No carbon limitation after lower crown loss in Pinus radiata

Abstract

Background and aims: Biotic and abiotic stressors can cause different defoliation patterns within trees. Foliar pathogens of conifers commonly prefer older needles and infection with defoliation that progresses from the bottom crown to the top. The functional role of the lower crown of trees is a key question to address the impact of defoliation caused by foliar pathogens.

Methods: A 2 year artificial defoliation experiment was performed using two genotypes of grafted Pinus radiata to investigate the effects of lower-crown defoliation on carbon (C) assimilation and allocation. Grafts received one of the following treatments in consecutive years: control-control, control-defoliated, defoliated-control and defoliated-defoliated.

Results: No upregulation of photosynthesis either biochemically or through stomatal control was observed in response to defoliation. The root:shoot ratio and leaf mass were not affected by any treatment, suggesting prioritization of crown regrowth following defoliation. In genotype B, defoliation appeared to impose C shortage and caused reduced above-ground growth and sugar storage in roots, while in genotype A, neither growth nor storage was altered. Root C storage in genotype B decreased only transiently and recovered over the second growing season.

Conclusions: In genotype A, the contribution of the lower crown to the whole-tree C uptake appears to be negligible, presumably conferring resilience to foliar pathogens affecting the lower crown. Our results suggest that there is no C limitation after lower-crown defoliation in P. radiata grafts. Further, our findings imply genotype-specific defoliation tolerance in P. radiata.

Keywords: A/C i curves; Pinus radiata; biomass; defoliation; foliar pathogens; growth; leaf area; non-structural carbohydrates; photosynthesis; root:shoot.

Fig. 1.

Flowchart illustrating the experimental design, measurements and dates. Shaded areas indicate destructive measurements. NSC_{ndl, root, stem}, non-structural carbohydrates sampled from needles, roots and stems, respectively; ETR, apparent electron transport rate measured by means of chlorophyll fluorescence; h, height; d, diameter; A, genotype A; B, genotype B; dates are expressed in mm/yyyy. Unless otherwise specified, the sample size is the same in both genotypes.

Fig. 2.

Response to defoliation of net photosynthesis as a function of intercellular CO₂ concentration 5 weeks (A and B) and 1 year following first-year defoliation (C and D), and 2 months following second-year defoliation (E and F), for genotype A (A, C and E) and genotype B (B, D and F).

Fig. 3.

Response to defoliation of stomatal conductance 5 weeks (A and B) and 1 year following first-year defoliation (C and D), and 2 months following second-year defoliation (E and f).

Fig. 4.

Response to defoliation of height (A and B) and diameter (C and D) for genotype A (A and C) and genotype B (B and D).

Fig. 5.

Response to defoliation of root:shoot ratio (A and B) and above-ground biomass including needle mass (C and D) for genotype A (A and C) and genotype B (B and D).

Fig. 6.

Response to defoliation of leaf area (A and B) and leaf mass (C and D) for genotype A (A and C) and genotype B (B and D). Leaf mass was directly measured. Leaf area was directly measured only before treatments, and then was predicted by leaf mass using regression equations.

Fig. 7.

Response to defoliation of soluble sugar concentrations in needles (A and B), stem (C and D) and roots (E and F) for genotype A (A, C and E) and genotype B (B, D and F).