Significance: Forests are experiencing growing risks of drought-induced mortality in a warming world. Yet, ecosystem dynamics following drought mortality remain unknown, representing a major limitation to our understanding of the ecological consequences of climate change. We provide an emerging picture of postdrought ecological trajectories based on field indicators of forest dynamics. Replacement patterns following mortality indicate limited short-term persistence of predrought dominant tree species, highlighting the potential for major ecosystem reorganization in the coming decades. The great variability of the observed dynamics within and among species reinforces the primary influence of drought characteristics and ecosystem legacies, modulated by land use, management, and past disturbances, on ongoing drought-related species turnover and their potential implications for future forest biodiversity and ecosystem services.

In this study, we investigated the potential of the photochemical resistance index (PRI) to track photosynthetic activity under water stress conditions by measuring PRI, leaf fluorescence, the xanthophyll cycle and photosynthetic activity in different forest tree species subjected to progressive drought. The PRI declined with pre-dawn water potential and a significant relationship between PRI and the xanthophyll de-epoxidation state (DEPS) was observed, although with large interspecific variability in the sensitivity of PRI to changes in DEPS. For single tree species, a strong relationship was observed on either PRI light saturated photosynthesis or PRI maximum photochemical efficiency of PSII (?F/Fm'); a larger variability in both relationships was apparent when data from different species were pooled together. However, an improved correlation was shown only in the former relationship by plotting the ?PRI (dawn PRI minus the midday PRI values). Thus, we conclude that PRI is able to provide a good estimate of maximum CO2 assimilation at saturating light and ?F/Fm' for single tree species, despite the severe drought conditions applied. PRI should be applied more cautiously when dealing with multispecific forests because of confounding factors such as the strong interspecific differences in the initial value of PRI and in the sensitivity of PRI to changes in DEPS in response to drought.

Old-growth mountain forests represent an ideal setting for studying long-term impacts of climate change. We studied the few remnants of old-growth forests located within the Pollino massif (southern Italy) to evaluate how the growth of conspecific young and old trees responded to climate change. We investigated two conifer species (Abies alba and Pinus leucodermis) and two hardwood species (Fagus sylvatica and Quercus cerris). We sampled one stand per species along an altitudinal gradient, ranging from a drought-limited low-elevation hardwood forest to a cold-limited subalpine pine forest. We used a dendrochronological approach to characterize the long-term growth dynamics of old (age > 120 years) versus young (age < 120 years) trees. Younger trees grew faster than their older conspecifics during their juvenile stage, regardless of species. Linear mixed effect models were used to quantify recent growth trends (1950-2015) and responses to climate for old and young trees. Climate sensitivity, expressed as radial growth responses to climate during the last three decades, partially differed between species because high spring temperatures enhanced conifer growth, whereas F. sylvatica growth was negatively affected by warmer spring conditions. Furthermore, tree growth was negatively impacted by summer drought in all species. Climate sensitivity differed between young and old trees, with younger trees tending to be more sensitive in P. leucodermis and A. alba, whereas older F. sylvatica trees were more sensitive. In low-elevation Q. cerris stands, limitation of growth due to drought was not related to tree age, suggesting symmetric water competition. We found evidence for a fast-growth trend in young individuals compared with that in their older conspecifics. Notably, old trees tended to have relatively stable growth rates, showing remarkable resistance to climate warming. These responses to climate change should be recognized when forecasting the future dynamics of old-growth forests for their sustainable management.

The oxygen isotope enrichment of bulk leaf water (Δ_b) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δ_b increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (e_a/e_i) and to stomatal conductance (g_s). The oxygen isotope enrichment of lamina water relative to source water \(({\bar{{\Delta}}}_{1}),\) which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δ_b (and \({\bar{{\Delta}}}_{1}),\) as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δ_b and \({\bar{{\Delta}}}_{1}\) with an L of 7.9 mm, much less than the total radial effective length L_r of 43 mm. The fitted values of L for \({\bar{{\Delta}}}_{1}\) of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.

Forest dieback and mortality events induced by drought stress are widely reported. However, few studies have jointly examined the role played by drought on growth and mortality in tree species inhabiting floodplain forests. Here, we focused on mortality events occurring since the early 2000s on large areas in a floodplain forest located within the Ticino regional park in Northwest Italy, where affected native (pedunculate oak, Quercus robur L.) and introduced tree species (black locust, Robinia pseudoacacia L.) coexist. We related growth with climate data and drought severity to discern if these species were similarly affected by drought. Then, we: (i) evaluated the presence of pathogens of the genus Phytophthora in recently dead oak trees since this was the most affected species and pathogens are often associated with oak decline cases; and (ii) compared xylem vessel diameter and tree-ring C isotope discrimination (δ¹³C) to highlight differences in water-use strategies between living and dead trees in both species. The radial growth of living and dead trees started diverging in the 1970s, although only after warm-drought periods occurred during 1990s did this divergence become significant. Growth of trees that died responded more negatively to drought than in the case of living trees. Moreover, trees that died formed smaller xylem vessels in the past than living trees and also showed more negative δ¹³C values in both tree species, indicating a higher intrinsic water-use efficiency in living than in dead trees. The pathogen Phytophthora cinnamomi Rands was only detected in one recently dead tree, suggesting that it is unlikely that dead oaks were predisposed to drought damage by the pathogen. We conclude that a climate shift from wet to warm-dry summer conditions in the early 1990s triggered forest dieback and induced mortality in both tree species. Temperate floodplain forests are susceptible to drought-induced dieback. The drought-sensitivity of both species could lead to successional shifts driven by a reduction of N inputs through N-fixing by black locust and the replacement of oak by drought-tolerant species.