Fredrik Lagergren

Transpiration response to soil moisture in pine and spruce trees in Sweden

Variation in transpiration and conductance between individual trees of Scots pine and Norway spruce was investigated in a mixed 50-year-old stand in central Sweden. Daily transpiration rates were measured by the tissue heat balance method on five trees of each species during a dry, warm growing season. Daytime averages of sapflow, climatic variables and soil water content were used to fit an empirical model of tree conductance for each tree. Conductance per unit needle area was about twice as high in pine as in spruce, while equal-sized trees transpired similarly in both species. Conductance generally decreased more steeply with increasing vapour pressure deficit and increased faster with increasing light in pine than in spruce, although one individual spruce behaved more like the pines. Inclusion of a linear or exponential function for air temperature improved the model for pine, but of the spruces, only one tree showed a clear temperature dependency. The response to decreasing soil water content varied widely; the spruces tended to be more sensitive to drought than the pines. When the drought was at its worst, no sapflow could be detected in some of the trees. On average, the reduction in transpiration began when ca. 80% of the extractable water in the rooting zone had been depleted.

Current Carbon Balance of the Forested Area in Sweden and its Sensitivity to Global Change as Simulated by Biome-BGC

Detailed information from the Swedish National Forest Inventory was used to simulate the carbon balance for Sweden by the process-based model Biome-BGC. A few shortcomings of the model were identified and solutions to those are proposed and also used in the simulations. The model was calibrated against CO2 flux data from 3 forests in central Sweden and then applied to the whole country divided into 30 districts and 4 age classes. Gross primary production (GPP) ranged over districts and age classes from 0.20 to 1.71 kg C m−2 y−1 and net ecosystem production (NEP) ranged from −0.01 to 0.44. The 10- to 30-year age class was the strongest carbon sink because of its relatively low respiration rates. When the simulation results were scaled up to the whole country, GPP and NEP were 175 and 29 Mton C y−1, respectively, for the 22.7 Mha of forests in Sweden. A climate change scenario was simulated by assuming a 4°C increase in temperature and a doubling of the CO2 concentration; GPP and NEP then increased to 253 and 48 Mton C y−1, respectively. A sensitivity analysis showed that at present CO2 concentrations NEP would peak at an increase of 5°C for the mean annual temperature. At higher CO2 levels NEP showed a logarithmic increase.

Forest management facing climate change : an ecosystem model analysis of adaptation strategies

To adapt to climate change, forest managers request information on management options for obtaining environmental, societal and economic goals. In this study, we assess the potential of adaptive forest management to influence the productivity and storm sensitivity of nemoral and boreal forest. The forest growth across Sweden over the 21st century was simulated by the ecosystem model LPJ-GUESS, comparing four management options: 1) default forest management, 2) shorter rotation period 3) increased fraction of broadleaved trees and 4) continuous cover forestry. The simulations indicated that a management strategy implemented by a majority of forest owners can have a large-scale effect on the standing volume and risk taking. The modelled risk of storm damage, expressed as the combined effect of tree properties, ground frost and wind load, was higher in the southern than in the northern part of the country due to latitudinal variations in all three components. We conclude that whereas the probability of a significant volume loss increase with the age of a forest, the calculated economic loss can be as high in young and mid-age forest stands. To reduce the risk of storm damage and fulfil a variety of management goals, a portfolio of adaptation strategies is needed. It should include active measures such as tree-species mixtures to spread the risks and shorter rotation periods of highly exposed stands, as well as reactive measures such as salvage and sanitary cutting to reduce the risk of subsequent spruce bark beetle outbreaks.

Does canopy mean nitrogen concentration explain variation in canopy light use efficiency across 14 contrasting forest sites

The maximum light use efficiency (LUE = gross primary production (GPP)/absorbed photosynthetic photon flux density (aPPFD)) of plant canopies has been reported to vary spatially and some of this variation has previously been attributed to plant species differences. The canopy nitrogen concentration [N] can potentially explain some of this spatial variation. However, the current paradigm of the N-effect on photosynthesis is largely based on the relationship between photosynthetic capacity (A(max)) and [N], i.e., the effects of [N] on photosynthesis rates appear under high PPFD. A maximum LUE-[N] relationship, if it existed, would influence photosynthesis in the whole range of PPFD. We estimated maximum LUE for 14 eddy-covariance forest sites, examined its [N] dependency and investigated how the [N]-maximum LUE dependency could be incorporated into a GPP model. In the model, maximum LUE corresponds to LUE under optimal environmental conditions before light saturation takes place (the slope of GPP vs. PPFD under low PPFD). Maximum LUE was higher in deciduous/mixed than in coniferous sites, and correlated significantly with canopy mean [N]. Correlations between maximum LUE and canopy [N] existed regardless of daily PPFD, although we expected the correlation to disappear under low PPFD when LUE was also highest. Despite these correlations, including [N] in the model of GPP only marginally decreased the root mean squared error. Our results suggest that maximum LUE correlates linearly with canopy [N], but that a larger body of data is required before we can include this relationship into a GPP model. Gross primary production will therefore positively correlate with [N] already at low PPFD, and not only at high PPFD as is suggested by the prevailing paradigm of leaf-level A(max)-[N] relationships. This finding has consequences for modelling GPP driven by temporal changes or spatial variation in canopy [N].

Winter respiratory C losses provide explanatory power for net ecosystem productivity

M. Haeni, R. Zweifel, W. Eugster, A. Gessler, S. Zielis, C. Bernhofer, A. 7 Carrara, T. Grünwald, K. Havránková, B. Heinesch, M. Herbst, A. Ibrom, A. 8 Knohl, F. Lagergren, B.E. Law, M. Marek, G. Matteucci, JH. McCaughey, S. 9 Minerbi, L. Montagnani, E. Moors, J. Olejnik, M. Pavelka, K. Pilegaard, G. 10 Pita, A. Rodrigues, M. J. Sanz Sánchez, M.-J. Schelhaas, M. Urbaniak, R. 11 Valentini, A. Varlagin, T. Vesala, C. Vincke, J. Wu, and N. Buchmann 12