Biotic and abiotic stressors can lead to different defoliation patterns within trees. Red needle cast (RNC) is a foliar disease caused by Phytophthora pluvialis affecting radiata pine (Pinus radiata) and Douglas fir (Pseudotsuga menziesii) forests and plantations in New Zealand and the US Pacific Northwest. Like most foliar diseases, RNC leads to defoliation starting in the lower half of the canopy. Little is known about the impact of the type of the needle loss associated with RNC on the photosynthesis, growth and resilience of the tree.
To investigate the effects of RNC-induced defoliation events on carbon assimilation, growth and resilience of radiata pine, 36 grafts from two different RNC-susceptible genotypes were artificially defoliated mimicking the impact of the RNC disease in winter 2016, while another 36 grafts served as control. In winter 2017, 18 of the defoliated grafts underwent a second defoliation, and in addition, 18 of the original controls were defoliated to allow comparison between a single and two consecutive defoliation events. Chlorophyll fluorescence was measured as a proxy for photosynthesis in current-year needles from the upper canopy of all plants along with recordings of height, diameter, woody biomass and root:shoot ratio before and after defoliation treatments.
Results/Conclusions
No treatment or genotype related differences in electron transport rate were determined at any time. This finding indicates that the remaining leaves do not upregulate photosynthesis in response to a defoliation event, suggesting a subsequent reduction in whole-canopy carbon assimilation.
Defoliation-induced growth losses occurred in only one of the two genotypes. While woody biomass accumulation had already decreased after the growing season following the first defoliation, the reduction in diameter and height growth became only apparent after two growing seasons. Surprisingly, the root:shoot ratio was not affected by any of the defoliation treatments, but was significantly higher in the genotype whose growth remained unaffected by defoliation.
The results suggest different strategies between RNC-susceptible genotypes to cope with defoliation. In some radiata pine genotypes, the contribution of the lower canopy to the whole-tree carbon uptake appears to be negligible presumably conferring resilience to foliar pathogens affecting the crown base. A general redundancy of lower canopy foliage in terms of carbon assimilation is shown, raising fundamental ecological questions on plant carbon budget.