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Conservation through utilization – multifunctional development of coppice forests in Rhineland-Palatinate

concluded 12/2011

Summary

 

Over-aged coppice forests occur all over Europe as a result of the abandonment of traditional harvesting practices at the beginning of the last century. With the increasing demand for bioenergy, coppicing, which typically aims at maximizing biomass production, could experience a renaissance. This thesis is focusing on the question if this traditional, but most widely abandoned forest management form, can in accordance to international criteria of sustainability regain the status of an excepted alternative of forest management.
 
For the sustainable management of these abandoned coppice forests, accurate estimates of their biomass potential are needed. Therefore biomass equations for the two most common tree species traditionally managed in Central European coppice systems were developed. In total, 24 sessile oak (Quercus petraea (Mattuschka) Liebl.) and 22 hornbeam (Carpinus betulus L.) trees from two different aged coppice stands in Rhineland-Palatinate (southwest Germany) were felled and separated into various biomass compartments which were directly weighed in the field. From every compartment, samples were taken to the laboratory to determine wood density and water content. Based on dendrometric parameters (diameter at breast height) and compartment dry mass, allometric equations were developed. Power functions provided the best fits for relationships between dbh and biomass in tree compartments and whole trees (R2= 0.97 and 0.92 for oak and hornbeam, respectively). These allometric equations for oak differ considerably from those developed for trees grown in high forests, pointing to the need to use specific equations for aged coppice forests.
 
If coppice forestry should in future be resumed to serve increasing fuel wood demands, improved knowledge concerning nutrient removals under different harvesting scenarios is crucial for the sustainable use of soil nutrients. The nutrient contents of various tree compartments of sessile oak and hornbeam from two sites differing in soil fertility were determined to calculate the nutrients removed under different harvesting intensities. Independent of site and species, the highest nutrient concentrations were found in twigs and stem bark, whereas stem wood and branches with a diameter ≥ 4 cm were characterized by low nutrient concentrations. Simple power functions to predict the nutrient content in the various tree parts showed mostly high coefficients of determination (R2 =0.51 – 0.98). Calculations of stand level nutrient content in tree compartments showed comparable values between both study sites. However, the comparison between the amount of nutrients stored in tree biomass and in soil indicated that sustainable nutrient management must consider different harvesting intensities according to site type. On sites with large soil nutrient pools, even whole trees can be harvested without causing deleterious effects. However, on sites with a low nutrient capital, current harvesting practices would result in high rates of nutrient export and therefore exploitation of soil pools. In these stands, harvesting intensity should be based on careful selection of the tree compartments removed to conserve nutrients on site.
 
Since there have been uncertainties about the capacity for vegetative regeneration (i.e. resprouting ability) of the oak stumps, the re-sprouting ability of sessile oak stumps in over 126 aged coppice forests was determined. We aimed to describe stump mortality and re-sprouting intensity in relation to three different harvesting methods (cutting heights), browsing and parent tree and stump parameters. We found that on average 16% of all sessile oak stools (i.e. grouped or single growing stumps) died within two vegetation periods after coppicing. Stump mortality was higher in unfenced areas compared to areas protected against browsing. No clear relationship was observed between stump mortality and harvesting method or stump and parent tree characteristics. Two vegetation periods after coppicing numerous new vigorous sessile oak stump sprouts were found. Growth of the new sprouts was mainly influenced by browsing. In unfenced areas average maximum sprout height was reduced by nearly 80%. Maximum sprout height (used as a representative parameter for re-sprouting intensity) was found to be unaffected by harvesting method. No relationship was found between stump height and parent tree characteristics and maximum sprout height either. We conclude that it is unjustified to reject the resumption of coppice management in 80-90 year old coppice forests due to a reduced re-sprouting ability of the oak stumps. Moreover the choice of the harvesting method applied is rather determined by economical conditions than by the re-sprouting intensity of the oak stumps. However in areas with a high abundance of browsers coppice cuts are only sustainable when they are fenced or otherwise protected.
 
Sorbus torminalis L. (Crantz) is all over Germany considered to be rare. However, the species is not uncommon in Central European oak coppice forests. There is generally very limited quantitative information on its regeneration and growth dynamics, although its timber is of high value. As mentioned above coppicing is no longer practiced in most parts of Central Europe. Therefore it is unclear whether S. torminalis can persist in high forest systems or if coppicing is needed for the conservation of this species. We quantified the frequency of S. torminalis on three 1 ha sample plots of former oak coppice forest. To determine whether S. torminalis regenerated continuously and how it might compete with the oaks, the age of 80 trees was determined and diameter and height growth was reconstructed for 20 trees by stem analysis. To assess its shade tolerance, photosynthesis was measured on leaves growing at high and low light conditions. Based on dendrochronological data we found that, over the last 80 years, continuous recruitment of S. torminalis occurred. Growth patterns and photosynthesis measurements suggest that S. torminalis is a very shade tolerant species that is able to survive long periods of intensive competition. We conclude that abandonment of coppicing in these forests does not threaten the status of S. torminalis. However, in order to grow trees into merchantable dimensions in a reasonable time, S. torminalis would have to be promoted through thinning. This research contributes to aspects of sustainable management of aged oak coppice forests. The various findings show that by following simple rules adapted to site and stand specific conditions (stocking volume, nutrient pools, browsing pressure, tree species diversity) a sustainable management of coppice forest can generally be achieved.

 

Homepage: http://www.niederwald-rlp.de/projekt.html
Supervisor:
Prof. Dr.  Jürgen Bauhus
Researcher: Pattrick Pyttel
Funding:  
Duration: 09/2008 - 12/2011
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