Limited Growth Recovery after Drought-Induced Forest Dieback in Very Defoliated Trees of Two Pine Species

Abstract

Mediterranean pine forests display high resilience after extreme climatic events such as severe droughts. However, recent dry spells causing growth decline and triggering forest dieback challenge the capacity of some forests to recover following major disturbances. To describe how resilient the responses of forests to drought can be, we quantified growth dynamics in plantations of two pine species (Scots pine, black pine) located in south-eastern Spain and showing drought-triggered dieback. Radial growth was characterized at inter- (tree-ring width) and intra-annual (xylogenesis) scales in three defoliation levels. It was assumed that the higher defoliation the more negative the impact of drought on tree growth. Tree-ring width chronologies were built and xylogenesis was characterized 3 years after the last severe drought occurred. Annual growth data and the number of tracheids produced in different stages of xylem formation were related to climate data at several time scales. Drought negatively impacted growth of the most defoliated trees in both pine species. In Scots pine, xylem formation started earlier in the non-defoliated than in the most defoliated trees. Defoliated trees presented the shortest duration of the radial-enlargement phase in both species. On average the most defoliated trees formed 60% of the number of mature tracheids formed by the non-defoliated trees in both species. Since radial enlargement is the xylogenesis phase most tightly related to final growth, this explains why the most defoliated trees grew the least due to their altered xylogenesis phases. Our findings indicate a very limited resilience capacity of drought-defoliated Scots and black pines. Moreover, droughts produce legacy effects on xylogenesis of highly defoliated trees which could not recover previous growth rates and are thus more prone to die.

Keywords: Pinus nigra; Pinus sylvestris; dendroecology; die-off; extreme climate event; forest resilience; xylem; xylogenesis.

Figure 1

Recent patterns in basal area increment of the two study pine species according to defoliation intensity (D, severe defoliation, black symbols; I, intermediate defoliation, gray symbols; N, scarce or no defoliation, empty symbols). Values are means ± SE (standard errors) and the bars show the annual number of measured trees (right y axis, sample depth) for each defoliation type (same colors are used for growth and sample depth data).

Figure 2

Climate-growth relationships (Pearson correlation coefficients) calculated for the two pine species and considering three defoliation degrees (N, scarce or no defoliation, empty symbols; I, intermediate defoliation, gray symbols; D, severe defoliation, black symbols). Correlations were obtained for 10- and 15-day long intervals considering the 1985–2008 period, and they are presented for mean maximum temperatures and total precipitation. The dashed lines indicate the 0.05 significance levels. The bars show averaged (temperatures) and summed (precipitation) values for each variable.

Figure 3

Number of cambium cells and tracheids observed in different maturation stages (radial enlargement, thickening, maturing) of P. sylvestris and P. nigra trees grouped in three defoliation classes (D, severe defoliation; I, intermediate defoliation; N, scarce or no defoliation). In the lowermost graph the bars correspond to the number of latewood tracheids. Means are given with standard errors (n = 5 trees for each defoliation class).

Figure 4

Estimated onset and cessation dates and durations (±SD) of the main phases of xylogenesis (E, radial enlargement; L, lignification and cell-wall thickening; M, mature tracheids; X, xylem formation) in Scots pine (P. sylvestris) and black pine (P. nigra) trees of different defoliation classes (D, severe defoliation; I, intermediate defoliation; N, scarce or no defoliation). Onset and cessation dates of selected xylogenesis phases are represented by diamond-crossed-by-a-line marks whose left (right) end of the line represents the minimum (maximum), the left (right) end of the diamond the first (third) quartile and the middle of the diamond corresponds to the median.