Little strokes fell great oaks: minor but chronic herbivory substantially reduces birch growth

Modern concepts of plant tolerance to herbivory are primarily based on studies of short-term severe damage, whereas the effects of minor chronic damage to long-lived woody plants, corresponding to background herbivory (215% annual loss of foliar biomass in boreal and temperate forests), remain poorly understood. In our experiment, the annual removal of 2, 4, 8 and 16% of the leaf area from naturally growing mountain birch Betula pubescens subsp. czerepanovii saplings during a seven-year period resulted in a pronounced reduction of plant vertical growth (30, 34, 45 and 78%, respectively). Leaf size decreased first (already after one year of the 16% treatment), resulting in the reduction of the total leaf area. This effect was followed by a considerable decrease in the length of long shoots in all treatments. Leaf number on the plant was maintained for a longer time, being reduced by the end of the experiment in 16% treatment only; no changes in specific leaf area or chlorophyll fluorescence were observed in either of the treatments. This pattern may indicate that the plant reallocates resources from the growth of the woody parts to the maintenance of the photosynthetic area, and can be seen as a strategy of tolerance to minor herbivory, whereas compensatory responses typical of severe herbivory (increased photosynthesis rates and shoot regrowth) have not been detected. The predicted 25% increase in background herbivory due to climate warming can potentially produce previously unrecognised negative impacts on tree growth. We conclude that in the long term, background herbivory is likely to impose stronger effects on the growth of woody plants than short-term devastating outbreaks of defoliators, thus contributing more to the development of plant evolutionary adaptations to herbivory than severe but episodic bouts of damage.