An increasing number of studies provide evidence that mixed-species stands can overyield monocultures. But it is still hardly understood, how the overyielding at the stand level emerges from the tree, canopy, and size structure. Analyses of 42 triplets with 126 mixed and mono-specific plots in middle-aged, two-species stands of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies [L.] Karst.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and European beech (Fagus sylvativa L.) in Central Europe revealed that mixed-species compared with mono-specific stands can have (1) higher tree numbers, higher right skewness and kurtosis of the size distribution, higher inequality of tree sizes, and thereby higher stocking densities and sum of crown projection areas, (2) growth–size relationships with stronger size asymmetry of growth and higher inequality of size growth, and (3) higher stand productivity coupled with higher maximum stand density, canopy space filling, and size asymmetry. These differences depend on the species assemblage. They suggest a deeper entrance of light into the canopy as well as a higher light interception and light-use efficiency as main causes of the overyielding and overdensity. We discuss implications for research and silviculture and draw conclusions for designing and managing resource-efficient production systems.

https://waldwachstum.wzw.tum.de/fileadmin/publications/Pretzsch_2015_Size_structure.pdf

Effect of tree species mixing on the size structure, density, and yield of forest stands

Mixture reduces climate sensitivity of Douglas-fir stem growth

The mixture of Douglas-fir and European beech produced more biomass compared to what would have been expected from a weighted average of pure stands Overyielding of the mixed stands improved with increasing stand age and under better site conditions
 The mixture of Douglas-fir and European beech has the intrinsic potential to be one of the most productive forest types in Central Europe The study investigated how the structure and productivity of mixed stands changed in comparison to pure ones It analyzed whether there is overyielding in mixed stands and if it was modified due to stand development or along an ecological gradient Throughout Germany, 18 research plot triplets with 1987 trees were established in seven different ecological regions from dry to moist site conditions at ages 30 to 120 years To investigate the growth of the stands, tree cores were collected from 1293 stems The study revealed significant overyielding of biomass in mixed stands of 6 % or 081 Mg ha−1 year−1 It was found that: (i) Overyielding in mixed stands was driven by an increase in Douglas-fir growth (ii) Both species modified their morphology in mixture Compared to the species in pure stands, Douglas-fir diameters in mixed stands were significantly larger, whereas European beech had a smaller diameter at breast height in the mixture The effect increased with the age (iii) The analyses revealed more pronounced overyielding in older stands and on better sites The findings show that overyielding of Douglas-fir and European beech in mixed stands results from a higher light interception by complementary space occupation

/pdf/improved-productivity-and-modified-tree-morphology-of-mixed-1i2i6wsaby.pdf

Improved productivity and modified tree morphology of mixed versus pure stands of European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) with increasing precipitation and age

Mixed mountain forests, primarily made up of Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), cover about 10 × 106 ha of submontane–subalpine altitudes in Europe. They provide invaluable ecosystem services, e.g. protection against avalanches, landslides or rockfall. However, pure Norway spruce stands have, since mediaeval times, been heavily promoted as productive stand types for salt works at sites naturally supporting mixed mountain forests. Damage to these secondary pure spruce stands has been steadily increasing in recent decades. Furthermore, due to their previous limitation due to low temperatures and a short growing season, forest ecosystems in higher elevations are expected to be strongly affected by climate warming. To address these problems, alternative management concepts are being intensively discussed. A possible option to improve the stability and resilience of the stand is the transformation from pure Norway spruce stands into site-appropriate, sustainable and stable mixed mountain forests. In this study, we have tested seven different transformation scenarios (e.g. slit, shelterwood and gap-coupes, strip clear-cutting, do-nothing) and their impact on five evaluation criteria (forest growth, economics, carbon sequestration, (stand) stability and biodiversity). As there are hardly any practical examples for some of the transformation scenarios available, we have used the forest growth simulator SILVA to assess whether the tested transformation scenarios differ in transformation success and to observe trade-offs between the criteria of evaluation. Of the investigated scenarios, we consider the ones with gap or slit-coupes with the most beneficial overall utility values for the portfolio of the five evaluation criteria. However, we showed with our results that it is possible, by means of several trajectories, to return destabilised forests to sustainable and stable systems. We showed that a transformation is realistic, even if sophisticated silvicultural concepts are not strictly pursued.

/pdf/assessing-transformation-scenarios-from-pure-norway-spruce-27z5uz38nx.pdf

Assessing transformation scenarios from pure Norway spruce to mixed uneven-aged forests in mountain areas

Phosphorus (P) is a virtually finite nutrient element that cannot be substituted by any other substance. To overcome limited supply of mineral P deposits and to contribute to its sustainable management, a better knowledge on P flows is needed. In this study, a substance flow analysis for P was conducted for the German region of Berlin-Brandenburg. The aim was to create a basis to improve P management in various societal sectors such as forestry, agriculture, human consumption, waste and wastewater management and urban soils. We found that within the system under study, agricultural soils showed the largest negative balance (−3617 t P), which was connected to low P fertilizer application and low livestock densities. Forest soils followed (−424 t P) possibly as compensating inputs by weathering and atmospheric deposition could not clearly be defined. From recent literature it was concluded that atmospheric deposition to soil pools has been overestimated in many P substance flow analyses and that forest productivity may become more P limited. The greatest P accumulations in soils were found in landfills (3492 t P) and urban soils (664 t P). The biggest flows came from agriculture, followed by human consumption. The efficiency of agricultural soils was high (127%), as reflected by a negative soil balance due to the low livestock density and input of mineral P. Agricultural soils were the largest contributor of P emissions to water bodies. For the region, evidence showed that weather variations were a major driver for P-removal by the main crops (50% range) influencing the overall P removal by about 46%. Based on climate change projections for the region, we discussed possible future implications for P flows of the region. The recovery efficiency of P from wastewater was very high (91.5%), but the recycling to agriculture was very low (11%), neglecting the large potential of P in wastewater. Also, the recycling of P into organic wastes has been hitherto insufficiently considered, as most organic wastes were not collected separately.

Phosphorus flows in Berlin-Brandenburg, a regional flow analysis

Die Verwendung von Holzrohstoffen ist in Deutschland in den letzten Jahrzehnten beträchtlich angestiegen (MANTAU, 2013). Insbesondere der Energieholzmarkt ist stark gewachsen, da durch die Entwicklung neuer Techniken und effizienter Logistik die Bereitstellungskosten für Energieholz gesunken und gleichzeitig die Preise aufgrund der Verknappung und Verteuerung fossiler Brennstoffe gestiegen sind (HAKKILA, 2004; VORHOLT, 2010). Vor diesem Hintergrund haben sich neue Märkte für die Verwendung von schwächeren Holzsortimenten und Reisig entwickelt. Hierdurch werden zunehmend auch solche Teile von Bäumen genutzt, die in den letzten Jahrzehnten noch im Wald und damit im Nährstoffkreislauf der Ökosysteme geblieben sind. Auch bei herkömmlichen Derbholzsortimenten ist die Nutzung aufgrund der zunehmenden Nachfrage, aber auch aufgrund des beschleunigten Wachstums der Wälder (KAHLE et al., 2008; PRETZSCH et al., 2014) erheblich angestiegen (SEINTSCH, 2010).

Nährstoffentzüge durch die Holz- und Biomassenutzung in Wäldern. Teil 1: Schätzfunktionen für Biomasse und Nährelemente und ihre Anwendung in Szenariorechnungen

Export of nutrients from forest ecosystems by harvesting timber and biomass. Part 1: Functions for estimating tree biomass and nutrient content and their application for scenario analyses.

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Nährstoffentzüge durch die Holz- und Biomassenutzung in Wäldern. Teil 1: Schätzfunktionen für Biomasse und Nährelemente und ihre Anwendung in Szenariorechnungen

Export of nutrients from forest ecosystems by harvesting timber and biomass. Part 1: Functions for estimating tree biomass and nutrient content and their application for scenario analyses.