When fungal infection comes to Pinus radiata, it can run through the bottom of the crown of the tree, removing its leaves. What effect does this defoliation have when it occurs? Mireia Gomez-Gallego and colleagues conducted a two-year defoliation experiment on two genotypes of Pinus radiata to find out.
“We used repeated artificial defoliation in Pinus radiata D. Don grafts to simulate the defoliation pattern associated with the Red needle cast (RNC) disease, a new foliar disease caused by Phytophthora pluvialis, whose infection peaks in winter,” write the authors. “The RNC disease typically affects lower-crown foliage other than current-year needles. We used grafts of two RNC-susceptible genotypes growing in the radiata pine plantation forests in New Zealand. In line with the timing of the RNC disease, we removed 1-year-old and older needles from the bottom half of the crown, during two consecutive winters, to check the effect of both a single and two defoliation events.”
The team note that a healthy tree will typically have far more ability to take in carbon (C) from the upper and outer parts of the tree crown, than the tree needs. Therefore one idea is the lower crown leaves are more for storing nutrients and carbon than for photosynthesis. If that’s the case then the scientists expected to see that defoliation caused no increase in photosynthesis, because there would be no sink to put the extra carbon into. Also they expected the defoliated trees to grow more slowly than their neighbours, again because they would lack the carbon reservoir that untouched trees had.
The test was done on seventy-two Pinus radiata D. Don grafts were grown in 45 litre bags under sheltered conditions in the Scion nursery (Rotorua, New Zealand). Ten were taken out at the start of the experiment and measured for biomass. The remaining sixty-two trees were split into two groups of thirty-one. In the first year of the experiment, one group had a defoliated lower crown, while the other did not. After that, eight pines were taken from each group and measured for biomass.
Then, of the remainder, each group was split into two again, in two groups of twelve, the trees underwent defoliation. In the other groups they did not. This meant that there were four groups. One that was defoliated twice, one that was defoliated in the second year, one that was defoliated in the first year and left to recover, and one that was never interfered with at all – until all the groups were then taken to be measured for biomass.
The team found that their hypothesis was correct, there was no upregulation of photosynthesis in the defoliated trees. This, they note, is in contradiction with many studies on angiosperms and gymnopserms. Gomez-Gallego and colleagues said they reason they got different results was probably due to the type of defoliation of trees. “[T]hose studies focused on either upper- or whole-crown defoliation, mainly simulating insect herbivory. Unlike lower-crown defoliation, removal of upper-crown foliage increases light penetration to lower-crown leaves, enhancing the photosynthetic activity of residual leaves.”
“Our findings put forward the specific proportion of the crown affected by defoliation as a new factor to consider within the framework of tree C dynamics,” say the authors. “The contribution of older, lower-crown needles to whole-tree C assimilation is smaller than that of current-year needles in the upper crown. Therefore, artificial removal of those leaves would have little impact on tree C balance. Why would a tree spend C and energy to maintain this ‘extra’ foliage?”
They provide some possible answers. One is that the leaves are nutrient and carbon stores. Another is the suggestion that the lower crown could be a buffer stopping pathogens from attacking the upper crown. Gomez-Gallego and colleagues add that the questions will remain open as there is currently a shortage of similar work to compare this research to. “Our experiment was a first approach to understand the role and function of the lower crown, as target foliage by pathogens, and raises the question of the ecological role of mild foliar pathogenic infections. Pathogen-induced defoliations have not been given sufficient attention because they are perceived as a non-lethal biotic stress.”