Canopy cover effects on mass loss, and nitrogen and phosphorus dynamics from decomposing litter in oak and pine stands in northern Lower Michigan

Patterns of litter decomposition and nitrogen (N) and phosphorus (P) release in relation to various levels of canopy cover were examined using litterbags placed on the forest floor of northern red oak (Quercus rubra L.) and red pine (Pinus resinosa Ait.) stands in northern Lower Michigan, USA. A series of experimental plots consisted of four levels of canopy cover treatments, i.e. clearcut, 25% (50% during first sampling year), 75%, and uncut. Mass loss from decomposing leaves was higher for oak leaves in red oak stands (approximately 60% loss of the original mass) than for pine needles in red pine stands (approximately 40% loss of the original mass) during the 2 year study period. Leaf mass loss in the clearcut red oak treatment was significantly higher than in the uncut red oak treatment. In contrast, no canopy cover effects on litter mass loss were found in red pine stands. Nitrogen concentrations in decomposing litter increased during the 2 year period in all canopy cover treatments in both stand types, but they did not differ significantly among canopy cover treatments. These results indicate that various levels of red oak and red pine canopy removal generally have a minor impact on litter decomposition and nutrient (N and P) release during the first 2 years following canopy manipulation, except in red oak clearcuts.     

Overstory-specific effects of litter fall on the microbial carbon turnover in a mature deciduous forest

Mature deciduous forests can serve as important carbon (C) sinks, but the C storage differs significantly in dependency on the tree species. To specify the significance of overstory-specific effects of litter fall on the soil microbial C turnover, we have investigated the ¹³C isotopic signature of microbial biomarker phospholipid fatty acids (PLFAs). Samples were taken under pure Fagus sylvatica and mixed overstory (F. sylvatica and Fraxinus excelsior or F. excelsior, Acer spp. and F. sylvatica) in a mature temperate deciduous forest in Central Germany 4 weeks prior to and 3 weeks after litter fall. Accordingly, the CO₂ emission from soil was measured before, during and after the litter fall to investigate the response of decomposition. At all sites and at both sampling dates the fungal biomarker PLFA 18:2ω6,9 had predominantly lower δ¹³C values (from −32 to −43‰) than the bacterial biomarker PLFAs (δ¹³C values from −23 to −39‰). This difference indicated that fungi generally used preferentially plant derived C, whereas the bacterial populations include groups which used SOM derived C, independent on the overstory trees. Under pure F. sylvatica overstory the δ¹³C values of microbial biomarker PLFAs were slightly decreased (up to 2‰ for 17:0br) or unchanged after litter fall. By contrast, under both variants of mixed overstory the δ¹³C values of biomarker PLFAs of fungi (18:2ω6,9) were increased after litter fall (+3.5 and +3.8‰). This might be explained partly by a faster initial decomposition of foliar litter from mixed overstory already during litter fall as confirmed by higher CO₂ emission under mixed F. excelsior, Acer spp. and F. sylvatica than under pure F. sylvatica in this period. However, the involved microbial populations differed overstory-specific. Bacterial biomarker PLFAs with strongest overstory-specific differences in the response on litter fall were 17:0br (Gram-positive bacteria), 18:1 and 19:0cy (Gram-negative bacteria). The present results indicate that a tree species conversion even exclusively between deciduous tree species might alter the soil microbial C turnover during litter decomposition and suggest that it would in the long-term change the SOM stability and C storage.     

Effects of enhanced ultraviolet-B radiation on water use efficiency, stomatal conductance, leaf nitrogen content and morphological characteristics of Spiraea pubescens in a warm-temperate deciduous broad-leaved forest

Spiraea pubescens, a common shrub in the warm-temperate deciduous forest zone which is distributed in the Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280–320 nm) radiation by artificially supplying a daily dose of 9.4 kJ/m² for three growing seasons, a level that simulated a 17% depletion in stratospheric ozone. The objective of this study was to explore the effects of long-term UV-B enhancement on stomatal conductance, leaf tissue δ ¹³C, leaf water content, and leaf area. Particular attention was paid to the effects of UV-B radiation on water use efficiency (WUE) and leaf total nitrogen content. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%). These changes were associated with a decrease in stomatal conductance (16.1%) and intercellular CO₂ concentration/ air CO₂ concentration (C _i /C _a) (4.0%), and an increase in leaf tissue δ ¹³C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%) and WUE (4.1%). The effects of UV-B on the plant were greatly affected by the water content of the deep soil (30–40 cm). During the dry season, differences in the stomatal conductance, δ ¹³C, and WUE between the control and UV-B treated shrubs were very small; whereas, differences became much greater when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period progressed over the three growing seasons. Correlation analysis showed that enhanced UV-B radiation decreased the strength of the correlation between soil water content and leaf water content, δ ¹³C, C _i/C _a, stomatal conductance, with the exception of WUE that had a significant correlation coefficient with soil water content. These results suggest that WUE would become more sensitive to soil water variation due to UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on morphological traits and growth of S. pubescens than hydro-physiological characteristics. __________ Translated from Journal of Plant Ecology, 2006, 30(1): 47–56 [译自: 植物生态学报]     

Tree rings indicate different drought resistance of a native (Abies alba Mill.) and a nonnative (Picea abies (L.) Karst.) species co-occurring at a dry site in Southern Italy

Climate changes induced by the anthropogenic alteration of the atmospheric radiative balance are expected to change the productivity and composition of forest ecosystems. In Europe, the Mediterranean is considered one of the most vulnerable regions according to climatic forecasts and simulations. However, although modifications in the inter-specific competition are envisaged, we still lack a clear understanding of the ability of the Mediterranean vegetation to adapt to climate changes. We investigated how two co-occurring tree species commonly used in afforestation programmes, the native Abies alba Mill. and the nonnative Picea abies L. Karst., adapt to climate change by assessing their growth performance and physiological responses in relation to past climate variability. Growth was addressed by analysing tree-ring width and carbon and oxygen stable isotopes. Statistical relationships between isotopic value and monthly climate data suggest that the two species underwent ecophysiological adaptation to Mediterranean climatic constraints. These adaptations are also expressed in the ring-width data. Based on the carbon isotope ratio reflecting the stomatal response to drought, we found that the precipitation in the first period of the growing season, i.e. early spring, is a major factor influencing the annual growth of A. alba, which although native, proved to be sensitive to drought. P. abies, on the other hand, showed a higher tolerance to summer drought stress. These findings should help define criteria for sustainability and effective forest conservation in the Mediterranean region.