My main research interests are in ecology and evolution of plant-herbivore interactions, relationship between biodiversity and ecosystem functioning, and meta-analysis and research synthesis in ecology.

Plant diversity has been long recognized as an important determinant of the abundance and species richness of organisms at higher trophic levels, such as herbivores. However, most of the research on plant diversity effects on herbivores has been conducted in agricultural ecosystems. The extent and the mechanisms of plant diversity effects on herbivores in natural ecosystems (e.g. grasslands and forests) have not been extensively investigated until recently. 

The focus of our research group within the recently funded EU collaborative project BACCARA (Biodiversity and Climate Change: a Risk Assessment) is on the effects of forest diversity on abundance and diversity of herbivores. We also try to elucidate the mechanisms behind these effects (e.g., differences between pure and mixed stands in tree growth, spatial structure, nutritional quality, abundance and diversity of herbivore natural enemies – parasitoids and predators).

We are using several long-term forest diversity experiments in boreal (Finland) and temperate (Germany) forest zones. In the first experiment established by our research team in 1999 in SW Finland, tree species diversity is manipulated by planting monocultures, 2-, 3- and 5-species mixtures of Scots pine (Pinus sylvestris), Norway spruce (Picea abies), Siberian larch (Larix sibirica), silver birch (Betula pendula) and black alder (Alnus glutinosa). Species mixtures are composed in such a way that they create a gradient from purely coniferous forest (pine, spruce and larch) through conifer/deciduous mixtures to purely deciduous forest (birch and alder) (Figure 1).

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Fig. 1. Example of plot layout at one of the three areas of the Satakunta experiment

In the second experiment, established in summer 2000 in SW Finland, tree genetic diversity within-species is manipulated by planting micropropagated plantlets of 8 clones of silver birch (Betula pendula) in single-clone stands and 2-, 4-, and 8-clone mixtures. The selected birch clones are known to display different degrees of resistance to pathogens and mammalian herbivores.

The third experiment has been established in Thuringia, Germany in 2004. In this experiment both tree species richness and functional diversity have been manipulated. Taken together, the above three experiments allow to examine the effects of three aspects of forest diversity (tree species richness, genetic diversity and functional similarity) on herbivores.

Our results indicate that effects of forest diversity on herbivores may depend both on herbivore type and tree species. For instance, stands composed of several tree species suffered less vole damage, but more moose damage than single species stands.

Fig. 2 . Effects of tree species diversity on moose and vole damage
from Vehviläinen and Koricheva 2006)

Among insect herbivores, those associated with birch were less numerous in mixed forest stands whereas herbivores associated with oak and alder were more abundant in monocultures of their host plants than in mixed stands. Therefore, birch displays associational resistance to herbivores whereas alder and oak display associational susceptibility.

Fig. 3 . Effects of tree species diversity on insect herbivory on birch, oak and alder
(from Vehviläinen et al. 2007). Negative effect sizes indicate lower herbivore
densities in mixed stands as compared to pure stands. )

The horse chestnut leaf miner (Cameraria ohridella) was first observed in Macedonia in the late 1970's, and was described as a new species in 1986. In 1989, it appeared unexpectedly in Austria and has since spread throughout central and eastern Europe. The larvae of this moth produce mines on leaves of horse chestnut (particularly white-flowering Aesculus hyppocastanum), feeding between the upper and lower leaf surfaces. On heavily infested trees with multiple attacks on the same leaf, the mines often merge together. This leads to browning and drying of the leaves, which eventually fall prematurely. Early leaf-fall can affect 70—100% of the leaves on a single tree. By August, the entire tree can appear autumnal.

Fig. 4 . Horse chestnut leaf heavily infested by Cameraria ohridella

Cameraria ohridella was first found in the UK in the London Borough of Wimbledon in July 2002. From this initial area of infestation, the moth has spread rapidly, and it is now present across most of south-central England, East Anglia and the Midlands. Horse chestnut trees around RHUL campus are heavily infested by the miner. 

Natural enemies such as hymenopteran parasitoids usually cause high mortality in native species of leafminers. However, studies in Europe have shown that although Cameraria ohridella is attacked by ca 15 species of generalist parasitoids, parasitism levels are low (usually <10%). No studies of parasitoid fauna associated with the horse chestnut leaf miner in UK have been conducted so far. There are also conflicting reports on the relative susceptibility of various species of Aesculus and Acer to Cameraria. Finally, there is a concern that horse chestnut trees affected by the miner may become more susceptible to infections by pathogens. In collaboration with Dr Nigel Straw (Forestry Commission, UK), we study natural enemies and host plants of the horse chestnut leafminer, as well as its association with horse chestnut pathogens such as the bleeding canker and the fungus Guignardia aesculi. If you are a second year RHUL student and are interested in conducting a final year project on Cameraria, please contact Julia Koricheva by e-mail.

Meta-analysis is a set of statistical procedures designed to combine results across independent studies that address a related set of research questions. It has been originally developed in medicine and social sciences and has a number of advantages over traditional narrative reviews and ‘vote-counting’ approach. Meta-analysis allows powerful quantitative analyses of the magnitude of effects and has a high degree of objectivity because it is based on a standardized set of statistical procedures. Another major beneficial feature of meta-analysis is that it offers a unique and highly improved control of type II error rates even if the number of studies in the meta-analysis is modest. Meta-analysis allows to estimate the magnitude of the effect combined across studies, to test whether it is significantly different from zero, and to examine which characteristics of studies influence the magnitude of the observed effect.

Meta-analysis has been introduced into ecology in early 1990s and is beginning to be widely used to test various ecological hypotheses and, in applied ecology, to assess the efficiency of different environmental management strategies. I have used the method of meta-analysis to review the results of studies testing the predictions of general plant-herbivore theories such as the plant stress hypothesis (Koricheva et al. 1998a), the carbon-nutrient balance hypothesis (Koricheva et al. 1998b), the induced defense hypothesis (Nykänen and Koricheva 2004), as well as studies testing for the existence of trade-offs between plant allocation to various defenses against herbivores and growth or reproduction (Koricheva 2002, Koricheva et al. 2004). More recently, I have used meta-analysis to review the efficiency of various methods of reedbed management (Valkama et al. 2008) and the possibility of using forest diversification as a strategy to reduce herbivore damage (Vehviläinen et al. 2007).

Currently, together with the NERC postdoctoral fellow Dr Kasey Barton, I am involved in meta-analysis of ontogenetic patterns of plant defences against herbivores. I am also the co-organizer of a working group on meta-analysis in ecology at the National Centre for Ecological Analysis and Synthesis and am currently co-editing with Prof. Jessica Gurevitch the first handbook on meta-analysis and research synthesis for ecologists.

Fig. 5 . Working group “Meta-analysis in ecology” at NCEAS, Santa Barbara, USA

The critical assumption of meta-analysis (as of all other types of research synthesis) is that the published studies included in the review are representative of all the studies conducted on a subject. This assumption may be violated if statistically non-significant results and/or results which contradict widely accepted hypotheses are less likely to be published. When such publication biases occur, meta-analysis based solely on published studies may considerably overestimate the magnitude of the effect. To assess the extent of publication bias against non-significant results, I have followed the publication fate of 187 manuscripts from 93 Finnish and Swedish doctoral dissertations on ecological topics defended between 1982 and 1998 (Koricheva 2003). Contrary to the expectations, the proportion of non-significant results was significantly higher in published manuscripts as compared to unpublished ones. This difference appears to be due to the tendency of ecologists to submit manuscripts with statistically significant findings to the journals with higher impact factors, which often have high rejection rates.

Publication bias against results which contradict widely accepted hypotheses appears to be more widespread in ecology than bias against non-significant results. We have revealed this kind of bias by examining temporal changes in the magnitude and direction of reported induced responses in plants. Cumulative meta-analyses revealed that studies conducted in the 1980s reported increase in phenolic concentrations, reduction in nutrient concentrations and negative effect on herbivore performance, consistently with the idea of induced resistance. In contrast, in the early 1990s when the idea that some types of plant damage may result in induced susceptibility was generally accepted, studies reported non-significant results or induced susceptibility, and smaller effects on herbivores.

Fig. 6 . Changes in the magnitude and direction of induced changes in phenolic
concentrations with the publication year (from Nykänen and Koricheva 2004)

Direction of the research findings in relation to the predictions of the hypothesis tested may also influence the place of publication. For instance, we have recently found that studies reporting results contrasting with the predictions of the carbon-nutrient balance (CNB) hypothesis are mostly published in journals with an impact factor of less than 1 whereas studies indicating a significant reduction in concentrations of carbon-based secondary compounds in response to nitrogen fertilization, as predicted by the CNB hypothesis, are published in journals with impact factors of more than 3.

Fig. 7 . Reported changes in carbon-based secondary compounds in response to nitrogen
fertilization as a function of the impact factor of the journal in which the study is published
(from Leimu and Koricheva 2004).

I am currently involved in the Ecobias working group at the National Center of Ecological Analysis and Synthesis in Santa Barbara, USA, which explores publication-related biases in ecology.

Ecobias working group at NCEAS, Santa Barbara, USA

Barton K.E., Koricheva J. 2009. The ontogeny of plant defense and herbivory: characterizing general patterns using meta-analysis. American Naturalist: in press

Aarssen L.W., C.J. Lortie, A.E. Budden, J. Koricheva, R. Leimu, T. Tregenza. 2009. Does publication in top-tier journals affect reviewer behaviour? PLoS ONE 4(7): e6283

Koricheva J., Gange A.C., Jones T. 2009. Effects of mycorrhizal fungi on insect herbivores: a meta-analysis. Ecology 90: 2088-2097

Bailey J.K., Schweitzer J.A., Ubeda F., Koricheva J., LeRoy C.D., Madritch M.D., Rehill B.J., Bangert R.K., Fisher D.G., Allen G.J., Whitham T.G. 2009. From genes to ecosystems: a synthesis of the effects of plant genetic factors across levels of organization. Philosophical Transactions of the Royal Society B 364: 1607-1616

Leimu R., Lortie C.J., Aarssen L., Budden A.E., Koricheva J., and Tregenza T. 2008. Does it pay to have a “bigwig” as a co-author? Frontiers in Ecology and the Environment 6: 410-411

Grod O.N., Budden A.E., Tregenza T., Koricheva J., Leimu R., Aarssen L.W., Lortie C.J. 2008. Systematic variation in reviewer practice according to country and gender in the field of ecology and evolution. PLoS ONE 3(9): e3202.

Vehviläinen H., Koricheva J., Ruohomäki K. 2008. Effects of stand tree species composition and diversity on abundance of predatory arthropods. Oikos 117: 935-943.

Aarssen L.W., Tregenza T., Budden A.E., Lortie C.J., Koricheva J., Leimu R. 2008. Bang for your buck: rejection rates and impact factors in ecological journals. The Open Ecology Journal 1: 14-19

Vehviläinen H., Koricheva J., Ruohomäki K. 2007. Tree species diversity influences herbivore abundance and damage: meta-analysis of long-term forest experiments. Oecologia 152: 287-298.

Kaitaniemi P., Riihimäki J., Koricheva J., Vehviläinen H. 2007. Experimental evidence for associational resistance against the European pine sawfly in mixed tree stands. Silva Fennica 41: 259-268.

Lortie C.J., Aarssen L.W., Budden A.E., Koricheva J., Leimu R., Tregenza T. 2007. Publication bias and merit in ecology. Oikos 116: 1247-1253.

Budden A.E., Tregenza T., Aarssen L., Koricheva J., Leimu R., Lortie C. 2007. Double-blind review favors increased representation of female authors. Trends in Ecology & Evolution: 23: 4-6; doi:10.1016/j.tree.2007.07.008

Valkama E., Lyytinen S., Koricheva J. What is the impact of reed management on reedbed wildlife? A meta-analytical review of European studies. Biological Conservation: 141: 364-374

Ossipov V., Ossipova S., Bykov V., Oksanen E., Koricheva J., Haukioja E. Application of metabolomics to genotype and phenotype discrimination of birch trees grown in a long-term open-field experiment. Metabolomics: 4: 39-51

Valkama E., Koricheva J., Oksanen E. 2007. Effects of elevated O3 alone, and in combination with elevated CO2, on tree leaf chemistry and insect herbivore performance: a meta-analysis. Global Change Biology 13: 184-201

Riihimäki J., Vehviläinen H., Kaitaniemi P., Koricheva J. 2006. Host tree architecture mediates the effects of predators on herbivore survival. Ecological Entomology 31: 227-235

Leimu R., Koricheva J. 2006. A meta-analysis of genetic correlations between plant resistances to multiple enemies. American Naturalist 168: E15-E37

Veteli T., Koricheva J., Niemelä P., Kellomäki S. 2006. Effects of forest management on the abundance of insect pests on Scots pine (Pinus sylvestris). Forest Ecology and Management 231: 214-217

Leimu R., Mutikainen P., Koricheva J., Fischer M. 2006. On the generality of positive relationship between plant population size, fitness, and genetic variation. Journal of Ecology 94: 942-952

Vehviläinen H., Koricheva J. 2006. Moose and vole browsing patterns in experimentally manipulated pure and mixed forest stands. Ecography 29: 497-506

Saikkonen K., Lehtonen P., Helander M.L., Koricheva J., Faeth SH. 2006. Conventional wisdom and model systems in ecology: dissecting the grass-endophyte literature. Trends in Plant Science 11: 428-433

Koricheva J, Vehviläinen H, Riihimäki J, Ruohomäki K, Kaitaniemi P, Ranta H (2006) Diversification of tree stands as a means to manage pests and diseases in boreal forests: myth or reality? Canadian Journal of Forest Research 36: 324-336

Leimu R, Koricheva J (2006) A meta-analysis of trade-offs between plant tolerance and resistance to herbivores: combining the evidence from ecological and agricultural studies. Oikos 112: 1-9 [Full text]

Vehviläinen H, Koricheva J, Ruohomäki K, Johansson T, Valkonen S (2006) Effects of stand tree species composition on insect herbivory of silver birch in boreal forests. Basic and Applied Ecology 7: 1-11

Valkama E, Koricheva J, Ossipov V, Ossipova S, Haukioja E, Pihlaja K (2005) Delayed induced responses of birch glandular trichomes and leaf surface lipophilic compounds to mechanical defoliation and simulated winter browsing. Oecologia 146: 385-393 [PubMed]

Leimu R, Koricheva J (2005) Does scientific collaboration increase the impact of ecological articles? BioScience 55: 438-443

Spehn EM et al. [36 co-authors including J. Koricheva] (2005) Ecosystem effects of biodiversity manipulations in European grasslands. Ecol. Monogr. 75: 37-63

Valkama E, Koricheva J, Salminen J-P, Helander M, Saloniemi I, Saikkonen K, Pihlaja K (2005) Leaf surface traits: overlooked determinants of birch resistance to herbivores, pathogens and endophytes? Trees 19: 191-197

Leimu R, Koricheva J (2005) What determines the citation frequency of ecological papers? Trends Ecol. Evol. 20: 28-32. [Full text]

Scherer-Lorenzen M, Potvin C, Koricheva J, Schmid B, Hector A, Bornik Z, Reynolds G, Schulze E-D (2005) The design of experimental tree plantations for functional biodiversity research. In: Scherer-Lorenzen M, Körner Ch, Schulze E-D (eds). Forest Diversity and Function: Temperate and Boreal Systems. Ecological Studies, vol. 176. Springer Verlag, Berlin Heidelberg, pp. 347-376

Riihimäki J, Kaitaniemi P, Koricheva J, Vehviläinen H (2005) Testing the enemies hypothesis in forest stands: the important role of tree species composition. Oecologia 142: 90-97 [PubMed]

Leimu R, Koricheva J (2004) Cumulative meta-analysis: a new tool for detection of temporal trends and publication bias in ecology. Proc. R. Soc. Lond. Ser. B-Biol. Sci. 271: 1961-1966 [PubMed]

Nykänen H, Koricheva J (2004) Damage-induced changes in woody plants and their effects on insect herbivore performance: a meta-analysis. Oikos 104: 247-268 [Full text]

Koricheva J, Nykänen H, Gianoli E (2004) Meta-analysis of trade-offs among plant antiherbivore defenses: are plants jacks-of-all-trades, masters of all? Am. Nat. 163: E64-E75 [Full text]

Koricheva J, Siipi H (2004) The phenomenon of biodiversity. In: M. Oksanen, J. Pietarinen (eds). Philosophy and Biodiversity. Cambridge University Press, pp. 27-53

Joshi J, Otway SJ, Koricheva J, Pfisterer AB, Alphei J, Roy BA, Scherer-Lorenzen M, Schmid B, Spehn E & Hector A (2004) Bottom-up effects and feed-backs in simple and diverse experimental grassland communities. In: Weisser WW, Siemann E (eds). Insects and Ecosystem Function. Ecological Studies, vol. 173. Springer Verlag, Berlin Heidelberg, pp. 115-134

Valkama E, Salminen J-P, Koricheva J, Pihlaja K (2004) Changes in leaf trichomes and epicuticular flavonoids during leaf development in three birch taxa. Ann. Bot. 94: 233-242 [Full text]

Valkama E, Salminen J-P, Koricheva J, Pihlaja K (2003) Comparative analysis of leaf trichome structure and composition of epicuticular flavonoids in Finnish birch species. Ann. Bot. 91: 643-655 [Full text]

Koricheva J (2003) Non-significant results in ecology: a burden or a blessing in disguise? Oikos 102: 397-401 [Full text]

Koricheva J (2002) Meta-analysis of sources of variation in fitness costs of plant antiherbivore defenses. Ecology 83: 176-190

Riipi M, Ossipov V, Lempa K, Haukioja E, Koricheva J, Ossipova S, Pihlaja K (2002) Seasonal changes in birch leaf chemistry: are there trade-offs between leaf growth and accumulation of phenolics? Oecologia 130: 380-390

Koricheva J (2002) The carbon-nutrient balance hypothesis is dead; long live the carbon-nutrient balance hypothesis? Oikos 98: 537-539

Raffaelli D, van der Putten W, Persson L, Wardle DA, Petchey O, Koricheva J, van der Heijden M, Mikola J, Kennedy T (2002) Multi-trophic dynamics and ecosystem processes. In: M. Loreau, S. Naeem, P. Inchausti (eds). Biodiversity and Ecosystem Functioning: Synthesis and Perspectives. Oxford University Press, New York, pp. 147-154

Koricheva J, Shevtsova A (2002) Pitfalls in interpretation of allelochemical data in ecological studies: implications for plant-herbivore and allelopathic research. In: Inderjit, AU Mallik (eds). Chemical Ecology of Plants: Allelopathy in Aquatic and Terrestrial Ecosystems. Birkhäuser Verlag, pp. 219-244

Lempa K, Martel J, Koricheva J, Haukioja E, Ossipov V, Ossipova S, Pihlaja K (2000) Covariation of fluctuating asymmetry, herbivory and chemistry during birch leaf expansion. Oecologia 122: 354-360

Koricheva J, Mulder CPH, Schmid B, Joshi J, Huss-Danell K (2000) Numerical responses of different trophic groups of invertebrates to manipulations of plant diversity in grasslands. Oecologia 125: 271-282

Haukioja E, Koricheva J (2000) Tolerance to herbivory in woody vs. herbaceous plants. Evol. Ecol. 14: 551-562