ageing coppices were less and less able to regenerate, thus, wood production was progressively decreasing. Hence, most of the forest was placed under a selective cutting system (SC) at the end of the 1970s. This was expected to resemble more closely the natural cycle, mimicking natural small-gap dynamics with a tree-by-tree harvesting method that preserves structural complexity and woody species diversity. In SC, pre-commercial thinning were conducted every 4 years (partial extraction of coppice wood: shrub species and unsuitable tree species at c. 23 m³/ha) and commercial felling every 8 years (selective cutting of individual mature trees at c. 10 m³/ha). CWS was maintained in some parts of the forest (commercial felling every 30 years, with the extraction of all coppice timber and 75% of standards at c. 200 m³/ha - the retained 25% consists at least of 80, 40, 15 and 5 trees/ha of 30, 60, 90 and 120 year-old trees, respectively). The size of the stands was on average 20 ha in SC and 8.5 ha in CWS.

Herbivore pressure on vegetation is mainly due to roe deer Capreolus capreolus, the only ungulate present (density reaching 30 individuals per 100 hectares).

Sample design and data collection: All 27 forest stands selected were located in similar abiotic conditions, with the same substrate (loess, thickness over 5 m), similar soils and topographic position (plateau). By comparing the composition and diversity of understorey vegetation, it was hoped to identify relationships between understorey plant diversity and silviculture practices and to identify functional response groups. A permanent block was randomly placed within the selected stands, consisting of a square 400 m² core plot, replicated at the four cardinal points (five plots per stand). The distance between the centre of the four replicates and the core plot was 50 m. Blocks were placed at a minimum distance of 10 m from forest roads, clearings etc., to minimize edge effects. CWS stands (n = 12) represented all stages of the coppice cycle, from post-logged to mature stands. Two ageing CWS stands (i.e. stands not logged for c. 50 years) were also included. For SC, the 15 stands were submitted to regular thinning every 4 years and felling (logging) every 8 years.

Vegetation surveys and other measurements were conducted during periods of peak vegetation cover (May–August) from 1999 to 2001. Vascular plants were assigned to one or more layers based on height: i) tree layer > 8 m; ii) shrub layer >< 2 m to 8 m high; iii) sub-shrub layer 0.5 m to 2 m high; and iv) herb layer < 0.5 m. Cover-abundance of vascular plant species was visually estimated as the vertically projected area using the Braun-Blanquet scale. The total cover of each layer was estimated as a percentage of the total plot area.

Structural attributes of species assemblages: There was a positive correlation between light levels and shrub cover, but increased light had little influence on the herb layer and did not correlate with herb cover. There were contrasting responses of vegetation cover and structural diversity to variations in soil nutrient levels.

SC stands had highest under shrub cover values, but they were associated with the lowest values for species diversity due to dominance of blackberry Rubus fruticosus. In this harvesting system, the high proportion of light reaching the forest floor induced a spectacular spread of blackberries which decreased species richness. It also caused shifts in guild composition, with graminoids and ferns growing strongly to the detriment of true forest species.

Herb layer cover was significantly higher and species richness greater in CWS stands. Within the CWS, there were consistent increases in herbaceous species richness in response to thinning. Within SC stands, species richness remained quite constant.

Species richness was highest in post-logged CWS stands and progressively decreased with stand age. Both vegetation cover and equitability reached a peak in 15-year-old stands, and subsequently decreased to stabilize in 25-year-old and older stands. Conversely, structural attributes of diversity remained quite constant and low in SC stands, close to those recorded in mature CWS stands.

Functional diversity: Functional diversity differed strongly between the two systems. Diversity was maintained in the coppice-with-standards system, but some guilds were lacking in selectively cut stands. The most negatively impacted guilds were tree and shrub saplings, prostrate ruderals, shade-tolerant perennials and vernal geophytes. The latter two comprise 'true forest species' which may also be considered as 'coppicing-maintained species'. To reach the same values of guild richness (i.e. number of guilds) or redundancy (i.e. proportion of the maximal species richness within each guild), larger areas were required in SC compared with CWS systems. The main plant functional type (PFT) responses were:

PFTs indifferent to the silvicultural system: ruderal annuals and tall light-requiring forbs.

PFTs associated with SC: ferns, tufted graminoids and late-flowering graminoids

PFTs associated with CWS: tree and shrub saplings, woody climbers, vernal geophytes, prostrated ruderals and shade-tolerant perenials.

Conclusions: These results suggest long-term negative effects of the selective cutting timber harvesting regime on both structural and functional plant diversity, compared to more traditional coppice-with-standards manegement. Cutting intervals are shorter than recovery times, so early successional species-dominated communities are maintained. Vernal geophytes and shade-tolerant perennials seem to be limited by the frequency of rather than by the severity of, disturbance. The authors conclude that, from a biodiversity viewpoint, the selective cutting system does not achieve the objective of sustainable forest management. The rotations currently in use do not match natural disturbances very closely. Retaining remnants of old coppice woodland and extending rotations to at least 50 years are recommended where biodiversity conservation is a goal of forest management.


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