Tuart canopy die-off during severe drought and heatwave

A severe and sudden die-off event, occurring in the regionally significant tuart (Eucalyptus gomphocephala) woodland in Rockingham Regional Park, coincided with extreme drought and heat conditions in early 2011

Within-tree distribution and survival of the eucalyptus longhorned borer Phoracantha semipunctata (Coleoptera: Cerambycidae) in a Mediterranean-type ecosystem

The attack patterns, infestation success and larval development of woodborers within living trees are complex and are largely shaped by host tree characteristics. Following a severe drought in a native eucalypt forest where outbreak densities of a native Australian beetle, the eucalyptus longhorned borer (Phoracantha semipunctata), occurred, a tree dissection study was conducted in Australia. This involved felling 40 trees each of jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) that were cut into 1-m sections and neonate larval galleries, larvae in pupal cells and adult borer emergence were measured and added to give total numbers per tree to determine the within-tree distribution and survival of P. semipunctata. There was a significant impact on larval survival in both species, in contrast, pupal survival remained high. Within-tree distribution of P. semipunctata was directional with borer emergence and incidence of larval galleries both negatively associated with tree section height above the ground and positively associated with section diameter and bark thickness, reaching a maximum towards the base of trees. High incidence and survival in lower thicker tree sections indicate a more conducive environment for larval development, in contrast to poor larval survival in smaller thinner sections at the top of trees. The dependence of larval survival on tree characteristics controlling the within-tree distribution of borer emergence is emphasized, and needs to be considered when estimating the spread of borer populations during outbreaks

Stem functional traits vary among co-occurring tree species and forest vulnerability to drought

Context: Stem functional traits are critical for tree hydraulic infrastructure and have important consequences for forest function, particularly concerning vulnerability to drought. Methods: Three stem traits, sapwood area, heartwood area, and bark area, were measured in two co-dominant forest species, Eucalyptus marginata Donn. Ex. Sm. and Corymbia calophylla (Lindl.) K.D.Hill & L.A.S.Johnson, in forest patches with low and high vulnerabilities to drought in south-western Australia. Patches of high drought vulnerability experienced die-off during a heatwave and drought in 2011, while patches of low vulnerability were largely not affected. Key results: Sapwood area was significantly higher in C. calophylla than in E. marginata, and C. calophylla maintained more sapwood per unit DBH than did E. marginata, especially in larger trees. There was a 29% smaller sapwood area in high drought-vulnerability patches than in low drought-vulnerability patches (including both species). The relationship between sapwood area and DBH varied by tree size. Small trees had a greater sapwood area in high drought-vulnerable patches, whereas larger trees had more sapwood in low drought-vulnerable patches. It is unclear whether sapwood area relationships reflect differences in leaf area or tree age. Conclusions: Observed differences in sapwood between species may help explain their differential tolerance to drought, whereas differences between drought-vulnerability sites may suggest adaptation in the studied species. Implications: Understanding the traits associated with drought vulnerability will increase our prediction of forest response to drying and warming. Strong relationships between stem traits and DBH, developed here, may help future efforts to model water-use in the Northern Jarrah Forest

What happens to fuels and fire potentials after drought-induced forest die-off?

Forest die-offs associated with drought and heat have recently occurred across the globe, raising concern that changes in fuels and microclimate accompanying die-off could affect subsequent fire behaviour. Despite widespread concern, little empirical data exist

Forest die-off following global-change-type drought alters rhizosphere fungal communities

Globally, forest die-off from global-change-type drought events (hotter droughts) are of increasing concern, with effects reported from every forested continent. While implications of global-change-type drought events have been explored for above-ground vegetation, below-ground organisms have received less attention, despite their essential contributions to plant growth, survival, and ecosystem function. We investigated rhizosphere fungal communities in soils beneath trees affected by a global-change-type drought in a Mediterranean climate-type ecosystem in southwestern Australia, quantifying how fungal richness, composition and functional groups varied along a drought impact gradient. Following a forest die-off three years previously, we collected soils beneath dead and alive trees within forest exhibiting high, minimal and relatively unaffected levels of forest die-off. Rhizosphere fungal DNA was extracted from soils, amplified and subjected to high throughput sequencing. Fungal community composition varied significantly (P < 0.001) along the drought impact gradient with less richness in drought affected stands. There was some evidence of community differentiation between dead versus alive trees (P = 0.09), and no difference in rarefied richness and diversity. When considered by functional group, die-off-impacted plots had more arbuscular mycorrhizal fungi (AM) and saprotrophs, and fewer ectomycorrhizal fungi (ECM), compared with living trees from the unaffected plots. Further, within die-off plots, dead versus alive tree rhizosphere samples contained more AM, saprotrophs and pathogens, and fewer ECM. Disruptions to rhizosphere fungal communities, such as altered functional groups, can have implications for ecosystem persistence and function, particularly in regions projected to experience increased global-change-type drought events

Chronic historical drought legacy exacerbates tree mortality and crown dieback during acute heatwave-compounded drought

Globally, combinations of drought and warming are driving widespread tree mortality and crown dieback. Yet thresholds triggering either tree mortality or crown dieback remain uncertain, particularly with respect to two issues: (i) the degree to which heat waves, as an acute stress, can trigger mortality, and (ii) the degree to which chronic historical drought can have legacy effects on these processes. Using forest study sites in southwestern Australia that experienced dieback associated with a short-term drought with a heatwave (heatwave-compounded drought) in 2011 and span a gradient in long-term precipitation (LTP) change, we examined the potential for chronic historical drought to amplify tree mortality or crown dieback during a heatwave-compounded drought event for the dominant overstory species Eucalyptus marginata and Corymbia calophylla. We show pronounced legacy effects associated with chronically reduced LTP (1951–1980 versus 1981–2010) at the tree level in both study species. When comparing areas experiencing 7.0% and 11.5% decline in LTP, the probability of tree mortality increased from low (0.55) in both species, and probability of crown dieback increased from high (0.74) to nearly complete (0.96) in E. marginata. Results from beta regression analysis at the stand-level confirmed tree-level results, illustrating a significant inverse relationship between LTP reduction and either tree mortality (F = 10.39, P = 0.0073) or dieback (F = 54.72, P < 0.0001). Our findings quantify chronic climate legacy effects during a well-documented tree mortality and crown dieback event that is specifically associated with an heatwave-compounded drought. Our results highlight how insights into both acute heatwave-compounded drought effects and chronic drought legacies need to be integrated into assessments of how drought and warming together trigger broad-scale tree mortality and crown dieback events

Rapid characterisation of vegetation structure to predict refugia and climate change impacts across a global biodiversity hotspot

Identification of refugia is an increasingly important adaptation strategy in conservation planning under rapid anthropogenic climate change. Granite outcrops (GOs) provide extraordinary diversity, including a wide range of taxa, vegetation types and habitats in the Southwest Australian Floristic Region (SWAFR). However, poor characterization of GOs limits the capacity of conservation planning for refugia under climate change. A novel means for the rapid identification of potential refugia is presented, based on the assessment of local-scale environment and vegetation structure in a wider region. This approach was tested on GOs across the SWAFR. Airborne discrete return Light Detection And Ranging (LiDAR) data and Red Green and Blue (RGB) imagery were acquired. Vertical vegetation profiles were used to derive 54 structural classes. Structural vegetation types were described in three areas for supervised classification of a further 13 GOs across the region.Habitat descriptions based on 494 vegetation plots on and around these GOs were used to quantify relationships between environmental variables, ground cover and canopy height. The vegetation surrounding GOs is strongly related to structural vegetation types (Kappa = 0.8) and to its spatial context. Water gaining sites around GOs are characterized by taller and denser vegetation in all areas. The strong relationship between rainfall, soil-depth, and vegetation structure (R2 of 0.8–0.9) allowed comparisons of vegetation structure between current and future climate. Significant shifts in vegetation structural types were predicted and mapped for future climates. Water gaining areas below granite outcrops were identified as important putative refugia. A reduction in rainfall may be offset by the occurrence of deeper soil elsewhere on the outcrop. However, climate change interactions with fire and water table declines may render our conclusions conservative. The LiDAR-based mapping approach presented enables the integration of site-based biotic assessment with structural vegetation types for the rapid delineation and prioritization of key refugia

Reviewing the use of resilience concepts in forest sciences

Purpose of the review Resilience is a key concept to deal with an uncertain future in forestry. In recent years, it has received increasing attention from both research and practice. However, a common understanding of what resilience means in a forestry context, and how to operationalise it is lacking. Here, we conducted a systematic review of the recent forest science literature on resilience in the forestry context, synthesising how resilience is defined and assessed. Recent findings Based on a detailed review of 255 studies, we analysed how the concepts of engineering resilience, ecological resilience, and social-ecological resilience are used in forest sciences. A clear majority of the studies applied the concept of engineering resilience, quantifying resilience as the recovery time after a disturbance. The two most used indicators for engineering resilience were basal area increment and vegetation cover, whereas ecological resilience studies frequently focus on vegetation cover and tree density. In contrast, important social-ecological resilience indicators used in the literature are socio-economic diversity and stock of natural resources. In the context of global change, we expected an increase in studies adopting the more holistic social-ecological resilience concept, but this was not the observed trend. Summary Our analysis points to the nestedness of these three resilience concepts, suggesting that they are complementary rather than contradictory. It also means that the variety of resilience approaches does not need to be an obstacle for operationalisation of the concept. We provide guidance for choosing the most suitable resilience concept and indicators based on the management, disturbance and application context

Drought and heat triggers sudden and severe dieback in a dominant Mediterranean-type woodland species

Ecosystems in Mediterranean climate regions are projected to undergo considerable changes as a result of shifting climate, including from extreme drought and heat events. A severe and sudden dieback event, occurring in regionally significant Eucalyptus gomphocephala woodland in Western Australia, coincided with extreme drought and heat conditions in early 2011. Using a combination of remote sensing and field- based approaches, we characterized the extent and severity of canopy dieback following the event, as well as highlighted potential predisposing site factors. An estimated 500 ha of woodland was severely affected between February and March 2011. Tree foliage rapidly discolored and died over this period. In the affected portion of the woodland, approximately 90% of trees greater than 20 cm DBH were impacted, while in the adjacent unaffected woodland 6% showed signs of damage. Tree density in the unaffected area had approximately 4.5 times more trees than the affected woodland. Precipitation drainage patterns are thought to explain the difference between affected and unaffected woodland. Dropping groundwater levels, a relatively shallow soil profile, and extreme drought and heat in 2010-2011 are thought to predispose water-shedding sites to drought-triggered canopy dieback during extended periods of dryness. Tracking forest health changes in response to severe disturbance is an important key to deciphering past and future vegetation change