Abstract
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.
Highlights
One major question is to forecast how forests will respond to Anthropocene conditions, including how native and tree invasive will coexist [1]
These will include a sustained rise in temperatures, more severe droughts, more widespread invasive tree species, and extensive incidence of pests and diseases, which have been identified as major drivers of tree mortality worldwide [2]
To quantify climate-growth associations, we focused on summer conditions since drought during that season has been shown to adversely affect oak growth in floodplain forests (e.g., [12])
Summary
One major question is to forecast how forests will respond to Anthropocene conditions, including how native and tree invasive will coexist [1] These will include a sustained rise in temperatures, more severe droughts, more widespread invasive tree species, and extensive incidence of pests and diseases (e.g., fungal pathogens), which have been identified as major drivers of tree mortality worldwide [2]. Such mortality events are often accompanied by growth decline, canopy dieback, drops in productivity, and other effects which alter the ecosystem services provided by forests [3]. Studies of growth decline and mortality in floodplain forests are scarce despite their particular conditions (water-saturated soils and high air humidity), which may explain why drought is not always considered a major driver of dieback there (but see [7,8]), as compared with other sites with a lower soil water availability [9,10,11]
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