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Process-based models are increasingly applied for simulating long-term forest developments in order to capture climate change impacts and to investigate suitable management responses. Regarding dimensional development, however, allometric relations such as the height/diameter ratio, branch and coarse root fractions or the dependency of crown dimension on stem diameter often do not account for environmental influences. While this may be appropriate for even-aged, monospecific forests, serious biases can be expected if stand density or forest structure changes rapidly. Such events occur in particular when forests experience disturbances such as intensive thinning or during early development stages of planted or naturally regenerated trees. We therefore suggest a calculation of allometric relationships that depends primarily on neighbourhood competition. Respective equations have been implemented into a physiology-based ecosystem model that considers asymmetric competition by explicit simulation of resource acquisition and depletion per canopy layer. The new implementation has been tested at two sites in Germany where beech (Fagus sylvatica) saplings have either been planted below a shelterwood of old spruces (Picea abies) or grown under clear-cut conditions. We show that the modified model is able to realistically describe tree development in response to stand density changes and is able to represent regeneration growth beneath a gradually decreasing overstorey of mature trees. In particular, the model could represent the faster crown size development in saplings until full ground coverage is established and a faster height growth afterwards. The effect enhances leaf area and thus assimilation per tree and increases carbon availability for stem growth at early development stages. Finally, the necessity to consider dynamic allometric relations with respect to climate change impacts is discussed, and further improvements are suggested.

Regarding the low availability of phosphorus in soil, tree internal cycling of phosphorus through re-translocation among needles would be a good strategy for conifers to cope with soil phosphorus deficiency and to support new needles in annual growth. Therefore, the relationship between the amount of plant-available phosphorus in the soil and the differences in concentrations of phosphorus among first- and second-year needles (P-difference Ny1 − Ny2) of adult Norway spruce was examined. No significant correlation could be detected between the stocks of available phosphorus extracted using citric acid and P-difference Ny1 − Ny2, even for trees with deficient nutritional status. The temporal variations of P-difference Ny1 − Ny2 at single plots showed the same order of magnitude as the variability between plots. The typically lower concentrations of phosphorus in second-year needles result mainly from an increase in needle weight of older needles. The net phosphorus re-translocation into younger needles appears to be of minor importance.

In the past few decades, high nitrogen emissions have reversed the nitrogen limitation of trees. Meanwhile, phosphorus (P) appears as a primary limiting nutrient of many forests in Europe. However, a simple and economic standard method for the determination of the plant-available phosphorus in forest soils, which drives the P supply of the trees, is not yet available. Therefore, we tested two soil extraction methods [citric-acid extract (Pcit), according to Hoffmann et al. (1991) and sodium bicarbonate extract (PHCO3), referring to Olsen et al. (1954)] for analyzing all Bavarian soil samples of the second German National Forest Soil Inventory (NFSI), resulting in a big data set with 342 forest sites and 2232 samples. We combined these results with the standardly gathered soil parameters and tested them as predictors for P nutrition indicated by the foliar P concentrations of the four most important tree species: Norway spruce (Picea abies), European beech (Fagus sylvatica), Scots pine (Pinus sylvestris) and pedunculate/sessile oak (Quercus robur/petraea). During the statistical process, we stepwise reduced the complexity of the prediction models. In our analyses, we achieved the best prediction of foliar P using soil information from the organic layer and mineral soil down to depths of 10 cm and 20 cm. Pcit stocks provided by far the best predictions in nearly all cases. Based on this parameter, we derived parametric regression functions to estimate the P nutrition (spruce: R2 = 0.52∗∗∗; beech: R2 = 0.49∗∗∗; pine: R2 = 0.39∗∗∗; oak: R2 = 0.35∗∗∗). Based on logistic regressions, threshold values were deduced for Pcit stocks down to 10 cm soil depth (spruce: 33 kg ha−1; beech: 43 kg ha−1; pine: 32 kg ha−1; oak: 41 kg ha−1), above which P deficiency becomes unlikely. Since these models are based on a large number of sites with diverse geologies and soil conditions, they can be used to estimate the P availability of most forests in Bavaria. Furthermore, the applied methods can be extended to other soil inventories. Estimated P availability based on such functions and thresholds can support sustainable nutrient management of temperate forests.

The climatic drought limit of common tree species depend on soil nutrient status In forest ecology, there is a huge experience in the ecological amplitude of tree species, which is commonly represented in so-called ecograms. However, the ecogram axes are purely qualitatively described and scaled. In the study presented here, we try to specify the drought limit in the ecogram for the ten most abundant and economically important tree species in Germany (Norway spruce, Scots pine, silver fir, European beech, pedunculate oak, sessile oak, sycamore maple, European elm, hornbeam and common ash) using generalized regression models. We use data on the occurrence of the tree species and on the soil of about 3300 plots of the second German forest soil condition survey (BZW II), combined with data from the ICP Forests Level I program (BioDiv and BioSoil) from 13 neighboring European countries as well as climate data from WorldClim. The focus is on the question to what extent the drought limit of the tree species depends on the soil nutrient status. As a predictor of nutrient status, we chose the base saturation type (BT) already introduced in practice. The warmth factor was included as Growing Degree Days, the water balance as climatic water balance in the forest vegetation period (KWB). The drought limit could be specified for all tree species except elm and pedunculate oak. It was found that not only in known nutrient-sensitive tree species (sycamore, ash, hornbeam), the drought limit depends on the BT, but also in beech and spruce. In the case of sycamore as an example of a nutrient-sensitive species, the dry climate limit on strongly acidic soils (BT 5) is already reached at a KWB of 20 l/m2, on base-rich (BT 1 and 0) only at a KWB of −150 l/m2. In beech, too, the drought limit on acidic soils has been reached more quickly than on base-rich sites (BT 5: −40 l/m2, BT 0: −120 l/m2). In contrast, the reaction of spruce is reversed (BT 5: −50 l/m2, BT 0: 10 l/m2). For pine, sessile oak and fir, the drought limit is independent of the base type and is −230 l/m2 (Scots pine), −150 l/m2 (sessile oak) and 0 l/m2 (silver fir). The drought limit specified by BT is a helpful quantity, especially in view of climate change, as it makes it possible to better estimate the potential of the different tree species.

Fungus-farming within galleries in the xylem of trees has evolved independently in at least twelve lineages of weevils (Curculionidae: Scolytinae, Platypodinae) and one lineage of ship-timber beetles (Lymexylidae). Jointly these are termed ambrosia beetles because they actively cultivate nutritional “ambrosia fungi” as their main source of food. The beetles are obligately dependent on their ambrosia fungi as they provide them a broad range of essential nutrients ensuring their survival in an extremely nutrient-poor environment. While xylem is rich in carbon (C) and hydrogen (H), various elements essential for fungal and beetle growth, such as nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), and manganese (Mn) are extremely low in concentration. Currently it remains untested how both ambrosia beetles and their fungi meet their nutritional requirements in this habitat. Here, we aimed to determine for the first time if galleries of ambrosia beetles are generally enriched with elements that are rare in uncolonized xylem tissue and whether these nutrients are translocated to the galleries from the xylem by the fungal associates. To do so, we examined natural galleries of three ambrosia beetle species from three independently evolved farming lineages, Xyleborinus saxesenii (Scolytinae: Xyleborini), Trypodendron lineatum (Scolytinae: Xyloterini) and Elateroides dermestoides (Lymexylidae), that cultivate unrelated ambrosia fungi in the ascomycete orders Ophiostomatales, Microascales, and Saccharomycetales, respectively. Several elements, in particular Ca, N, P, K, Mg, Mn, and S, were present in high concentrations within the beetles’ galleries but available in only very low concentrations in the surrounding xylem. The concentration of elements was generally highest with X. saxesenii, followed by T. lineatum and E. dermestoides, which positively correlates with the degree of sociality and productivity of brood per gallery. We propose that the ambrosia fungal mutualists are translocating essential elements through their hyphae from the xylem to fruiting structures they form on gallery walls. Moreover, the extremely strong enrichment observed suggests recycling of these elements from the feces of the insects, where bacteria and yeasts might play a role.