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Research Highlight discussing: Fajardo, A., Moreno‐Meynard, P. & Soto, D. P. (2024). Forest stand dynamics of a short‐stature tree species: Ecological knowledge for sustainable forest management. Journal of Applied Ecology, https://doi.org/10.1111/1365‐2664.14662.Oliver stand dynamics conceptual model of tree stand succession was designed for tall trees but is not necessarily applicable to short‐stature tree species, which represent a large proportion of forests/woodlands worldwide.The South American short tree/shrub Nothofagus antarctica is a persuasive example of a short tree species that can benefit from an alternative conceptual model on stand dynamics. It also illustrates the high variability in stand dynamics observed in many short‐stature trees.Stand dynamics of short‐stature trees remain generally understudied, hindering their management and silvicultural use. Although generalisations may not be feasible due to the continuum of growth forms and succession dynamics of short trees, there is an urgent need to increase our knowledge for their sustainable management. [ABSTRACT FROM AUTHOR]

Rising temperature has been associated with increased occurrence of herbivorous insect outbreaks, explained by several direct and indirect mechanisms. Whereas natural enemies are known key drivers of forest‐defoliating insect cycles, indirect effects of temperature on insect's ability to defend against pathogens and parasitoids (e.g., immunocompetence), as well as the interaction with other mechanisms (e.g., diet), remain less explored. The aim of this study is to evaluate the effect of temperature and diet on the performance and immune response of the model lepidopteran system Ormiscodes amphimone (Saturniidae) and its host plants Nothofagus spp. (Nothofagaceae).Larvae of O. amphimone were reared under two temperature conditions (ambient 18:6°C and warmed, 21:6°C; light: dark, 14:10 h) and on leaves of two of their preferred Nothofagus host plants, which vary in quality (lower N. antarctica–higher N. pumilio). We measured developmental time, female pupal weight as a proxy of fitness, relative growth rate, nutritional indices and melanisation of a monofilament as a proxy of immune response.Results showed that an average rise of 2°C favours larval immunocompetence, potentially decreasing mortality exerted by parasitoids. Moreover, depending on diet, an increase in temperature can either maintain (on more nutritious N. pumilio leaves) or enhance (on less nutritious N. antarctica leaves) larval nutritional efficiency, performance and female pupal weight.Hence, an increase in temperature could enhance O. amphimone population growth, through attenuating differences caused by diet and enhancing immunocompetence, favouring outbreak frequency, severity and area. [ABSTRACT FROM AUTHOR]

Intentionally allowing or promoting invasion by non‐native trees into areas characterized by treeless vegetation could contribute to climate‐change mitigation by increasing carbon (C) sequestration. In some areas of the world, incentives exist to retain invasive non‐native trees in natural systems as a mechanism for increasing ecosystem C storage and reducing atmospheric carbon dioxide levels. Although this novel opportunity for C sequestration holds appeal, such an approach is problematic for several reasons: (1) invasive trees do not always increase net C sequestration due to greater occurrence of fire or reduced soil C; (2) lower albedo in invaded areas can increase absorption of solar radiation, thereby offsetting potential C sequestration; and (3) tree invasions often also have negative effects on biodiversity, economic opportunities, and water yield. Such drawbacks are sufficient to raise doubts about the widespread use of non‐native tree invasions in treeless areas as a tool to ameliorate climate change. [ABSTRACT FROM AUTHOR]

Questions: Increased wildfire activity is resulting in plant community‐type conversions worldwide. In some regions, fire‐sensitive forests are being replaced by flammable fire‐resilient communities, increasing the likelihood of reburning due to positive fire feedbacks. Here we evaluated whether fire severity affects post‐fire plant community flammability attributes that lead to community‐type conversions and changes in the likelihood of reburning. Specifically, we assessed how fire severity, the dominant pre‐fire vegetation, and distance to unburned remnant forest may shape post‐fire community properties and flammability trajectories in northwestern Patagonia. Location: Northwestern Patagonia, Argentina. Methods: We repeated sampling of Nothofagus pumilio, Nothofagus dombeyi, and Austrocedrus chilensis forests and native shrublands (composed of multiple shrub species) two and 18 years after fires that burned at low‐ to very high‐severity levels, and measured community structure and flammability attributes. Results: Eighteen years after fire, forests that burned at moderate to very high severity were unable to recover and were replaced by more flammable shrublands and grasslands. Following low‐severity fire, fine‐fuel density was lower and forest recovery was enhanced by greater survival of remnant seed trees. Burnt shrublands increased in abundance across all severity classes but attained highest fine‐fuel production after moderate‐ to very high‐severity fire. Conclusions: Low fire severity, by enabling regeneration of forests that are less flammable than shrublands, diminishes the likelihood of reburning, thus establishing a negative feedback favorable to forest perpetuation. Conversely, moderate to very high fire severity leads to a positive feedback by promoting conversion to shrublands and greater fine‐fuel accumulation. This increases the likelihood of reburning, reinforcing the persistence of pyrophytic communities and favoring landscape‐scale loss of fire‐sensitive forests. This fire severity‐mediated positive feedback may in many regions worldwide further amplify warming‐related wildfire activity increases, posing serious threats to the persistence of fire‐sensitive ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: In the context of ongoing climatic warming, forest landscapes face increasing risk of conversion to non‐forest vegetation through alteration of their fire regimes and their post‐fire recovery dynamics. However, this pressure could be amplified or dampened, depending on how fire‐driven changes to vegetation feed back to alter the extent or behaviour of subsequent fires. Here we develop a mathematical model to formalize understanding of how fire–vegetation feedbacks and the time to forest recovery following high‐severity (i.e. stand‐replacing) fire affect the extent and stability of forest cover across landscapes facing altered fire regimes. We evaluate responses to increasing burn rates while varying the direction (negative vs. positive) of fire–vegetation feedbacks under a continuum of values for feedback strength and post‐fire recovery time. In doing so, we determine how interactions among these variables produce thresholds and tipping points in landscape responses to changing fire regimes. Where the early‐seral vegetation was less fire‐prone than older forests, negative feedbacks limited the reductions in forest cover in response to higher fire frequency or slower forest recovery. By contrast, positive feedbacks (more flammable early‐seral vegetation) produced a tipping point beyond which increases in burn rates or a slowing of forest recovery drove extensive forest loss. With negative feedbacks, the rates of forest loss and expansion in response to variation in fire frequency were similar. However, where feedbacks were positive, the conversion from predominantly forested to non‐forested conditions in response to increasing fire frequency was faster than the re‐expansion of forest cover following a return to the initial burn rate. Strengthening the positive feedbacks increased this asymmetry. Synthesis. Our analyses elucidate how fire–vegetation feedbacks and post‐fire recovery rates interact to affect the trajectories and rates of landscape response to altered fire regimes. We illustrate the vulnerability of ecosystems with positive fire–vegetation feedbacks to climate change‐driven increases in fire activity, especially where post‐fire recovery is slow. Although negative feedbacks initially provide resistance to forest loss with increasing burn rates, this resistance is eventually overwhelmed with sufficient increases to burn rates relative to recovery times. [ABSTRACT FROM AUTHOR]

Ecological memory, often determined by the extent and type of retained biological legacies present following disturbance, may produce persistent landscape patterns. However, after fire, the persistence or switch to an alternative state may depend on the complex interplay of ecological memory (biological legacies) and potential effects of new external factors influencing the post-fire environment. The current study assesses both the strength of ecological memory resulting from biological legacies of pre-burn vegetation types as well as post-fire effects of livestock., Following a severe fire in 1999, we set up a network of long-term exclosures to examine the effects of legacies and cumulative herbivory by cattle on fuel types, amounts, distribution, flammability and microenvironmental conditions in two post-fire communities representing alternative fire-driven states: pyrophobic Nothofagus pumilio subalpine forests and pyrophytic Nothofagus antarctica tall shrublands in northwestern Patagonia, Argentina., Our results show that the retained post-disturbance legacies of tall shrublands and subalpine forests largely determine fuel and flammability traits of the post-fire plant communities 16 years after fire. The importance of biological legacies retained from the unburned plant communities was reflected by the substantially higher amounts of total fine fuel, greater vertical and horizontal fuel continuity and the higher temperatures reached during experimental tissue combustion at post-fire shrubland compared to post-fire forest sites., We show that herbivores may produce antagonistic effects on flammability by decreasing tissue ignitability, total fine fuel and litter depth, and disrupting the vertical and horizontal fine fuel continuity, therefore reducing the probability of fire propagation. However, cattle can increase ratios of dead to live fine fuels, reduce soil moisture, and inhibit tree height growth to canopy size, consequently impeding the development of a closed pyrophobic forest canopy., Synthesis. Our results support the hypothesis that biological legacies, most importantly the dominance by pyrophytic woody plants that resprout vigorously vs. the dominance by pyrophobic obligate seeders, favour fuel and flammability characteristics at the community level which reinforce the mechanisms maintaining pyrophytic shrublands vs. pyrophobic forests. Herbivory by introduced cattle can partially blur sharp pyrophobic/pyrophytic state boundaries by promoting the development of novel post-fire transitional states. [ABSTRACT FROM AUTHOR]

Fire is a major disturbance affecting forests worldwide with significant economic, social, and ecological impacts. The southernmost forests on Earth extend continuously along the Andes from mid- to subantarctic latitudes in South America. In this region, warming and drying trends since the mid-20th century have been linked to a positive trend in the Southern Annual Mode (SAM), the leading mode of extratropical climate variability in the Southern Hemisphere. Due to the scarcity of documentary fire records and the lack of tree-ring fire histories, little is known about how wildfire activity responds to shifts in the westerly circulation pattern and associated climatic variability in the Andean region south of ~44° S. For the first time, we applied dendrochronological techniques to reconstruct fire history from the angiosperm Nothofagus pumilio at 16 sites distributed from ~44° to 50° S to determine relationships between fire occurrence and the two primary drivers of wildfire activity: climate variability and human activities. Partial cross-sections with fire scars were collected from 363 trees in Argentina and Chile. Chronologies of annually resolved fire-scar dates start in 1791 and show a pattern of higher fire frequency during the 20th century, concurrent with the human occupation and colonization processes in southern Patagonia. Years of widespread fire occurring synchronously in two or more disjunct sites are associated with broad-scale climatic anomalies. Intense droughts inferred from extreme departures in temperature, precipitation, and the Standardized Precipitation-Evapotranspiration Index (SPEI) during the growing seasons of 1944 and 1962 are consistent with the two most severe fires at northern sites. Extended droughts, reflected by the association of fire occurrence with six-month cumulative precipitation and SPEI, create conditions for widespread fires at the southern sites (south of ~46° S). Regional fires were concurrent with significant positive departures of SAM during the austral spring-summer. This tree-ring fire record reveals the influences of both climate variability and human activities on fire in the N. pumilio forests across the Andes, and also establishes the feasibility of using this tree species as a natural archive of fire history. [ABSTRACT FROM AUTHOR]

Question Under climate change and increased ignitions by humans, burning of forests in which severe fires were naturally infrequent may result in environmental changes that increase the probability that they will burn again. On the eastern slopes of the northern Patagonian Andes, after fire-resistant Nothofagus pumilio forests burn they are typically replaced by fire-prone shrublands dominated by resprouting shrubs. We examine fuel properties and microclimatic conditions at the community level as potential fire feedback mechanisms mediating switches from fire-resistant N. pumilio forests to fire-prone shrublands. Location Northwestern Chubut province, Patagonia, Argentina. Methods We characterized the volume and vertical distribution of fine fuels, understorey woody and semi-woody plant composition, stand structure and microclimatic conditions in unburned and burned N. pumilio forest and shrublands 14-29 yr after severe fire. Results Fuel amount and arrangement in unburned N. pumilio forests are unfavourable for fire activity compared with post-fire N. pumilio forests and shrublands. Unburned N. pumilio forests presented vertical discontinuities in fine fuel distribution and lesser amounts of fine fuels near the ground in comparison to fuels in shrublands. Floristic understorey composition of unburned and burned shrublands was very similar, while composition of unburned and burned N. pumilio forests showed clear differences. Additionally, microclimatic conditions following burning of N. pumilio forests and shrublands were significantly warmer and drier than in the unburned forest, and more frequently exceeded thresholds associated with fire activity in this region. Conclusions Positive feedbacks from initial burning of otherwise fire-resistant N. pumilio forest will accelerate the rate of fire-induced conversion of forests to non-forest assemblages. Once transformed to the alternative state of shrublands, return to a forest cover is unlikely due to increased probability of burning in shrublands, as well as the unfavourable effects of warmer and drier conditions on tree establishment. [ABSTRACT FROM AUTHOR]

Research from the Patagonian-Andean region is used to explore challenges and opportunities related to the integration of research on wildfire activity into a broader earth-system science framework that views the biosphere and atmosphere as a coupled interacting system for understanding the causes and consequences of future wildfire activity. We examine how research in disturbance ecology can inform land-use and other policy decisions in the context of probable future increases in wildfire activity driven by climate forcing. Climate research has related recent warming and drying trends in much of Patagonia to an upward trend in the Southern Annular Mode which is the leading pattern of extratropical climate variability in the southern hemisphere. Although still limited in spatial extent, tree-ring fire history studies are beginning to reveal regional patterns of the top-down climate influences on temporal and spatial pattern of wildfire occurrence in Patagonia. Knowledge of relationships of fire activity to climate variability in the context of predicted future warming leads to the hypothesis that wildfire activity in Patagonia will increase substantially during the first half of the 21st century. In addition to this anticipated increase in extreme fire events due to climate forcing, we further hypothesize that current land-use trends will increase the extent and/or severity of fire events through bottom-up (i.e. land surface) influences on wildfire potential. In particular, policy discussions of how to mitigate impacts of climate warming on fire potential need to consider research results from disturbance ecology on the implications of continued planting of flammable non-native trees and the role of introduced herbivores in favouring vegetation changes that may enhance landscape flammability. [ABSTRACT FROM AUTHOR]

In the temperate forests of the southern Andes, Nothofagus pumilio, the dominant species of the most extensive forest type, experiences severe defoliation caused by caterpillars of the Ormiscodes genus (Lepidoptera: Saturniidae). This study uses tree rings to reconstruct the history of Ormiscodes outbreaks for the 1850-2005 period and examines relationships between outbreaks and climate variability. We used local climate records to compare outbreak-climate relationships in the northern Patagonian Andes (c. 41°S) and the cooler southern Patagonian Andes (c. 49°S). We also examined relationships between outbreak events and regional climate variability driven by variability in the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation. Although relationships between Ormiscodes outbreaks and climate proved to be complex, in northern Patagonia defoliation events are associated with drier and warmer than average growing seasons. Warming and drying trends in Patagonia during the latter part of the 20th century have been linked to a positive trend in SAM. During the post-1976 period of accelerated warming in Patagonia, widespread defoliation outbreaks have occurred in both northern and southern Patagonia but the increase in frequency of events has been greater in the south. In southern Patagonia the increases in frequency of outbreaks in the late 20th century appear to be unprecedented over the c. 150 year tree-ring record of reconstructed outbreaks. These results are consistent with the greater magnitude of recent warming in southern Patagonia, and suggest that under predicted warmer and drier climates in the 21st century, defoliator outbreaks may continue to increase in frequency. This study is the first systematic reconstruction of past insect outbreaks in South America and provides a preliminary understanding of how climate variability affects defoliator outbreaks in Patagonian Nothofagus forests. [ABSTRACT FROM AUTHOR]

The article presents a summary of the thesis entitled "Insect defoliator outbreaks and environmental heterogeneity in Nothofagus forests in souther South America." It mentions that present changes in the spatiotemporal patterns of herbivorous insect outbreaks in the area have caused public concern and research activity regarding its impact on global warming trends. It offers an understanding of how climate variability can affect complex ecological processes involving herbivorous insects.