Different post-culture dynamics in abandoned chestnut orchards and coppices
Marco Conedera1, Pietro Stanga1, Bernhard Oester2and Peter Bachmann3
1 WSL Swiss Federal Institute of Forest, Snow and Landscape Research, Sottostazione Sud delle Alpi, CH-6504 Bellinzona, Switzerland. conedera@wsl.ch
2 WSL Swiss Federal Institute of Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland
3 Swiss Federal Institute of Technology, CH-8092 Zürich, Switzerland
Abstract
Humans spread the sweet chestnut (Castanea sativaMiller) in Europe, thereby masking the orig - inal range of the species. They cultivated it as a monoculture in mainly two management regimes:
coppice and orchard. In the absence of management the chestnut stands tend to be invaded by other tree species, giving way to post-cultural evolutionary dynamics. Comparison of aerial photo- graphs from 1959/60 and 1995 indicates a clear general trend toward mixed and shaded forests and the existence of different evolutionary patterns of orchard and coppice stands.
Keywords: Castanea sativa, post-cultural evolution, abandonment, southern Switzerland
1 Introduction
The distribution of chestnut (Castanea sativaMiller) in Europe has always been closely tied to human activities (PITTE 1986). Its diffusion and active management have masked the original range of the species, resulting in the chestnut’s presence in areas at the limits of its ecological possibilities (BERNETTI1995). Humans, especially in mountainous regions, learnt how to manage the chestnut in extremely advantageous and diversified ways. High forests are of limited importance in traditional chestnut culture (BERNETTI1995). There are, in fact, two main management regimes applied to chestnut forests:
– coppice: pure chestnut forests regenerated from adventitious or dormant buds. The main product is wood (poles and firewood);
– orchard: grafted chestnut trees in open stands. The main product is the fruit, additional products are pasture or forage, litter and timber.
The chestnut orchards and coppices are artificially kept as monocultures. However, in the absence of management, they tend to be invaded by other tree species, giving way to evol - utionary dynamics toward climax forests, as observed in many European regions in the last decades (PACI1992, ARNAUDet al.1997).
PACIet al.(2000) identified different evolutionary trends, which depend on climate, soil, potential vegetation, silvicultural regime, stand density, pathogens, and the time since aban- donment. The open-structured orchards are more susceptible to invasion by other tree species than coppice stands (ROMANEet al.1995, CONEDERAet al.2000, PACIet al.2000).
In this study we used aerial photographs to analyse the post-culture natural dynamics of chestnut stands in southern Switzerland in order to give a quantitative estimate of the ongoing evolution and to detect the main factors regulating the different dynamics in orchard and in coppice stands.
2 Material and methods
Study area
The climate in the hilly regions of southern Switzerland is warm-temperate and rainy (insubric), with a mean annual precipitation of 1600–2000 mm (of which 800 mm occurs during the vegetation period) and a mean annual temperature of 12 °C. The soils are generally classified as haplic podzol (cryptopodzol) on cristalline bedrock. The chestnut was introduced by the Romans nearly 2000 years ago (TINNERet al.1999). Due to human activity, chestnut stands became the dominant forests on acid soils. The chestnut belt stretches over more than 20 000 ha from 200 to 700–1000 m a.s.l., depending on the aspect. The abandonment of the chestnut culture in southern Switzerland took place in the second half of the 20thcentury because of chestnut blight and also as part of the general abandonment of rural areas (CONEDERA1996). Recently palaeoecological and phyto - sociological studies have given rise to the hypothesis that Fagus sylvatica, Tiliaspp., Abies alba, and Quercusspp. play a central role in the potential vegetation of the region (TINNERet al.1999, DIONEA2001).
Diachronic analysis of aerial photographs
During the flowering period, chestnut trees can be easily distinguished from other tree species in aerial photographs (STANGA 1997). The historical development of the chestnut stands was assessed on a meso-scale by comparing aerial photographs from two different periods (1959/60 and 1995). Black and white photographs (approx. 1:20 000) taken in 1959/60 (IFRF 1959) were compared with aerial colour photos (scale: 1:10 000) taken in 1995. 10 sample areas covering a total surface of about 600 ha with 1125 stands were selected. The following main characteristics were analysed (for more details, see STANGA1997):
– chestnut stand type(orchard, coppice, high forest). Minimum stand size = 0.2 ha;
– degree of mixture(chestnut crown cover vs.other species, in 20%-classes). Stands with less than 20% chestnut in 1959/60 were not considered. Stands with less than 20% chestnuts in 1995 were classified as “other species”;
– degree of cover(cover percentage of the crowns of all tree species in the stand, in 20%-classes).
For each stand, additional descriptive features were recorded:
– site conditions(poor, medium, good) based on 1:25 000 national maps and using the key for sites on crystalline rocks proposed by KELLER(1979);
– fire events between 1959/60 and 1995(0, 1–2, >2); identified in the Forest Fire Data Base (CONEDERAet al. 1993);
– presence of other species in the stand: no other species, pioneer species (birch) and other widely disseminating species (ash, lime, etc.), or climax species with heavy seeds (beech, oak);
– dominant species in neighbouring stands in 1959/60: chestnut stands or other species (degree of mixture of chestnut <20%).
Statistical analysis
The enlarged set of explanatory variables was used to investigate the factors of major influence in the ongoing dynamics. The evolution of the degree of mixture (Y = 1 when the proportion of chestnut decreased, Y = 2 when the proportion of chestnut remained unchanged or increased) was defined as a binary response variable. In order to reduce the degrees of freedom of the model, the classes of some explanatory variables were regrouped in 2: this is the case for the degree of mixture (“pure” if >80%; “mixed” if between 20–80%) and cover (“close” if >0.8;
“open” if 0.2–0.8). For the statistical analysis, a model describing the probability that the response variable assumes the value 1 (decrease in proportion of chestnut) was used, where P(Yi=1) is a function of the explanatory variables, xi1, xi2, xi3, ..., xim. The probability was logarithmically transformed to avoid values exceeding 1, so that the following logistic regression model was obtained:
log ⎛⎜ = h(xi(1), ...., xi(m)) = α+ β1xi(1) + β2xi(2) + ... + βmxi(m)
⎝
The analysis was performed using the CATMOD (CATegorical data MODeling) of the SAS statistical package.
3 Results
In the 10 areas 1125 forest stands with a total of 606.3 ha were analysed. In 1959/60 575.2 ha (94.9%) were classified as chestnut stands, 3.4 ha (0.6%) as stands of other species (chestnut presence <20%) and 27.7 ha (4.5%) as open areas. Table 1 shows that by 1995 121.4 ha (20.0%) of chestnut stands had been lost. Only 0.5 ha of new chestnut high forests were noted in the formerly mixed stands and 8.3 ha in the formerly open areas. For further analysis coppices and high forests were combined.
Both chestnut stand types tend to become shaded as shown by the increase in the degree of cover (Fig. 1).
To find the factors which had a major influence on the ongoing dynamics in forests and orchards, we set up two separate models. Table 2 shows the sources giving the best fit for both models, whereas in Table 3 the maximum likelihood estimates of the model sources are reported.
The ecological and silvicultural interpretation of the models can be summarised as follows: pure orchards (chestnut presence >80%) have a greater probability of a diminution in the degree of mixture than mixed orchards; pure coppice stands, on the other hand, are less susceptible to invasion by other species. In the case of 100% pure orchards, open stands are more rapidly colonised by other species than closed stands. On the other hand, closed orchard stands are more sensitive to the evolution toward mixed stands if other species are present initially. These factors are not significant for coppices and high forests. The type of the colonising species plays a significant role in the evolution of orchards. It is slower when the initial species have a climax character. In coppice stands, site conditions seem to play a significant role: coppice stands are less susceptible to invasion by other species when they are located on medium sites. But they tend to be invaded when they are on either good or poor sites. This trend is especially strong for pure stands.
Table 1. Evolution of the forest types.
1960 1995
chestnut other open
orchards coppices high forests species areas
forest type ha % ha % ha % ha % ha % ha %
chestnut
orchards 122.1 100.0 88.8 72.7 8.0 6.6 0.0 0.0 23.5 19.2 1.8 1.5 coppices 441.7 100.0 0.9 0.2 345.4 78.2 4.6 1.0 85.2 19.3 5.6 1.3 high forests 11.4 100.0 0.0 0.0 1.6 14.0 4.5 39.5 5.3 46.5 0.0 0.0 other species 3.4 100.0 0.0 0.0 0.0 0.0 0.5 14.7 2.9 85.3 0.0 0.0 open areas 27.7 100.0 0.0 0.0 0.0 0.0 8.3 29.3 8.0 28.9 11.4 41.2
New chestnut forests (8.8 ha) Former chestnut forests (121.4 ha) P(Yi= 1)
1 – P(Yi= 1) ⎛
⎜⎝
Table 2. Maximum likelihood analysis of variance.
Symbol Source Classes DF Chi-square Pobs
Orchards
Intercept 1 39.91 <0.0001
MX Initial mixture 1 = pure (>80%) 1 19.25 <0.0001
2 = mixed (20–80%)
OS Presence of other species 1 = none 2 34.15 <0.0001
2 = pioneer and other species 3 = climax species
CO(OS) Cover as function of 1 = close (>0.8) 3 17.32 0.006 presence of other species 2 = open (0.2–0.8)
Likelihood ratio 5 8.82 0.1164
Coppices and high forests
Intercept 1 87.33 <0.0001
MX Initial mixture 1 = pure (>80%) 1 47.45 <0.0001
2 = mixed (20–80%)
ST(MX) Site condition as function 1 = good 4 99.16 <0.0001
of initial mixture 2 = medium 3 = poor
CO(MX) Cover as function of 1 = close (>0.8) 2 4.10 0.1290
initial mixture 2 = open (0.2–0.8)
Likelihood ratio 4 –2.09 0.7186
1959/60 1995 orchards
degree of cover
1959/60 1995
coppices and high forests
degree of cover a) %
90 80 70 60 50 40 30 20 10
0 >0.8 0.6–0.8 0.4–0.6 0.2–0.4
b) % 90 80 70 60 50 40 30 20 10
0 >0.8 0.6–0.8 0.4–0.6 0.2–0.4
Fig. 1. Evolution of the degree of cover between 1959/60–1995.
a) orchards
b) coppices and high forests
(data expressed in % of the total area in 1959/60)
Table 3. Analysis of maximum likelihood estimates.
Effect Parameter Estimate Standard error Chi-square Pobs orchards
Intercept 0.9072 0.1436 39.91 <0.0001
MX 1 1.0577 0.2411 19.25 <0.0001
OS 1 –1.8033 0.3309 29.70 <0.0001
2 1.0868 0.2242 23.50 <0.0001
CO(OS) 1 when OS = 1 –0.5245 0.1384 14.37 0.0002
1 when OS = 2 0.0628 0.1531 0.17 0.6817
1 when OS = 3 0.5726 0.3435 2.78 0.0955
coppices and high forests
Intercept 0.6060 0.0648 87.33 <0.0001
MX 1 –0.4467 0.0648 47.45 <0.0001
ST(MX) 1 when MX = 1 0.3149 0.1416 4.94 0.0262
2 when MX = 1 –1.0326 0.1093 89.28 <0.0001
1 when MX = 2 0.2955 0.1083 7.45 0.0064
2 when MX = 2 –0.0599 0.0978 0.37 0.5405
CO(MX) 1 when MX = 1 0.1181 0.0724 2.66 0.1027
1 when MX = 2 0.0871 0.0728 1.43 0.2314
4 Discussion
In our study we have shown that there is a tendency for chestnut groves to disperse in the absence of cultural inputs as was also shown in earlier studies (PACIet al.2000, MALTONI
and PACI 2001). The process is rapid. In 35 years a clear shift took place toward mixed stands, with an overall increase in the degree of cover. The main reasons for this evolution may be the artificial stand structures and their reduced initial biomass (GUIDIet al.1994), combined with the general low regeneration capacity of chestnut (BACILIERIet al.1993).
As suggested by PACIet al.(2000), the silvicultural regime plays a prominent role in regu- lating the post-culture evolution of chestnut stands. Orchards are more sensitive to invasion by other species because of their open structure and reduced biomass. The main factors regu lating the rapidity of the colonisation by other species, besides the initial degree of mix- ture and of cover of the stand, may be external factors such as the ecological properties of the present species (climax, pioneer or other species). The initial mixture also plays an important role in coppice stands, but in an opposite direction: due to the general greater density of coppice stands, pure stands tend to be more resistant to colonisation by other species. It is worth noting that once the colonising trees have become established (chestnut
<80%), the old orchard trees show a better resistance than coppice stools, which tend to become overgrown faster.
For coppice stands, site conditions are the second most important factors regulating the natural evolution, probably as an indirect expression of the type of species that may potentially colonise the stand. Medium sites tended to be the least susceptible to invasion, probably because of lack of any very competitive natural species, while pioneer species on poor sites and any valuable broad-leaved species on good sites are more competitive than chestnut coppices.
For both silvicultural regimes, the introduction of the explanatory variable “dominant species in neighbouring stands” did not improve the model. This could be a consequence of the stronger effect of the species already present in the stands in 1959/60 or of the quite broad classes considered. With the available data, it was not possible to analyse the role of neighbouring mother trees as recommended by GUIDIet al.(1994), or the effects of forest fires, although DELARZEet al.(1992) showed that fire tends to stop natural evolution.
In conclusion, our results show clearly that orchard and coppice stands have different evolutionary patterns. A better understanding of these ongoing processes should contribute to developing a better approach to more sustainable landscape management.
Acknowledgments
We express our thanks to Martin Hägeli and Carmen Frank of the Spatial Information Handling Section at the WSL in Birmensdorf for technical support and to Dr Daniel Mandallaz of the Chair of Forest Inventory and Planning of the Swiss Federal Institute of Technology Zurich for advise on statistics and the Swiss National Science Foundation for financial support (grant no. 31- 39323.93).
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Accepted 8.4.02
