Size Matters a Lot: Drought-Affected Italian Oaks Are Smaller and Show Lower Growth Prior to Tree Death

Abstract

Hydraulic theory suggests that tall trees are at greater risk of drought-triggered death caused by hydraulic failure than small trees. In addition the drop in growth, observed in several tree species prior to death, is often interpreted as an early-warning signal of impending death. We test these hypotheses by comparing size, growth, and wood-anatomy patterns of living and now-dead trees in two Italian oak forests showing recent mortality episodes. The mortality probability of trees is modeled as a function of recent growth and tree size. Drift-diffusion-jump (DDJ) metrics are used to detect early-warning signals. We found that the tallest trees of the anisohydric Italian oak better survived drought contrary to what was predicted by the theory. Dead trees were characterized by a lower height and radial-growth trend than living trees in both study sites. The growth reduction of now-dead trees started about 10 years prior to their death and after two severe spring droughts during the early 2000s. This critical transition in growth was detected by DDJ metrics in the most affected site. Dead trees were also more sensitive to drought stress in this site indicating different susceptibility to water shortage between trees. Dead trees did not form earlywood vessels with smaller lumen diameter than surviving trees but tended to form wider latewood vessels with a higher percentage of vessel area. Since living and dead trees showed similar competition we did not expect that moderate thinning and a reduction in tree density would increase the short-term survival probability of trees.

Keywords: Quercus frainetto; anisohydric species; drought-induced dieback; growth; mortality; tree rings; wood anatomy.

FIGURE 1

Box plots showing the significant differences in tree height between living (white boxes) and dead (gray boxes) Italian oak trees at the SP and OR study sites.

FIGURE 2

Growth (A) quantified as basal area increment (BAI) of living (white symbols) and dead (filled symbols) Italian oak trees at the SP and OR study sites and growing-season drought severity (B). In the uppermost plots, the black bars show the number of dead trees classified according to their last formed ring which was considered the death year (right y axes). The drought severity was quantified using the May SPEI drought index calculated at a 10-month long scale (big red and yellow symbols show very dry years; the 2000 year is highlighted with a red symbol because it showed the driest May). The gray filled areas indicate periods when the BAI of dead trees was significantly (P < 0.05) lower than that of living trees Values are means ± SE.

FIGURE 3

Drought-growth associations differed between living and dead trees at the SP site but not at the OR site. The box plots show the correlations between BAI and the May SPEI drought index calculated at 10-months long time scales. Correlations were calculated for 32-year long periods prior (1950–1981) and overlapping (1982–2013) with the drought-induced growth decline and increase in mortality rate of the studied Italian oaks. Asterisks indicate significant (P < 0.05) differences found between living and dead trees at the SP site. Empty and filled boxes correspond to living and dead trees, respectively. The dashed and dotted horizontal lines indicate significance levels.

FIGURE 4

Latewood anatomical variables measured in living (empty symbols) and dead (filled symbols) Italian oak trees at SP and OR study sites. The gray filled areas indicate periods when the variables significantly (P < 0.05) differ between living and dead trees (Wilcoxon signed-rank test). Since most dead trees did not form the 2014 ring we did not present values for them and for this year. Values are means ± SE.

FIGURE 5

Comparison of the main non-parametric drift-diffusion-jump (DDJ) metrics calculated for the basal-area increment series (1970–2014 period) of the Italian oak dead trees sampled at the SM (A) and OR (C) sites vs. the living trees from SM (B) and OR (D) sites. The DDJ metrics include the total variance, the conditional variance, the diffusion and the jump. The red lines show the variance medians among trees and light and dark blue shadings indicate the 10–90% and 40–60% confidence intervals, respectively. The y axes show variance components and they were standardized to the 0–1 range.