Early-Warning Signals of Individual Tree Mortality Based on Annual Radial Growth

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter- annual growth variability and a decrease in growth synchrony in the last similar to 20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.Peer reviewe

Low growth resilience to drought is related to future mortality risk in trees

Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality.Fil: DeSoto, Lucía. Consejo Superior de Investigaciones Científicas; España. Universidad de Coimbra; PortugalFil: Cailleret, Maxime. Eidgenössische Technische Hochschule Züric; Suiza. Université Aix-marseille; Francia. Swiss Federal Institute for Forest, Snow and Landscape Research; SuizaFil: Sterck, Frank. University of Agriculture Wageningen; Países BajosFil: Jansen, Steven. Universitat Ulm; AlemaniaFil: Kramer, Koen. University of Agriculture Wageningen; Países Bajos. Land Life Company; Países BajosFil: Robert, Elisabeth M. R.. Creaf; España. Vrije Unviversiteit Brussel; Bélgica. Royal Museum for Central Africa; BélgicaFil: Aakala, Tuomas. University of Helsinki; FinlandiaFil: Amoroso, Mariano Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Recursos Naturales, Agroecología y Desarrollo Rural. - Universidad Nacional de Rio Negro. Instituto de Investigaciones en Recursos Naturales, Agroecología y Desarrollo Rural; ArgentinaFil: Bigler, Christof. Eidgenössische Technische Hochschule Züric; SuizaFil: Camarero, J. Julio. Consejo Superior de Investigaciones Científicas; EspañaFil: Čufar, Katarina. University 0f Ljubljana; EsloveniaFil: Gea Izquierdo, Guillermo. Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria; EspañaFil: Gillner, Sten. Technische Universität Dresden; AlemaniaFil: Haavik, Laurel J.. Servicio Forestal de los Estados Unidos; Estados UnidosFil: Hereş, Ana Maria. Basque Centre For Climate Change; España. Transilvania University of Brasov; RumaniaFil: Kane, Jeffrey M.. Humboldt State University; Estados UnidosFil: Kharuk, Vyacheslav I.. Siberian Federal University; Rusia. Siberian Division of the Russian Academy of Sciences; RusiaFil: Kitzberger, Thomas. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Klein, Tamir. Weizmann Institute of Science; IsraelFil: Levanič, Tom. Slovenian Forestry Institute; EsloveniaFil: Linares, Juan C.. Universidad Pablo de Olavide; EspañaFil: Mäkinen, Harri. Natural Resources Institute Finland; FinlandiaFil: Oberhuber, Walter. Universidad de Innsbruck; AustriaFil: Papadopoulos, Andreas. Geoponiko Panepistimion Athinon; GreciaFil: Rohner, Brigitte. Eidgenössische Technische Hochschule Zürich; Suiza. Swiss Federal Institute for Forest, Snow and Landscape Research; SuizaFil: Sangüesa Barreda, Gabriel. Universidad de Valladolid; EspañaFil: Stojanovic, Dejan B.. University of Novi Sad; SerbiaFil: Suarez, Maria Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; ArgentinaFil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Martínez Vilalta, Jordi. Universitat Autònoma de Barcelona; España. Creaf; Españ

Do Native Insects and Associated Fungi Limit Non-Native Woodwasp, Sirex noctilio, Survival in a Newly Invaded Environment?

Sirex noctilio F. (Hymenoptera: Siricidae) is an introduced pest of pines (Pinus spp.) in several countries in the Southern Hemisphere. Although S. noctilio is established in North America (first discovered in 2004), it has not been a destructive pest there so far, where forest communities more closely resemble those in its native Eurasian range—where it is not a pest. To investigate the influence of the existing community of associated insects (competitors + natural enemies) and fungi (vectored by insects) on S. noctilio survival in North America, we examined stage-specific mortality factors and their relative importance, generating life tables drawn from experimentally-manipulated and natural cohorts of Sirex spp. (mostly S. noctilio, but some native S. nigricornis F.). For both natural and experimentally-manipulated cohorts, factors which acted during the earliest Sirex life stages, most likely tree resistance and/or competition among fungal associates, were paramount in dictating woodwasp survival. Experimentally-manipulated life tables revealed that protection from the community of associates resulted in a significantly, and substantially larger (>15x) S. noctilio F1 generation than exposure to it. Seventy percent of generation mortality in the exposed cohort was due to tree resistance or unknown causes early in larval development, which could have included competition among other bark- or wood-inhabiting insects and/or their fungal associates. Only 46% of generation mortality in the protected cohort was due to tree resistance and/or unknown causes. Parasitoids, particularly endoparasitoids (Ibalia spp.), showed limited ability to control S. noctilio, and reduced the experimentally-established cohort by only 11%, and natural cohorts an average of 3.4%. The relative importance of tree resistance vs. competition with bark- and wood-borers in reducing S. noctilio survival remains unclear. Tree resistance and/or competition likely contribute more than natural enemies in maintaining the S. noctilio population in North America below damaging levels

Suitability of eastern pines for oviposition and survival of Sirex noctilio F.

The European woodwasp, Sirex noctilio F., is a pest of pines in many areas around the world. Since its introduction to North America, the distribution of S. noctilio overlaps with a known host (Pinus sylvestris) and hosts native to North America. Direct comparisons of suitability for oviposition and larval survival among these pines have not been made. We tested the relative suitability of four common pine species in northeastern North America (P. sylvestris, P. resinosa, P. banksiana, and P. strobus) as hosts for S. noctilio in a controlled, but in situ experiment. In a mixed pine forest in northern Ontario, we caged S. noctilio mating pairs on 10 freshly cut pine logs of each species, and estimated oviposition, counted adult S. noctilio (F1 generation) that emerged from logs, and calculated survivorship from egg to adult. Pinus sylvestris and P. resinosa were optimal hosts according to all three metrics of S. noctilio performance. Pinus strobus was a suitable larval host, but was not perceived as such by females, as evidenced by lower oviposition. Pinus banksiana was perceived as a suitable host by females, but was the least suitable larval host. Our results suggest that P. sylvestris and P. resinosa are more suitable hosts, at least in cut logs, than P. strobus and P. banksiana for S. noctilio in eastern North America

Suitability of eastern pines for oviposition and survival of <i>Sirex noctilio</i> F.

<div><p>The European woodwasp, <i>Sirex noctilio</i> F., is a pest of pines in many areas around the world. Since its introduction to North America, the distribution of <i>S</i>. <i>noctilio</i> overlaps with a known host (<i>Pinus sylvestris</i>) and hosts native to North America. Direct comparisons of suitability for oviposition and larval survival among these pines have not been made. We tested the relative suitability of four common pine species in northeastern North America (<i>P</i>. <i>sylvestris</i>, <i>P</i>. <i>resinosa</i>, <i>P</i>. <i>banksiana</i>, and <i>P</i>. <i>strobus</i>) as hosts for <i>S</i>. <i>noctilio</i> in a controlled, but <i>in situ</i> experiment. In a mixed pine forest in northern Ontario, we caged <i>S</i>. <i>noctilio</i> mating pairs on 10 freshly cut pine logs of each species, and estimated oviposition, counted adult <i>S</i>. <i>noctilio</i> (F1 generation) that emerged from logs, and calculated survivorship from egg to adult. <i>Pinus sylvestris</i> and <i>P</i>. <i>resinosa</i> were optimal hosts according to all three metrics of <i>S</i>. <i>noctilio</i> performance. <i>Pinus strobus</i> was a suitable larval host, but was not perceived as such by females, as evidenced by lower oviposition. <i>Pinus banksiana</i> was perceived as a suitable host by females, but was the least suitable larval host. Our results suggest that <i>P</i>. <i>sylvestris</i> and <i>P</i>. <i>resinosa</i> are more suitable hosts, at least in cut logs, than <i>P</i>. <i>strobus</i> and <i>P</i>. <i>banksiana</i> for <i>S</i>. <i>noctilio</i> in eastern North America.</p></div

Survivorship curves from (a) experimentally-manipulated and (b) natural <i>Sirex</i> cohorts.

<p>Different symbols correspond to different (a) treatments or (b) sites, and represent an estimate of the total population density of <i>Sirex</i> recovered from (a) 9 logs (0.07 m³ of wood) or (b) one tree (0.006–0.21 m³ of wood each), which were/was collected in October (egg—mid-sized larval stages) or in June (adult stage).</p

Number of logs and different trees (<i>Pinus sylvestris</i>) collected for <i>Sirex</i> life tables and the actual number of surviving insects recovered from each life stage.

<p>E and N refer to experimentally-manipulated and natural cohorts, respectively.</p><p>Number of logs and different trees (<i>Pinus sylvestris</i>) collected for <i>Sirex</i> life tables and the actual number of surviving insects recovered from each life stage.</p

Boxplots of estimated number of eggs oviposited by <i>Sirex noctilio</i> in each pine species.

<p>n = 20 logs per <i>Pinus</i> species. Boxes are bounded by the first and third quartiles; the internal solid line represents the median.</p