Biodiversity effects and transgressive overyielding

Aims The potential for mixtures of plant species to produce more biomass than every one of their constituent species in monoculture is still controversially discussed in the literature.Here we tested how this socalled transgressive overyielding is affected by variation between and within species in monoculture yields in biodiversity experiments.Methods We use basic statistical principles to calculate expected maximum monoculture yield in a species pool used for a biodiversity experiment.Using a real example we show how between-and withinspecies variance components in monoculture yields can be obtained.Combining the two components we estimate the importance of sampling bias in transgressive overyielding analysis.Important Findings The net biodiversity effect(difference between mixture and average monoculture yield)needed to achieve transgressive overyielding increases with the number of species in a mixture and with the variation between constituent species in monoculture yields.If there is no significant variation between species,transgressive overyielding should not be calculated using the best monoculture,because in this case the difference between this species and the other species could exclusively reflect a sampling bias.The sampling bias decreases with increasing variation between species.Tests for transgressive overyielding require replicated species’monocultures.However,it can be doubted whether such an emphasis on monocultures in biodiversity experiments is justified if an analysis of transgressive overyielding is not the major goal.

Toward managing mixed-species stands： from parametrization to prescription

A better understanding and a more quantitative design of mixed-species stands will contribute to more integrative and goal-oriented research in mixed-species forests. Much recent work has indicated that the structure and growth of mixed species forests may fundamentally differ from monocultures. Here we suggest how to progress from the present accumulation of phenomenological findings to a design of mixed-species stands and advanced silvicultural prescriptions by means of modelling. First, the knowledge of mixing effects on the structure and growth at the stand, species, and individual tree level is reviewed, with a focus on those findings that are most essential for suitable modelling and silvicultural designs and the regulation of mixed stands as opposed to monocultures. Then, the key role of growth models, stand simulators, and scenario assessments for designing mixed species stands is discussed The next section illustrates that existing forest stand growth models require some fundamental modifications to become suitable for both monocultures and mixed-species stands. We then explore how silvicultural prescriptions derived from scenario runs would need to be both quantified and simplified for transfer to forest management and demonstrated in training plots. Finally, we address the main remaining knowledge gaps that could be remedied through empirical research.

Tree species richness enhances stand productivity while stand structure can have opposite effects, based on forest inventory data from Germany and the United States of America

Background： in recent studies, mixed forests were found to be more productive than monocultures with everything else remaining the same. Methods： To find out if this productivity is caused by tree species richness, by a more heterogeneous stand structure or both, we analyzed the effects of forest structure and tree species richness on stand productivity, based on inventory data of temperate forests in the United States of America and Germany. Results： Having accounted for effects such as tree size and stand density, we found that： （I） tree species richness increased stand productivity in both countries while the effect of tree size heterogeneity on productivity was negative in Germany but positive in the USA; （11） productivity was highest at sites with an intermediate amount of precipitation; and （111） growth limitations due water scarcity or low temperature may enhance structural heterogeneity. Conclusions： In the context of forest ecosystem goods and services, as well as future sustainable forest resource management, the associated implications would be：

Biodiversity, productivity, and temporal stability in a natural grassland ecosystem of China

Understanding the effect of biodiversity on ecosystem function is critical to promoting the sustainability of ecosystems and species conservation in natural ecosystems. We observed species composition, species richness and aboveground biomass,and simulated the competitive assemblages in a natural grassland ecosystem of China, aiming to test some assumptions and predictions about biodiversity–stability relationships. Our results show that aboveground productivity and temporal stability increased significantly with increasing species richness, and via a combination of overyielding, species asynchrony, and portfolio effects. Species interactions resulted in overyielding caused by trait-independent complementarity, and were not offset by a negative dominance effect and trait-dependent complementarity effect. Therefore, the mechanisms underlying the biodiversity effect shifted from the selection effect to the complementarity effect as diversity increased, and both effects were coexisted but the complementarity effect represent a mechanism that facilitates long term species coexistence in a natural grassland ecosystem of China.

Leaf area increases with species richness in young experimental stands of subtropical trees

Aims Most biodiversity-ecosystem functioning research has been carried out in grassland ecosystems,and little is known about whether forest ecosystems,in particular outside the temperate zone,respond similarly.Here,we tested whether productivity,assessed as leaf area index(LAI),increases with species richness in young experimental stands of subtropical trees,whether this response is similar for early-season leaf area(which is dominated by evergreens)and seasonal leaf area increase(which is dominated by deciduous species),and whether responses saturate at high species richness.Methods We used a planted tree biodiversity experiment in south-east China to test our hypotheses.LAI was determined three times by digital hemispheric photography in 144 plots that had been planted with 400 trees each,forming communities with 1,2,4,8 or 16 tree species.Important Findings LAI increased significantly with tree species richness in the fifth year of stand establishment.Similar,but weaker,statistically non-significant trends were observed 1 year before.We did not observe leaf area overyielding and the presence of particularly productive and unproductive species explained large amounts of variation in leaf area,suggesting that selection-type effects contributed substantially to the biodiversity effects we found in this early phase of stand establishment.Effects sizes were moderate to large and comparable in magnitude to the ones reported for grassland ecosystems.Subtropical(and tropical)forests harbor substantial parts of global net primary production and are critical for the Earth’s carbon and hydrological cycle,and our results suggest that tree diversity critically supports these ecosystem services.

HOW MULTISPECIES INTERCROP ADVANTAGE RESPONDS TO WATER STRESS:A YIELDCOMPONENT ECOLOGICAL FRAMEWORK AND ITS EXPERIMENTAL APPLICATION

Absolute yield and land use efficiency can be higher in multicrops.Though this phenomenon is common,it is not always the case.Also,these two benefits are frequently confused and do not necessarily occur together.Cropping choices become more complex when considering that multicrops are subject to strong spatial and temporal variation in average soil moisture,which will worsen with climate change.Intercropping in agroecosystems is expected to buffer this impact by favoring resistance to reduced humidity,but there are few empirical/experimental studies to validate this claim.It is not clear if relatively higher multicrop yield and land use efficiency will persist in the face of reduced soil moisture,and how the relation between these benefits might change.Here,we present a relatively simple framework for analyzing this situation.We propose a relative multicrop resistance(RMR)index that captures all possible scenarios of absolute and relative multicrop overyield under water stress.We dissect the ecological components of RMR to understand the relation between higher multicrop yield and land use efficiency and the ecological causes of different overyield scenarios.We demonstrate the use of this framework with data from a 128 microplot greenhouse experiment with small annual crops,arranged as seven-species multicrops and their corresponding monocrops,all under two contrasting watering regimes.We applied simple but robust statistical procedures to resulting data(based on bootstrap methods)to compare RMR,and its components,between different plants/plant parts.We also provide simple graphical tools to analyze the data.

Effects of monoculture-conditioned soils on common tallgrass prairie species productivity

Aims Within biodiversity-ecosystem function experiments,it is widely understood that yields of some species rapidly decline when planted in monoculture.This effect may arise due to decreased access to soil nutrients or an increase in detrimental soil patho-gens within monoculture plantings.To determine whether or not soil conditioning affects tall grass prairie species biomass produc-tion,we conducted a field experiment to assess species growth in conspecifically and heterospecifically conditioned soils and a greenhouse experiment to elucidate how conspecific soil biota affected species growth.Methods To test for species-specific soil effects,seedlings of the legume Astragulus canadensis,the cool-season grass Elymus canaden-sis,the forb Helianthus maximiliani and the warm-season grass Panicum virgatum were grown in field plots that had either been conspecifically or heterospecifically conditioned over 2 years.Plant growth was recorded over a single growing season,and soils were assessed for differences in their nematode(mesofauna)communities.Seedlings of these species were additionally grown over a 6-week period in conspecifically conditioned soil that was either untreated,heated to 60°C,sterilized(autoclaved at 120°C)or heated to 60°C and reinoculated with conspecific soil biota.The two heating treatments were used to compare growth responses between a low-and high-temperature soil treatment.The reinoculation treatment was used to assess the effect of soil biota in light of any nutrient changes that may have occurred with soil heating.Important Findings Elymus canadensis,H.maximiliani and P.virgatum growth was improved in field plots conditioned by the legume A.canadensis com-pared with their growth in conspecifically conditioned(home)soils.Despite variation(grass versus nongrass)in their soil nematode com-munities,there was no evidence to suggest that these three species were inhibited by conspecific or functionally conspecific soil condi-tioning in the field.Astragulus canadensis was the only species whose growth was reduced in conspecifically conditioned field soil.In the greenhouse,E.canadensis growth increased in all of the heat-treated soils,likely a response to a fertilization effect associated with soil heat-ing.Panicum virgatum growth also increased among the heated soils.However,its growth decreased in heated soils where conspecific soil mesofauna were reintroduced,indicating that this grass may be inhib-ited by soil mesofauna.Finally,A.canadensis growth decreased in soils treated to fully remove soil biota and was not affected by rein-troduction of soil mesofauna,suggesting that this species negatively responds to soil changes that occur with extreme heating.At least for the suite of tallgrass prairie species evaluated within this experiment,it appears that changes in soil chemistry and generalist soil biota,as opposed to increasing species-specific soil pathogens,more strongly contribute to temporal disparities in their performance.