The aim here was to asses to what degree and over what time span an increased conversion of existing broadleaved forests to planted coniferous forests could cover an increased demand for wood by studying the effects of implementing Option (b) above.
Methods
Total forest resources (142 Mha) of 25 EU countriesii were examined using the EFISCEN model. EFISCEN is an area-based matrix model that assesses the availability of wood and projects forest resource development. It is especially suitable for large-scale forest scenario projections at a country or a regional level (Sallnäs 1990; Nabuurs et al., 2006; Schelhaas et al. 2007). The same forest resource database as underlying the UNECE Forest Sector Outlook studies has been used (UNECE/FAO 2011). The model simulates the development of the forest resources in terms of integrating data on wood increment, growing stock, forested area, tree species and age class distribution in time steps of five years, usually for periods of 50-60 years. A detailed description of the model is given by Schelhaas et al. (2007).
In EFISCEN, the state of the forest is described by distribution of age and volume classes, using inventory data for the forest area available for wood supply. Transitions of area data between matrix cells during simulation reflect natural processes and are influenced by management regime and change in forested area. Growth dynamics are simulated by shifts in area between matrix cells. In each 5-year time step, the area in each matrix cell moves up one age class to simulate ageing. Part of the area in each cell also moves to a higher volume class. Growth dynamics are estimated by mathematical functions with coefficients based on inventory data or yield tables.
Management was specified at two levels in the model. Firstly, a basic forest management regime defined the period during which thinnings could take place and a minimum age for final felling. These regimes can be regarded as constraints on the total harvest level. Thinnings were simulated by shifting the specified area to a lower volume class. Final felling was simulated by shifting the specified area outside the matrix to a separate "bare-forest land" class. Shifting bare-forest land back into the matrix simulated replanting or regeneration. Secondly, the demands for wood from thinnings and from final felling were specified separately, and EFISCEN simulated the felling of the required wood volume, if it is available.
The study was confined to forest tree species and management regimes that are already specified in EFISCEN for particular countries. A typical conversion of land to fast-growing coniferous forest (e.g. Pinus and Picea spp.) with rotation length of 40-80 years was simulated rather than a more extreme conversion to short-rotation Eucalyptus or Populus spp. forest. This conversion method was selected as the most logical and economical change. Furthermore, few countries have poplar or eucalypt species parameterised in EFISCEN thus hampering a large-scale conversion to these species in the model. Further, although poplar and willow are interesting from bioenergy and growth-rate points of view, they mostly grow in floodplains where no new areas are available, as these are in intensive use for agriculture. Assumptions for all Scenarios were that forest owners would be willing to harvest at the specified felling age.
The following three scenarios were examined:
1) Business as Usual (BAU): European Forest Sector Outlook Reference Scenario based on the The European forest sector outlook study II. 2010-2030 (UNECE/FAO 2011).
No species change.
Increase in wood demand of 0.6% y-1.
Increased growth rates due to climate change.
2) High demand + Change of tree species (Plantation)
At final felling of most broadleaved species, 50% of the area is converted to a faster-growing coniferous species.
Higher increase in wood demand of 2% y-1 during the first time step of 5 years. This equals 64 million m3 per 5 years. In order to avoid an exponential increase in demand, we increase demand in subsequent periods with the same 64 million m3 5y-1.
3) High demand + Change of tree species and shortening rotation length (Plantation+)
As for (2) but the rotation length of the broadleaved species is shortened to 20 years in order to increase the rate of future conversion to conifer plantations.
Results
Simulation of the BAU Scenario indicated that European forests could supply almost all of the increase in demand (0.6% y-1) until 2065 with minimal change in management (Figure 4). Coniferous forests (just over half of the total forest area) could supply 473 million m3 y-1 in 2065, an increase of 103 million m3 y-1.
In the Plantation+ Scenario, conversion of 50% of the forested area to coniferous species after shortening the rotation length of broadleaved forest was found to increase wood supply to 561 million m3 y-1 by 2065, (Figure 5). However, this form of management would not be enough to satisfy the high demand level in this Scenario.
The supply of coniferous wood under the Plantation Scenario (556 million m3 y-1 in 2065) would be similar to that under Plantation+ Scenario. This result suggests that shortening the rotation length of broadleaved forest in order to speed up the conversion would not increase wood supply. Lack of increase in coniferous wood supply between 2030 and 2045 (Figure 5) reflects the amount of time needed for coniferous species to reach harvestable age. High demand would exhaust the reservoir of coniferous wood that exists between 2010 and 2020.
Other results show that due to the higher achieved felling level in the Plantation+ Scenario, the growing stock rises less under this regime. Namely, the growing stock amounts to 211 m3 ha-1 in 2065 under the plantation+ versus 242 m3 ha-1 in 2065 under the BAU scenario.
Because the specified conversion method for both the Plantation and Plantation+ Scenarios involved felling of broadleaved forest, a demand for broadleaved wood is required to stimulate the conversion to coniferous forests. By doing the conversion this way, the conversion becomes rather slow. This inertia in the system is also visible in Figure 6, where net annual increment for all forests is depicted. Only after 2045, the increment in both the plantation scenarios starts to increase compared to BAU, due to an increase in area of faster growing plantations. In 2065, in both plantation scenarios, net annual increment could rise to 7.4 m3 ha-1 y-1, while remaining at 6.8 m3 ha-1 y-1 under BAU scenario.