The Oaks of the North Are Unexpectedly Well-Defended

Normally you would expect plant defences to increase, the closer you get to the equator. This is where insect life is more active, and so more defence against herbivores are needed. But is this always the case? Xoaquín Moreira and colleagues looked at seed defences in oak, and found that in Europe, the seeds in the north have higher chemical defences than the south.

The team of scientists started their work on seeds after their previous work on leaves contradicted some assumptions about leaf herbivory. “We previously found that pedunculate oak Quercus robur (Fagaceae) populations found at lower latitudes exhibited higher levels of insect leaf herbivory and had lower concentrations of chemical defences in leaves, suggesting that latitudinal variation in herbivory was driven by an inverse gradient in chemical defences”, write the authors.

“In addition, we further found that abiotic factors influenced leaf defences and, in doing so, indirectly influenced insect leaf herbivory, suggesting a scenario whereby abiotic variation shaped plant defences, and these in turn shaped latitudinal variation in herbivory for this oak species. We here expand on this previous work and test for latitudinal variation in insect seed predation and seed traits putatively related to insect attack across Q. robur populations.

“If present, latitudinal clines in seed predation may be equally important or even more so than patterns of leaf herbivory, as seed predation has direct impacts on plant fitness and seedling recruitment and seed traits may therefore be under strong selection to increase defence against seed predation.”

To find out if there was a similar defensive shift across latitude in oaks, the team sampled 36 oak populations between northern Spain and southern Finland. They quantified insect seed predation, as well as seed defensive and nutritional traits thought to be associated with insect herbivory in these trees.

“We found strong but contrasting latitudinal clines in insect seed predation and seed traits for Q. robur, whereby populations found at lower latitudes had higher levels of seed predation and lower concentrations of seed total phenolics and phosphorus,” say Moreira and colleagues. “Our findings also indicated a significant negative association between seed predation and seed total phenolics and phosphorus, as well as a strong direct association between temperature, but not precipitation, and seed predation, whereby the proportion of attacked seeds increased towards warmer climates.”

“In addition, temperature was negatively associated with seed chemical defences and nutrients, with oak populations found at warmer sites exhibiting lower levels of total phenolics and phosphorus.”

The result that trees in warmer locations suffered more from insect attack was not a surprise. But, given the danger, it was a surprise that the seeds in those locations were not better defended.

While the authors believe their results are interesting, they also acknowledge there is much more to be done. An example they give is questioning whether chemistry is the primary defence against weevils, or if seed masting to overwhelm herbivores, is more important. This, they say means that surveys work better over multiple years, to account for difference between years in seed predation. They also question if uninfested seeds are over-represented with infested seeds dropping from the trees and being more difficult to sample.

“[O]ur results point at several interesting lines of research to assess the mechanisms underlying clinal variation in seed–predator interactions,” Moreira and colleagues conclude.