Graph-theoretic connectivity measures: what do they tell us about connectivity?

Graph-theoretic connectivity analyses have received much attention in connectivity evaluation during the last few years. Here, we explore the underlying conceptual differences of various graph-theoretic connectivity measures. Based on connectivity analyses from three reserve networks in forested landscapes in Central Finland, we illustrate how these conceptual differences cause inconsistent connectivity evaluations at both the landscape and patch level. Our results also illustrate how the characteristics of the networks (patch density) may affect the performance of the different measures. Many of the connectivity measures react to changes in habitat connectivity in an ecologically undesirable manner. Patch prioritisations based on a node removal analysis were sensitive to the connectivity measure they were based on. The patch prioritisations derived from different measures showed a disparity in terms of how much weight they put on patch size versus patch location and how they value patch location. Although graphs operate at the interface of structure and function, there is still much to do for incorporating the inferred ecological process into graph structures and analyses. If graph analyses are going to be used for real-world management and conservation purposes, a more thorough understanding of the caveats and justifications of the graph-theoretic connectivity measures will be needed.     

Hotspots in cold climate: Conservation value of woodland key habitats in boreal forests

The concept of Woodland Key Habitats (WKH, small-scaled presumed hotspots of biodiversity) has become an essential component of biodiversity conservation in Fennoscandian and Baltic forests. There have been debates over the importance of WKHs in relation to the conservation of biodiversity in production forests. We applied a systematic review protocol and meta-analysis to summarize knowledge on comparisons of biodiversity qualities, such as dead wood and species richness, between WKHs and production forests in relevant countries. We also summarized the knowledge on the impact of edge effects by comparing WKHs surrounded by production forests to WKHs surrounded by clear cuts. Studies had been conducted in Finland, Norway and Sweden. Based on our meta-analysis, WKHs seem to be relative hotspots for dead wood volume, diversity of dead wood, number of species and number of red-listed species. There were some differences also between countries in these biodiversity qualities. Only two studies compared WKHs surrounded by production forests and clear cuts, respectively. Hence, the capability of WKHs to maintain their original species composition and support species persistence over time remains to be addressed, as well as their role in relation to other conservation tools.     

Cost-effective strategies to conserve boreal forest biodiversity and long-term landscape-level maintenance of habitats

Setting aside parcels of land is the main conservation strategy to reduce the rate of biodiversity loss worldwide. Because funding for biological conservation is limited, it is important to distinguish the most efficient ways to use it. Here, we assess implications of alternative measures to conserve biodiversity in managed boreal forest landscapes. We calculated four alternative spatio-temporal scenarios and compared these to the current management regime over 100-year time period. In the alternative scenarios, a fixed amount of funding was invested in (1) permanent large reserves (each tens of ha in size), (2) permanent small reserves (each a few ha in size), (3) temporary small reserves (based on 10-year contracts with private land owners), and (4) green-tree retention (small groups of trees retained on clear-cuts). To assess biodiversity implications, we used habitat suitability indices to calculate overall habitat availability for five groups of red-listed and habitat-specific species associated with decaying spruce logs. The possibilities for timber harvests did not differ among the scenarios, but biodiversity performance was different. The scenarios with permanent reserves tended to outperform other scenarios, suggesting that conservation policies based on permanent reserves are the most cost-efficient in the long term. Results, however, varied among time scales and species groups. In the short term, a strategy of investment in temporary small reserves was the most efficient. Habitat for species associated with old spruce dead-wood and preferring shade was rare throughout all simulations, and therefore, it is likely that these species cannot be sustained in managed forests. Species that live on fresh dead-wood and are associated with forest edges coped well in all scenarios suggesting that such species will persist in managed landscapes without additional conservation efforts. Explicit definition of conservation objectives and time frames for conservation action are thus prerequisites for successful conservation planning.     

Effects of forest restoration treatments on the abundance of bark beetles in Norway spruce forests of southern Finland

Restoration of protected areas in boreal forests frequently includes creating substantial volumes of dead wood. While this benefits a wide range of dead wood dependent invertebrate species, some of these are regarded as forest pests. Therefore, the risk of elevated levels of tree mortality in surrounding commercial forests must be considered. In a large-scale field experiment in southern Finland, we studied the effects of restoration treatments on the abundance of bark beetles within and in the vicinity of restored areas, in particular focusing on Ips typographus and Pityogenes chalcographus. The treatments applied to managed Norway spruce forests were controlled burning and partial harvesting combined with retaining 5, 30 or 60 m³/ha of cut down wood. We found that the abundance of bark beetles increased by both burning and harvesting with down wood retention, being highest where burning and harvesting had been combined. The actual volume of down wood retention had no significant effect. The effect of burning on the number of bark beetles along host tree boles was negative which suggests that burnt spruces provided a less suitable resource for bark beetles than unburnt dead spruces. The abundance of bark beetles along host trees also decreased with increasing volume of down wood retention. The abundance of P. chalographus was slightly elevated up to 50 m outside restored areas but the abundance was very low compared to that within the areas. The abundance of I. typographus was extremely low outside restored areas. We conclude that restoration treatments increase the abundance of bark beetles via increased availability of resources, but that the effect of burning is likely to be counteracted by decreased resource quality. Thus, burning might be the “safest” way to produce large quantities of dead wood. Furthermore, the fact that only few beetles were collected in adjacent areas suggests that restored areas pose little threat of serving as refugia in which bark beetle populations increase in sufficient numbers to attack live trees in adjacent forests. However, restoration actions repeated at consecutive years within a small area might enable the populations to grow to outbreak levels.     

Methanogen activity in relation to water table level in two boreal fens

We studied methanogen activity—measured by in vitro methane production potential and by detection of the messenger RNA (mRNA) of a functional gene—in two boreal fens under high and deep water table (WT) level conditions resulting from a rainy growing season and a dry growing season. The depth of the highest CH₄-producing layers differed between the years. In the wet year, the highest CH₄ production rate was around 20 cm below the mean WT. In the dry year, the highest rates were measured close to the peat surface, well above the mean WT. The distribution of activity in the peat profiles of the two fens appeared to be site specific. Under deep-WT conditions, CH₄ production potential was generally lower than that under high-WT conditions. Detection of the mRNA of the methanogen-specific mcrA gene indicated in situ methanogenesis in both water-saturated peat (below the WT) and unsaturated peat (above the WT). Analyses of DNA-derived and mRNA-derived methanogen community structures showed greater similarity between those two in water-saturated peat than in unsaturated peat. This suggested that favorable conditions promoted the activity of most members in methanogen communities, but unfavorable conditions showed differences between distinct community members in adaptation to adverse conditions.     

The use of ecological traits in extinction risk assessments: A case study on geometrid moths

Identifying ecological traits that make some species more vulnerable than others is vital for predictive conservation science. By identifying these predisposing traits we can predict which species are most prone to decline and gain an understanding of the reasons behind the decline. The aim of this study was to determine the ecological traits that best predict extinction risk and distribution change in Finnish geometrid moths and to develop an understanding of the biological connections between these traits and threats. We found that larval specificity, overwintering stage and flight period length predicted distribution change and extinction risk. There was also an interaction effect between larval specificity and body size on both distribution change and extinction risk. In monophagous species the host plant distribution predicted extinction risk. Even though ecological traits are known to be important determinants of extinction risk, the IUCN red list categorization system is exclusively based on quantitative measures of populations and ignores the ecological traits. Here, we propose that taxon specific ecological data should also be used to predict extinction risk at least on a regional scale to improve the accuracy of the IUCN extinction risk classification.     

Woodland key habitats evaluated as part of a functional reserve network

Woodland key habitats (WKHs) represent a potentially cost-efficient means to protect biodiversity in managed forests. The Forest Act of Finland defines 13 habitat types of WKHs, which enjoy legal protection. It has been argued that WKHs are too small-sized and scattered in occurrence to be actually important in the maintenance of forest biodiversity. However, from the species’ perspective, WKHs form a network together with nature reserves. We evaluated the value and role of WKHs as a part of the whole reserve network using a graph-theoretical connectivity approach in three areas (ca. 500 km² each) located in Central Finland. The networks were formed separately for different habitat types and dispersal distances (ranging from 200 m to 25 km). We compared networks with and without WKHs, and thereby quantified the contribution of WKHs to overall network connectivity. We also examined the role of WKHs in the networks based on patch importance and network centrality measures. The results showed that the connectivity contributions of WKHs are tightly linked with the dispersal abilities of threatened species: WKHs enhance habitat connectivity, especially for species with an intermediate dispersal ability. For species with a poor dispersal ability, the protection of large set-asides would be a more efficient way to increase habitat connectivity than WKHs. WKH-based conservation seems to improve the connectivity of naturally rare and scattered habitat types relatively more than common habitat types, but in sparse networks a greater dispersal ability is required to gain enhanced connectivity than in dense ones. The connectivity value of WKHs can be understood as an emergent and scale-dependent property, appearing at the level of the entire functional network. Provided that the site characteristics of WKHs can be safeguarded, they can be a valuable and efficient addition to the reserve network.     

Volatile monoterpenes in soil atmosphere under birch and conifers: Effects on soil N transformations

The aim of this study was to examine the occurrence and concentrations of volatile organic compounds (VOCs), in particular, volatile monoterpenes, in soil atmosphere under silver birch (Betula pendula L.) and two conifers, Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), and to determine the effects of the most relevant monoterpenes on transformations of soil N. The study site was a 70-year-old tree species experiment in Kivalo, northern Finland. VOCs were collected using two methods, passive air samplers and a chamber method. In soil atmosphere under spruce and especially under pine, the concentrations of monoterpenes were high, α- and β-pinene, Δ-3-carene and myrcene being the most abundant compounds, whereas concentrations of monoterpenes in soil atmosphere under birch were negligible. Samples of humus layer from the birch stand incubated in vitro and exposed to vapors from monoterpenes typical of coniferous forest soil showed decreased rates of net N mineralization but simultaneously increased rates of C mineralization. The response of soil microbial biomass C and N to different monoterpenes varied, but some monoterpenes considerably decreased soil microbial biomass. Altogether these results suggest that these compounds have negative effects on soil N transformations, but may serve as carbon and energy source for part of soil microbes.