Bidirectional Nitrogen Transfer and Plant Growth in a Mixed Plantation of N2-Fixing Species and Eucalyptus urophylla × E. grandis under Different N Applications

N2-fixing species play a crucial role in mixed-plantations as they improve stand productivity. To quantify the N transfer from N2-fixing species to Eucalyptus (Eucalyptus urophylla × E. grandis) in N2-fixing species/Eucalyptus plantations, we established a pot experiment and confirmed the occurrence of this process under natural conditions. The 15N was traced in labeled species as well as in neighboring tree species after labeling, and the growth was evaluated in short-term natural trials. Our results showed that a bidirectional N transfer occurred. The amount of net N transfer was 21.8–127.0 mg N plant−1, which was equal to 1.5–21.2% of the total nitrogen (TN) that accumulated in Eucalyptus plants under pot conditions, was transferred from Dalbergia odorifera to Eucalyptus. The amount of N transferred significantly decreased with the increasing N application rate but increased with time after labeling. Compared with the results for the Eucalyptus monocrop, the soil N concentration (including NO3−-N and NH4+-N) greatly improved when D. odorifera was introduced together with Eucalyptus under both field and pot conditions. Furthermore, the results under field conditions were consistent with the results of the pot experiment. The dry matter (DM) yield (14.5–16.4%) and the N content (5.1–9.6%) in Eucalyptus increased when mixed together with D. odorifera, but the N content in and DM yield of D. odorifera slightly decreased. It is concluded that the N transfer between Eucalyptus and D. odorifera is a much more important dynamic process than previously recognized, and Eucalyptus and legume intercropping is a successful management practice because N transfer provides a significant amount of N required for Eucalyptus productivity.

Eucalyptus globulus Coppices in Portugal: Influence of Site and Percentage of Residues Collected for Energy

Studies that quantify forest bioenergy potentials hardly address the questions of site quality, proportion and type of residues removed from the stands, and environmental impacts of those removals. However, those factors are important for energy-potential results and forest sustainability. This study compares, in terms of residual biomass availability for energy production and of sustainability, different locations, site indices, and forest management strategies in Eucalyptus globulus stands for pulp and paper in northern and central Portugal. A growth and production simulator was used to calculate the availability of residues and the area needed to supply a biomass-fired power plant under a variety of scenarios. Regions with more rainfall generate more residues, but site index and quantity and type of residues harvested are the most important factors. Under the different scenarios analyzed, the amount of residues potentially harvested range from 0.7 to 4.3 Mg ha−1 a−1, the upper bound corresponding to a scenario where stumps are valorized. The maximization of residue removal maximizes the bioenergy produced but has to be considered prudently. Studies indicate that stump removal has limited effect on Eucalyptus globulus stand productivity, diversity, and system sustainability, but impacts of residue removals increase with a decrease in site index.

Tree diversity effects on forest productivity: disentangling the effects of tree species addition vs. substitution

Mixture effect on stand productivity is usually apprehended through a substitutive approach, whereby productivity in mixed stands is compared to productivity in monocultures, at equivalent stand density. This approach has proved that in many cases mixed stands perform better than monospecific forests, however, we do not yet have a solid theory about species behaviour in the mixture or even guidelines for combining species. The addition of a second tree species to an existing mono-specific stand has received much less consideration. Yet, this approach has the potential to separate the facilitation effect from the complementarity effect. We compared the effect of tree species substitution vs. addition on the productivity of maritime pine and silver birch in a young tree diversity experiment implemented in 2008 in SW France. Substituting pines with birches to create two-species mixtures resulted in an increase of tree productivity at stand level beyond what was expected from monocultures (i.e., overyielding). In contrast, creating mixture through the addition of birches to pine stands had no effect on the maritime pine stand productivity (transgressive mixture effect not significant). This absence of effect is produced by two distinct density-dependence responses at an individual level. Our results allow clarifying the cases in which a mixed stand can be considered as an alternative to a monoculture of a productive species. In particular, the addition of a pioneer and soil low-demanding species during young developmental stages is a possibility to diversify the stand and potentially to increase ecosystem services without altering the productivity of the target species.

Nutrient Balance as a Tool for Maintaining Yield and Mitigating Environmental Impacts of Acacia Plantation in Drained Tropical Peatland—Description of Plantation Simulator

Responsible management of Acacia plantations requires an improved understanding of trade-offs between maintaining stand production whilst reducing environmental impacts. Intensive drainage and the resulting low water tables (WT) increase carbon emissions, peat subsidence, fire risk and nutrient export to water courses, whilst increasing nutrient availability for plant uptake from peat mineralization. In the plantations, hydrology, stand growth, carbon and nutrient balance, and peat subsidence are connected forming a complex dynamic system, which can be thoroughly understood by dynamic process models. We developed the Plantation Simulator to describe the effect of drainage, silviculture, fertilization, and weed control on the above-mentioned processes and to find production schemes that are environmentally and economically viable. The model successfully predicted measured peat subsidence, which was used as a proxy for stand total mass balance. Computed nutrient balances indicated that the main growth-limiting factor was phosphorus (P) supply, and the P balance was affected by site index, mortality rate and WT. In a scenario assessment, where WT was raised from −0.80 m to −0.40 m the subsidence rate decreased from 4.4 to 3.3 cm yr−1, and carbon loss from 17 to 9 Mg ha−1 yr−1. P balance shifted from marginally positive to negative suggesting that additional P fertilization is needed to maintain stand productivity as a trade-off for reducing C emissions.

Carbon sequestration in mixed deciduous forests: The importance of mid-storey trees for forest productivity

<p>Forests play an important role in climate regulation due to carbon sequestration. However, a deeper understanding of forest carbon flux dynamics are often missing due to a lack of information about forest structure and species composition, especially for non-even-aged and mixed forests. In this study, we combined field inventory data of a mixed deciduous forest in Germany with an individual-based forest gap model to investigate daily carbon fluxes and to examine the role of tree size and species composition for the overall stand productivity. Simulation results show that the forest model is capable to reproduce daily eddy covariance measurements (R<sup>2</sup> of 0.73 for gross primary productivity and of 0.65 for ecosystem respiration). The simulation results showed that the forest act as a carbon sink with a net uptake of 3.2 t<sub>C</sub> ha<sup>-1</sup> yr<sup>-1</sup>  (net ecosystem productivity) and an overall gross primary productivity of 18.2  t<sub>C</sub> ha<sup>-1</sup> yr<sup>-1</sup>. At the study site, medium sized trees (30-60cm) account for the largest share (66%) of the total productivity. Small (0-30cm) and large trees (>60cm) contribute less with 8.5% and 25.5% respectively. Simulation experiments showed, that species composition showed less effect on forest productivity. Stand productivity therefore is highly depended on vertical stand structure and light climate. Hence, it is important to incorporate small scale information’s about forest stand structure into modelling studies to decrease uncertainties of carbon dynamic predictions. Experiments with such a modelling approach might help to investigate large scale mitigation strategies for climate change that takes local forest stand characteristics into account.</p>

Analyzing the effect of silvicultural management on the trade-off between stand structural heterogeneity and productivity over time

This study combined an empirically based simulation with an analysis of the trade-off between structural heterogeneity and stand productivity depending on time, spatial scale, and silvicultural management, whereas volume growth and tree species diversity have been examined in detail, the role of forest structure and its interdependencies with stand productivity has only lately become a stronger research focus. We used the growth simulator SILVA to examine the development of stand structural heterogeneity and its trade-off with stand productivity in age-class versus uneven-aged pure and mixed spruce and beech stands at different spatial scales over 100 years. Those stands were based on typical forest types in Bavaria and were representative of forests in Central Europe. We examined how stand structure and its trade-off with productivity were modified by a multifunctional, a production-oriented, and a set-aside management scenario. The production-oriented management scenario applied to uneven-aged stands led to a reduction in structural heterogeneity per unit of productivity over time. In age-class stands, the production-oriented scenario was able to maintain the initial structural heterogeneity. The structural heterogeneity per unit of productivity increased more strongly with increasing spatial scale in age-class stands compared to uneven-aged stands. Combining forest stand simulation with scenario analyses is an exemplary method for testing the effect of silvicultural management alternatives on forest structure. This approach can later be connected to climate models considering long-term changes in growing conditions and support the planning of multifunctional forests.

Positive response of tree productivity to warming is reversed by increased tree density at the Arctic tundra-taiga ecotone

The transition zone between the northern boreal forest and the arctic tundra, known as the tundra-taiga ecotone (TTE) has undergone rapid warming in recent decades. In response to this warming, tree density, growth, and stand productivity are expected to increase. Increases in tree density have the potential to negate the positive impacts of warming on tree growth through a reduction in the active layer and an increase in competitive interactions. We assessed the effects of tree density on tree growth and climate-growth responses of Cajander larch (<i>Larix cajanderi</i>) and on trends in the normalized difference vegetation index (NDVI) in the TTE of Northeast Siberia. We examined 19 mature forest stands that all established after a fire in 1940 and ranged in tree density from 300 to 37,000 stems ha-1. High density stands with shallow active layers had lower tree growth, higher stand productivity, and more negative growth responses to growing season temperatures compared to low density stands with deep active layers. Variation in stand productivity across the density gradient was not captured by Landsat derived NDVI, but NDVI did capture annual variations in stand productivity. Our results suggest that the expected increases in tree density following fires at the TTE may effectively limit tree growth and that NDVI is unlikely to capture increasing productivity associated with changes in tree density.