Cohorts Of Seedlings Research Articles

Factors determining fine-scale spatial genetic structure within coexisting populations of common beech (Fagus sylvatica L.), pedunculate oak (Quercus robur L.), and sessile oak (Q. petraea (Matt.) Liebl.)

Key messageNaturally regenerating populations of common beech, pedunculate, and sessile oaks develop strong spatial genetic structures at adult and seedling stages. Significant genetic relationship occurs between individuals growing up to 60 m apart. This indicates the minimum distance separating trees from which seeds used for reforestation should be harvested to avoid the adverse effects of excessive relatedness among offspring.ContextSpatial genetic structure is an inherent characteristic of naturally regenerating plant populations and has practical implications in forests for the management of genetic resources.AimsWe investigated the extent of spatial genetic structure in three broad-leaved forest tree species (common beech—Fagus sylvatica L.; pedunculate oak—Quercus robur L.; and sessile oak—Q. petraea (Matt.) Liebl.) coexisting in the same nature reserve, explored its variation among species and different life stages (adults/offspring), and tested its possible determinants.MethodsWe explored patterns of spatial distribution of individuals, and using microsatellites, we estimated parameters of spatial genetic structure based on kinship relationships, considering possible sources of variation.ResultsIn adults, the strongest spatial genetic structure was found for Q. petraea (Sp = 0.0187), followed by F. sylvatica (Sp = 0.0133), and the weakest in Q. robur (Sp = 0.0080). It was uniform across different age classes in pedunculate oak but decreased with age in sessile oak. No apparent relationship between age and spatial genetic structure was found in beech. Offspring exhibited significant spatial genetic structure (ranging from 0.0122 in beech to 0.0188 in sessile oak). The cohorts of seedlings having both parents present within the study site had stronger spatial genetic structures than cohorts of seedlings with only one local parent.ConclusionSpatial genetic structure is strong in naturally regenerating populations of heavy-seeded forest trees. Pollen immigration from outside of a local forest stand can significantly decrease the extent of spatial genetic structure in offspring generations.

The Genetics and Ecology of Post-Fire Eucalyptus globulus Recruitment in an Isolated Stand in Central Portugal

Eucalyptus globulus Labill. is a widespread forest tree species, reported as naturalized across the introduced range, often showing abundant natural regeneration after wildfires. This paper studies a post-fire cohort of seedlings derived from a small, isolated E. globulus stand in central Portugal. The aim is to better understand the genetic dynamics and dispersal mechanisms of naturally established E. globulus populations in the introduced range. The seedling density at 55 m from adult trees was 12,000 ha−1, the farthest seedling being registered at 101 m. Post-fire expansion occurred in a southward direction, in accordance with predominant wind. Seedlings had significantly lower levels of genetic diversity (Ae = 5.8.; He = 0.8) than adult trees (Ae = 6.5; He = 0.8). Crossings were strongly unbalanced, with only eight trees contributing to the sampled seedlings, and one single tree contributing to 52% of these seedlings. Moreover, the co-ancestry amongst seedlings more than doubled (from θ = 0.03 to 0.076), and the population status number was around one third of the value registered for the adult population (from Ns = 16.2 to 6.6). Despite its isolation, external pollen was detected in the stand, and appears to have travelled at least 700 m, contributing to 22% of the offspring. Overall, the seedling cohort is much less diverse than the parent trees, with expected lower resilience and persistence to environmental stresses.

Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

The pioneer effect advantage in plant invasions: site priming of native grasslands by invasive grasses

AbstractEvaluating the mechanisms that drive plant invasions in grassland ecosystems can provide insight into subtle, yet critical, drivers of ecosystem function. Common hypotheses for invader success are that (1) an invader’s physiology may allow better use of resources and competitively exclude resident natives; (2) the lack of invader‐specific herbivores and diseases may result in increased growth and competitive advantage over natives; (3) invaders may bring pathogens or herbivores that negatively impact native plants directly; (4) the invader may be able to establish earlier or later than natives, giving it a priority effect or phenological niche separation; (5) the invader may produce more seeds, and propagule pressure may allow it to eventually dominate; and (6) once the invader occurs on a site, its plant detritus or litter can offer a legacy effect advantage. Yet, a widely neglected aspect of invasion is that invaders may prime the conditions in places other than where they are currently established, giving them a pioneer effect advantage. These pioneer effects can come from plant materials that move from invaded to uninvaded areas through three pathways—pollen, litter, seeds, and their associated microbiomes—each of which can favor the invader directly or indirectly. In turn, unsuccessful cohorts of seedlings can change the biotic community through root exudates as well as any biotic load they bring with them, along with taking up nutrients and potentially subverting the litter decay cycle. When seedlings of invaders occur at times when the native species have evolved not to germinate, site priming impacts may increase or accelerate. We propose that monitoring of uninvaded areas adjacent to invaded areas is needed to assess the emerging significance of pioneer effects and site priming in advancing plant invasions, especially in grassland systems worldwide.

Expansion of the rare Eucalyptus risdonii under climate change through hybridization with a closely related species despite hybrid inferiority.

Hybridization is increasingly recognized as an integral part of the dynamics of species range expansion and contraction. Thus, it is important to understand the reproductive barriers between co-occurring species. Extending previous studies that argued that the rare Eucalyptus risdonii was expanding into the range of the surrounding E. amygdalina by both seed and pollen dispersal, we here investigate the long-term fitness of both species and their hybrids and whether expansion is continuing. We assessed the survival of phenotypes representing a continuum between the two pure species in a natural hybrid swarm after 29 years, along with seedling recruitment. The performance of pure species as well as of artificial and natural hybrids was also assessed over 28 years in a common garden trial. In the hybrid zone, E. amygdalina adults showed greater mortality than E. risdonii, and the current seedling cohort is still dominated by E. risdonii phenotypes. Morphologically intermediate individuals appeared to be the least fit. Similar results were observed after growing artificial first-generation and natural hybrids alongside pure species families in a common garden trial. Here, the survival, reproduction, health and growth of the intermediate hybrids were significantly less than those of either pure species, consistent with hybrid inferiority, although this did not manifest until later reproductive ages. Among the variable progeny of natural intermediate hybrids, the most E. risdonii-like phenotypes were the most fit. This study contributes to the increasing number of reports of hybrid inferiority in Eucalyptus, suggesting that post-zygotic barriers contribute to the maintenance of species integrity even between closely related species. However, with fitness rapidly recovered following backcrossing, it is argued that hybridization can still be an important evolutionary process, in the present case appearing to contribute to the range expansion of the rare E. risdonii in response to climate change.

Ectomycorrhizal fungal community assembly on seedlings of a Neotropical monodominant tree

AbstractEctomycorrhizal tree species may benefit from positive plant–soil feedbacks, where soil environments near adult trees enhance conspecific seedling growth and survival. In tropical monodominant forests, seedling survival is particularly important, as seedling banks help maintain stand‐level dominance over generations. Positive plant–soil feedbacks may be mediated by diverse ectomycorrhizal fungal communities, which improve nutrient acquisition of heavily shaded seedlings. Despite the potential importance of these fungi, little is known about ectomycorrhizal fungal community development on seedlings of tropical monodominant trees. In Guyana, we sequentially monitored percent colonization and species composition of ectomycorrhizal fungi on an even‐age cohort of seedlings of the tropical monodominant tree Dicymbe corymbosa (Fabaceae subfamily Detarioideae). Ectomycorrhizal fungi found on D. corymbosa seedlings over a 12‐month period of early development were compared to those of conspecific adults and four other ectomycorrhizal tree species in the region. Species turnover was high (80%) between 6‐ and 12‐month‐old seedlings, though the /russula‐lactarius, /clavulina, and /tomentella‐thelephora lineages were species‐rich on seedlings at all ages. The number of ectomycorrhizal morphotypes per seedling increased with age, but extent of fungal colonization did not. Seedling ectomycorrhizal fungi were shared with sympatric conspecific adults (55%) and, to a lesser extent, regional heterospecific adults (27%), but numerous species were previously unrecorded for Guyana. Over their development D. corymbosa seedlings did not rely strictly on adult trees for their mycobionts but appeared to foster unique assemblages of ectomycorrhizal fungi.

Incorporating Shrub Neighborhood Dynamics to Predict Forest Succession Trajectories in an Altered Fire Regime

As fire regimes shift in western North America, patches of stand-replacing fire are becoming larger. Forest succession in these patches is not well understood. There is concern that competition with rapidly reestablishing shrubs, combined with dispersal limitation, may delay or impede conifer recovery and/or shift tree composition toward shade-tolerant species. However, tree–shrub interactions and shrub neighborhood dynamics have not been closely examined. To investigate the patterns and processes determining forest recovery after severe wildfire, we developed a data-driven simulation model that we use to predict conifer emergence above the shrub canopy. Our model results showed that ponderosa pine (Pinus ponderosa) emerged at a faster rate than white fir (Abies concolor) under whitethorn ceanothus (Ceanothus cordulatus) and deerbrush (Ceanothus integerrimus), and at a similar rate under greenleaf manzanita (Arctostaphylos patula). Across all shrub species, ponderosa pine had a relative advantage over fir in the period between conifer establishment and peak shrub competition, requiring a mean of 16 years for 50% of individuals to emerge compared to 22 years for fir. Fir emergence rates then surpassed those of pine, leading to similar overall emergence by simulation end: 83% ± 7% for pine and 82% ± 7% for fir. These results show that, on balance, shrub neighborhood dynamics do not produce an ecological filter favoring firs for the initial cohort of established seedlings, but emergence patterns are sensitive to shrub species. Further applications of this data-driven simulation framework could improve understanding of other important components of post-fire succession.

Recruitment of a genotyped Quercus robur L. seedling cohort in an expanding oak forest stand: diversity, dispersal, and performance across habitats

Few studies have linked the origin of dispersed tree seeds with their post-dispersal fate. We show that habitat-dependent mortality in a pedunculate oak ( Quercus robur L.) seedling cohort reshapes the effective fecundity of individual mother trees but has little effect on the cohort’s genetic diversity. Initial tree recruitment plays a key role in forest regeneration, yet little is known on how patterns of recruit mortality feed back on the fecundity of reproducing trees. To investigate how among-habitat variation in seedling arrival and survival alters initial patterns of genetic diversity and maternal reproductive success. We genotyped a pedunculate oak seedling cohort (n = 809) and monitored it over 3 years. The mother trees of 81% of the seedlings were identified through parentage analysis. Seedlings were assigned to one of three habitats (broadleaved forest, pine plantation, or open area). Broadleaved forest received most seedlings (≈ 65%) but their survival was reduced by a third compared with pine plantations or open areas. Thus, mother trees dispersing many descendants to broadleaved forest suffered a disproportionate reduction of their reproductive success. Genetic diversity did not vary among habitats, nor over the monitoring period. The quality of seed dispersal, in terms of delivery sites, can considerably influence the reproductive success of individual mother trees without affecting the overall genetic diversity of the recruits.

Increased Genetic Diversity via Gene Flow Provides Hope for Acacia whibleyana, an Endangered Wattle Facing Extinction

In this paper we apply a conservation genomics approach to make evidence-based management recommendations for Acacia whibleyana, an endangered shrub endemic to Eyre Peninsula, South Australia. We used population genomic analysis to assess genetic connectivity, diversity, and historical inbreeding across all known stands of the species sampling remnant stands, revegetated stands of unknown origin, and a post-fire seedling cohort. Our results indicate a degree of historical connectivity across the landscape, but habitat loss and/or pollinator community disruption are potential causes of strong genetic structure across the remnant stands. Remnant stands had low genetic diversity and showed evidence of historical inbreeding, but only low levels of intra-stand relatedness indicating that risks of contemporary inbreeding are low. Analysis of a post-fire first generation cohort of seedlings showed they likely resulted from intra-stand matings, resulting in reduced genetic diversity compared to the parents. However, admixed seedlings in this cohort showed an increase in heterozygosity relative to likely sources and the non-admixed seedlings of the same stand. Assisted inter-stand gene flow may prove an effective management strategy to boost heterozygosity and corresponding increases in adapting capacity in this endangered species.

Edge Effects on Seedling Diversity Are Mediated by Impacts of Fungi and Insects on Seedling Recruitment but Not Survival

Growing awareness of the role of enemies in plant community dynamics has led to ecologists exploring how plant-enemy interactions change in human-modified systems. Proximity to forest edges was found to weaken the role of two groups of plant enemies—insect herbivores and fungal pathogens—in increasing plant diversity during the seed-to-seedling transition. However, it is less clear whether edge effects similarly compromise the diversifying effects of fungi and insects on established seedlings. We examined this question in a human-modified wet tropical forest in the Western Ghats of southern India. Over an annual cycle of recruitment, in 730 seedling plots (1 × 1 m each) arrayed at distances 0–100 m from the forest edge across 15 locations in a 30 km2 landscape, we suppressed the activity of fungal pathogens and insect herbivores by applying fungicide and insecticide to soil, seeds, and seedlings in a subset of plots. Suppressing fungi and insects reduced diversity mainly for seedling recruits and not for seedlings that had already established. However, pesticide effects were only apparent at 90–100 m from forest edges. Specifically, in the interior sites, fungi and insect activity increased recruit diversity, which helped maintain local seedling diversity even though diversity of established seedlings declined with annual mortality. By comparison, canopy openness affected neither the diversity of survivors from the initial seedling cohort nor total seedling diversity after an annual cycle of recruitment. Our results indicate that insects and fungi promote diversity more prominently during early seedling establishment rather than through impacts on post-establishment seedling survival. Thus, edge effects can weaken the diversifying effects of plant-insect and plant-fungal interactions during recruitment and thereby modify the seedling template available for the future tree community in human-modified forests.

A 20-year experiment on the effects of deer and hare on eastern hemlock regeneration

Eastern hemlock (Tsuga canadensis (L.) Carrière) is a shade-tolerant, slow-growing tree once common in forests across the Great Lakes region. It was heavily exploited in the late 19th and early 20th centuries and now experiences limited regeneration across much of its range. This failure to regenerate has been ascribed to poor seedbed conditions, insufficient canopy openings, warmer climate, and browsing by white-tailed deer (Odocoileus virginianus (Zimmermann, 1780)) or snowshoe hare (Lepus americanus Erxleben, 1777). To test whether deer or hare limit hemlock regeneration, we studied >2000 hemlock seedlings inside and adjacent to experimental deer exclosures at 59 sites randomly distributed across hemlock and hemlock-component stands in northern Wisconsin and the western Upper Peninsula of Michigan in the United States. We monitored local deer and hare abundance, seedling growth, and seedling survival for 20 years. Two First Nations reservations showed lower deer density and greater survival and growth of unprotected seedlings than three national forests of the United States. Cohorts of hemlock seedlings protected from deer survived at a rate four times higher than those exposed to deer (59.3% vs. 15.0%) and shared a combined height 5.2 times greater. Hare densities significantly affected seedling survival only within exclosures. This extensive, long-term study identifies deer as the primary factor constraining hemlock regeneration in the region.

Riding on the wind: volatile compounds dictate selection of grassland seedlings by snails.

Seedling herbivory is an important selective filter in many plant communities. The removal of preferred food plants by both vertebrate and, more commonly, invertebrate herbivores can destroy entire seedling cohorts, and consequently dictate plant community assembly. Nevertheless, our understanding of how and why some seedlings are more prone to herbivore attack than their neighbours remains limited. For seedlings, where even minor tissue damage is fatal, avoiding contact with herbivores is probably advantageous and, on this basis, volatile organic compounds (VOCs) are strong candidates to fulfil a primary defensive role. We quantified seedling selection by snails (Cornu aspersum) for 14 common, European grassland species. Seedling acceptability was subsequently compared with species-specific expression of constitutive secondary defence metabolites (CSDMs), and VOCs to determine their relative influence on seedling selection. We found no relationship between seedling acceptability and CSDMs, but seedling selection was strongly associated with VOC profiles. Monoterpenes (specifically β-ocimene) were identified as likely attractants, while green leaf volatiles (GLVs) (3-hexen-1-ol acetate) were strongly associated with low seedling acceptability. By elucidating a relationship between VOCs and seedling acceptability, we contradict a long-held, but poorly tested, assumption that seedling selection by herbivores in (semi-)natural plant communities centres on CSDMs. Instead, our results corroborate recent work showing how GLVs, including 3-hexen-1-ol acetate, deter crop seedling selection by molluscs. Although our failure to establish any early-ontogenetic relationship between VOCs and CSDMs also suggests that the former do not 'advertise' possession of the latter, we nevertheless reveal the role that VOCs play in defending seedlings against herbivory before lethal damage occurs.

Regulating acorn germination and seedling emergence in Quercus pagoda (Raf.) as it relates to natural and artificial regeneration

Dormancy break and germination requirements for Quercus pagoda (Raf.) acorns were determined, as well as the effects of acorn pretreatment and post-germination temperatures on epicotyl emergence, seedling development, and seedling biomass accumulation. There was an inverse linear relationship between length of cold stratification (5/1 °C for 0–12 weeks) and cumulative germination percentages in all incubation temperatures (15/6, 20/10, 25/15, 30/20 °C). Acorns required 12 weeks of cold stratification to break dormancy. Gibberellic acid substituted for cold stratification; although, it was not as effective as 12 weeks cold stratification. At 16 weeks of cold stratification, 20% of acorns had germinated, and the remaining 80% of ungerminated acorns reached ≥ 97 ± 1% cumulative germination within 4 days in all incubation temperatures. Post-germination time to epicotyl emergence and to leaf flush was a function of temperature, and time decreased with increased temperatures. With light held constant (50 μmol m−2 s−1), seedlings accumulated greater biomass in temperatures > 20/10 °C. Q. pagoda acorns possess Type 2 nondeep physiological dormancy, and this allows for artificial manipulation of timing and duration of germination. Extending cold stratification 4 weeks beyond dormancy break (i.e., 16 weeks) yields more uniform germination across a range of temperatures. Epicotyl emergence, seedling development, and biomass accumulation may be regulated by manipulating growing temperature. In this study, the most uniform seedling cohort with the greatest total biomass was produced when acorns received 16 weeks of cold stratification, followed by transfer of germinants to 30/20 °C.

Shifting demographic conflicts across recruitment cohorts in a dynamic post-disturbance landscape.

Seed dispersal effectiveness, which measures the number of adult plant individuals produced by seed dispersal, is the product of the number of seeds dispersed and the probability a seed produces an adult. Directed dispersal to certain habitat types may enhance some stages of recruitment but disfavor others, generating demographic conflicts in plant ontogeny. We asked whether temporal changes in habitat features may affect the distribution of seedlings recruited from dispersed acorns, and whether this could induce shifts in the life-stage conflicts experienced by successive cohorts of naturally recruited plants. As early successional habitats are characterized by rapid change, we used a burnt pine stand in southern Spain to monitor the recruitment and performance of a major tree species (Quercus ilex) across 7 yr in four types of post-fire habitats. These differed in structure and included patches of unburnt forest and three management alternatives of burnt trees: logging, partial cutting, and nonintervention. Young oaks that resprouted after the fire were mainly located near acorn sources, while new seedlings initially emerged mostly in habitats with standing snags due to habitat selection by European jays, Garrulus glandarius, for dispersal. The dead pines gradually collapsed and attracted less dispersal, so subsequent seedling cohorts mainly recruited within patches of unburnt pines. These live pines enhanced the survival of the oaks located beneath their canopy but greatly reduced their growth as compared to the other post-fire habitats, thus representing a demographic conflict that was absent elsewhere. As a consequence of the directional shift in the habitat where seedlings recruited, successive seedling cohorts experienced a gradual improvement in their likelihood of survival but a reduction in growth. The progressive intensification of this life-stage conflict hinged on the reduction of vertical structures in the habitat with standing burnt pines. Recruitment success thus involved temporal variation in the habitat where recruitment occurred, likely resulting from changes in the direction of seed dispersal, and spatial variation in habitat suitability for seedling establishment and growth. Temporal changes in habitat structure can indirectly change the environment in which recruitment occurs, and consequently seed dispersal effectiveness, by shifting the direction of seed dispersal.

The canary in the coal mine: Sprouts as a rapid indicator of browse impact in managed forests

Forest managers are frequently confronted with sustaining vegetation diversity and structure in landscapes experiencing high ungulate browsing pressure. Often, managers monitor browse damage and risk to plant communities using vegetation as indicators (i.e., phytoindicators). Although useful, the efficacy of traditional phytoindicators is sometimes hampered by limited distribution and abundance, variable browse susceptibility, and lagged responses. In contrast, sprouts possess traits which make them readily available and attractive to browsers, yet fairly resilient to tissue loss. Here, we experimentally evaluate whether hardwood tree stump sprouts are effective and sensitive phytoindicators of deer browse pressure. We measured sprout abundance and height in fenced and unfenced plots at 17 shelterwood harvested sites scattered across a 6500km2 region where deer densities varied by nearly an order of magnitude. We found browsing did not alter the proportion of stumps sprouting and sprout density; however, browse pressure varied among the four most abundant species. Acer rubrum and Acer saccharum were heavily browsed, although browse pressure on A. saccharum decreased in areas with greater canopy openness. Fagus grandifolia and Prunus serotina were less preferred. Differences in palatability altered size hierarchies. Averaged across all species, browsing reduced sprout height by 39%, relative to protected sprouts. Under ambient browsing, P. serotina was 60–100% taller than other species and significantly taller than A. saccharum and F. grandifolia. However, within fences A. saccharum and A. rubrum doubled in size, relative to browsed individuals, and were as tall as P. serotina. Deer impact on sprout height within unfenced forest stands was negatively correlated with estimated deer densities (R2=0.46). Thus, we suggest sprout surveys can provide a measure of impact across much larger areas. Our results demonstrate that sprouts, particularly those of Acer species, offer an abundant, easily measured, and reliable indicator of browse pressure. Moreover, browse impacts on sprouts emerged before impacts were detected on seedling abundance, height, or biomass. We argue sprouts can warn of imminent browse risk to seedlings (and perhaps non-woody vegetation) and thereby allow managers to take actions to mitigate or avert losses to the regenerating seedling cohort.

Investigation into the mortality of mangroves at St. Lucia Estuary

Lake St. Lucia is the largest estuarine lake in South Africa and is a UNESCO World Heritage Site. The St. Lucia Estuary resides in a poor state due to drought and human impacts which have resulted in the closure of the mouth to the sea since 2002 and changes in freshwater inflow particularly from the Mfolozi River. This provides a unique opportunity to study the responses of the mangroves that usually grow in an intertidal environment. High water levels and inundation caused mangroves to die in The Narrows in 2013/2014 which caused concern. The aim of this study was to investigate the response of the mangroves to fluctuating environmental conditions. This was done by measuring the population structure and sediment characteristics at four sites in 2010, 2013 and 2014. The reconnection of the Mfolozi River to St. Lucia in 2013 via the Beach Canal and Back Channel as well as good rains led to an increase in water level which caused permanent inundation of pneumatophores (aerial roots of the white mangrove Avicennia marina) and anoxic conditions resulting in the mortality of mangroves fringing the main estuary channel (Site 3 on The Narrows). Self-thinning is a natural process that has occurred at Sites 1 and 2 while Site 4, which was previously considered a dying population, had a recruitment of a new seedling cohort in 2014. The dieback of mangroves along the water's edge is part of the natural dynamics because this would have occurred under the natural state for this system when the mouth closed to the sea, water level was high and the estuary was fresher. This study has shown that site-specific differences will allow for recruitment and overall survival of mangroves at St. Lucia Estuary.

The relationship between seedling survival rates and their genetic relatedness to neighboring conspecific adults

Seedling genotype is one of the major factors affecting seedling survival when the rate of damage by natural enemies depends on a host genotype. However, to our knowledge, no previous study has examined the mortality of seedlings adjacent to conspecific adults, considering intraspecific variation in seedling genotypes. On the basis of the assumption that natural enemies adapt to adult trees having a unique genotype, we tested the hypothesis that seedling survival decreases when they are more closely genetically related to neighboring adults, by measuring the mortality rate of seedlings of Fagus crenata growing in a cool-temperate forest in Japan. We estimated the genetic relatedness of seedlings (2 and 6 years old) to the neighboring adult F. crenata using microsatellite analysis and measured seedling survival. We determined that there was a non-significant negative correlation between seedling survival and genetic relatedness in the 2-year-old seedling cohort, and a non-significant but positive correlation in the 6-year-old seedlings. Our results call for further studies on the potential relationship between seedling survival and genetic relatedness to neighboring adults.

Forest fuels and potential fire behaviour 12 years after variable-retention harvest in lodgepole pine

Variable-retention harvesting in lodgepole pine offers an alternative to conventional, even-aged management. This harvesting technique promotes structural complexity and age-class diversity in residual stands and promotes resilience to disturbance. We examined fuel loads and potential fire behaviour 12 years after two modes of variable-retention harvesting (dispersed and aggregated retention patterns) crossed by post-harvest prescribed fire (burned or unburned) in central Montana. Results characterise 12-year post-treatment fuel loads. We found greater fuel load reduction in treated than untreated stands, namely in the 10- and 100-h classes (P = 0.002 and 0.049 respectively). Reductions in 1-h (P < 0.001), 10-h (P = 0.008) and 1000-h (P = 0.014) classes were greater in magnitude for unburned than burned treatments. Fire behaviour modelling incorporated the regenerating seedling cohort into the surface fuel complex. Our analysis indicates greater surface fireline intensity in treated than untreated stands (P < 0.001), and in unburned over burned stands (P = 0.001) in dry, windy weather. Although potential fire behaviour in treated stands is predicted to be more erratic, within-stand structural variability reduces probability of crown fire spread. Overall, results illustrate trade-offs between potential fire attributes that should be acknowledged with variable-retention harvesting.

Aerial sowing stopped the loss of alpine ash (Eucalyptus delegatensis) forests burnt by three short-interval fires in the Alpine National Park, Victoria, Australia

Multiple fires in quick succession can cause an abrupt switch from forest to non-forest vegetation in some biomes. A prime example of such type conversion concerns alpine ash forests (Eucalyptus delegatensis subsp. delegatensis), which are endemic to the Australian Alps bioregion. These forests are long lived and can only regenerate following fire disturbance through the release of seed from the tree canopy, but complete regeneration failure of the eucalypt overstorey can occur if immature stands are subsequently reburnt. Wildfires in 2003, 2007 and 2009 burnt over 87% of the Alpine ash forest in Victoria, with some areas being burnt a second or third time within a decade by the 2013 Harrietville-Alpine Bushfire. Using aerial and field surveys we demonstrated that an area burnt two or three times in the Alpine National Park in Victoria would have resulted in loss of alpine ash forest were it not for an aerial sowing program. This sowing program used reduced sowing rates compared to those typically used in forestry silviculture operations in Victoria. Seedling establishment during the first year was lower on sown than natural seedfall areas but fell within the range of acceptable silvicultural stocking rates. Analysis of cohorts of seedlings over their first summer showed mortality was highest in the 2–3months following emergence, especially during hot dry periods, but there was no difference in mortality rates between artificially and naturally sown areas. While the aerial sowing intervention established a cohort of alpine ash seedlings, the challenge for land managers is to protect these seedlings from wildfires for the next two decades to avoid loss of these forests.

Biomass growth response to spatial pattern of variable‐retention harvesting in a northern Minnesota pine ecosystem.

Variable-retention harvesting (VRH) is an approach for sustaining complex structure in managed forests. A criticism of VRH is that ecological benefits may come at a cost of reduced growth of regeneration, due to competition with residual trees. However, the spatial pattern of retention, i.e., dispersed or aggregated, in VRH systems can be manipulated to minimize suppression of regeneration, and resource limitation to regeneration might be mitigated by reduction of woody shrubs. Continued growth of the residual cohort will compensate for growth reduction of regeneration, although this may differ with retention pattern. We examined aboveground whole-stand biomass growth of trees in a VRH experiment in Pinus resinosa forest in Minnesota, USA. Treatments included dispersed retention, aggregated retention, and an uncut control, as well as a shrub treatment (reduced density or ambient). We addressed the following hypotheses: (1) biomass growth of a cohort of planted pine seedlings will be highest with aggregated rather than dispersed retention, (2) biomass growth of the planted seedlings will increase with shrub reduction, and (3) biomass growth of the residual overstory will be higher with dispersed rather than aggregated retention. Aboveground biomass growth of the planted pines ranged from 0.4 kg·ha−1·yr−1 in the overstory-control–ambient-shrub treatment to 23 kg·ha−1·yr−1 in the aggregated-retention–shrub-reduction treatment. The difference between the control and the retention treatments was significant (P < 0.0001), but not between dispersed and aggregated retention (P = 0.97). Thus, our first hypothesis was not supported. In all treatments, biomass growth was significantly higher (>100% increase) with shrub reduction (P = 0.001), supporting our second hypothesis. Biomass growth of residual trees ranged from 2404 kg·ha−1·yr−1 in the uncut-control–ambient-shrub treatment to 1043 kg·ha−1·yr−1 in the aggregated-retention–shrub-reduction treatment. Differences were significant between the control and retention treatments (P = 0.003), and marginally higher with dispersed vs. aggregated retention (P = 0.09), lending support to our third hypothesis. Our results suggest that managers have flexibility in application of VRH and can expect similar stand-level biomass growth of planted regeneration regardless of retention pattern, but somewhat higher stand-level biomass growth of retained trees with dispersed retention.