How does Holly Oak compensate for losing leaves?

When a plant reproduces, a lot of resources can get directed to producing the reproductive organs. Stores expended on packaging seeds are spent on vegetative growth or defence. Iris Le Roncé and colleagues note that plants are modular, and so resource allocation strategies to reproduction can also vary among hierarchical levels. To find out how resources are allocated, they carried out a defoliation experiment on Quercus ilex.

Q. ilex is the ‘evergreen oak’, or ‘holly oak’, a tree found in forests around the Mediterranean. A a major part of the forests, learning how it copes with adversity is useful, so Le Roncé and colleagues put it to the test. “The originality of our study is to cover the main steps of the reproductive cycle, at different scales within the tree and with a large range of defoliation intensities, from 0 to over 80% of defoliation, and to quantify its impact not only on the allocation to reproduction of the same year but also on allocation to vegetative and reproductive organs the following year.” they write in their article.

They split the branches of eight trees into three groups. The first was the control group, These branches had no defoliation, so establish a baseline. The next group was 50% defoliated. The last group was the unlucky group. These branches lost 85% of their leaves.

“As we expected, defoliation reduced the number of catkins per shoot, the number of staminate flowers per catkin and the number of female flowers per shoot in the following year, while it increased the number of leaves,” write the authors. “This allocation shift did not impact the number of stamens per flower, which might be more developmentally constrained. We observed a reduction, but not an interruption of female flowers production the year following defoliation. This contrasts with findings by Wiley et al. (2017) after whole tree defoliation in Quercus velutina, probably because, in our case, defoliated branches could rely on resources from non-defoliated branches further away.”

Another feature of the defoliated branches was that they produced fruits, like the control group. The botanists didn’t see increased photosynthesis in the remaining leaves, so they believe the carbon for the fruit either came from leaves further away on other branches, from distant reserves, or from local reserves in shoots. The problem with local carbon as a source is that authors say the shoots on the branches wouldn’t have had the reserves to produce the fruit.

The arrival from carbon from elsewhere suggest that the allocation of resources is determines at a larger scale than the shoot, the authors say. “Most importantly, our results also reveal the complexity in resource allocation strategies to the different plant functions over two consecutive years depending on the resource availability,” they add. This finding shows how climate events in one summer could have long-term consequences for some trees.