Measuring and managing plant species diversity in the chestnut (Castanea sativa Mill.) ecosystems of the Cevennes
François Romane1, Hélène Gondard1, Michel Grandjanny1, Anna Grossmann1, Alain Renaux1, Zuheir Shater2
1 CNRS – CEFE, 1919 Route de Mende, F-34293 Montpellier cedex 5, France
2 Département d’Ecologie et de Foresterie, Faculté d’Agronomie, Université de Tichrine, Lattaquie, Syrie, France
francois.romane@cefe.cnrs-mop.fr
Abstract
Forest managers are sometimes “stressed” by having to measure the plant species diversity in forests. Nevertheless, it is necessary if this parameter is to be included later in management. From an applied point of view, two plot designs (square or circle) that we used to assess diversity in various types of managed chestnut ecosystems (grove, forest, coppice, and reafforestation) are discussed.
The results suggest that both designs were efficient. Moreover for each of these two designs we also report findings about changes in species richness: (i) species richness is considerably influ- enced by the management type in some cases (coppicing favours species richness for few years after the clear-cut, whereas abandonment induces a loss of species richness, etc.); (ii) biodiversity in previous chestnut stands modified by tree plantation is quite low and is not much influenced by the origin of the tree species used (native or introduced).
Keywords: biodiversity, Castanea sativaMiller, Cevennes, grove abandonment, chestnut, France
1 Introduction
Accumulated evidence strongly suggests that ecological complexity, the complex sophisti - cated interactions among various forms of life, together with intricate heterogeneous habitat structure, plays a key role in promoting the diversity of biological traits and functions in nature (HIGASHI 1992). Ecological complexity enhances biodiversity, which is often con - sidered essential in guaranteeing the stability of an ecosystem. Even though this is more a postulate than a demonstrated truth confirmed by experimentation, more and more scientists and managers nowadays are convinced that biodiversity must be taken into account in man- ageing of any ecosystem sustainably. But biodiversity, is a quite complicated concept and there is diversity at all levels of biological organisation (PAUSASand AUSTIN2001). This complexity means that scientists and managers have only been able to study biodiversity partially and have mainly focussed on those aspects which are “easy” to observe.
In forest management it is now largely accepted that knowledge of diversity is needed, even if only for the economic purpose of eco-labelling. The “easiest” form of diversity to observe is often the plant species richness in the lowest vegetation stratum (approximately 0 – 1 m) and the biodiversity can be measured using the Shannon-Weaver index (MAGUR -
RAN1988).
In the framework of the Working group 4 of the COST Action G4 (“Multidisciplinary Chestnut Research”), we tested two plot designs used to collect information about species richness. We did not compare the two designs to assess their efficiency, but only comment on
each of them and on the results obtained with them. Evaluating effenciency would be quite a complicated task, particularly because it would be difficult to choose the “reference” design to describe the biodiversity. The concept of “structure” can only be discussed when the method has been chosen to describe and measure that structure (BOUDON1968). Similarly, it would probably be necessary to choosea priori the method to estimate “biodiversity”
before discussing biodiversity (HUBÁLEK2000). The plot designs were set up to study, at the population level, the species richness (number of plant species in a given area) and to calcu- late the Shannon-Weaver biodiversity index, which requires an abundance estimation of each species.
Nomenclature of plant species according to Flora Europaea(TUTINet al.1964–1980).
2 The square design
Objectives
Since the beginning of the 20th century the management of these chestnut ecosystems has changed dramatically as most of the orchards and groves have been abandoned and turned into coppice stands. The main stages of this succession are, according to GONDARDet al.
(2000, 2001): (i) the cultivated grove (CG); (ii) the abandoned grove (AG) more or less invaded by shrubs, and with a dying back of the stems (mainly due to Cryphonectria parasit- icaattacks); the shoots resprouting from the stool lead progressively to the next stages; (iii) a young coppice (YC) about 15-years old, which turns into: (iv) medium coppice (MC) about 40 years old; and (v) old coppice (OC) older than 55.
We expected an increasing plant species richness along such a gradient of abandonment due to the more “natural” management.
Experimental plots and methods
The diversity was estimated in this study according to the number of plant species, i.e. the species richness, present in the plots (MAGURRAN1988, HUBÁLEK2000).
The plot design of a 10 x 10 m square was “imposed” because we re-used the design of a previous experiment set up by the foresters who gave us information about the forest struc- ture to be related later to the plant species diversity. In this 100 m2square (five replicates for each abandonment stage) we listed the plant species to indicate the plant species richness.
The abundance of the species was also estimated from 100 “point quadrat” (GOODALL1952) along the square axis, allowing estimates of other biodiversity indices, not discussed here, such as the Shannon-Weaver index (MAGURRAN1988, HUBÁLEK2000).
This “square design” was implemented in the five abandonment stages, as described above, to assess the relationships between the biodiversity and the abandonment of chestnut groves in the Cevennes Mountains. In these mountains the countryside has been extensively covered by chestnut (Castanea sativaMill.) orchards and groves for centuries.
The studied area was in a transition climate between Mediterranean and Oceanic. It is about 600 m a.s.l., mainly on schist bedrock, with about 1400 mm annual rainfall.
Results
The results clearly showed (Fig. 1) that during the abandonment period the plant diversity decreased at least during the early stages, i.e. for the first few decades, probably due to the higher cover of the canopy which had previously been regularly opened by the farmers. It is possible that the plant species richness will increase later, i.e. when the old coppice stands become older and canopy gaps occur.
Discussion
Empirically and a posteriori, it appeared to us that this “square” design was efficient.
Moreover we used the same design for a direct analysis, using permanent plots, of the veg - etation changes after clear-cutting in a 22-year-old chestnut coppice stand. The results were also highly statistically significant, i.e. a marked increase in the number of species during the first few years after the clear-cut. These results suggest that this square design is appropriate for measuring species diversity.
Nevertheless we met two main difficulties: 1) The size of the squares (100 m2) was probably too small to obtain a complete list of the species. It is quite difficult to solve this problem with any design shape (square and circle) as the vegetation in the Cevennes has a very “fine coarse” pattern. An increase in the square size would result in many squares including non-homogeneous vegetation 2) It was difficult to set up squares on steep slopes invaded by shrubs, which are very common in the Cevennes. The use of another shape might be more appropriate.
3 The circle design
Objectives
This study attempted to assess the consequences for plant species diversity of tree plantation in old chestnut groves. In particular, we compared the effect on biodiversity of “natural”
forests with “native” species (Pinus sylvestrisL., Pinus nigrasubsp.salzmanniiDunal includ- ing the original species Castanea sativa) with afforested plots with “exogenous” species (Pinus pinaster Aiton, Pseudotsuga menziesii [Mirbel] Franco, Pinus nigraArnold subsp.
laricioMaire, Cedrus atlantica[Endl.] Carrière). We expected that the biodiversity would be reduced by the introduction of exogenous tree species (PETERKEN2001).
Experimental plots and methods
Given the difficulties found using the square design described above, we attempted in this study to set up a circle design. For that we used a telescopic graduated pole from the centre of the plot, which is a more efficient way of setting up a plot than making squares. We used concentric circles depending on the tree density as in the French National Inventory (IFN 1994): (i) 6 m radius (i.e. 133.04 m2) if the number of tree per hectare is more than 1 500 stems ha-1(DBH > 7.5 cm), or (ii) 9 m (254.00 m2) if it is less than 1500 stems. The number of plant species (species richness) was recorded in the corresponding circle. We also
CG AG YC MC OC
Abandonment gradient
Plant species richness
50 40 30 20 10 0
Fig. 1. Plant species richness along an abandon- ment gradient of chestnut groves in southern France (confidence limits at P = 0.05; 5 replicates for each stage)
(CG = Cultivated grove; AG = Abandoned grove; YC = Young coppice; MC = Medium coppice; OC = Old Coppice). Modified from ROMANEet al.2001.
added a third circle (11.28 m radius and 400 m2) to attempt to obtain an exhaustive list (the data pro cessing was done separately for each recorded circle). At these sites we generally did not have any trouble with the vegetation pattern, which had probably been homogenised by the plantation. The abundance of the species was estimated from the Braun-Blanquet’s scale.
Results and discussion
We found, to our surprise, no statistically significant difference in the plant species richness between the plantations with native and introduced trees (Fig. 2). The only differences occurred with two of the exogenous species: Pseudotsuga menziesiiseemed to induce a very low species richness and Cedrus atlanticato induce a richness higher than in the native species.
Thus the plant species richness in the understorey was not generally dependent on the tree species. This could be due to the age of the afforested plots we sampled being about 30 to 40 years, when the canopy cover of the planted trees is probably the most dense. It is, how- ever, at present impossible to investigate older plots because they do not yet exist, as the afforestations occurred generally only a few decades ago.
These results imply that if forest managers want to avoid a drastic decrease of plant species richness they should not plant too many stands at the same time.
We recommend using the circular design as it is efficient and easy to apply in the forest.
CS PS PNs
N = 11
PP PM PNI CA
N = 16 N = 8
N = 14 N = 16
N = 16
N = 10 Native species
Exogenous species
Mean species richness
35 30 25 20 15 10 5 0
Fig. 2. Comparison of the plant species richness between “native” forests and afforested plots with exogenous tree species (confidence limits at P = 0.05; N = sample size for each species); CS = Castanea sativa; PS = Pinus sylvestris; PNs = Pinus nigra subsp. salzmannii; PP = Pinus pinaster; PM = Pseudotsuga menziesii;PNl = Pinus nigrasubsp. laricio;CA = Cedrus atlantica)
4 Concluding remarks
The two types of design used to estimate plant diversity were not compared statistically.
Nevertheless, our impression is that both are efficient. Both types were quite easy to set up, but the circle was a little bit easier to use than the square where the point quadrat technique took considerable time. The main difficulty is the plant species identification, which requires a very good knowledge of methodological local flora. A second methodological constraint is the shortness of the period where all the plants are present (spring time in Mediterranean countries).
We recommend that plant species diversity, evaluated in terms of species number or richness, could be progressively and easily introduced as a criterion in chestnut forest management because it varies greatly according to the type of management used.
Acknowledgements
We would like to thank the COST Programme (COST G4), the European Union (DG XII:
contracts “CHESUD”, “CASCADE”, and “MANCHEST”), the French Ministères de l’Environnement et de l’Agriculture, the Parc National des Cévennes, the Office National des Forêts, and the Région Languedoc-Roussillon for their support. We also warmly thank Marie Maistre and Philippe Perret for their help in the field.
5 References
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Accepted 4.3.02
