Post-fire Seedling Establishment Research Articles

Spatial and temporal drivers of post-fire tree establishment and height growth in a managed forest landscape

BackgroundIn seed-obligate conifer forests of the western US, land managers need a better understanding of the spatiotemporal variability in post-fire recovery to develop adaptation strategies. Successful establishment of post-fire seedlings requires the arrival of seeds and favorable environmental conditions for germination, survival, and growth. We investigated the spatiotemporal limitations to post-fire seedling establishment and height growth in dry to moist mixed conifer forests with and without post-fire forest management treatments (salvage logging, grass seeding) in areas burned from low to high severity. In 2011, we measured post-fire seedling establishment year, juvenile density (seedlings and saplings), and height growth (annual and total) in 50 plots with six conifer species in the School Fire (2005), Blue Mountains, WA, USA. In 2021, we remeasured the plots for post-fire juvenile density and height growth.ResultsPost-fire juvenile tree densities appeared sufficient for self-replacement of forest (> 60 stems ha−1) in 96% of plots in 2021 (median 3130 stems ha−1), but densities were highly variable (range 33–100,501 stems ha−1). Annual seedling establishment was positively correlated with cooler, wetter climate conditions during the summer of germination (July–September) and the growing season of the subsequent year (April–September) for multiple tree species. We found lower juvenile densities at greater distances to seed sources and with higher grass cover, while salvage logging had no effect. Annual height growth was shorter on warmer, drier topographic positions for three species, whereas annual height growth was associated with climate variability for one species. Shifts in height class structure from 2011 to 2021 were, in part, explained by differences among species in annual height growth.ConclusionsAbundant and widespread tree seedling establishment for multiple conifer species after fire was strong evidence that most burned sites in the present study are currently on a trajectory to return to forest. However, post-fire establishment may be constrained to brief periods of cooler, wetter climate conditions following future fires. Long-term monitoring of post-fire recovery dynamics is needed to inform management activities designed to adapt forests to climate change and future disturbances, which will collectively shape future forest structure and composition.

Postfire Seedling Establishment of Desert Peach (Prunus fasciculata) and Joshua Tree (Yucca brevifolia) from Simulated Seed Caches in the Mojave Desert

Desert rodents in the western USA cache enormous numbers of seeds. Seed caches represent a reliable, although highly variable, source of seedling recruitment for numerous desert plants. Prefire caching of refractory seeds (i.e., those capable of forming persistent seed banks) is an important source of postfire seedling recruitment, but it is likely that prefire caches of short-lived seeds also may contribute to postfire regeneration. The primary objective of this study was to quantify seedling recruitment from artificial caches of 2 species with short-lived seeds planted after a stand-replacing wildfire in a singleleaf pinyon (Pinus monophylla) forest. I investigated the survival of 180 artificial caches of Yucca brevifolia and Prunus fasciculata. Caches were composed of 2, 4, or 8 seeds. Also, because soils of the burned area have unusually high coarse fragment volumes (59%), I examined, in greenhouse trials, how soil coarse fragment volumes (CFVs) impacted seedling establishment of these 2 species from caches. In greenhouse trials, the number of P. fasciculata seedlings increased with increasing CFVs. In contrast, Y. brevifolia seedling numbers decreased with increasing CFVs. In field transects, 17% of cached P. fasciculata seeds produced seedlings, whereas 12% of Y. brevifolia seeds recruited seedlings in the first year postfire. The advantage of P. fasciculata in greenhouse CFV trials was not replicated in field seedling recruitment. Seedling mortality over the first 3 years postfire was not statistically different among the 3 cache sizes for either species, nor did seedling sizes differ significantly by cache size and survey date. I conclude that, although the overall recruitment from postfire caches was low (12%–17%), prefire caches of short-lived seeds probably recruit postfire seedlings, but only in specific circumstances.

Seed size-seed number trade-offs: influence of seed size on the density of fire-stimulated persistent soil seed banks.

Does the seed size-seed number allocation trade-off model apply to long-term persistent soil seed banks? This trade-off between seed size versus number of seeds produced is usually applied at a single population on an annual basis. Our question is how this model might apply to close relatives that produce dormant seed forming long-term persistent soil seed banks. These two criteria allow a focus on divergent evolution of conspecifics and permits us to isolate seed size in the spectrum of life history traits that may be influencing seed traits, and on how seed size influences accumulation and persistence in the soil. In California, Arctostaphylos species only produce physiologically dormant seed that are fire-stimulated and that vary in seed size permitting seed size-seed bank density relationship as a test of the seed size-seed number allocation model. Soil seed banks of 10 species of Arctostaphylos were sampled with fruit volumes ranging from 21-1063 mm3 . Seed bank density was determined by hand extraction from soil samples. We found that seed bank densities were significantly negatively related to fruit or seed size. Rather than an issue of allocational trade-off between size and number, we interpret these results as reflecting seed predation and postfire seedling establishment. Seed bank densities, even after decades, generally were less than one or two-year's seed production, suggesting intense seed predation. Burial by scatter-hoarding rodents provided sufficient seeds deep enough for survival of fire. Variation on seed size suggests seedling establishment constraints, but it needs further research.

Limited overall impacts of ectomycorrhizal inoculation on recruitment of boreal trees into Arctic tundra following wildfire belie species-specific responses.

We tested whether post-fire seedling establishment of common boreal tree and expanding shrub species at treeline and in Arctic tundra is facilitated by co-migration of boreal forest mycorrhizal fungi. Wildfires are anticipated to facilitate biome shifts at the forest-tundra ecotone by improving seedbed conditions for recruiting boreal species; at the same time fire alters the composition and availability of mycorrhizal fungi critical to seedling performance. To determine the role of root-associated fungi (RAF) in post-fire seedling recruitment and future biome shifts, we outplanted four dominant boreal tree and shrub species inoculated with one of three treatments at treeline and in tundra: burned boreal forest, unburned boreal forest, or a control treatment of sterilized inoculum. We compared survivorship, growth, and physiological performance of the seedlings in relation to mycorrhizal inoculum treatment and among host species, characterized the RAF communities based on ITS-rDNA sequencing of individual root tips sampled from surviving seedlings, and tested for correlations between RAF composition and the inoculation treatments, host species, and duration of the experiment. We explored correlations between RAF composition and seedling metrics. Both live and sterile autoclaved inoculation treatments had similar effects on seedling survivorship and growth for all species. RAF composition did not vary by treatment, suggesting that most colonization was due to local fungi. However, seedling traits and growth were correlated with RAF species composition, colonization, and the relative abundance of specific RAF taxa. Picea sp. performance in particular showed strong co-variation with RAF metrics. Our results suggest that mycorrhizal co-migration is not a primary limiting factor to boreal seedling recruitment because the experimental provision of inoculum did not affect seedling recruitment; yet, RAF did influence seedling performance, particularly resident RAF at treeline and in tundra, suggesting that mycorrhizal fungi are important to vegetation processes at the treeline-tundra ecotone.

Fire recurrence and emergency post-fire management influence seedling recruitment and growth by altering plant interactions in fire-prone ecosystems

Projections of future wildfire regimes forecast an increased frequency of large high-severity fires that create very harsh environmental conditions and constitute a challenge to post-fire ecosystem regeneration. Under these new circumstances, better knowledge of the plant interaction mechanisms underlying post-fire seedling establishment success would aid restoration management to achieve the intended targets. We evaluated the combined effect of recurrent large stand-replacing fires and conventional post-fire restoration activities (salvage logging after a single large fire, and direct seeding and linear subsoiling plus seedling planting after two subsequent large fires) on tree seedling recruitment and performance (development, annual growth, and biomass) in the early stages of succession in fire-prone maritime pine (Pinus pinaster Ait.) ecosystems. We quantified plant facilitative/competitive interactions between naturally recruited pine seedlings, neighbouring seedlings and potential nurse shrubs with different post-fire regeneration strategies (obligate seeders vs resprouters), by computing the relative interaction index (RII). The results evidenced that fire recurrence altered plant species composition and conditioned initial pine seedling recruitment and establishment, prevailing over the expected negative impact of salvage logging and positive impact of seeding. Seedling recruitment was sufficient to ensure natural tree regeneration after a single fire event and undermined by repeated fires. Both delaying burned timber removal during salvage logging operations and retaining immature dead trees without commercial value onsite in subsoiled stands enhanced seedling recruitment via facilitative interactions. Higher seedling growth and height under shrubs than in open ground resulted in lower aerial and root biomass, indicating elongation in response to shade, and net competition for resources. Inter-specific competition between naturally regenerated seedlings and shrubs was aggravated by intra-specific competition with neighbouring seedlings and by mechanical site preparation in subsoiled stands. All in all, post-burn increased soil fertility most likely counterbalanced the environmental stress created by fire, shifting the net outcome of plant interactions from positive (facilitation) to negative (competition). We recommend alternative post-fire management actions that decrease plant competition and take advantage of facilitation by residual burned wood, to ultimately accelerate ecosystem recovery after large stand-replacing fires.

Effects of fire on geomorphic factors and seedling site conditions within the alpine treeline ecotone, Glacier National Park, MT

This paper evaluates the geomorphic effects of fire on the alpine treeline ecotone and how those effects influence post-fire seedling establishment conditions. Forest fires are becoming more common in high elevations, and ecosystem responses in these areas are not well studied. Results indicated that burned areas experienced increased soil loss, complete loss of the duff layer, less compact soils, larger clast sizes, and increased rock spalling, and altered seedling site conditions. Seedlings showed distinct preferences for micro-sites with deeper, less compact soils, and a greater cover of rock. The effects of fire within the alpine treeline ecotone will affect treeline position and geomorphic processes within these higher elevations.

Limitations to Postfire Seedling Establishment: The Role of Seeding Technology, Water Availability, and Invasive Plant Abundance

Seeding rangeland following wildfire is a central tool managers use to stabilize soils and inhibit the spread of invasive plants. Rates of successful seeding on arid rangeland, however, are low. The objective of this study was to determine the degree to which water availability, invasive plant abundance, and seeding technology influence postfire seedling establishment. Across four fire complexes, whole plots were either seeded using a rangeland drill, seeded by hand where seeds could be placed at an exact depth, or left as unseeded controls. Irrigation and weeding treatments were applied to subplots within whole plots in an incomplete factorial design. In three of the four fires, seeding method was the single factor limiting establishment with seedling density over sevenfold higher in the hand-seeded compared to the drill-seeded treatments. In contrast to our hypotheses, water and weeding had no positive effect on seedling establishment in any of the four fires; however, background weed density was relatively low. The native community recovered at all sites with minimal bunchgrass mortality. These results strongly suggest a need for a decision framework that evaluates postfire seeding needs relative to natural recovery. Based on these initial results, it appears modest improvements in seeding technology may yield substantial increases in seeding success.

Relationship between post‐fire regeneration and leaf economics spectrum in Mediterranean woody species

Summary 1Recent work has identified global-scale relationships between key leaf traits (leaf economics spectrum). However, it is important to determine whether this approach can be applied at local scale with smaller subsets of species facing similar environments. Since fire is a key process in Mediterranean shrubland dynamics we analyze whether fire-related life-history traits influence the pattern of correlation between the leaf economic spectrum and leaf moisture traits. 2Using structural equation modelling and exploratory path analysis, we developed alternative models to test how interspecific leaf traits are related to the seasonal variation of water content (leaves and shoots) and to the type of post-fire regeneration of Mediterranean woody species. 3This study demonstrates that for these species seasonal variation in water content and fuel moisture would be better predicted by the presence or absence of a trait describing post-fire seedling establishment than by the leaf economic spectrum traits. However, leaf dry matter content (LDMC) is influenced by both the leaf economic spectrum and the post-fire regenerative type. 4Seeder species (those that recruit via seeds immediately after fire) present lower LDMC and higher relative seasonal variation of relative water content (RWCrsv) than non-seeders. We hypothesize that since seeder species mostly evolved under the Mediterranean climate, they developed a particular strategy of drought tolerance (without causing an effect to the relation between the volume occupied by cytoplasm relative to the volume occupied by cell walls), which is the cause of the observed relation between LDMC and RWCrsv. 5This study suggests that the leaves of Mediterranean woody species would follow the general leaf economics spectrum (Wright et al. 2004) but that specific selective forces, such as disturbance regime, acting at regional scale also play a relevant role to explain leaf traits related to water content.

Regeneration and Seedling-habitat Relationships of the Marginal Population of Pitch Pine (Pinus rigida) After Prescribed Burning, Eastern Ontario, Canada

Post-fire seedling establishment in relation to microsite conditions and relationships of seedlings (saplings) to older tree individuals was studied in two treatments (i.e., burned and unburned sites) in an area known as the pitch pine (Pinus rigida) ridge on Hill Island, St. Lawrence Islands National Park, Eastern Ontario. Pinus rigida is a rare tree species in Canada at the northern limit of its range. Pinus strobus (eastern white pine) and Quercus alba (white oak) are also common species of the site. The negative correlation between density of mature P. rigida trees and total canopy cover and hardwood density was con.rmed in unburned plots on the ridge. On the other hand, P. strobus density was positively correlated with those parameters. The density of P. rigida seedlings is higher (by 48%) and P. strobus seedlings lower (by 82%) in burned plots than in unburned plots. Seedling recruitment was also dependent on the occurrence of seed trees of both species. The density of P. strobus seedlings was generally higher than that of P. rigida seedlings, especially in unburned plots. The long absence of .re, high seed production of P. Strobus, and relatively low number of P. rigida seed-producing trees, which have a low amount of fertile cones, are probably the main factors in.uencing seedling recruitment on the ridge. Discriminant analysis con.rmed different seedbed requirements of both species. Pinus strobus seedlings occurred within both treatments, especially on deeper soil pro.les with thicker litter depth, although, for seed regeneration, a mineral soil seedbed or a greatly reduced organic layer was also important. Pinus rigida seedling recruitment on burned and unburned plots was comparatively higher on mineral soil, thinner litter layer, and moss and lichen cushions.

Factors affecting post-fire seedling establishment of selected mallee understorey species

Factors affecting the survival of post-fire germinants in mallee communities, in central western New South Wales, were examined. Experiments compared the relative effects of native and introduced herbivores (kangaroos, goats, rabbits), after small- and large-scale fires (20–50 and > 10 000 ha, respectively), with particular emphasis on edge effects, seedling clustering, topography and eucalypt canopy presence. The experiments (1985–1997) focused on common understorey species Acacia rigens Cunn. ex Don, A. wilhelmiana F.Muell. and Triodia scariosa N.T.Burb. subsp. scariosa, in mallee dominated by Eucalyptus species. Following a large fire (1985), high spring rainfall and rabbit grazing on A. rigens only, survival of Acacia species and T. scariosa remained relatively high 4 years later (60–70%). After small burns (1987, 1988), low spring rainfall and grazing by rabbits and kangaroos, survival of Acacia species declined to between 0 and 30% of the germinants by the second summer. In most cases, local extinction had occurred within 8 years. After small burns (1988, 1989) and low spring rainfall, the survival of T. scariosa declined to between 0 and 35% of germinants by the second summer (effect of grazing unknown). No consistent effect of edge, topography and eucalypt canopy was found. Survival of clustered Acacia seedlings was between 10 and 20% lower than unclustered seedlings. Given the high frequency of low rainfall and its interaction with grazing, prescribed burning of mallee for wildfire control and nature conservation may require the local elimination of rabbits and a reduction in kangaroo numbers, especially in the first spring and summer following seedling germination.

Relative Importance of Factors Contributing to Postfire Seedling Establishment in Maritime Chaparral

Several mechanisms have been proposed to explain the postburn flush of seedlings in the chaparral; these include direct effects of fire, such as heating of the soil and seed bank, and indirect effects of fire, such as a temporary reduction in competition by shrubs or herbivory by mammals. I conducted an experiment in burned and adjacent unburned chaparral to assess the relative importance of these mechanisms to seedling emergence and mortality of four functional plant groups: shrubs, subshrubs, perennial herbs, and annual herbs. I assessed direct effects of burning by comparing experimental plots in a burned area to plots in adjacent, unburned chaparral that were cleared of shrubs. In the unburned chaparral, competition with shrubs was manipulated by removing aboveground vegetation; herbivory by large mammals was manipulated using fence exclosures. The direct effects of burning enhanced emergence of shrub and subshrub seedlings, but reduced emergence of annual herbs. Shrub removal alone did not affect seedling emergence, but did improve survivorship of annuals. Exclusion of mammalian herbivores improved the survivorship of annual herbs and all seedlings combined. No single factor explained the high abundance of perennial herbs in the burn area; my results suggest that the combined removal of shrubs and herbivores following fire enhanced their growth and establishment. Thus, both the direct and indirect effects of fire contributed to the high postburn recruitment of seedlings in maritime chaparral, but different mechanisms were responsible for the increase in specific plant groups.

Factors Contributing to Postfire Seedling Establishment in Chaparral: Direct and Indirect Effects of Fire

1 In the chaparral significant seedling establishment occurs only following fire. Mechanisms proposed to explain this abundant recruitment after fire include: direct heating of the soil and seed bank, and a temporary reduction in competition or herbivory. 2 I tested hypotheses regarding the relative importance of these mechanisms to different plant functional groups using field experiments conducted in burned and nearby unburned chaparral. Competition was manipulated by removing shrubs from mature chaparral, and mammalian herbivory manipulated using small cage exclosures. 3 Burning and reduction in herbivory were primarily responsible for the postburn 'flush' of seedlings, although plant functional groups differed in their responses to fire. 4 Shrub seedling density was enhanced only by herbivore exclusion. The abundance of subshrubs and annual herbs was positively affected only by burning. Perennial herbs increased both with burning and herbivore exclusion. 5 The effects of variation in fire intensity or soil heating were also investigated, by comparing unmanipulated plots in the burn to areas in the burn which had been cleared of shrubs prior to the fire. 6 Areas in the burn with reduced soil heating had higher overall seedling densities, cover and biomass; perennial herbs were the only functional group unaffected by variation in fire intensity. 7 Comparison of the results of this study to those from similar experiments in maritime chaparral suggest that inconsistencies between the responses of chaparral communities to fire may be due primarily to differences in species composition.

Significant Potential Levels of Hierarchical Phenotypic Selection in a Woody Perennial Plant, Cistus ladanifer

Cistus ladanifer L. is a Mediterranenn woody perennial species (with two varieties: var. albiflorus and var. maculatus) that follows an obligatory seeder strategy for post-fire seedling establishment. Large numbers of seeds are produced inside fruit capsules split up into compartments. The capsules are the result of apical meristems and appear in different shoot modules and branches throughout the plant. Seed production, used here as a measure of fitness, can thus be analyzed hierarchically in the different structural levels of the plant. A nested analysis of variance was used to test the heterogeneity in the reproductive output of this species in the hierarchical structural arrangement of the individual plants, in order to provide empirical evidence on the potential of hierarchical phenotypic selection, a theoretical framework recently developed for the evolutive processes in modular organisms

Factors controlling postfire seedling establishment in southern California chaparral

This paper discusses the interactions among prefire shrub abundance, soil moisture, and plant and animal species on postfire seedling establishment in mixed chparral in southern California. Postfire germination and survival of seedlings in a stand dominated by a facultative seeder (Adenostoma fasciculatum) and by an obligate seeder (Ceanothus greggii) were monitored for 2 years. Relative to prefire abundance, germination of C. greggii was higher than that of A. fasciculatum. Survival during the first year was also higher in C. greggii than in A. fasciculatum. During the second year, however, mortality of C. greggii was greater than that of A. fasciculatum, mostly due to a psyllid infection. Germination of A. fasciculatum was negatively related to prefire shrub abundance. C. greggii germination was not associated with prefire shrub abundance. Seedling mortality of both species was very strongly related to the depletion of soil moisture the first few months after germination. A. fasciculatum was more sensitive than C. greggii to the drying of the soil, especially in the upper levels. C. greggii seedlings had longer roots, greater root/shoot biomass ratios, higher water potentials, and a later peak in seasonal growth activity compared to A. fasciculatum. Herbs promoted greater survival of A. fasciculatum. Our results indicate that the obligate seeder species, C. greggii, is better adapted to establish seedling in chaparral by producing greater relative germination and greater seedling survival than the facultative seeder species A. fasciculatum. The greater adaptability of C. greggii to the physical environment is counteracted by interspecific interference by plant and animal interactions which tend to favor A. fasciculatum over C. greggii.

Fire intensity and herbivory effects on postfire resprouting of Adenostoma fasciculatum in southern California chaparral

Resprouting is the main regeneration mechanism after fire in Mediterranean-type ecosystems. Herbivores play an important role in controlling postfire seedling establishment, but their influence on regeneration by resprouting is less well known. To study the effects of fire intensity on resprouting of Adenostoma fasciculatum in southern California chaparral, and its interaction with herbivory, we conducted an experimental burn at three levels of fire intensity. We found that increasing fire intensity increased plant mortality, reduced the number of resprouts per plant, and delayed the time of resprouting. Herbivory increased with fire intensity, and was related to the time of resprouting. Plants resprouting later in the season and out of synchrony with the main flush were attacked more readily by herbivores. Post-resprouting mortality also increased with fire intensity and was significantly associated with herbivory in the higher fire intensity plots. Fire intensity effects on chaparral regeneration by resprouting may be farreaching through effects on the population structure, resprout production, and growth of Adenostoma fasciculatum.

A survey of the regeneration of some problem shrubs and trees after wildfire in western New South Wales.

The regeneration of several shrub and tree species in western New South Wales was recorded after widespread natural fires in the summer of 1974175. At 25 locations, plots were established in the wake of the fires and observations were recorded of the rue damage, the regrowth of the plants and post-fire seedling establishment. Interest was centred on species regarded as woody weeds. Fourteen months after the fires, the average survival of seven of the most common species was: Callitris columellaris (white cypress pine) 275, Acac~a aneura (mulga) 16%, Dodonaea attenuafa (narrow-leaved hopbush) 26%, Cassia eremophila var. platvpoda (punty) 48%, Acacia homalophylla (yarran) 87%, Eremophila mitchell~i (budda) 88% and Eremophila sturtii (turpentine) 87%. These results are for plants whose leaves were totally scorched or burnt. For Dodonaea attenuafa and Cassia eremophila there was a large difference in recovery between locations, a difference which was not explicable in term of fire intensity or shrub size. Post-fue seedling establishment occurred with these two shrubs, particularly with D. attenuata, which had high seedling numbers on eight of the eleven locations.