George Matusick

Context: Forest ecosystems experience compositional and structural changes as species’ environmental envelopes shift with climate change. Extreme climate events and pests/pathogens are driving these ecosystem changes. Determining which of the two potential drivers is causing a particular forest die-off can be challenging. In south-western Australia, widespread forest die-off in 2011 coincided with extremely hot and dry conditions. It occurred in a forest ecosystem that has historically experienced Phytophthora cinnamomi root disease (Phytophthora dieback). Aims: To determine whether the causal agent of Phytophthora dieback, P. cinnamomi, was associated with forest die-off in the Northern Jarrah Forest. Methods: A combination of direct (isolation of pathogen) and indirect (survey of susceptible indicator plant species) measurements were taken inside and outside patches of forest experiencing the die-off. Key results: There was no consistent association between die-off patches and the presence of P. cinnamomi. P. cinnamomi was isolated from 3 of 33 control plots and 3 of 33 die-off plots. Although several plant species susceptible to P. cinnamomi were absent from die-off plots, the findings were inconsistent across species. This may be explained by plant tolerance to high temperatures and drought. Conclusions: P. cinnamomi was not the proximate cause of the observed die-off in the Northern Jarrah Forest in 2011. Implications: Novel disturbance caused by extreme climate events can mimic damage caused by certain pests/pathogens. More research is needed to determine the tolerances of plants to extreme temperature and drought conditions to disentangle abiotic and biotic drivers of tree die-off.

The frequency and intensity of forest disturbances, such as drought and fire, are increasing globally, with an increased likelihood of multiple disturbance events occurring in short succession. Disturbances layered over one another may influence the likelihood or intensity of subsequent events (a linked disturbance) or impact response and recovery trajectories (a compound disturbance), with substantial implications for ecological spatiotemporal vulnerability. This study evaluates evidence for disturbance interactions of drought followed by wildfire in a resprouting eucalypt‐dominated forest (the Northern Jarrah Forest) in southwestern Australia. Sites were stratified by drought (high, low), from previous modeling and ground validation, and fire severity (high, moderate, unburnt), via remote sensing using the relative difference normalized burn ratio (RdNBR). Evidence of a linked disturbance was assessed via fine fuel consumption and fire severity. Compound disturbance effects were quantified at stand scale (canopy height, quadratic mean diameter, stem density) and stem scale (mortality). There was no evidence of prior drought influencing fine fuel consumption or fire severity and, hence, no evidence of a linked disturbance. However, compound disturbance effects were evident; stands previously affected by drought experienced smaller shifts in canopy height, quadratic mean diameter, and stem density than stands without prior drought impact. At the stem scale, size and fire severity were the strongest determinants of stem survival. Proportional resprouting height was greater in high drought sites than in low drought sites (p < 0.01), meaning, structurally, the low drought stands decreased in height more than the high drought stands. Thus, a legacy of the drought was evident after the wildfire. Although these resprouting eucalypt forests have been regarded as particularly resilient, this study illustrates how multiple disturbances can overwhelm the larger tree component and promote an abundance of smaller stems. We suggest that this is early evidence of a structural destabilization of these forests under a more fire‐prone, hotter, and drier future climate.

Many public land management programs in the southeastern United States have been restoring the longleaf pine forest for more than 20 years, which includes intensive treatment with fire, thinning, chemical control of competition, and tree planting. A shift to more passive management (prescribed burning alone) is anticipated once a critical level of longleaf pine has been established. It remains unclear whether this longleaf pine threshold has been reached and whether intensive management should continue at Fort Benning, Georgia. Using the Landis-II forest landscape model, changes in tree species and forest types were estimated from 2017 to 2117 under four forest management scenarios, ranging from passive (“burn only”) to intensive (“proactive”). The desired future condition includes 75% of upland forest dominated by longleaf pine (>49.5% composition). The proactive scenario resulted in the desired future forest condition, whereas reactive and passive scenarios did not. These results suggest a critical threshold of longleaf pine forest has not been reached at Fort Benning and therefore intensive management approaches are still required. This study shows that even well-established populations of longleaf pine on public lands require maintenance and continued intensive restoration to reach desired forest-wide conditions.

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.

Significance: Forests are experiencing growing risks of drought-induced mortality in a warming world. Yet, ecosystem dynamics following drought mortality remain unknown, representing a major limitation to our understanding of the ecological consequences of climate change. We provide an emerging picture of postdrought ecological trajectories based on field indicators of forest dynamics. Replacement patterns following mortality indicate limited short-term persistence of predrought dominant tree species, highlighting the potential for major ecosystem reorganization in the coming decades. The great variability of the observed dynamics within and among species reinforces the primary influence of drought characteristics and ecosystem legacies, modulated by land use, management, and past disturbances, on ongoing drought-related species turnover and their potential implications for future forest biodiversity and ecosystem services.

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.

In recent decades, conservation objectives have driven changes to the management of some pine forests in the southeastern United States. Forest thinning and frequent burning of old‐field and plantation pine forests have resulted in an open loblolly–shortleaf pine forest community which resembles the original longleaf pine forest. It is, however, unclear how the structure, composition, and function of the loblolly–shortleaf forest compare to natural longleaf pine forest, and whether it represents an alternative stable state, or simply a transitional state. Understanding the stability of open loblolly–shortleaf pine forest is critical, particularly because several threatened and endangered species are now reliant on it for habitat. The structure and composition of loblolly–shortleaf forest and natural longleaf pine forest were compared using data from permanent forest plots at Fort Benning, Georgia, USA. To assess the stability of the loblolly–shortleaf pine forest and determine whether it is an alternative stable or transitional state, the LANDIS‐II forest landscape simulation model was used to simulate changes in forest type cover under no disturbance, and a frequent‐fire regime at Fort Benning. Under both management scenarios, nearly all loblolly–shortleaf pine forest converted to mixed hardwood forest over the course of the simulation, with most conversion occurring within 60 yr. In contrast, longleaf pine forest cover increased under frequent fire. Several important structural and compositional differences may have contributed to the instability of loblolly–shortleaf pine forest compared to longleaf pine forest. These include, among other factors, higher densities of resprouting hardwood trees and shrubs in loblolly–shortleaf pine forest, including sweetgum, a resilient broadleaf species capable of transforming ecosystem structure. These results highlight the instability of the open loblolly–shortleaf pine forest community and confirm that is a transitional state, destined for mixed hardwood forest in the coming decades under either no disturbance or frequent fire alone. Future forest planning should consider an active transition from the loblolly–shortleaf pine forest in the coming decades if open pine forest is to be conserved for wildlife and conservation objectives.

Prolonged drought and intense heat‐related events trigger sudden forest die‐off events and have now been reported from all forested continents. Such die‐offs are concerning given that drought and heatwave events are forecast to increase in severity and duration as climate change progresses. Quantifying consequences to carbon dynamics and storage from die‐off events are critical for determining the current and future mitigation potential of forests. We took stand measurements five times over 2+ years from affected and unaffected plots across the Northern Jarrah Forest, southwestern Australia, following an acute drought/heatwave in 2011. We found a significant loss of live standing carbon (49.3 t ha⁻¹), and subsequently a significant increase in the dead standing carbon pool by 6 months post‐die‐off. Of the persisting live trees, 38% experienced partial mortality contributing to the rapid regrowth and replenishment (82%–88%) of labile carbon pools (foliage, twigs, and branches) within 26 months. Such regrowth was not substantial in terms of net carbon changes within the timeframe of the study but does reflect the resprouting resilience of this forest type. Dead carbon generated by the die‐off may persist for centuries given low fragmentation and decay rates resulting in low biogenic emission rates relative to other forest types. However, future fire may threaten persistence of both dead and live pools via combustion and mortality of live tissue and impaired regrowth capacity. Resprouting forests are commonly regarded as resilient systems, however, a changing climate could see vulnerable portions of forests become carbon sources rather than carbon sinks.