Long-term standardized forest phenology in Sweden: a climate change indicator.

Because climate change alters patterns of vegetative growth, long-term phenological measurements and observations can provide important data for analyzing its impact. Phenological assessments are usually made as records of calendar dates when specific phase changes occur. Such assessments have benefits and are used in Citizen Science monitoring. However, these kinds of data often have low statistical precision when describing gradual changes. Frequent monitoring of the phenological traits of forest trees and berries as they undergo gradual change is needed to acquire good temporal resolution of transitions relative to other factors, such as susceptibility to frosts, insects, and fungi, and the use of berries as a food resource. Intensive weekly monitoring of the growth of apical and branch buds and the elongation of shoots and leaves on four tree species, and the abundance of flowers and berries of bilberry and lingonberry, has been performed in Sweden since 2006. Here, we present quantitative methods for interpolating such data, which detail the gradual changes between assessments in order to describe average rates of development and amount of interannual variation. Our analysis has shown the active growth period of trees to differ with latitude. We also observed a change in the timing of the maximum numbers of ripening berries and their successive decline. Data from tree phenology assessments can be used to recommend best forestry practice and to model tree growth, while berry data can be used to estimate when food resources for animals are most available.

Limited vertical CO2 transport in stems of mature boreal Pinus sylvestris trees.

Several studies have suggested that CO2 transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused 13C-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO2 efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue 13C enrichment was also evaluated at these positions. Most of the 13C label was lost by diffusion within a few metres of the infusion point indicating rapid CO2 loss during vertical xylem transport. No 13C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO2 diffused radially to the atmosphere. Our results support the notion that xylem CO2 transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO2 derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

The Net Landscape Carbon Balance-Integrating terrestrial and aquatic carbon fluxes in a managed boreal forest landscape in Sweden.

The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO2 ), methane (CH4 ), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO2 , CH4 , DOC, DIC, and harvested C obtained from tall-tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2 ) in Sweden to estimate the NLCB across the land-water-atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m-2  year-1 ) with the landscape-atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH4 , the landscape was a GHG sink of 237 g CO2 -eq m-2  year-1 , thus providing a climate-cooling effect. The CH4 flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear-cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall-tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change.

Photosynthetic refixation varies along the stem and reduces CO2 efflux in mature boreal Pinus sylvestris trees.

Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23-27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.

Ecophysiological variation of transpiration of pine forests: synthesis of new and published results.

Canopy transpiration (EC ) is a large fraction of evapotranspiration, integrating physical and biological processes within the energy, water, and carbon cycles of forests. Quantifying EC is of both scientific and practical importance, providing information relevant to questions ranging from energy partitioning to ecosystem services, such as primary productivity and water yield. We estimated EC of four pine stands differing in age and growing on sandy soils. The stands consisted of two wide-ranging conifer species: Pinus taeda and Pinus sylvestris, in temperate and boreal zones, respectively. Combining results from these and published studies on all soil types, we derived an approach to estimate daily EC of pine forests, representing a wide range of conditions from 35° S to 64° N latitude. During the growing season and under moist soils, maximum daily EC (ECm ) at day-length normalized vapor pressure deficit of 1 kPa (ECm-ref ) increased by 0.55 ± 0.02 (mean ± SE) mm/d for each unit increase of leaf area index (L) up to L = ~5, showing no sign of saturation within this range of quickly rising mutual shading. The initial rise of ECm with atmospheric demand was linearly related to ECm-ref . Both relations were unaffected by soil type. Consistent with theoretical prediction, daily EC was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. Our finding facilitates the estimation of daily EC of wide-ranging pine forests using remotely sensed L and meteorological data. We advocate an assembly of worldwide sap flux database for further evaluation of this approach.

Species-specific activation time-lags can explain habitat restrictions in hydrophilic lichens.

Photosystem II (PSII) activation after hydration with water or humid air was measured in four hydrophilic and a generalist lichen to test the hypothesis that slow activation might explain habitat restriction in the former group. For the hydrophilic species, activation was after 4 h nearly completed in Lobaria amplissima and Platismatia norvegica, while only c. 50% for Bryoria bicolor and Usnea longissima. The generalist Platismatia glauca was activated instantaneously. The effect of this on lichen field performance was investigated using a dynamic model separating the two water sources rain and humid air. Model simulations were made using the species-specific characteristics and climate data from 12 stream microhabitats. For U. longissima, slow PSII activation could reduce realized photosynthesis by a factor of five. Bryoria bicolor was almost as severely affected, while P. norvegica displayed moderate reductions. Lobaria amplissima displayed longer realized activity periods even in unfavourable microclimates, possibly because of a higher water loss resistance. Both close proximity to streams and presence of turbulent water had a positive impact on realized activity among the slowly activated species, coinciding with observed distribution patterns of hydrophilic species. The results presented here may thus partly explain observed habitat restrictions of rare hydrophilic lichens.

Modelling hydration and photosystem II activation in relation to in situ rain and humidity patterns: a tool to compare performance of rare and generalist epiphytic lichens.

A dynamic water and activity model was developed to assess how efficiently lichens can exploit in situ rain and humid air. The capacity to rehydrate and activate photosynthesis [i.e. photosystem II (PSII)] by these water sources was compared among four hydrophilic and one generalist epiphytic lichen. Hydration status, potential (instant activation) and realized (delayed activation) day-light activity were simulated using a model based on species-specific hydration, PSII activation characteristics and in situ water content for Platismatia norvegica in three microclimatic scenarios. The results showed that delayed PSII activation could have profound effects on lichens' ability to exploit environmental water sources. During rain, realized activity was reduced by 19, 34 and 56% compared to simulations assuming instant activation for three hydrophilic lichens in the driest microclimate. During humid air, the reduction was 81% for the most extreme species and scenario, because of slow hydration and low equilibrium water content. Many and brief hydration events may thus hamper species with slow activation and fast desiccation kinetics. No evidence of compensation by a 'water-holding' morphology was observed among studied species. The developed model may provide a tool for identifying suitable habitats for long-term persistence of lichens with physiological constraints.

Fractional contributions by autotrophic and heterotrophic respiration to soil-surface CO2 efflux in Boreal forests.

Soil-surface CO2 efflux ('soil respiration') accounts for roughly two-thirds of forest ecosystem respiration, and can be divided into heterotrophic and autotrophic components. Conventionally, the latter is defined as respiration by plant roots. In Boreal forests, however, fine roots of trees are invariably covered by ectomycorrhizal fungi, which by definition are heterotrophs, but like the roots, receive sugars derived from photosynthesis. There is also a significant leaching of labile carbon compounds from the ectomycorrhizal roots. It is, therefore, more meaningful in the context of carbon balance studies to include mycorrhizal fungi and other mycorrhizosphere organisms, dependent on the direct flux of labile carbon from photosynthesis, in the autotrophic component. Hence, heterotrophic activity becomes reserved for the decomposition of more complex organic molecules in litter and other forms of soil organic matter. In reality, the complex situation is perhaps best described as a continuum from strict autotrophy to strict heterotrophy. As a result of this, and associated methodological problems, estimates of the contribution of autotrophic respiration to total soil respiration have been highly variable. Based on recent stand-scale tree girdling experiments we have estimated that autotrophic respiration in boreal forest accounts for up to 50-65% of soil respiration during the snow-free part of the year. Girdling experiments and studies of the delta(13)C of the soil CO2 efflux show that there is a lag of a few days between the carbon uptake by photosynthesis and the release by autotrophic soil respiration of the assimilated carbon. In contrast, estimates of 'bomb 14C' and other approaches have suggested that it takes years to decades between carbon uptake via photosynthesis and the bulk of soil heterotrophic activity. Temperature is normally used as a driver in models of soil processes and it is often assumed that autotrophic soil activity is more sensitive to temperature than is heterotrophic activity, but this is questionable. It is inherently difficult to make a precise separation of autotrophic and heterotrophic respiration from soils. The partitioning between these two components is highly variable in space and time, and taxonomic autotrophs and heterotrophs may perform the function of the other group to some degree. Care should be taken to disturb as little as possible the delicate plant-microbe-soil system, and this speaks for non-intrusive isotopic methods. There are, however, problems in modelling the flux of isotopes through this complex system. Girdling of tree stands is a very robust alternative approach to make the distinction between autotrophic and heterotrophic activities, but ultimately kills the trees and cannot, therefore, always be used. A further development would be to block the phloem sugar transport reversibly. We propose that thus assumption needs further critical testing.