Multiyear defoliations in southern New England increases oak mortality

After decades of multiyear defoliation episodes in southern New England, Lymantria dispar dispar (previously gypsy moth) populations diminished with the appearance of the L. dispar fungus in 1989. Multiyear defoliations did not occur again until 2015-2018. To assess the impact of the return of multiyear defoliations, we examined 3095 oaks on 29 permanent study areas in Connecticut and Rhode Island that were established at least eleven years before the latest outbreaks. Pre-defoliation stand level oak mortality averaged 2% (three-year basis). Post-defoliation mortality did not differ between managed and unmanaged stands, but was much higher in severely defoliated stands (36%) than in stands with moderate (7%) or low-no defoliation (1%). Pre-defoliation mortality of individual trees differed among species, was lower for larger diameter trees and on unmanaged than managed stands. Post-defoliation mortality on plots with no to moderate defoliation was similar to pre-defoliation mortality levels. Following multiyear defoliations, white oak mortality was higher than for northern red and black oak. There was weak evidence that mortality was elevated on stands with higher basal area following severe defoliation. Natural resource managers should not assume that oaks that survived earlier multiyear defoliations episodes will survive future multiyear outbreaks, possibly because trees are older.

Antagonistic Potential of Native Trichoderma spp. against Phytophthora cinnamomi in the Control of Holm Oak Decline in Dehesas Ecosystems

Phytophthora root rot caused by the pathogen Phytophthora cinnamomi is one of the main causes of oak mortality in Mediterranean open woodlands, the so-called dehesas. Disease control is challenging; therefore, new alternative measures are needed. This study focused on searching for natural biocontrol agents with the aim of developing integrated pest management (IPM) strategies in dehesas as a part of adaptive forest management (AFM) strategies. Native Trichoderma spp. were selectively isolated from healthy trees growing in damaged areas by P. cinnamomi root rot, using Rose Bengal selective medium. All Trichoderma (n = 95) isolates were evaluated against P. cinnamomi by mycelial growth inhibition (MGI). Forty-three isolates presented an MGI higher than 60%. Twenty-one isolates belonging to the highest categories of MGI were molecularly identified as T. gamsii, T. viridarium, T. hamatum, T. olivascens, T. virens, T. paraviridescens, T. linzhiense, T. hirsutum, T. samuelsii, and T. harzianum. Amongst the identified strains, 10 outstanding Trichoderma isolates were tested for mycoparasitism, showing values on a scale ranging from 3 to 4. As far as we know, this is the first report referring to the antagonistic activity of native Trichoderma spp. over P. cinnamomi strains cohabiting in the same infected dehesas. The analysis of the tree health status and MGI suggest that the presence of Trichoderma spp. might diminish or even avoid the development of P. cinnamomi, protecting trees from the worst effects of P. cinnamomi root rot.

Predisposing factors’ effects on mortality of oak (Quercus) and hickory (Carya) species in mature forests undergoing mesophication in Appalachian Ohio

Background Mature oak (Quercus spp.) and hickory (Carya spp.) trees are gradually being replaced by more shade-tolerant tree species across the eastern U.S., likely due to fire suppression and increased precipitation. Oaks and hickories are highly valuable to wildlife; therefore, studying their mortality patterns can provide information on the longevity of habitat quality for many animal species. Oak mortality has most often been studied following large oak decline events, but background mortality rates in forests with aging oak and hickory canopies warrant equal attention, especially in the context of widespread oak and hickory regeneration failure. Methods We studied background mortality rates of five oak and one hickory species over a 23–25 year time period (1993–1995 to 2018), using 82 1/20th hectare permanent plots on the Marietta Unit of the Wayne National Forest in southeastern Ohio. We calculated mortality rates based on remeasurement of individual trees for white oak (Quercus alba), chestnut oak (Quercus montana), northern red oak (Quercus rubra), black oak (Quercus velutina), scarlet oak (Quercus coccinea), and pignut hickory (Carya glabra). For each of these species other than scarlet oak, we also modeled the relationships of mortality probability with a priori topographic, soil, stand structural, and individual tree covariates, using a mixed-effects logistic regression framework. Results The species with the highest mortality rate was scarlet oak (61.3%), followed by northern red oak (41.4%), black oak (26.7%), pignut hickory (23.9%), white oak (23.4%), and chestnut oak (19.1%). In our models, northern red oak mortality was associated with more mesic slope positions, shallower solums, more acidic soils, and older stand ages. Pignut hickory and chestnut oak mortality rates were associated with higher basal areas on the plot, while white oak mortality showed the opposite pattern. Conclusions Our data suggest that red oak subgenus trees in mature forests of our area will become increasingly uncommon relative to white oak subgenus trees, as the result of higher mortality rates likely related to the shorter lifespans of these species. Particularly vulnerable areas may include more mesic topographic positions, shallower or more acidic soil, and older stands. Since maintaining oak subgenus diversity is beneficial to wildlife diversity in the eastern U.S., managers in areas with extensive mature mixed-oak forests could choose to favor the red oak subgenus when conducting silvicultural treatments.

Pedunculate oak decline in southern Belgium: a long-term process highlighting the complex interplay among drought, winter frost, biotic attacks, and masting

Since 2013, pedunculate oak (Quercus robur L.) mortality has been observed in the Ardennes region of Belgium. We aimed to understand the current decline by retrospectively (1945–2015) studying radial growth patterns of trees classified by three health statuses (reference, declining, and dying) and by linking them to abiotic and biotic hazard history, which we recorded and quantified. Our results show that oak mortality in the Ardennes is a long-term process, with 1987 as a tipping point for growth trajectories of declining and dying trees. That year was preceded by two growth crises (1976–1981 and 1984–1987), and it falls within the last major episode of oak decline in Belgium. Among hazards, very cold winters and caterpillar outbreaks have significant impacts on growth-pattern differentiation. Apart from 1976, extreme drought is still rare; however, mild spring droughts, especially in the years n − 1 and n − 2, explain some of the growth loss relative to the reference trees. Finally, masting appears to be an important contributing factor for the death of weakened trees. Given the direct and delayed impacts of the extreme drought of 1976 and subsequent water balance impairment due to winter frosts and mild spring droughts, the health of pedunculate oak is giving cause for concern in the context of climate change.

A GIS approach to identify the Cork Oak Decline in M’Sila Forest (North-West of Algeria)

The decline of cork oak trees in North-West of Algeria was supposed to be caused by a combination of primary or predisposing factors, and secondary or opportunistic factors. There is a broad consensus about the role of water scarcity and nutrients in the primary causes, given the similarity of symptoms of cork oak decline and of chronic water deficits. To identify and characterize the cork oak decline in M’sila forest, a cork oak mortality index was determined for each transience area using a GIS approach and a linear regression model formulated by analyzing the interactions between the attributes of station (competition of superior and inferior floor); and allowing a justification of cork oaks mortality. The mortality index is significantly high in the invasive shrub plots and where the Aleppo pine density is very high; in which competition for soil water between the trees and the inferior floor increases. The lowest mortality value characterized by a lower density of Aleppo pine and herbaceous inferior floor.

Phytophthora Species Detected in Two Ozark Forests with Unusual Patterns of White Oak Mortality

Widespread decline and mortality of white oaks (Quercus alba) occurred in Missouri Ozark forests between 2011 and 2017. Symptoms included rapid crown death with bronzing of leaves, retention of dead leaves, crown dieback and thinning, and loss of large limbs within one year of death. Decline and mortality were associated with hillside drainages and fit descriptions of European oak forests predisposed to decline by pathogenic Phytophthora species. A survey was performed at two locations in 2014 and 2015 to assess the distribution of dead and declining white oaks, and the occurrence and distribution of Phytophthora species. Multiple Phytophthora species were detected, including P. cinnamomi, P. cactorum, P. europaea, and P. pini. P. cinnamomi was the most common and widely distributed species among plots at both locations. The detection of P. cinnamomi at the base of white oaks was not associated with poor crown vigor. However, more quantitative survey techniques are necessary to clearly evaluate this relationship. P. cinnamomi kills fine roots of white and red oaks in North America and has been associated with the decline of white oaks in the United States (Ohio) and other countries. Further studies are needed to determine the importance of P. cinnamomi in oak decline within the Ozark highlands.