Genotype × herbivore effect on leaf litter decomposition in Betula Pendula saplings: ecological and evolutionary consequences and the role of secondary metabolites.

Tarja Silfver,Juha Mikola,Mira Rasehorn,Ulla Paaso,Matti Rousi

doi:10.1371/journal.pone.0116806

Abstract

Plant genetic variation and herbivores can both influence ecosystem functioning by affecting the quantity and quality of leaf litter. Few studies have, however, investigated the effects of herbivore load on litter decomposition at plant genotype level. We reduced insect herbivory using an insecticide on one half of field-grown Betula Pendula saplings of 17 genotypes, representing random intrapopulation genetic variation, and allowed insects to naturally colonize the other half. We hypothesized that due to induced herbivore defence, saplings under natural herbivory produce litter of higher concentrations of secondary metabolites (terpenes and soluble phenolics) and have slower litter decomposition rate than saplings under reduced herbivory. We found that leaf damage was 89 and 53% lower in the insecticide treated saplings in the summer and autumn surveys, respectively, which led to 73% higher litter production. Litter decomposition rate was also affected by herbivore load, but the effect varied from positive to negative among genotypes and added up to an insignificant net effect at the population level. In contrast to our hypothesis, concentrations of terpenes and soluble phenolics were higher under reduced than natural herbivory. Those genotypes, whose leaves were most injured by herbivores, produced litter of lowest mass loss, but unlike we expected, the concentrations of terpenes and soluble phenolics were not linked to either leaf damage or litter decomposition. Our results show that (1) the genetic and herbivore effects on B. pendula litter decomposition are not mediated through variation in terpene or soluble phenolic concentrations and suggest that (2) the presumably higher insect herbivore pressure in the future warmer climate will not, at the ecological time scale, affect the mean decomposition rate in genetically diverse B. pendula populations. However, (3) due to the significant genetic variation in the response of decomposition to herbivory, evolutionary changes in mean decomposition rate are possible.

Highlights

Aboveground herbivores typically consume 5–20% of plant biomass in terrestrial ecosystems [1, 2]
Our insecticide treatment decreased leaf damage by 53–89%, which shows that the herbivore load was significantly reduced, but our prediction was falsified: the litter produced under reduced herbivore pressure had generally higher concentrations of terpenes and soluble phenolics and the insecticide effect on litter mass loss varied from negative to positive among the genotypes
0.357 0.956 0.047 < 0.001 0.025 metabolites remained in the litter and decelerated litter decomposition [13,14], our results suggest that higher herbivore pressure may decrease the production of terpenes and soluble phenolics in B. pendula saplings and further, that there is no clear link between these compounds and the decomposition rate of B. pendula litter

Summary

Introduction

Aboveground herbivores typically consume 5–20% of plant biomass in terrestrial ecosystems [1, 2] They can affect ecosystem processes through various direct and indirect mechanisms [3,4,5,6] and depending on how plants respond to herbivory, herbivores can either accelerate or decelerate nutrient cycling [5,6,7]. Deceleration of nutrient cycling can occur when the preferred, N-rich plant species cannot tolerate the herbivory and the well-defended species that produce litter of low quality increase in dominance [5,6, 9,10]. Because leaf litter fall forms an important stock of nutrients in terrestrial ecosystems [15,16], such herbivore-induced changes in the quantity and quality of litter can have significant effects on nutrient cycling rates [6]

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Results

Conclusion