What Drives Decomposition Rates of Coarse Woody Debris (CWD)?

Currently increasing efforts are made to manage CWD as a habitat component and a carbon store in forest ecosystems. For this a basic understanding of patterns and rates of dead wood decomposition in different forests is crucial. The decomposition rate of CWD is mainly dependent on climatic (wood temperature, wood moisture) and substrate specific (tree species, decay stage, diameter) variables. Here, we analysed the influence of these factors using a combined approach. 1) We assessed the decay rate of Fagus sylvatica, Picea abies and Pinus sylvestris in three diameter classes (10-20 cm, 20-40 cm, \u3e40 cm) along a climatic/altitudinal gradient (temperature, precipitation) retrospectively in the field. 2) We analysed under controlled conditions the effect of varying wood temperature (5, 10 and 20 ∞C) and moisture (three steps) on the current respirational carbon loss of CWD of Fagus sylvatica, Picea abies and Pinus sylvestris in relation to decay stage (1, 3 and 5 related to a 5-class decay classification system). 3) We measured CWD respiration continuously over one year in the field on a F. sylvatica and P. abies log and analysed the effect of substrate specific (tree species, decay stage, diameter), micro-climatic (wood moisture and temperature) as well as environmental variables (ground contact or suspended). A highly significant effect of wood temperature and moisture on respirational carbon loss regardless of decay stage was observed under controlled conditions as well as in the field. In both cases the respirational C loss of F. sylvatica CWD was about twice that of P. abies. Suggestions will be provided, how C loss from CWD might be represented in decomposition and ecosystem C models

Toward a methodical framework for comprehensively assessing forest multifunctionality

Biodiversity-ecosystem functioning (BEF) research has extended its scope from communities that are short-lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger-scale and longer-time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long-lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above? and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized

Carbon allocation in a mixed-species plantation of 'Eucalyptus globulus' and 'Acacia mearnsii'

Aboveground biomass was twice as high in mixtures of 'Eucalyptus globulus' and 'Acacia mearnsii' when compared to 'E. globulus' monocultures after 11 years. This was attributed to increased nutrient availability and accelerated rates of N and P cycling in mixtures. This study examined whether the increase in aboveground biomass production was associated with an increase in total productivity (both above- and belowground), a change in C partitioning (from below to aboveground) or both. Total annual belowground C allocation (TBCA) was determined during year 11 in a mixed-species trial near Cann River, southeastern Australia. Monocultures of 'E. globulus' (100%E) and 'A. mearnsii' (100%A) and mixtures of these species (50%E:50%A) were planted in a replacement series. Using a conservation of mass approach, TBCA was estimated as soil carbon dioxide (CO₂) efflux C minus the C input from aboveground litter plus changes in the C stored in soil, roots and the forest floor litter layer. Aboveground net primary production (ANPP) was also estimated to enable comparison of ratios of above and belowground fluxes between treatments. TBCA ranged from 14.6 to 16.3 Mg C ha⁻¹ year⁻¹ and was not significantly different in 100%E, 50%E:50%A and 100%A. Higher ratios of ANPP:TBCA in the mixtures (0.41) than in either monoculture (100%A:0.28 100%E:0.31) indicated that trees in mixture partitioned a lower proportion of assimilated C belowground than those in monocultures. Since the mixture was as productive as monocultures belowground but more productive aboveground, it appears to be more productive overall and thus have the potential to increase C sequestration above that of monocultures

Nutrient cycling in a mixed-species plantation of 'Eucalyptus globulus' and 'Acacia mearnsii'

A doubling of aboveground biomass production has been observed in mixtures of 'Eucalyptus globulus' Labill. and 'Acacia mearnsii' de Wildeman when compared with monocultures after 11 years of growth. This study examined to what extent increased nitrogen (N) availability and accelerated rates of nutrient cycling may contribute to increased growth in mixtures. Monocultures of 'E. globulus' (E) and 'A. mearnsii' (A) and mixtures of these species were planted in a species replacement series: 100% E, 75% E + 25% A, 50% E + 50% A, 25% E + 75% A, and 100% A. Litterfall mass increased with aboveground biomass production and was highest in 50:50 mixtures and lowest in monocultures. Owing to higher N concentrations of 'A. mearnsii' litter, N contents of annual litterfall were at least twice as high in stands containing A. mearnsii (32-49 kg·ha¹·year–¹) as in 'E. globulus' monocultures (14 kg·ha¹·year¹). Stands with 'A. mearnsii' also cycled higher quantities of phosphorus (P) in annual litterfall than 'E. globulus' monocultures. This study demonstrated that mixing 'A. mearnsii' with 'E. globulus' increased the quantity and rates of N and P cycled through aboveground litterfall when compared with 'E. globulus' monocultures. Thus, mixed-species plantations appear to be a useful silvicultural system to improve nutrition of eucalypts without fertilization

On the success and failure of mixed-species tree plantations: lessons learned from a model system of 'Eucalyptus globulus' and 'Acacia mearnsii'

Mixed plantations of a 'Eucalyptus' species with a nitrogen-fixing tree species can produce significantly higher quantities of aboveground biomass than monocultures. However, if species or sites are not chosen correctly, one species may suppress the growth of the other and mixtures may be less productive than monocultures. Based on a study of 'Eucalyptus globulus' and 'Acacia mearnsii', this paper discusses the species attributes and site factors that should be considered to improve the probability of increasing biomass production using mixed-species plantations. In an 11-year-old mixed-species trial of 'E. globulus' and 'A. mearnsii' in southeastern Australia aboveground biomass production was twice as high in mixtures containing 50% 'E. globulus' and 50% 'A. mearnsii' than in 'E. globulus' monocultures. There are three main types of interactions that led to this growth outcome: competition, competitive reduction and facilitation. Facilitation occurred as 'A. mearnsii' fixed significant quantities of N, both in monoculture and when mixed with 'E. globulus'. In addition, not only rates of N but also those of P cycling through litterfall were significantly higher in mixed stands than 'E. globulus' monocultures, pointing to the importance of selecting a nitrogen-fixing species that is capable of N fixation and subsequent fast nutrient cycling through litterfall. Mixed stands developed stratified canopies, such that 'E. globulus' eventually overtopped 'A. mearnsii' after 9 years. This resulted in an increase in light capture at the stand level and a reduction in competition for light for 'E. globulus', a relatively shade intolerant species. This illustrates the importance of selecting species based on their height growth dynamics and relative shade tolerances, to ensure that neither species is suppressed by the other and that the less tolerant species is not overtopped by the more shade tolerant species. In addition to species attributes, site factors, such as soil nitrogen, phosphorus and water availability, play an important role in the interactions and processes occurring in mixtures. In a pot trial containing monocultures and mixtures of 'E. globulus' and 'A. mearnsii', mixtures produced more biomass than monocultures of either species at low levels of N fertiliser. However, at high levels of N fertiliser 'E. globulus' suppressed 'A. mearnsii' and the biomass production of mixtures was not significantly different to that of 'E. globulus' monocultures. This suggests that mixtures should only be planted on sites where the processes and interactions between species will increase the availability of, or reduce competition for, a major limiting resource for growth at that site. The accurate prediction of successful mixed-species combinations and sites is difficult due to the limited number of studies on mixtures. A mechanistic approach is required to examine the interactions and processes that occur in mixtures and to demonstrate why certain combinations are successful on some sites and not others

Mixed-species plantations of 'Eucalyptus' with nitrogen-fixing trees: A review

Mixed-species plantations of 'Eucalyptus' with a nitrogen (N₂) fixing species have the potential to increase productivity while maintaining soil fertility, compared to 'Eucalyptus' monocultures. However, it is difficult to predict combinations of species and sites that will lead to these benefits. We review the processes and interactions occurring in mixed plantations, and the influence of species or site attributes, to aid the selection of successful combinations of species and sites. Successful mixtures, where productivity is increased over that of monocultures, have often developed stratified canopies, such that the less shade-tolerant species overtops the more shade-tolerant species. Successful mixtures also have significantly higher rates of N and P cycling than Eucalyptus monocultures. It is therefore important to select N₂-fixing species with readily decomposable litter and high rates of nutrient cycling, as well as high rates of N₂-fixation. While the dynamics of N₂-fixation in tree stands are not well understood, it appears as though eucalypts can benefit from fixed N as early as the first or second year following plantation establishment. A meta-analysis of 18 published studies revealed several trials in which mixtures were significantly (

Productivity of Three Young Mixed-Species Plantations Containing N₂-Fixing 'Acacia' and Non-N₂-Fixing 'Eucalyptus' and 'Pinus' Trees in Southeastern Australia

Mixed species plantations have the potential to exceed the biomass production of monocultures. This study examined the productivity of three mixed species plantations in southeastern Australia. Two of these trials contained a 'Eucalyptus' sp. ('E. saligna' Smith or 'E. nitens' [Deane & Maiden] Maiden) planted with 'Acacia mearnsii' De Wild., and the other contained 'Pinus radiata' D. Don with 'A. mearnsii', 'A. decurrens' Willd., 'E. benthamii' Maiden & Cambage, or 'E. smithii' R. Baker. Each trial contained both monocultures and mixtures, and was replicated three or four times. Tree diameters or heights were smaller in mixture than monocultures for some species (P. radiata diameters of 5.9 cm and 7.0 cm in 2:1 mixtures with 'A. mearnsii' and monocultures, respectively) but tended to increase (not significantly) for other species ('E. nitens' diameters of 10.6 cm and 8.5 cm and 'A. mearnsii' diameters of 9.2 cm and 8.8 cm in 1:1 mixtures and monocultures, respectively). As a result, mixtures were intermediate in aboveground biomass production between monocultures of the mixed species in each trial, or they were not significantly different from the monocultures. Competition for resources other than nitrogen (N), such as light, soil moisture, or other nutrients, appeared to balance any positive effects that might have occurred, such as through increased N availability. For example, foliar N concentrations of 'E. saligna' were higher in mixture (23.1 mg g⁻¹) than monoculture (17.7 mg g⁻¹); however, this did not result in greater aboveground tree biomass. The range of different growth responses from mixing different species in this study and in other studies shows that a fundamental understanding of the underlying processes is required to enable a greater predictive capacity of the circumstances under which mixtures can be successful

Effects of Changing the Supply of Nitrogen and Phosphorus on Growth and Interactions between 'Eucalyptus globulus' and 'Acacia mearnsii' in a Pot trial

Significant increases in aboveground biomass production have been observed in mixed plantations of 'Eucalyptus globulus' and 'Acacia mearnsii' when compared to monocultures. However, this positive growth response may be enhanced or lost with changes in resource availability. Therefore this study examined the effect of the commonly limiting resources soil N, P and moisture on the growth of 'E. globulus' and 'A. mearnsii' mixtures in a pot trial. Pots containing two 'E. globulus' plants, two 'A. mearnsii' plants or one of each species were treated with high and low levels of N and P fertiliser. After 50 weeks, 'E. globulus' plants grew more aboveground biomass in mixtures than monocultures. 'A. mearnsii' were larger in mixtures only at low N, where both species were similar in size and the combined aboveground biomass of both species in mixture was greater than that of monocultures. At high N and both high and low levels of P fertiliser 'E. globulus' appeared to dominate and suppress 'A. mearnsii'. In these treatments, the faster growth of 'E. globulus' in mixture did not compensate the reduced growth of 'A. mearnsii', so mixtures were less productive than (or not significantly different from) E. globulus monocultures. The greater competitiveness of 'E. globulus' in these situations may have resulted from its higher N and P use efficiency and greater growth response to N and P fertilisers compared to 'A. mearnsii'. This trial indicates that the complex interactions between species in mixtures, and thus the success of mixed plantations, can be strongly influenced by site factors such as the availability of N and P

Assessing nitrogen fixation in mixed- and single-species plantations of 'Eucalyptus globulus' and 'Acacia mearnsii'

Mixtures of 'Eucalyptus globulus' Labill. and 'Acacia mearnsii' de Wildeman are twice as productive as 'E. globulus' monocultures growing on the same site in East Gippsland, Victoria, Australia, possibly because of increased nitrogen (N) availability owing to N₂ fixation by 'A. mearnsii'. To investigate whether N2 fixation by 'A. mearnsii' could account for the mixed-species growth responses, we assessed N₂ fixation by the accretion method and the 15N natural abundance method. Nitrogen gained by 'E. globulus' and 'A. mearnsii' mixtures and monocultures was calculated by the accretion method with plant and soil samples collected 10 years after plantation establishment. Nitrogen in biomass and soil confirmed that 'A. mearnsii' influenced N dynamics. Assuming that the differences in soil, forest floor litter and biomass N of plots containing 'A. mearnsii' compared with 'E. globulus' monocultures were due to N₂ fixation, the 10-year annual mean rates of N₂ fixation were 38 and 86 kg ha⁻¹ year⁻¹ in 1:1 mixtures and 'A. mearnsii' monocultures, respectively. Nitrogen fixation by 'A. mearnsii' could not be quantified on the basis of the natural abundance of 15N because such factors as mycorrhization type and fractionation of N isotopes during N cycling within the plant confounded the effect of the N source on the N isotopic signature of plants. This study shows that 'A. mearnsii' fixed significant quantities of N₂ when mixed with 'E. globulus'. A decline in δ15N values of 'E. globulus' and 'A. mearnsii' with time, from 2 to 10 years, is further evidence that N2 was fixed and cycled through the stands. The increased aboveground biomass production of 'E. globulus' trees in mixtures when compared with monocultures can be attributed to increases in N availability