Impact of mechanized harvesting on compaction of sandy and clayey forest soils : results of a meta-analysis

Nowadays, harvest operations are predominantly performed fully mechanized using heavy tractors or forestry machines. The resulting soil compaction may negatively affect the soil ecosystem. We wanted to draw general conclusions concerning the impact of mechanized harvesting on forest soil bulk density and the influencing factors. Therefore, we combined the data of several studies using a meta-analysis approach. The impact decreased from the surface towards deeper soil layers. At 0-10 cm depth, the impact on clayey soils was highest although not significantly different from the impact on sandy soils. Higher initial bulk densities, i.e., on already compacted forest soils, generally led to smaller extra increases of bulk density after machine traffic. For sandy soils, the impact was also significantly smaller when machines were lighter. No significant relationship was observed between the compaction degree and traffic intensity. We observed clear compaction on both clayey and sandy soils, especially in case of low initial soil compaction degrees and heavy machines. The compacted initial state of many forest soils, the long recovery period, and the generally high impact of the first passes that is frequently mentioned in literature all count in favour of designated skid trails and an adjustment of the machine type to the job

Understorey phylogenetic diversity in thermophilous deciduous forests : overstorey species identity can matter more than species richness

A

Effects of soil compaction on growth and survival of tree saplings: a meta-analysis

Soil compaction due to mechanized harvesting operations in forests can have profound effects on forest soils and, hence, can have a detrimental effect on subsequent forest regeneration. We performed a meta-analysis to quantify the effect of soil compaction on height growth, diameter growth, and survival of tree saplings. These effects were predominantly insignificant, varied strongly and were thus not unambiguously negative. Only on silty soils, growth and survival were significantly reduced by soil compaction, which contrasted with sandy and loamy soils, where the effect of soil compaction was negligible or even slightly positive. A weighted analysis revealed an overall decrease of height growth on the compacted area, but this result should be interpreted with caution due to the limited number of observations. Although results did not show an overall negative effect of soil compaction, harvesting activities should focus on minimizing soil compaction degree and extent to prevent a decrease of soil productivity. From a methodological point of view we suggest providing more basic statistics in the articles and to include more shade-tolerant tree species in future experimental designs. These species are currently underrepresented

Effects of charcoal hearth soil on forest regeneration : evidence from a two-year experiment on tree seedlings

Production of wood charcoal is a traditional form of forest use that lasted for millennia in most temperate regions, vanishing only some decades ago in the Mediterranean countries. Here, the abandoned charcoal hearths form a network of microhabitats with peculiar vegetation and soil conditions. Previous observational studies showed that establishment of woody species at these sites is severely hindered for unknown reasons. To test the effects of charcoal hearth soil on tree growth we used a common garden experiment with three major Euro-Mediterranean forest trees with different traits and ecology, one evergreen (Quercus ilex, holm oak) and two deciduous (Fagus sylvatica, beech, and Quercus cerris, Turkey oak). These were sown on control and charcoal-enriched soil collected in forest hearths abandoned since decades. Seed germination, seedling growth, photosynthetic efficiency and mortality were measured over a period of two years. Some responses were species-specific, while others were possibly associated to key traits such as evergreen vs. deciduous habit. Although charcoal soil effects were mainly positive on growth rate (height increase), they were mostly negative on germination of beech seeds, survival of holm oak seedlings, and photosynthetic efficiency. Although total biomass was not significantly affected, the root:shoot ratio was increased as a possible effect of physiological drought on hearth soil. These results support field-based evidence that the long persistence of charcoal remains in the soil may be not a favourable condition for forest regeneration. Management implications concern the use of biochar practices to promote forest restoration, which should be further tested on a wide range of species in different life-stages before applications in the field, also considering its long-term consequences

The functional role of temperate forest understorey vegetation in a changing world

Temperate forests cover 16% of the global forest area. Within these forests, the understorey is an important biodiversity reservoir that can influence ecosystem processes and functions in multiple ways. However, we still lack a thorough understanding of the relative importance of the understorey for temperate forest functioning. As a result, understoreys are often ignored during assessments of forest functioning and changes thereof under global change. We here compiled studies that quantify the relative importance of the understorey for temperate forest functioning, focussing on litter production, nutrient cycling, evapotranspiration, tree regeneration, pollination and pathogen dynamics. We describe the mechanisms driving understorey functioning and develop a conceptual framework synthesizing possible effects of multiple global change drivers on understorey-mediated forest ecosystem functioning. Our review illustrates that the understorey's contribution to temperate forest functioning is significant but varies depending on the ecosystem function and the environmental context, and more importantly, the characteristics of the overstorey. To predict changes in understorey functioning and its relative importance for temperate forest functioning under global change, we argue that a simultaneous investigation of both overstorey and understorey functional responses to global change will be crucial. Our review shows that such studies are still very scarce, only available for a limited set of ecosystem functions and limited to quantification, providing little data to forecast functional responses to global change

Biotic predictors complement models of bat and bird responses to climate and tree diversity in European forests

Bats and birds are key providers of ecosystem services in forests. How climate and habitat jointly shape their communities is well studied, but whether biotic predictors from other trophic levels may improve bird and bat diversity models is less known, especially across large bioclimatic gradients. Here, we achieved multi-taxa surveys in 209 mature forests replicated in six European countries from Spain to Finland, to investigate the importance of biotic predictors (i.e., the abundance or activity of defoliating insects, spiders, earthworms and wild ungulates) for bat and bird taxonomic and functional diversity. We found that 9 out of 12 bird and bat diversity metrics were best explained when biotic factors were added to models including climate and habitat variables, with a mean gain in explained variance of 38% for birds and 15% for bats. Tree functional diversity was the most important habitat predictor for birds, while bats responded more to understorey structure. The best biotic predictors for birds were spider abundance and defoliating insect activity, while only bat functional evenness responded positively to insect activity. Accounting for potential biotic interactions between bats, birds and other taxa of lower trophic levels will help to understand how environmental changes along large biogeographical gradients affect higher-level predator diversity in forest ecosystems