First report of Albizia lebbeck dieback caused by Scytalidium dimidiatum in Oman

Over 200 Albizia lebbeck trees at Sultan Qaboos University campus wilted and died. The symptoms were dieback of large branches due to infection by Scytalidium dimidiatum. The fungus has also infected Ficus benghalensis, F. carica, F. retusa, Thespesia populnea, Delonix regia and Peltophorum petrocarpum. This is the first report of the fungus in Oman and on A. lebbeck, T. populnea, D. regia and P. petrocarpum. The strain of this fungus has not been found to cause human disease in Oman. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microbial activities in soils of a healthy and a declining reed stand

Microbial processes were investigated in the soil of a declining, more eutrophic (Romberk West) and a healthy looking, less eutrophic (Romberk East) freshwater reed stand. Soil was sampled monthly from June to September 1997. Glucose induced carbon dioxide (CO₂) production in oxic and anoxic conditions, methane (CH₄) production, nitrification and denitrification activities were measured in laboratory conditions in suspensions prepared from homogenised soil samples. Within a stand the proportion of anaerobic (as opposed to aerobic) microbial activity was greatest in June. Potential methanogenesis was highest in June and decreased later in both stands. Methane production was approximately the same in June at both stands but it was higher at Romberk East than at Romberk West stand in later months. Denitrifying activity was higher in August than July at both stands. Nitrifying activity was undetectable at both stands over the entire study period. Generally Romberk West was more anaerobic than Romberk East, with lower redox potential, higher amounts of oxygen-consuming organic matter and a lower ratio of CO₂ production in oxic conditions to CO₂ production in anoxic conditions. Microbial activity was apparently restricted at Romberk West stand in comparison to Romberk East. The shift from aerobic to anaerobic microbial metabolism and a coinciding restriction of metabolic activities at Romberk West are thought to be indicative of a strengthened oxygen stress in the soil, associated with accumulation of metabolites toxic to both the microorganisms and the reed. Possible links between eutrophication, microbial characteristics and reed performance are discussed.     

Secondary buds in Scots pine trees infested with <Emphasis Type="Italic">Gremmeniella abietina</Emphasis>

In winter 2000–2001, there was a serious outbreak of Gremmeniella abietina Morelet in southeastern Norway. During the outbreak, we noted that injured Scots pine trees (Pinus sylvestris L.) developed secondary buds in response to the fungus attack, and we decided to study the relationship between injury, appearance of secondary buds and recovery of the trees thereafter. For this purpose, 143 trees from 10 to 50 years of age were chosen and grouped into crown density classes. Injury was assessed in detail, and buds were counted before bud burst in the spring of 2002. In addition, a subset of 15 trees was followed through the summer of 2002 to assess recovery. All injured trees developed secondary buds, with a clear overweight of dormant winter buds in proportion to interfoliar buds. Healthy control trees did not develop secondary buds at all. The secondary buds appeared predominantly on the injured parts of the tree; interfoliar buds in particular developed just beneath the damaged tissue. Most of the secondary buds died during the winter of 2001–2002, mainly because the fungus continued to spread after the first outbreak. Many of the remaining buds developed shoots with abnormal growth during the summer. Secondary buds may help trees to recover from Gremmeniella attacks, but this strategy may fail when the fungus continues to grow and injure the newly formed buds and shoots.     

Rosellinia aquila Among Fungi on Branches of Sessile Oak with Symptoms of Decline

Rosellinia aquila was found for the first time in crowns of sessile oaks, Quercus robur (Mattuschka) Liebl., with symptoms of decline. The fungus occurred mostly on diseased branches but also colonized twigs with healthy appearance. Its anamorph and teleomorph stages were produced abundantly on dead branches after long incubation in moist conditions. Colonization of oak trees by R. aquila was likely to have been facilitated by higher humidity and temperature in the oak stands.     

Fire‐induced tree mortality in a neotropical forest: the roles of bark traits,tree size,wood density and fire behavior

Large‐scale wildfires are expected to accelerate forest dieback in Amazônia, but the fire vulnerability of tree species remains uncertain, in part due to the lack of studies relating fire‐induced mortality to both fire behavior and plant traits. To address this gap, we established two sets of experiments in southern Amazonia. First, we tested which bark traits best predict heat transfer rates (R) through bark during experimental bole heating. Second, using data from a large‐scale fire experiment, we tested the effects of tree wood density (WD), size, and estimated R (inverse of cambium insulation) on tree mortality after one to five fires. In the first experiment, bark thickness explained 82% of the variance in R, while the presence of water in the bark reduced the difference in temperature between the heat source and the vascular cambium, perhaps because of high latent heat of vaporization. This novel finding provides an important insight for improving mechanistic models of fire‐induced cambium damage from tropical to temperate regions. In the second experiment, tree mortality increased with increasing fire intensity (i.e. as indicated by bark char height on tree boles), which was higher along the forest edge, during the 2007 drought, and when the fire return interval was 3 years instead of one. Contrary to other tropical studies, the relationship between mortality and fire intensity was strongest in the year following the fires, but continued for 3 years afterwards. Tree mortality was low (≤20%) for thick‐barked individuals (≥18 mm) subjected to medium‐intensity fires, and significantly decreased as a function of increasing tree diameter, height and wood density. Hence, fire‐induced tree mortality was influenced not only by cambium insulation but also by other traits that reduce the indirect effects of fire. These results can be used to improve assessments of fire vulnerability of tropical forests.     

Future vulnerability mapping based on response to extreme climate events: Dieback thresholds in an endemic California oak

Aim

This study presents a bioclimate modelling approach, using responses to extreme climate events, rather than historical distributional associations, to project future species vulnerability and refugia. We aim to illustrate the compounding effects of groundwater loss and climate on species vulnerability.

Location

California, USA.

Methods

As a case study, we used the 2012–2015 California drought and resulting extensive dieback of blue oak (Quercus douglasii). We used aerial dieback surveys, downscaled climate data and subsurface water change data to develop boosted regression tree models identifying key thresholds associated with dieback throughout the blue oak distribution. We (1) combined observed dieback–climatic threshold relationships with climate futures to anticipate future areas of vulnerability and (2) used satellite‐derived measurements of subsurface water loss in drought/dieback modelling to capture the mediating effect of groundwater on species response to climatic drought.

Results

A model including climate, climate anomalies and subsurface water change explained 46% of the variability in dieback. Precipitation in 2015 and subsurface water change accounted for 62.6% of the modelled probability of dieback. We found an interaction between precipitation and subsurface water in which dieback probability increased with low precipitation and subsurface water loss. The relationship between precipitation and dieback was nonlinear, with 99% of dieback occurring in areas that received <363 mm precipitation. Based on a MIROC_rcp85 future climate scenario, relative to historical conditions, 13% of the blue oak distribution is predicted to experience more frequent years below this precipitation threshold by mid‐century and 81% by end of century.

Main conclusions

As ongoing climate change and extreme events impact ecological processes, the identification of thresholds associated with observed dieback may be combined with climate futures to help identify vulnerable populations and refugia and prioritize climate change‐related conservation efforts.