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Tree regeneration is a key demographic process influencing long‐term forest dynamics. It is driven by climate, disturbances, biotic factors and their interactions. Thus, predictions of tree regeneration are challenging due to complex feedbacks along the wide climatic gradients covered by most tree species. The stress gradient hypothesis (SGH) provides a framework for assessing such feedbacks across species ranges, suggesting that competition between trees is more frequent under favourable conditions, whereas reduced competition (i.e. positive interactions) is more likely under climatic stress. Moreover, tree life‐history strategies (LHS) may shed light on how and whether the SGH explains regeneration of different tree species.To address these topics, we developed statistical models based on >50,000 recruitment events observed for 24 tree species in an extensive permanent plot network (6540 plots from 299 unmanaged European temperate, boreal and subalpine forests) covering a wide climatic gradient.We found that the effects of Leaf Area Index (as a proxy for competition) on tree recruitment changed along climatic gradients but in a species‐specific manner. Competition predominates, with its intensity decreasing under stressful conditions for most species, as predicted by the SGH. However, positive interactions were only evident for a few species. Additionally, the ability of the SGH to explain patterns of competition and positive interactions across the gradients differed among species, with some differences and exceptions that may be related to varying LHS.Synthesis. Our study shows that competition between trees toward climatic stress decreases systematically but depends on species stress tolerance to climate and shade. These findings explain within‐ and between‐species differences in tree recruitment patterns in European temperate forests. Moreover, our findings imply that projections of forest dynamics along wide climatic gradients and under climate change must accommodate both competition and positive interactions, as they strongly affect rates of community turnover. [ABSTRACT FROM AUTHOR]

Soils and forest ecosystems vary predictably along a 145-km transect in northern Lower Michigan. In the east, Entisols support open jack pine stands. In the central transect, weak Spodosols have formed under oak-pine-aspen forests. In the Lake Michigan snowbelt on the west, strongly developed Spodosols occur beneath mesic northern hardwoods. We hypothesized that increasing amounts of snowfall, coupled with decreasing fire frequencies, promote soil development and enhance soil C dynamics at western sites. We also hypothesized that enhanced soil development facilitated greater proportions of broadleaf tree establishment, which in turn accelerates snowmelt rates and further facilitates soil development by enhancing deeper C translocation. Along the transect, we described, sampled, and characterized twelve soils. Soil development increases east to west along the transect, changing most rapidly at the inner margins of the snowbelt, near the coniferous-broadleaf forest ecotone. Coincident with strong soil development in the snowbelt is an increase in soil C storage and cycling. Depth profiles of C, 13C, and Fe- and Al-humus complexes all suggest that snowmelt percolation drives these patterns. Hardwoods produce and cycle more C than coniferous stands to the east and have thicker snowpacks. In the snowbelt, late-lying snowpacks limit spring fires, and large pulses of snowmelt water drive the fresh, soluble C from O horizons deeper, enhancing soil development and fostering ecosystem productivity. Although the current snowbelt, climate, and fire patterns across the peninsula might date only to ≈7,000 cal yr BP, they have nonetheless affected pedogenesis to the point that a major Entisol-to-Spodosol continuum has formed. [ABSTRACT FROM AUTHOR]

Abstract: Land degradation as a result of wind (aeolian processes) is one of the most significant causes of soil loss in northern Iran. In order to gain a broader understanding of the specific effects of aeolian disturbance on soils in this region, research was conducted to evaluate soil microbial respiration and nitrogen mineralization in places where differences in microtopography occur due to the uprooting of trees by windthrow events, thereby causing variations in microtopography hereby referred to as “pit–mound positions”. A twenty-hectare study site situated within the Tarbiat Modares University Experimental Forest Station located in the Mazandaran province of northern Iran was chosen for this research, with thirty-four uprooted trees selected for detailed study. A classification of five microsites was developed to distinguish differences in microtopography in the immediate vicinity of these uprooted trees: mound top, mound wall, pit bottom, pit wall and closed canopy. Soil was sampled in the 0–15, 15–30 and 30–45cm depths at all microsites using a core soil sampler with an 81cm2 cross section. Soil reaction, organic carbon, total nitrogen, carbon-to-nitrogen ratio, moisture, abundance and biomass of earthworms, soil microbial respiration and net N mineralization for all samples were measured in the laboratory. Statistical comparisons revealed that the highest soil microbial respiration and net N-mineralization occurred in pit bottoms. Mound tops showed the lowest levels of soil microbial respiration and net N mineralization. Measurements of soil microbial respiration and net N mineralization were shown to decrease in relation to increased soil depth, with significant differences depending on soil depth. Indications of aeolian processes indicate that windthrow events create different microsites, thereby influencing the specific micro-scale soil characteristics. These micro-scale characteristics should be taken into account when performing forest soils surveys and to inform forest management practices. [Copyright &y& Elsevier]