Forest and woodland replacement patterns following drought-related mortality

Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern post drought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.Peer reviewe

Contrasting effects of fog frequency on the radial growth of two tree species in a Mediterranean-temperate ecotone

International audienceThe performance and persistence of rear-edge tree populations are relevant issues for conserving biodiversity because these stands harbor high intraspecific biodiversity and play a key role during periods of climate change. The occurrence of these populations is associated with the influence of heterogeneous topography, creating suitable refugia with regionally rare environmental conditions. Climate is changing at a global-scale, but little is known about the long-term impact on local climatic singularities and the associated taxa. We analyzed tree-ring growth chronologies of the two species (Fagus sylvatica and Quercus ilex) forming the evergreen-deciduous forest ecotone, constitutive of the rear-edge of F. sylvarlca distribution. The study area is a coastal range with frequent fog immersion, which has been hypothesized to favor the persistence of F. sylvatica in Mediterranean peninsulas. We analyzed the long-term effect of fog on tree growth along a topographical gradient and the sensitivity of growth to rainfall and temperature. The annual number of foggy days has decreased by 62% over the last four decades, concomitant with increasing temperatures. Fog frequency was a relevant factor determining tree growth; fog during summer had positive effects on F. sylvatica growth mainly through a temperature buffering effect. The positive effect of fog on the growth of Q. ilex, however, was likely caused by a collinearity with rainfall. Q. ilex growth was less sensitive to climate than F. sylvatica, but growth of both species was enhanced by a positive early-summer water balance. Our results indicate that a decrease in fog frequency and an increase in temperature may generally benefit Q. ilex in this forest ecotone. Although future changes in rainfall and temperature matter most for the fate of rear-edge tree populations, local climatic singularities such as fog should also be considered. Those can have complementary effects that can swing the balance in ecotones and rear-edge tree populations such as those studied here

Forgetting Fire: Traditional Fire Knowledge in Two Chestnut Forest Ecosystems of the Iberian Peninsula and its Implications for European Fire Management Policy

Human beings have used fire as an ecosystem management tool for thousands of years. In the context of the scientific and policy debate surrounding potential climate change adaptation and mitigation strategies, the importance of the impact of relatively recent state fire-exclusion policies on fire regimes has been debated. To provide empirical evidence to this ongoing debate we examine the impacts of state fire-exclusion policies in the chestnut forest ecosystems of two geographically neighbouring municipalities in central Spain, Casillas and Rozas de Puerto Real. Extending the concept of 'Traditional Ecological Knowledge' to include the use of fire as a management tool as 'Traditional Fire Knowledge' (TFK), we take a mixed-methods and interdisciplinary approach to argue that currently observed differences between the municipalities are useful for considering the characteristics of >pre-industrial anthropogenic fire regimes> and their impact on chestnut forest ecosystems. We do this by examining how responses from interviews and questionnaire surveys of local inhabitants about TFK in the past and present correspond to the current biophysical landscape state and recent fire activity (based on data from dendrochronological analysis, aerial photography and official fire statistics). We then discuss the broader implications of TFK decline for future fire management policies across Europe particularly in light of the published results of the EU sponsored Fire Paradox research project. In locations where TFK-based >pre-industrial anthropogenic fire regimes> still exist, ecosystem management strategies for adaptation and mitigation to climate change could be conceivably implemented at a minimal economic and political cost to the state by local communities that have both the TFK and the adequate social, economic and cultural incentives to use it. © 2015 Elsevier Ltd.This research was made possible by an Academic Outreach Engagement Grant from Middlebury College. The following Middlebury College and New York University students volunteered as interviewers during the Fall of 2012 and Spring of 2013 semesters: Kimberly Sable, Kaelin Stone, Charlotte O’Herron, Forrest Carroll, Zuzana Vuova, Peter Elbaum, Jillian Mock, Jessica Davis, Cody Beaudreau, Aidan McGrath, Gabrielle Fromer, Fran Bullard, William Marrs, Rosie Mazzarella, Emily Duh, Shaun Devlin, Martin Kim, Samuel Schwartzbad, Priyanka Jhaveri, Renee Antoine, Phillip Origlio, Michael Cutrone, Louis Bedford, Lindsey Skolnik, Nino Kakauridze, Giovanni Barcenes, Ann Yang and Rachel Rinehart. Francisco Seijo would like to thank the Fundación “Equo” for its help in finding local volunteers for the project and the municipal governments of Rozas de Puerto Real and Casillas – and particularly David Saugar and Daniel Moreno – for their kind and disinterested collaboration in the deployment of the survey questionnaire. FS would also like to express his gratitude to Beatriz Pérez Ramos from the Universidad de Castilla-La Mancha and Peter Fule of Northern Arizona University for their helpful comments and contributions to this paper and to Captain Jorge Garcia Rodriguez of the Spanish army for allowing us access to military aerial photographs. Jorge Lozano is being supported by a Prometeo Fellowship from the SENESCYT, a national agency for Education and Science of the Government of Ecuador. James Millington would like to acknowledge the Leverhulme Trust for his Early Career Fellowship (ECF/2010/0378) which funded his fieldwork in the study area. G. Sangüesa-Barreda and J.J. Camarero contributions to this study were supported by projects CGL2011-26654 (Spanish Ministry of Economy and Competitiveness) and 1032S/2013 (OAPN, Spanish Ministry of Agriculture and Environment). All authors would also like to express their gratitude to the people of Rozas and Casillas for their hospitality and, especially, patience in responding to our questions.Peer Reviewe

A synthesis of radial growth patterns preceding tree mortality

Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1–100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks