Biomass equations for sessile oak (Quercus petraea (Matt.) Liebl.) and hornbeam (Carpinus betulus L.) in aged coppiced forests in southwest Germany

Abstract

Over-aged coppice forests (older than 40 years) occur all over Europe as a result of the abandonment of traditional harvesting practices during the last 60–100 years. With the increasing demand for bioenergy, there is renewed interest in coppicing, which typically aims at maximizing biomass production. For the sustainable management of these 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 this study.

In total, 24 oak (Quercus petraea) and 24 hornbeam (Carpinus betulus) 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 (dbh)) 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 equations that are specific to silvicultural systems, in this case for aged coppice forests.

Introduction

Coppicing is a traditional silvicultural management system applied all over the world. It is mostly used for the production of firewood and various non-timber forest products [1], [2], [3] whereas short rotation coppice systems are specifically employed to produce woody biomass for energy purposes only [4]. In many European countries, this practice was abandoned in the first half of the last century due to socio-economic changes [4], [5]. Managed coppiced forests (i.e. younger than 40 years) cover only 750 km² of Germany which represents 0.7% of the total forest area [6]. However, the area of abandoned, older coppice forests is assumed to amount to several thousand km². Because most privately owned forests are not inventoried, the actual area is not certain. These aged oak coppice forests represent a potential biomass source of unknown magnitude.

In rural environments, coppice stands often represent important elements of the cultural landscapes until today. The abandonment of periodic coppicing leads to a process of transformation during which the stands lose their typical coppice characteristics like multiple stem structure and increasingly resemble high forests in which individual trees have originated from seed. Subsequently, the specific ecological value of coppice forests [7], [8], [9] decreases and an important element of the cultural landscape gradually disappears. One way of preserving the ecological, cultural and historical value of aged coppice forests would be to resume coppicing, which would additionally benefit light and warmth demanding species and can increase biodiversity [7], [9]. Hence, the reintroduction of coppice management may also be motivated by concerns about biodiversity and landscape values [9].

Ongoing initiatives by the European Union (EU) call for a substantial increase in the use of renewable energy sources. It is aimed that by 2020 one fifth of the European energy consumption will be from renewable sources (European Parliament 2009: directive 2009/28/EG [10]). Of all renewable energy currently consumed in the EU, 47% is generated from forest biomass (i.e. wood and wood waste [11]). The resulting demand for biomass as an energy source has stimulated interest to resume coppicing of forests that had undergone this management in the past [12], [13]. However, aged coppice forests have often not been properly inventoried, particularly not on private land, and hence their potential contribution to biomass supply is difficult to quantify. One problem associated with the inventory of biomass in this type of forests is the lack of suitable biomass equations.

Based on strong relationships between tree weight and dendrometric values, like diameter at breast height (dbh), tree height or green crown height, biomass equations have been developed for many tree species around the world e.g. [14], [15], [16], [17]. However, only very few equations exist for trees originating from coppice. These trees differ from those managed in high forests in terms of crown size and stem form [18]. Owing to the origin of stems and since coppice forests are commonly not thinned, many trees have small crowns and multiple stems [19]. Accordingly, to calculate biomass of coppice trees, equations developed for trees grown in high forests may be less accurate.

There are no published biomass equations for aged oak coppice stands in Central Europe. It is also questionable, if biomass equations for younger coppice forests or those from other European regions [20], [21], [22], [23] could be used instead. Extrapolation of biomass equations to other regions may lead to inaccurate predictions (e.g. [24]). These differences can be explained by a number of factors, like e.g. silvicultutral practices, genetic differences and the influence of different site conditions.

The objective of this study was to develop easily applicable equations for the calculation of single tree biomass of the most common coppice forest species in south-western Germany, namely sessile oak (Quercus petraea) and hornbeam (Carpinus betulus). To facilitate the assessment of biomass removal by different harvesting intensities, separate equations were developed for different tree compartments such as for example stem with and without bark, stem and branches with a diameter larger than 7 cm and the whole tree (without leaves). In addition, we compared the allometric equations for oak from this study with equations from the literature to assess how these might differ between regions and silvicultural systems and how large the error for biomass estimates might be, if equations from the literature are applied.