Mid Sweden University

In the mainland Nordic countries and the Baltic States, the delineation and set-aside of woodland key habitats (WKHs) has been one important approach to conserving biodiversity outside traditional protected areas. Though the specifics of the key habitat concept differ from country to country, the intent is to set aside forest areas that (1) exhibit a low degree of exploitation, (2) host or potentially host red-listed species, and/or (3) contain old-growth characteristics (e.g. dead wood, large old trees) or other qualities considered valuable for maintaining biodiversity. However, it is still uncertain to what extent WKHs actually retain quantities and qualities of coarse woody debris (CWD) that are characteristic of old-growth forests. The Biodiversity Monitoring Programme (BMP) recently conducted a detailed inventory of 491 WKHs across Sweden, providing a large dataset with which to evaluate the effectiveness of the WKH program with respect to CWD. In the present study we analyze the BMP data and compare CWD volume and composition between WKHs, mature managed (stand age 81-120 years), overmature managed (age 121-140 years), and published findings from old-growth forests. The national average volume of CWD (standing and downed combined, m3/ha) was higher in WKHs (19.5) than the mature managed (9.3) and the overmature managed forest (12.2), yet was markedly lower than that reported from old-growth forests. CWD volumes in spruce-dominated WKHs had been reduced by 50-63% in the southern and middle boreal regions to 43-64% in the northern boreal region when compared to old-growth forests. In general, CWD amount, variability and quality were greater within WKHs in the boreal regions as opposed to the nemoral and boreonemoral regions in the south of Sweden. The majority of the WKHs (64%) contained key elements (very large and/or decayed dead wood known to be crucial habitat for many threatened wood-dependent species). Considering that these structures are largely absent from managed forests, WKHs have better retained some of the important features of old-growth forests when compared to the surrounding managed forest. WKHs are therefore valuable habitats for dead wood dependent species and representative focal areas for continued and future forest restoration and landscape planning. Our results confirm that large sun-exposed and burned dead wood are underrepresented CWD components within WKHs and emphasize the need to broaden the WKH definition to include locations containing these structures.

Questions: Can small and isolated high-conservation value forests (e.g. designated woodland key habitats) maintain old-growth forest characteristics and functionality in fragmented landscapes? To what extent have past disturbances (natural and anthropogenic) influenced the development of old-growth characteristics of these forests? How long does it take for selectively cut stands to attain conditions resembling old-growth forests?

Location: Southern boreal zone of central Sweden.

Methods: We linked multiple lines of evidence from historical records, biological archives, and analyses of current forest structure to reconstruct the forest history of a boreal landscape, with special emphasis on six remaining core localities of high-conservation value forest stands.

Results: Our reconstructions revealed that several of these stands experienced wildfires up to the 1890s; all had been selectively harvested in the late 1800s; and all underwent substantial structural and compositional reorganization over the following 100-150 years. This time interval was sufficient to recover considerable amounts of standing and downed dead wood (mean 60.3 m3 ha−1), a range of tree ages and sizes (mean basal area 32.6 m2 ha−1), and dominance of shade-tolerant spruce. It was insufficient to obtain clearly uneven tree age structures and large (>45 cm diameter) living and dead trees. Thus, these forests contain some, but not all, important compositional and structural attributes of old-growth forests, their abundance being dependent on the timing and magnitude of past natural and anthropogenic disturbances. Our landscape-level analysis showed marked compositional and structural differences between the historical forest landscape and the present landscape, with the latter having a greater proportion of young forests, introduction of non-native species, and lack of large trees and dead wood.

Conclusions: The remnant high-conservation value stands were not true representatives of the pre-industrial forests, but represent the last vestige of forests that have regenerated naturally and maintained a continuous tree cover. These traits, coupled with their capacity for old-growth recovery, make them valuable focal areas for conservation.

Long-term experimental forest fragmentation studies remain uncommon, despite their critical role in the advancement of ecological theory and conservation planning. In 1986 five circular forest fragments (1/16-1 ha) were exposed through clearcutting within an old-growth Norway spruce (Picea abies) forest in northern Sweden. Initial responses to fragmentation (1986-1991) showed very high tree mortality and structural degradation of the fragments. In the present study we re-inventoried these fragments to evaluate tree mortality patterns and structural changes occurring over a longer time period (1991-2004). The fragments can readily be viewed as harvest retention patches or 'woodland key habitats' (i.e., set-aside patches of high conservation value), allowing us to make inferences about the effectiveness of these novel conservation tools. Tree mortality rates dropped markedly (to 1.2-3.9%/year) compared to the initial responses, yet remained elevated over those of control plots in the nearby unfragmented forest (0.7%). Mortality increased with tree diameter, resulting in smaller-diameter, more homogenous stands. Mortality also generally increased with decreasing fragment size and was dependent of tree location within fragments. Standing death (45% of dead trees, 1991-2004) replaced uprootings (71%, 1986-1991) as the dominant mode of mortality. Numbers of dying and standing dead trees increased during the second sampling period, further adding to structural change and reduced stand density. Elevated tree mortality resulted in uncharacteristically high volumes of coarse woody debris. Results clearly show that adverse edge-related changes to forest structure and function persist up to two decades after fragmentation. Fragments of this size largely fail as remnants intended to maintain forest interior conditions and late-successional forest structure. However, when embedded within a harvested landscape, they: (1) provide abundant coarse woody debris and snags for deadwood-dependent species that risk extirpation in the surrounding matrix and (2) retain important structures for the developing stands.

1. Wood-decaying fungi are ubiquitous and functionally important organisms within forest ecosystems world-wide, but remarkably little is known of their population dynamics in relation to the dynamics of their host substrates. Living in transient environments, where local extinctions are caused by gradual substrate (patch) destruction or deterioration due to wood decomposition, the long-term persistence of these species requires successful colonization of new patches.2. During a 6-year period, we examined the colonization-extinction dynamics of wood-decaying fungi (Aphyllophorales: Polyporaceae and Corticiaceae) in relation to the spatiotemporal distribution of host logs within a boreal old-growth Norway spruce (Picea abies) forest.3. The dynamics of the species studied were strongly influenced by both local log characteristics (within patch) and connectivity (between patch). Several species (e.g. Asterodon ferruginosus, Phellinus ferrugineofuscus, P. viticola, Phlebia centrifuga) showed a positive effect of connectivity, mainly colonizing logs in the vicinity of previously occupied logs. This implies that some wood-decaying fungi may be dispersal limited in terms of successful colonizations. The relative importance of patch conditions and connectivity was however, highly species specific.4. Our results further illustrate the importance of life-strategies adopted by species that are present during different stages of wood decomposition. Early colonizers were primarily affected by the stage of decomposition; secondary colonizers were affected by a variety of within patch and/or between patch variables, maintaining high species coexistence within intermediate stages of decay. Phellinus nigrolimitatus was the dominant polyporous decayer at the final stages of decomposition, clearly gaining a competitive advantage from specializing on highly decomposed wood and having very low mean annual mortality rates.5. Local extinction rates were higher on small diameter logs than large diameter logs, and generally increased as decay proceeded, illustrating the importance of deterministic patch destruction due to wood decomposition.6. Synthesis. The fungi-log study system was highly dynamic, illustrating that both characteristics and spatiotemporal availability of logs are important in explaining the distribution patterns and population dynamics of wood-decaying fungal communities. The result implies that the dynamics of some wood-decaying fungi can be characterized as patch-tracking metapopulations, with connectivity-dependent colonizations and local extinctions caused by the turnover of the patches.