M. Mund

The influence of climate and fructification on the inter-annual variability of stem growth and net primary productivity in an old-growth, mixed beech forest

The periodic production of large seed crops by trees (masting) and its interaction with stem growth has long been the objective of tree physiology research. However, very little is known about the effects of masting on stem growth and total net primary productivity (NPP) at the stand scale. This study was conducted in an old-growth, mixed deciduous forest dominated by Fagus sylvatica (L.) and covers the period from 2003 to 2007, which comprised wet, dry and regular years as well as two masts of Fagus and one mast of the co-dominant tree species Fraxinus excelsior (L.) and Acer pseudoplatanus (L.). We combined analyses of weather conditions and stem growth at the tree level (inter- and intra-annual) with fruit, stem and leaf production, and estimates of total NPP at the stand level. Finally, we compared the annual demand of carbon for biomass production with net canopy assimilation (NCA), derived from eddy covariance flux measurements, chamber measurements and modelling. Annual stem growth of Fagus was most favoured by warm periods in spring and that of Fraxinus by high precipitation in June. For stem growth of Acer and for fruit production, no significant relationships with mean weather conditions were found. Intra-annual stem growth of all species was strongly reduced when the relative plant-available water in soil dropped below a threshold of about 60% between May and July. The inter-annual variations of NCA, total NPP and leaf NPP at the stand level were low (mean values 1313, 662 and 168 g C m(-2) year(-1), respectively), while wood and fruit production varied more and contrarily (wood: 169-241 g C m(-2) year(-1); fruits: 21-142 g C m(-2) year(-1)). In all years, an annual surplus of newly assimilated carbon was calculated (on average 100 g C m(-2) year(-1)). The results suggest that stem growth is generally not limited by insufficient carbon resources; only in mast years a short-term carbon shortage may occur in spring. In contrast to common assumption, stem growth alone is not a sufficient proxy for total biomass production or the control of carbon sequestration by weather extremes.

Linking plant nutrition and ecosystem processes

Mineral nutrients are a major part of all the physiological and biogeochemical processes in forest ecosystems. This is especially true for forests across Europe, which were deprived of nutrients due to intensive wood and litter use, and which experienced deposition of acids, nitrogen and sulphur over the second half of this century, resulting in significant nutrient imbalances for growth (Schulze 1989). Decreased nutrient availability can lead to a reduction of leaf size (Linder 1987), resulting in an almost instantaneous decrease in current year growth. In this way, the nutrient status of long-lived conifer needles might influence net primary production (NPP) long after a transient nutrient shortage, caused, e.g. by one dry season, has occurred. In natural forest ecosystems nutrient uptake from soil solution and nutrient release through litterfall and fine root turnover should balance each other such that the turnover time of nutrients within the system meets the requirements for stand growth (Gorham et al. 1979; Miller 1986; Attiwill and Adams 1993) and keeps the ecosystem nutrient cycle tight. Any deviation from this cycle due to anthropogenic influence (e.g. Vitousek et al. 1997) or natural disturbance (e.g. Foster et al. 1997) could alter one or more processes within the nutrient cycle with long-lasting effects on forest functioning.

Dissolved Organic Carbon from European Beech Logs: Patterns of Input to and Retention by Surface Soil

Abstract: The flux of dissolved organic carbon (DOC) from aboveground litter into the soil is generally considered an important pathway for carbon transport. However, the extent to which dead wood, a highly concentrated source of carbon (C), may contribute not only to this flux but also to the accumulation of soil organic carbon (SOC) is still unknown. Here, concentrations and fluxes of DOC in solution beneath 5 logs of Fagus sylvatica were quantified using tension lysimeters. Soil samples beneath and adjacent to an additional 18 logs were analyzed for SOC. Concentrations of stable C isotopes were determined in wood of logs, DOC, and SOC to follow the fate of C from logs to the soil. Mean DOC concentrations in soil solution beneath logs were highly variable and ranged between 11.6 ± 5.8 mg·L-1 (± SD) and 696 ± 654 mg·L-1, while beneath litter without logs the DOC concentrations had an average value of 10 ± 3 mg·L-1. Peak DOC concentrations beneath logs reached 4317 mg·L-1. At 0–20 cm soil depth, SOC concentrations and SOC pools beneath logs were not higher than for control soils. The difference in the composition of stable C isotopes between wood (-25.5 ± 1.0‰) and litter (-28.4 ± 0.2‰) was maintained in DOC and SOC beneath respective substrates. A calculated amount of 20.5 ± 13.6% of the original SOC within 0–20 cm mineral soil was exchanged over a period of 17 ± 8 y by C from logs. However, despite the increased DOC fluxes, SOC pools beneath logs did not increase.

Remotely-sensed detection of effects of extreme droughts on gross primary production.

Severe droughts strongly impact photosynthesis (GPP), and satellite imagery has yet to demonstrate its ability to detect drought effects. Especially changes in vegetation functioning when vegetation state remains unaltered (no browning or defoliation) pose a challenge to satellite-derived indicators. We evaluated the performance of different satellite indicators to detect strong drought effects on GPP in a beech forest in France (Hesse), where vegetation state remained largely unaffected while GPP decreased substantially. We compared the results with three additional sites: a Mediterranean holm oak forest (Puéchabon), a temperate beech forest (Hainich), and a semi-arid grassland (Bugacpuszta). In Hesse, a three-year reduction in GPP following drought was detected only by the Enhanced Vegetation Index (EVI). The Photochemical Reflectance Index (PRI) also detected this drought effect, but only after normalization for absorbed light. In Puéchabon normalized PRI outperformed the other indicators, while the short-term drought effect in Hainich was not detected by any tested indicator. In contrast, most indicators, but not PRI, captured the drought effects in Bugacpuszta. Hence, PRI improved detection of drought effects on GPP in forests and we propose that PRI normalized for absorbed light is considered in future algorithms to estimate GPP from space.

Species-specific and generic biomass equations for seedlings and saplings of European tree species

AbstractBiomass equations are a helpful tool to estimate the tree and stand biomass production and standing stock. Such estimations are of great interest for science but also of great importance for global reports on the carbon cycle and the global climate system. Even though there are various collections and generic meta-analyses available with biomass equations for mature trees, reports on biomass equations for juvenile trees (seedlings and saplings) are mainly missing. Against the background of an increasing amount of reforestation and afforestation projects and forests in young successional stages, such equations are required. In this study we have collected data from various studies on the aboveground woody biomass of 19 common tree species growing in Europe. The aim of this paper was to calculate species-specific biomass equations for the aboveground woody biomass of single trees in dependence of root-collar-diameter (RCD), height (H) and the combination of the two (RCD2 H). Next to calculating species-specific biomass equations for the species available in the dataset, we also calculated generic biomass equations for all broadleaved species and all conifer species. The biomass equations should be a contribution to the pool of published biomass equations, whereas the novelty is here that the equations were exclusively derived for young trees.

Silviculture and its interactions with biodiversity and the carbon balance of forest soils

It is well known that intensive forest management practices can have significant effects on the biogeochemistry and biodiversity of forest ecosystems. For example, planting and thinning affects the structural biodiversity. Planting of nursery trees also determines the species (including mycorrhizae) and genetic diversity. Fertilization changes the nutrient balance, and thus competitive interactions. Clear-cutting combined with intensive soil preparation causes soil erosion, soil compaction, and losses of soil organic carbon and cations, which in turn affects biodiversity (e.g., Heinsdorf and Krauß 1974; Bormann and Likens 1979; Covington 1981; Heinsdorf 1986; Black and Harden 1995; Apps and Price 1996; Nyland 1996; Jurgensen et al. 1997; Rollinger et al. 1998; Worrell and Hampson 1997; Prescott et al. 2000b; Quesnel and Curran 2000; Johnson and Curtis 2001; Block et al. 2002). However, our knowledge about the interactions of biodiversity with silviculture and site-specific factors and the role of biodiversity in biogeochemical cycles is still very limited. The Kyoto-Protocol (UN 1997) and the “Bonn agreement” (UN 2001), in particular, raised the question if and which forest management practices influence the carbon balance of forest ecosystems. It is evident that increased decomposition of dead organic matter after clear-cutting results in a net loss or a zero carbon balance of the forest ecosystem over about 5–6 years afterwards, even when successful regeneration occurs (Pypker and Fredeen 2002; Rannik et al. 2002). The time period of net carbon release can be prolonged to 14–20 years if growth of the regenerating stands is reduced or if large amounts of dead wood remain on site (e. g., Cohen et al. 1996; Schulze et al. 1999). Nevertheless, the relative contribution of decomposing dead wood, organic-layer material or soil organic matter (SOM) to the net ecosystem carbon balance is still unclear. Also, the mechanisms that could cause the large discrepancies observed between different case studies investigating the