Bark wounding triggers gradual embolism spreading in two diffuse-porous tree species

Abstract

Xylem transport is essential for the growth, development and survival of vascular plants. Bark wounding may increase the risk of xylem transport failure by tension-driven embolism. However, the consequences of bark wounding for xylem transport are poorly understood. Here, we examined the impacts of the bark wounding on embolism formation, leaf water potential and gas exchange in the terminal branches of two diffuse-porous tree species (Acer platanoides L. and Prunus avium L.). The effects of bark removal were examined on field-grown mature trees exposed to increased evaporative demands on a short-term and longer-term basis (6 h vs 6 days after bark wounding). Bark removal of 30% of branch circumference had a limited effect on the xylem hydraulic conductivity when embolized vessels were typically restricted to the last annual ring near the bark wound. Over the 6-day exposure, the non-conductive xylem area had significantly increased in the xylem tissue underneath the bark wound (from 22-29% to 51-52% of the last annual ring area in the bark wound zone), pointing to gradual yet relatively limited embolism spreading to deeper xylem layers over time. In both species, the bark removal tended to result in a small but non-significant increase in the percent loss of hydraulic conductivity compared with control intact branches 6 days after bark wounding (from 6 to 8-10% in both species). The bark wounding had no significant effects on midday leaf water potential, CO2 assimilation rates, stomatal conductance and water-use efficiency of the leaves of the current-year shoot, possibly due to limited impacts on xylem transport. The results of this study demonstrate that bark wounding induces limited but gradual embolism spreading. However, the impacts of bark wounding may not significantly limit water delivery to distal organs and leaf gas exchange at the scale of several days.

Keywords: drought; gas exchange; hydraulic conductivity; tree injury; water potential; xylem.

Figure 1

(A) Scheme showing consecutive analyses performed on selected trees during both sampling dates. Seven terminal branches were selected on each tree at the beginning of the experiments. The branch designed for maximum vessel length measurements was excised before the first sampling date. In each sampling date, a pair of terminal branches was used for hydraulic measurements and one branch was used for dye perfusion analyses. Each pair of branches for hydraulic measurements consisted of one terminal branch with wounded bark (i.e., wounded; in gray) and one intact terminal branch (i.e., control; in black). Bark was also wounded in the terminal branch used for dye perfusion analyses. All terminal branches were positioned on the same mature branch close to each other. Midday leaf water potential and leaf gas exchange were measured repeatedly during both sampling dates on the branches designed for hydraulic measurements harvested on the second sampling date (denoted by asterisks). (B) Scheme of branch segment harvested for hydraulic measurements or dye perfusion analyses. The arrow points to the position of the bark wound. Dotted lines delineate the position of the segment used for hydraulic measurements or dye perfusion. The leaf used for gas exchange and water potential measurements (in gray) was situated approximately in the middle of the current-year shoot length. (C) A photograph of a branch of A. platanoides with wounded bark (arrow).

Figure 2

Courses of air temperature (A), relative air humidity (B) and VPD (C) on the experimental site after the bark wounding. The sums of precipitation are shown as blue bars in graph A. Light green area (8–10 a.m. on 15 June) represents a period when the bark was wounded. Gray areas (2–5 p.m.) represent the first and second sampling of branches for destructive analyses on 15 and 21 June.

Figure 3

Percent loss of xylem conductivity measured on the control and wounded terminal branches of A. platanoides (A) and P. avium (B) at two sampling dates. Columns and error bars represent means and one SE (n = 8–10). Split-plot analysis of variance was used for statistical testing. The factors were treatment (T; control vs wounded) and sampling date (S; 15 vs 21 June).

Figure 4

The cross-section of terminal branches of A. platanoides (A) and P. avium (B) used for the analysis of the conductive xylem area. Six days after bark wounding, the branch segments were perfused with a solution of crystal violet staining conductive xylem vessels in violet. The embolism primarily spreads radially from the bark wound zone, while the tangential spreading toward the xylem underneath the intact bark was minimal. In A. platanoides, the non-conductive xylem occupies slightly more than half of the last annual ring area, representing a typical progress in embolism spread observed in both species after 6-day exposure. In only a few cases, the non-conductive xylem area spanned the annual ring boundary, as shown in the section of P. avium. White arrowheads point to annual ring boundaries. Abbreviations: b, bark; bw, bark wound zone; cx, conductive xylem; nx, non-conductive xylem; p, pith. Scale bars = 1 mm.

Figure 5

Relative proportions of the non-conductive xylem area in the last annual ring measured on terminal branches of A. platanoides (A) and P. avium (B) at two sampling dates. The non-conductive xylem area was standardized to the area of the last annual ring underneath the intact bark region (with bark) and region with wounded bark (without bark; see Figure S2B available as Supplementary data at Tree Physiology Online for details). Non-conductive xylem area in the xylem zone with bark represents maturating xylem, while the area in the xylem zone without bark consists of both immature and embolized xylem. Columns and error bars represent means and one SE (n = 7–10). Split-plot analysis of variance was used for statistical testing. Letters indicate statistically significant differences within individual species at P ≤ 0.05. The factors were treatment (T; with bark vs without bark) and sampling date (S; 15 vs 21 June).

Figure 6

Gas exchange-related parameters measured on leaves of control and wounded terminal branches of A. platanoides (A, C and E) and P. avium (B, D and F) during midday hours at two sampling dates. (A and B) CO₂ assimilation rates. (C and D) Stomatal conductance. (E and F) Intrinsic WUE. Columns and error bars represent means and one SE (n = 9–10). Split-plot analysis of variance was used for statistical testing. The factors were treatment (T; control vs wounded) and sampling date (S; 15 vs 21 June).

Figure 7

Midday leaf water potential measured on leaves of control and wounded terminal branches of A. platanoides (A) and P. avium (B) at two sampling dates. Columns and error bars represent means and one SE (n = 9–10). Split-plot analysis of variance was used for statistical testing. The factors were treatment (T; control vs wounded) and sampling date (S; 15 vs 21 June).