Dominik Hessenmöller

Old-growth forests as global carbon sinks

Old-growth forests remove carbon dioxide from the atmosphere at rates that vary with climate and nitrogen deposition. The sequestered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil. Old-growth forests therefore serve as a global carbon dioxide sink, but they are not protected by international treaties, because it is generally thought that ageing forests cease to accumulate carbon. Here we report a search of literature and databases for forest carbon-flux estimates. We find that in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive. Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral. Over 30 per cent of the global forest area is unmanaged primary forest, and this area contains the remaining old-growth forests. Half of the primary forests (6 × 108 hectares) are located in the boreal and temperate regions of the Northern Hemisphere. On the basis of our analysis, these forests alone sequester about 1.3 ± 0.5 gigatonnes of carbon per year. Thus, our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net ecosystem productivity. Old-growth forests accumulate carbon for centuries and contain large quantities of it. We expect, however, that much of this carbon, even soil carbon, will move back to the atmosphere if these forests are disturbed.

Quantifying land use and disturbance intensity in forestry, based on the self‐thinning relationship

Forest management and disturbances affect forest soils and biomass carbon stocks, emissions to the atmosphere, and radiative forcing. The conventional approaches to quantifying the intensity of land management based on net primary productivity (NPP) are limited because they lack a sound ecological basis. These limitations are especially evident for forests. This study proposes a new way of characterizing the degree of management and disturbance in forest stands that do not experience a change in classification, i.e., they remain forests. The proposed index, called land use and disturbance intensity (LUDI), uses the relationship between stand density and diameter at breast height for a relatively unmanaged/pristine baseline forest and different management schemes, in conjunction with the self-thinning relationship, to calculate the difference between potential and actual biomass storage. The LUDI distinguishes between the long and short timescales associated with management and disturbance. This capacity sets it apart from the conventional NPP-based approaches currently used to quantify the disturbance intensity. The study uses a semi-qualitative validation approach to demonstrate that the proposed index reproduces textbook knowledge on management and disturbance intensity. However, our quantitative approach that distinguishes between timescales adds insight into the effects of forest management on the living biomass stock. Further, data from the German national forest inventory were used to demonstrate that the proposed index does not require knowledge about the management scheme. Although LUDI is constrained to aboveground living biomass, we believe that the approach is useful in the context of broadening our capability in large-scale management reconstruction and through this understanding the effects of land use on the carbon cycle, beyond effects of deforestation and afforestation.

Richness of lichen species, especially of threatened ones, is promoted by management methods furthering stand continuity

Lichens are a key component of forest biodiversity. However, a comprehensive study analyzing lichen species richness in relation to several management types, extending over different regions and forest stages and including information on site conditions is missing for temperate European forests. In three German regions (Schwäbische Alb, Hainich-Dün, Schorfheide-Chorin), the so-called Biodiversity Exploratories, we studied lichen species richness in 631 forest plots of 400 m2 comprising different management types (unmanaged, selection cutting, deciduous and coniferous age-class forests resulting from clear cutting or shelterwood logging), various stand ages, and site conditions, typical for large parts of temperate Europe. We analyzed how lichen species richness responds to management and habitat variables (standing biomass, cover of deadwood, cover of rocks). We found strong regional differences with highest lichen species richness in the Schwäbische Alb, probably driven by regional differences in former air pollution, and in precipitation and habitat variables. Overall, unmanaged forests harbored 22% more threatened lichen species than managed age-class forests. In general, total, corticolous, and threatened lichen species richness did not differ among management types of deciduous forests. However, in the Schwäbische-Alb region, deciduous forests had 61% more lichen species than coniferous forests and they had 279% more threatened and 76% more corticolous lichen species. Old deciduous age classes were richer in corticolous lichen species than young ones, while old coniferous age-classes were poorer than young ones. Overall, our findings highlight the importance of stand continuity for conservation. To increase total and threatened lichen species richness we suggest (1) conserving unmanaged forests, (2) promoting silvicultural methods assuring stand continuity, (3) conserving old trees in managed forests, (4) promoting stands of native deciduous tree species instead of coniferous plantations, and (5) increasing the amount of deadwood in forests.

Differential Responses of Herbivores and Herbivory to Management in Temperate European Beech

Forest management not only affects biodiversity but also might alter ecosystem processes mediated by the organisms, i.e. herbivory the removal of plant biomass by plant-eating insects and other arthropod groups. Aiming at revealing general relationships between forest management and herbivory we investigated aboveground arthropod herbivory in 105 plots dominated by European beech in three different regions in Germany in the sun-exposed canopy of mature beech trees and on beech saplings in the understorey. We separately assessed damage by different guilds of herbivores, i.e. chewing, sucking and scraping herbivores, gall-forming insects and mites, and leaf-mining insects. We asked whether herbivory differs among different forest management regimes (unmanaged, uneven-aged managed, even-aged managed) and among age-classes within even-aged forests. We further tested for consistency of relationships between regions, strata and herbivore guilds. On average, almost 80% of beech leaves showed herbivory damage, and about 6% of leaf area was consumed. Chewing damage was most common, whereas leaf sucking and scraping damage were very rare. Damage was generally greater in the canopy than in the understorey, in particular for chewing and scraping damage, and the occurrence of mines. There was little difference in herbivory among differently managed forests and the effects of management on damage differed among regions, strata and damage types. Covariates such as wood volume, tree density and plant diversity weakly influenced herbivory, and effects differed between herbivory types. We conclude that despite of the relatively low number of species attacking beech; arthropod herbivory on beech is generally high. We further conclude that responses of herbivory to forest management are multifaceted and environmental factors such as forest structure variables affecting in particular microclimatic conditions are more likely to explain the variability in herbivory among beech forest plots.

The use of forest inventory data for placing flight-interception traps in the forest canopy

Forest canopies are an important part of forest ecosystems and comprise the majority of arthropod diversity. As tree crowns provide various microhabitats, a suitable trapping standard for canopy‐dwelling arthropods is required. However, vertical trap position is often not standardized. We developed six vertical placement strategies for flight‐interception traps based on information on tree height and crown base obtained from forest inventory data. Strategies differed in how crown base and height of trees were weighted in the calculations of trap height. Forest inventory data from almost 1600 plots in three regions of Germany. Were used to calculate the theoretical suitability of the strategies. The strategy to place traps at the canopy centre was predicted to result in very few trees for which placement would not be possible because trees are too short or crown base is too high, and this strategy by definition resulted in the lowest deviation of trap position from the canopy centre. The strategy Centre was tested in the field by installing three flight‐interception traps each in 150 deciduous or coniferous stands of various age structures. On average, 53.8% of the traps were installed at exactly the desired height and for 86.9% of the traps the difference was <10% from the desired height. The strategy worked less well in thickets and pole woods. Overall, our results show that (1) forest inventory data are highly suitable to derive a priori trap placement strategies, even though these data are generally collected in only a small circle in each forest stand, (2) placing traps in the canopy centre largely avoids the problem of not being able to place traps at the desired height in the field, and (3) calculations of trap height should ideally be based on plot‐specific data, but data from many stands of one forest type also provide a reasonable fit.

A silvicultural strategy for managing uneven-aged beech-dominated forests in Thuringia, Germany: a new approach to an old problem

ABSTRACT Maintaining a permanent forest canopy cover and eventually harvesting wood in a final harvest according to predefined dimensions is often considered as prototype for future management of deciduous forests. An uneven-aged structure is considered by the public to resemble “natural” conditions, and by forest engineers it is considered as being more resilient to disturbances. In the Hainich-Dün region of Thuringia, Germany, beech-dominated selection forests covering about 10,000 ha have been managed for almost 1000 years, initially by irregular use, but as regular selection system since about 200 years. Managing these stands remains difficult, due to the lack of yield tables and a quantification of harvest of uneven-aged stands considering differences in site conditions and handling of over-sized trees. It is the objective of the present study to develop tables of target stand volumes, increments, and harvest for different diameter ranges of uneven-aged stands according to site conditions. The present study is based on repeated grid-based inventories of about 2150 plots, which were partly re-inventoried 3 times over the past 20 years. The recommended target wood volumes vary between 296 and 388 m3 ha−1. Stand growth rates of different yield classes were estimated to range between 6.7 and 7.7 m3 ha−1 yr−1 which is 30% lower than for age class forest. Nevertheless, the economic returns are higher. Thus, selective cutting with single tree selection remains a viable silvicultural system, but it may change over time into small-scale shelter-woods for improving growth of regeneration.