Drought and Phytophthora Are Associated With the Decline of Oak Species in Southern Italy

Abstract

Forest decline induced by climate change is a global phenomenon that affects many tree species, mainly in drought-prone areas as the Mediterranean region. In southern Italy, several oak species have shown decline symptoms and elevated mortality since the 2000s due to drought stress. However, it remains to be answered whether decline occurred alone or whether a pathogen was also involved. To this aim, we compared two coexisting oak species in a forest located in southern Italy which are assumed to be less (Quercus cerris) and more tolerant to drought (Quercus pubescens). We sampled fifteen couples of neighboring declining (D) and non-declining (ND) trees of both species. Wood cores were taken from all trees to perform dendrochronological analyses to detect the decline onset and link it to potential climatic drivers. Carbon isotope ratios (d¹³C) were analyzed in wood of the two vigor classes to compare their water-use efficiency. Phytophthora presence was also assessed in soil samples from ten D-ND couples of trees per species. The oak species most affected by drought-induced decline in terms of leaf shedding and mortality was Q. cerris, i.e., the least tolerant to drought. In both species, the D trees showed a reduced growth rate compared with ND trees from 2000 onward when drought and warming intensified. Q. pubescens showed higher growth sensitivity to precipitation, temperature and drought than Q. cerris. This sensitivity to climate was magnified in D trees whose growth decreased in response to warm and dry conditions during the prior winter and the late summer. The Q. pubescens D trees were more efficient in their water use than ND trees before the growth divergence between D and ND trees amplified. In the studied area, Phytophthora quercina was isolated from 40% of the sampled trees, and tended to be more frequent amongst ND than amongst D trees. Our data suggests that droughts and warm summer conditions triggered oak decline. The high prevalence of P. quercina in the studied area warrants further study as a potential predisposing factor.

Keywords: Phytophthora species; Quercus cerris; Quercus pubescens; carbon isotopes; dendroecology; drought stress; forest dieback; water-use efficiency.

FIGURE 1

Climate trends and drought patterns in the study area. The selected climate variables correspond to monthly mean maximum (TMx) and minimum (TMn) temperatures (A) and precipitation [(B) Prec] showing the most pronounced trends for the 1950–2016 period as assessed through the tau (τ) statistic (D). The drought severity was assessed using the May Standardized Precipitation Evapotranspiration Index (SPEI) for 11-months long scales (C), which showed high associations to tree growth (see Figure 4).

FIGURE 2

Radial-growth patterns from 1900 to 2016 of non-declining (ND) and declining (D) trees in (A) Q. pubescens and Q. cerris and (B) residual ring-width chronologies for the best-replicated period (1950–2016). The right y axes show the sample depth. In the plot (A) the gray areas indicate periods when tree-ring width of ND trees was significantly (P < 0.05) higher than in D trees according to Wilcoxon rank-sum tests. Values are mean tree-ring widths ± SE. The inset shows one of the sampled couples of ND and D Q. cerris trees.

FIGURE 3

Climate-growth relationships (Pearson correlations) in declining (D, filled bars) and non-declining (ND, hatched bars) trees of Quercus pubescens and Quercus cerris. The dashed and dotted horizontal lines indicate the 0.05 and 0.01 significance levels.

FIGURE 4

Drought-growth relationships (Pearson correlations) in declining (D) and non-declining (ND) trees of Quercus pubescens and Quercus cerris. The color scale shows the correlations between ring-width indices and the Standardized Precipitation Evapotranspiration Index (SPEI) calculated at 1–24 months long scales (x axes) from January to December (y axes). Significance levels are as in Figure 2.