Aims Understanding the linkage of soil respiration (Rs) with forest development is essential for long-term C cycle models. We estimated the variation and temperature sensitivity (Q 10 value) of Rs and its hetero-, (Rh) and autotrophic (Ra) components in relation to abiotic and biotic factors in Norway spruce stands of different ages, and the effect of trenching on microbial and soil characteristics. Methods Trenching method was used to partition Rs into Rh and Ra. Ingrowth core method was used to estimate fine root production. Soil microbial biomass was measured using manometric respirometers. Results Rs varied in differently aged stands demonstrating non-linear response to development stage. The variation of Rs was explained by changes in biotic factors rather than by changes in soil microclimate. Rh was more sensitive to Ts than Rs or Ra. After 4 years of trenching soil pH, N, SOM and dehydrogenase activity were significantly changed in trenched plots compared to control plots. Conclusions Different Q 10 values of Rh and Ra in stands of different ages indicate the importance of Rs partitioning. Trenching should be used during a limited number of years because of the possible changes in chemical characteristics of soil and in the activity of soil microbial community.

Light- and nitrogen-use change was examined along productivity gradients in natural grasslands at Laelatu, western Estonia, both at community level and in most abundant species. Aboveground biomass (M) ranged from 341 to 503 g m−2 in wet (W) and from 248 to 682 g m−2 in dry (D) community. Aboveground leaf area ratio (aLAR) decreased with rising M in D site, while it increased in W site. In a high-aLAR W community (significantly higher compared to D), adjustment of leaf morphology through an increase in specific leaf area is responsible for an increase in aLAR with rising productivity. In low-aLAR stand, by contrast, adjustment of biomass allocation due to decrease in aboveground leaf mass fraction is primarily responsible for the tendency of aLAR to decline. In conclusion, a decrease in aLAR is not a universal response to increasing M. We hypothesise that there exists an optimum of light acquisition efficiency (ΦM) along a productivity gradient independent of community type. Aboveground nitrogen-use efficiency (aNUE) decreased in high-aLAR, W community with increasing M, while in low-aLAR, D site, there was no relationship along a gradient, although aNUE increased along six plots dominated by graminoids. A trade-off was established between leaf nitrogen content per unit leaf area (N A) and aLAR.

We investigated changes in chlorophyll a fluorescence from alternate leaf surfaces to assess the intraleaf light acclimation patterns in combination with natural variations in radiation, leaf angles, leaf mass per area (LMA), chlorophyll content (Chl) and leaf optical parameters. Measurements were conducted on bottom- and top-layer leaves of Tilia cordata Mill. (a shade-tolerant sub-canopy species, sampled at heights of 11 and 16 m) and Populus tremula L. (a light-demanding upper canopy species, sampled at canopy heights of 19 and 26 m). The upper canopy species P. tremula had a six times higher PSII quantum yield (Φ(II)) and ratio of open reaction centres (qP), and a two times higher LMA than T. cordata. These species-specific differences were also present when the leaves of both species were in similar light conditions. Leaf adaxial/abaxial fluorescence ratio was significantly larger in the case of more horizontal leaves. Populus tremula (more vertical leaves), had smaller differences in fluorescence parameters between alternate leaf sides compared with T. cordata (more horizontal leaves). However, optical properties on alternate leaf sides showed a larger difference for P. tremula. Intraspecifically, the measured optical parameters were better correlated with LMA than with leaf Chl. Species-specific differences in leaf anatomy appear to enhance the photosynthetic potential of leaf biochemistry by decreasing the interception of excess light in P. tremula and increasing the light absorptance in T. cordata. Our results indicate that intraleaf light absorption gradient, described here as leaf adaxial/abaxial side ratio of chlorophyll a fluorescence, varies significantly with changes in leaf light environment in a multi-layer multi-species tree canopy. However, this variation cannot be described merely as a simple function of radiation, leaf angle, Chl or LMA, and species-specific differences in light acclimation strategies should also be considered.

Soil respiration resulting from microbial and root respiration is a major component of the forest carbon cycle. The response of soil respiration to varying environmental factors (soil temperature and soil moisture) was studied in a Norway spruce chronosequence composed of four age classes (4, 27, 36, and 84 year old) on Gleyic Podzol. Soil respiration was measured monthly with closed dynamic chamber system, soil temperature and soil moisture were measured simultaneously. Mean soil respiration rate averaged over three years was 3.3 μmol CO2 m-2s-1, ranging from 0.6 to 5.4 μmol CO2 m-2s-1, with the maximum occurring in August and the minimum in December. Stand age had a significant effect on soil respiration: the highest respiration rate was found in 27-year-old stand. Over three years an exponential relationship between soil respiration and soil temperature accounted for 68-81% of the seasonal variation, Q10 (the factor by which the respiration rate differs for a temperature interval of 10 °C) for the individual stands ranged between 4.4 and 5.4. The influence of soil moisture content on soil respiration was weak and revealed in dry conditions only. The results of this study can be used to help understand and predict the effect of harvest on soil respiration and how the respiration might respond to changing climate conditions.

The effect of elevated CO2 and O3 on apparent quantum yield (f), maximum photosynthesis (Pmax), carboxylation efficiency (Vcmax) and electron transport capacity (Jmax) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O3 tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO2 alone did not affect f or Pmax, and increased Jmax in the O3-sensitive, but not in the O3-tolerant clone. Elevated O3 decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O3 increased through time. Significant interaction effect, whereby the negative impact of elevated O3 was exaggerated by elevated CO2 was seen in Chl, N and Jmax, and occurred in both O3-tolerant and O3-sensitive clones. The clonal differences in the level of CO2 O3 interaction suggest a relationship between photosynthetic acclimation and background O3 concentration. 2009 Elsevier Ltd. All rights reserved.

A facility for free air humidity manipulation (FAHM) was established to investigate the effect of increased air humidity on trees' performance and their canopy functioning with respect to rising air humidity predicted for Northern Europe. The FAHM system enables air relative humidity (RH) to be increased up to 18 units (%) over the ambient level during mist fumigation, depending on the wind speed inside the experimental stand. Water was dispersed inside 14 x 14m experimental plots in the form of mist with an average particle size of 50 mu m from June to August in 2008, and from May to September in 2009. The average increase in RH was 7 units (%) over the whole period of humidification in 2008 (P<0.05). The average diurnal stem sap flux density per unit projected leaf area (F) in silver birch (Betula pendula Roth.) trees was 24.8% (P<0.05) and 27.2% (P<0.01) higher in control (C) plots compared to humidification (H) plots during misting in 2008 and 2009, respectively. However, the difference between C and H plots was statistically insignificant (P>0.05) in silver birch on the days without misting. In hybrid aspen (Populus tremula L x P. tremuloides Michx.) the average difference in F between C and H plots was 61.1% (P<0.001) during mist fumigation in the summer of 2009. Nevertheless, the difference was considerable (38.8%; P<0.001) also on the days without misting, reflecting the impact of plant inner factors on F as a result of long-term acclimation to fumigation. The leaves of silver birch in a humidified plot demonstrated up to 2.4 degrees C lower (P<0.05) leaf temperature (T-L) compared to the control plot in 2009. The decline in T-L decreased the humidity gradient between leaf and air by about 1/3, whereas 2/3 of the effect was caused directly by changes in air humidity in the leaf boundary layer. Our preliminary data suggest that the FAHM experimental facility enables water fluxes through a deciduous tree canopy to be reduced and this effect is attributable both to the increased air humidity and decreased leaf temperature. Changes in these two basic factors may create considerable differences in the physiology, anatomy and nutrition of a whole tree, also affecting forest functioning in the light of global climate change. (C) 2010 Elsevier BM. All rights reserved.

The aim of this study was to evaluate the use of ground-based canopy reflectance measurements to detect changes in physiology and structure of vegetation in response to experimental warming and drought treatment at six European shrublands located along a North–South climatic gradient. We measured canopy reflectance, effective green leaf area index (green LAIe) and chlorophyll fluorescence of dominant species. The treatment effects on green LAIe varied among sites. We calculated three reflectance indices: photochemical reflectance index PRI [531nm; 570nm], normalized difference vegetation index NDVI680 [780nm; 680nm] using red spectral region, and NDVI570 [780nm; 570nm] using the same green spectral region as PRI. All three reflectance indices were significantly related to green LAIe and were able to detect changes in shrubland vegetation among treatments. In general warming treatment increased PRI and drought treatment reduced NDVI values. The significant treatment effect on photochemical efficiency of plants detected with PRI could not be detected by fluorescence measurements. However, we found canopy level measured PRI to be very sensitive to soil reflectance properties especially in vegetation areas with low green LAIe. As both soil reflectance and LAI varied between northern and southern sites it is problematic to draw universal conclusions of climate-derived changes in all vegetation types based merely on PRI measurements. We propose that canopy level PRI measurements can be more useful in areas of dense vegetation and dark soils.

We examined the effect of elevated CO2 on photosynthesis and the allocation of leaf nitrogen to photosynthetic and non-photosynthetic pools in two poplar species (Populus alba, genotype 2AS11 and P. nigra, genotype can Pourtet) at the POP-EUROFACE site. In P. alba. the light-saturated photosynthetic rate at growth CO2 concentration (A(sat)) over two years was significantly higher (by 42%) in trees exposed to CO2 enrichment. In P. nigra, there was no significant difference, though on average A(sat) was 29% higher in the CO2 enrichment treatment. Stomatal conductance was significantly reduced by CO2 enrichment in both species: by 22% in P alba and by 18% in P nigra. Neither maximum carboxylation rate (V-max) nor maximum rate of electron transport (J(maxi) was reduced by CO2 enrichment. A change in the partitioning of leaf nitrogen between photosynthetic and non-photosynthetic pools was detected in P. nigra: partitioning into non-photosynthetic nitrogen increased by 15% in elevated CO2 conditions. No differences were detected between the sun and shade leaves of either studied species in terms of their photosynthetic responses to elevated CO,. The greater allocation of leaf nitrogen into the non-photosynthetic pool in elevated CO2 conditions displayed by P. nigra in comparison with P. alba may explain the non-significant photosynthetic stimulation in the former.

Ecosystems that develop on mine spoil can serve as significant sinks for CO2. The aim of this study was to estimate the rate of carbon accumulation and its distribution along forest ecosystem partitions in young Scots pine ( Pinus sylvestris L.) plantations in the Narva oil shale opencast, Estonia. The tree layer was measured in 2004 in 13 stands afforested with 2-year-old seedlings during 1968 to 1994. Three stands (afforested in 1990, 1983, and 1968) were selected for detailed analysis of the carbon sequestration. Soil profiles were sampled in these stands in 2005. Radiocarbon analysis combined with a simple model of litter production was used to differentiate between plant-derived recent carbon and carbon stemming from fragments of oil shale. Total carbon accumulated since afforestation in vegetation, forest floor, and A horizon was 7.8 t·ha–1in the stand established in 1990, 34.5 t·ha–1in that established in 1983, and 133.4 t·ha–1in that established in 1968. Most of the sequestered carbon was allocated to tree stems; their portion increasing with age from 28% to 51%. The portion of recent soil organic carbon increased from 5% to 23%, which shows that soils contribute significantly to carbon accumulation during early forest succession on degraded land.

Shrublands constitute significant and important parts of European landscapes providing a large number of important ecosystem services. Biogeochemical cycles in these ecosystems have gained little attention relative to forests and grassland systems, but data on such cycles are required for developing and testing ecosystem models. As climate change progresses, the potential feedback from terrestrial ecosystems to the atmosphere through changes in carbon stocks, carbon sequestration, and general knowledge on biogeochemical cycles becomes increasingly important. Here we present carbon and nitrogen balances of six shrublands along a climatic gradient across the European continent. The aim of the study was to provide a basis for assessing the range and variability in carbon storage in European shrublands. Across the sites the net carbon storage in the systems ranged from 1,163 g C m −2 to 18,546 g C m −2 , and the systems ranged from being net sinks (126 g C m −2 a −1 ) to being net sources (−536 g C m −2 a −1 ) of carbon with the largest storage and sink of carbon at wet and cold climatic conditions. The soil carbon store dominates the carbon budget at all sites and in particular at the site with a cold and wet climate where soil C constitutes 95% of the total carbon in the ecosystem. Respiration of carbon from the soil organic matter pool dominated the carbon loss at all sites while carbon loss from aboveground litter decomposition appeared less important. Total belowground carbon allocation was more than 5 times aboveground litterfall carbon which is significantly greater than the factor of 2 reported in a global analysis of forest data. Nitrogen storage was also dominated by the soil pools generally showing small losses except when atmospheric N input was high. The study shows that in the future a climate‐driven land cover change between grasslands and shrublands in Europe will likely lead to increased ecosystem C where shrublands are promoted and less where grasses are promoted. However, it also emphasizes that if feedbacks on the global carbon cycle are to be predicted it is critically important to quantify and understand belowground carbon allocation and processes as well as soil carbon pools, particularly on wet organic soils, rather than plant functional change as the soil stores dominate the overall budget and fluxes of carbon.

Needle nitrogen partitioning and photosynthesis of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current- and previous-year shoots were sampled from upper and lower canopy positions in four stands, ranging in age from 13 to 82 years. A/c i curves were determined to obtain maximum carboxylation rate (V cmax) and maximum rate of electron transport (J max), whereas needle nitrogen partitioning into carboxylation (P R), bioenergetics associated with electron transport (P B) and thylakoid light harvesting components (P L) was calculated from the values of V cmax, J max and leaf chlorophyll concentration. The greatest changes in studied needle characteristics took place between tree ages of 13 and 26 years, and this pattern was independent of needle age and canopy position. Needle mass per projected area (LMA) was lowest in the 13-year-old stand and mass-based nitrogen concentration (NM) was generally highest in that stand. The values of LMA were significantly higher and those of NM lower in the 26-year-old stand. Mass-based V cmax and J max were highest in the 13-year-old stand. Area-based photosynthetic capacity was independent of tree age. The proportion of photosynthetic nitrogen (P R, P B and P L) was highest and that of non-photosynthetic nitrogen lowest in the 13-year-old stand. Current-year needles had lower LMA and P L, but higher photosynthetic capacity compared to 1-year-old foliage. Needles from lower canopy positions exhibited lower LMA, area-based nitrogen concentration and photosynthetic capacity than needles from upper canopy. The period of substantial reductions in needle photosynthetic capacity and changes in nitrogen partitioning coincides with the onset of reproductive phase during tree ontogeny.

The effect of elevated CO2 and O3 on apparent quantum yield (ϕ), maximum photosynthesis (Pmax), carboxylation efficiency (Vcmax) and electron transport capacity (Jmax) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O3 tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO2 alone did not affect ϕ or Pmax, and increased Jmax in the O3-sensitive, but not in the O3-tolerant clone. Elevated O3 decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O3 increased through time. Significant interaction effect, whereby the negative impact of elevated O3 was exaggerated by elevated CO2 was seen in Chl, N and Jmax, and occurred in both O3-tolerant and O3-sensitive clones. The clonal differences in the level of CO2 × O3 interaction suggest a relationship between photosynthetic acclimation and background O3 concentration.

We studied acclimation patterns in leaf dry mass per area (MA), nitrogen (NA) and chlorophyll (ζA) content per area, and chlorophyll to nitrogen ratio (ζ/N) along vertical light gradients in natural temperate mixed herbaceous canopy and deciduous tree canopy. In the deciduous tree canopy, all leaves are formed at approximately the same time, and the light gradient during the rest of the growing season reflects the differences in light availability during leaf development, whereas in the herbaceous canopy, leaf production continues during most of the growing season and major changes in light conditions occur after leaf maturation. MA and NA increased strongly with increasing current light availability (ID) in the tree canopy. In the herbaceous canopy, MA and NA were generally unrelated to ID. Depending on species, the correlation between chlorophyll content per leaf area (ζA) and ID was positive, negative, or non-significant. Path analyses revealed two opposite effects of ID on the amount of leaf chlorophyll. In the tree canopy, increasing ID enhanced ζA through changes in MA and NA, whereas the direct effect of light was negative in both canopies. The overall correlation network between foliage structural and chemical traits and the relationships with ID were significantly stronger in the tree canopy, suggesting limited re-acclimation potential in the mixed herbaceous canopy. Within-species acclimation patterns reflected the patterns within the main functional types. These data demonstrate that the relationships of current light availability vs. leaf dry mass per area, leaf nitrogen and chlorophyll contents, and chlorophyll to nitrogen ratio differ among multi-species herbaceous canopies and deciduous tree canopies due to contrasting canopy development.

We Studied Light Acclimation Of Leaf Chlorophyll (Chl A) And Nitrogen (Na) Contents Per Unit Area, And Leaf Dry Mass Per Unit Area (Lma) Along A Vertical Light Gradient In Natural Herbaceous And Tree Canopies. We Found That Lma Increased Strongly With Increasing Irradiance In The Tree Canopy But Not In The Herbaceous Canopy, And That Chl A Was Much More Variable In The Herbaceous Canopy (Coefficient Of Variation 82%) Compared To The Tree Canopy (16%). This Finding Can Indicate That In The Herbaceous Canopy Mature Leaves Had Undergone Re-Acclimation To Changed Light Conditions, While In The Tree Canopy Measured Light Conditions Reflected The Light Gradient During Leaf Development. Path Analysis Revealed That The Acclimation Of Leaf Chlorophyll Content To Increased Light Availability Consists Of Two Opposing Responses, Which Operate In Different Time Scales: (I) An Increasing Effect, Which Is Mediated Via Changes In Lma And N A, And Is Therefore Related To Light Conditions During Leaf Development; (Ii) And A Decrease, Which Is A Rapid Photoprotective Response.

Stomatal density and size were measured along the light gradient of a Betula pendula Roth. canopy in relation to microclimatic conditions. The theoretical stomatal conductance was calculated using stomatal density and dimensions to predict to what degree stomatal conductance is related to anatomical properties and relative stomatal opening. Stomatal density was higher and leaf area smaller in the upper canopy, whereas epidermal cell density did not change significantly along the canopy light gradient, indicating that stomatal initiation is responsible for differences in stomatal density. Stomatal dimensions - the length of guard cell on the dorsal side and the guard cell width - decreased with declining light availability. Maximum measured stomatal conductance and modelled stomatal conductance were higher at the top of the crown. The stomata operate closer to their maximum openness and stomatal morphology is a more important determinant of stomatal conductance in the top leaves than in leaves of lower canopy. As stomata usually limit photosynthesis more in upper than in lower canopy, it was concluded that stomatal morphology can principally be important for photosynthesis limitation in upper canopy.

The long-term response of leaf photosynthesis to rising CO2 concentrations [CO2] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO2] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand.After 6 yr of exposure to free-air CO2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar (Populus) short-rotation coppice.Net photosynthetic rate (A(growth)) was 49% higher in poplars grown in elevated [CO2], independently of the canopy position. J(max) significantly increased (15%), whereas leaf carboxylation capacity (V-cmax), leaf nitrogen (N-a) and chlorophyll (Chl(a)) were unaffected in elevated [CO2]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown.These results suggest that the photosynthetic stimulation by elevated [CO2] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.

The quickly rising atmospheric carbon dioxide (CO2)-levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO2 increase. Here, we report the likely impact of future increases in atmospheric CO2 on woody biomass production of three poplar species (Populus alba L. clone 2AS-11, Populus nigra L. clone Jean Pourtet and Populusxeuramericana clone I-214). Trees were growing in a high-density coppice plantation during the second rotation (i.e., regrowth after coppice; 2002-2004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO2 (FACE; free air carbon dioxide enrichment of 550 ppm). Half of each plot was fertilized to study the interaction between CO2 and nutrient fertilization. At the end of the second rotation, selective above- and belowground harvests were performed to estimate the productivity of this bio-energy plantation. Fertilization did not affect growth of the poplar trees, which was likely because of the high rates of fertilization during the previous agricultural land use. In contrast, elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above- and belowground parts. The increased initial stump size resulting from elevated CO2 during the first rotation (1999-2001) could not solely explain the observed final biomass increase. The larger leaf area index after canopy closure and the absence of any major photosynthetic acclimation after 6 years of fumigation caused the sustained CO2-induced biomass increase after coppice. These results suggest that, under future CO2 concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of C-neutral energy.