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Seed heat tolerance and germination of six legume species native to a fire-prone longleaf pine forest

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Recognition of spatial heterogeneity of fire at fine scales is emerging, particularly in ecosystems characterized by frequent, low-intensity fire regimes. Differences in heat flux associated with variation in fuel and moisture conditions create microsites that affect survivorship and establishment of species. We studied the mechanisms by which fire affects seed germination using exposure of seeds to fire surrogates (moist and dry heat). Tolerance (survival) and germination responses of six perennial, herbaceous legume species common to the fire-prone longleaf pine–wiregrass ecosystem of the southeastern USA were examined the following heat treatments. Moist heat was more effective in stimulating germination than dry heat flux for most species examined. We also compared intrinsic seed properties (relative seed coat hardness, percent moisture, and seed mass) among species relative to their heat tolerance and heat-stimulated germination responses. Seed coat hardness was closely associated with the probability of dry and moist heat-stimulated germination. Variation among species in optimal germination conditions and degree of heat tolerance likely reflects selection for specific microsites among a potentially diverse suite of conditions associated with a low-intensity fire regime. Fire-stimulated germination, coupled with characteristics of seed dormancy and longevity in the soil, likely fosters favorable recruitment opportunities in restoration situations aimed at reintroducing a frequently prescribed burn regime to a relict longleaf pine site. In a restoration context in which externally available seed pool inputs are limited, this regenerative mechanism may provide a significant source of recruitment for vegetative recovery in a post-fire landscape.
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Seed heat tolerance and germination of six legume species
native to a fire-prone longleaf pine forest
M. Scott Wiggers .J. Kevin Hiers .Analie Barnett .
Robert S. Boyd .L. Katherine Kirkman
Received: 16 March 2016 / Accepted: 1 November 2016 / Published online: 17 November 2016
ÓSpringer Science+Business Media Dordrecht (outside the USA) 2016
Abstract Recognition of spatial heterogeneity of fire
at fine scales is emerging, particularly in ecosystems
characterized by frequent, low-intensity fire regimes.
Differences in heat flux associated with variation in fuel
and moisture conditions create microsites that affect
survivorship and establishment of species. We studied
the mechanisms by which fire affects seed germination
using exposure of seeds to fire surrogates (moist and dry
heat). Tolerance (survival) and germination responses
of six perennial, herbaceous legume species common to
the fire-prone longleaf pine–wiregrass ecosystem of the
southeastern USA were examined the following heat
treatments. Moist heat was more effective in stimulating
germination than dry heat flux for most species exam-
ined. We also compared intrinsic seed properties
(relative seed coat hardness, percent moisture, and seed
mass) among species relative to their heat tolerance and
heat-stimulated germination responses. Seed coat hard-
ness was closely associated with the probability of dry
and moist heat-stimulated germination. Variation
among species in optimal germination conditions and
degree of heat tolerance likely reflects selection for
specific microsites among a potentially diverse suite of
conditions associated with a low-intensity fire regime.
Fire-stimulated germination, coupled with characteris-
tics of seed dormancy and longevity in the soil, likely
fosters favorable recruitment opportunities in restora-
tion situations aimed at reintroducing a frequently
prescribed burn regime to a relict longleaf pine site. In a
restoration context in which externally available seed
pool inputs are limited, this regenerative mechanism
may provide a significant source of recruitment for
vegetative recovery in a post-fire landscape.
Keywords Fabaceae Hard seed coat
Heat-stimulated germination Longleaf pine
ecosystem Seed dormancy Seed survival
Historically, fire has been the dominant disturbance
regime in many ecosystems worldwide (Bond et al.
Communicated by Devan Allen McGranahan.
M. S. Wiggers (&)
U.S. Fish and Wildlife Service, Mississippi Field Office,
Jackson, MS 39213, USA
J. K. Hiers
Tall Timbers Research Station, Tallahassee, FL 32312,
A. Barnett
The Nature Conservancy, Atlanta, GA 30303, USA
R. S. Boyd
Department of Biological Sciences, Auburn University,
Auburn, AL 36849, USA
L. K. Kirkman
J.W. Jones Ecological Research Center, Newton,
GA 39870, USA
Plant Ecol (2017) 218:151–171
DOI 10.1007/s11258-016-0674-x
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Likewise, laboratory studies have indicated that moist heat could promote germination more than dry heat in some species (e.g. hard-seeded legumes) (Wiggers et al., 2017), revealing the complex influences of seed characteristics on heat-stimulated germination. These characteristics, as well as climate adaptions (Zomer et al., 2022) and fire history (Valbuena et al., 2012), might drive the large intra-specific variability of heat-stimulated germination that have been detected for many Mediterranean species (Tavşanoglu and Pausas, 2018). ...
... Among them, we highlight the absence of fertile stems of G. lutea over the short-term after fire, as signifies the interruption of the reproductive cycle and the total absence of seed production. This pause has been seen above all in long-lived species with delayed reproduction (Lloret et al., 1996) as G. lutea, and it occurs because these species allocate resources to vegetative growth (Whelan, 1995). Although this is the first work analyzing fire effects on G. lutea populations, the results found by Mayorova et al. (2015) agreed with our findings, as they found flowering of G. lutea in the Ukrainian Carpathians to be always interrupted under strong stress conditions. ...
... Guerra, 1998;Manrique-Revuelta et al., 2014). Compared to burning, clearing has no negative effects on underground tissues (Whelan, 1995;Zedler, 2007;Fernández et al., 2013). Likewise, mechanical clearing might favor the stimulation of new buds formation (Klimesová and Klimes, 2009;Hartmann et al., 2018) because adventitious bud primordia can develop from wound callus tissue, which proliferates from injured root surfaces (Hartmann et al., 2018). ...
Overexploitation, land-use change, and climate warming are compromising the conservation of many plant species across the globe, as well as the benefits they provide to people. The yellow gentian (Gentiana lutea L.) is a hemicryptophyte used for pharmacological and food industries harvested in the mountain systems of Southern Europe, which may be subject of management measures. Here we identify the influence of two common management strategies, burning and clearing, on G. lutea populations, and provide insights on the role of burning on seed germination by analyzing the effect of thermal shocks from a populational approach. To achieve these goals, we selected areas affected by low intensity burning and mechanical clearing over the short term (1–2 years) and over the medium term (6–8 years) in the Cantabrian Mountains (NW Iberian Peninsula), where we sampled the number of basal rosettes, fertile stems and cover of G. lutea, and the total cover of woody species. Likewise, we collected seeds from three populations, which were used to quantify several seed traits and to analyze the germination dynamics after heat shocks alone and combined with a dormancy break treatment (gibberellic acid). Results showed decreases in G. lutea population variables immediately after burning and a full recovery over the medium term. On the contrary, clearing did not show effects over the short-term but largely increased all population variables over the medium term. Among other factors, management effects are mediated by their influence on woody species cover, which constrains G. lutea populational growth. Thermal shocks did not break the dormancy of G. lutea seeds, and thermal shocks combined with forced dormancy break revealed no clear effects of heating. Populational differences in the germinative response were detected, as heavier and less moist seeds require longer time to germinate. Based on our results, we propose mechanical clearing as a suitable management strategy to sustain G. lutea populations in NW Iberian Peninsula, but we recommend further analysis to extrapolate our advice to sites with distinct environmental conditions.
... Location, position, and traits of buds for surviving heat from fire are critically important when considering potential desired fire effects. While tree bark thickness (Varner et al. 2016), litter characteristics , and seeds (Wiggers et al. 2017) have been researched for numerous pyrophytes, tree buds have received less attention for their ability to survive fire. Clarke et al. (2012) investigated resprouting as a primary functional trait in response to fire-induced injury and suggested that bud position is often ignored when vegetative resprouting potential is described and defined for a given species. ...
... With the buds in place, this frame was then fixed upon a 55.9 cm × 76.2 cm × 76.2 cm wooden, adjustable rack at one of two heights above a propane gas tube burner (Tejas Smokers, Houston, TX, USA, model PBM125-44; 3.2 cm inner diameter, 3.8 cm outer diameter, 55.9 cm length): 30 cm and 60 cm (Fig. 3B). Three times were used for bud heat exposure: 15, 45, and 75 s, with heat doses generally following Wiggers et al. (2017). The mass of propane gas used per experimental burn (0.32 g s −1 ) was measured using a large scale. ...
... This experiment was designed for and conducted in a laboratory with calculated and expected differences in BPM based upon differences in height above a propane burner and differences in heat exposure time, similar to seed trials (Wiggers et al. 2017). The heights and exposure times selected were chosen to mimic potential prescribed burn scenarios in the southeastern United States used to topkill hardwood species in the dormant season (in terms of both heat exposure times and distance to flames). ...
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Background Traits of mature trees, such as bark thickness and texture, have been documented to promote resistance or resilience to heating in fire-prone forests. These traits often assist managers as they plan and promote prescribed fire management to accomplish specific land management objectives. Species are often grouped together as pyrophobes or pyrophytes as a result of these features. Nonetheless, little is known about species-specific traits of other structures, such as bud diameter, length, mass, moisture content, and surface area, that might be related to heat tolerance. Many prescribed fires are utilized in the eastern United States to control regeneration of less desired species, which could apply a more mechanistic understanding of energy doses that result in topkilling mid-story stems. In this study, we investigated potential relationships between terminal bud mortality from lateral branches of midstory stems and species-specific bud features of six eastern US deciduous trees. Characterized at maturity as either pyrophytes or pyrophobes, each was exposed to different heat dosages in a laboratory setting. Results Bud diameter, length, mass, moisture content, and surface area differed by species. Bud percent mortality at the first heat flux density (0.255–0.891MJm ⁻² ) was highest for two pyrophobes, chestnut oak ( Quercus montana Willd.) and scarlet oak ( Quercus coccinea Münchh). For the second heat flux density (1.275–1.485MJm ⁻² ), bud percent mortality was highest for these species and red maple ( Acer rubrum L.). Principal component analysis suggested that bud surface area and length differentiated species. Red maple, chestnut oak, and scarlet oak produced clusters of buds, which may explain their more pronounced bud mortality. Yellow-poplar ( Liriodendron tulipifera L.) was also present in that cluster, suggesting that its unique bud architecture of pre-emergent leaves may have elicited responses most similar to those of the clustered buds. Conclusions Contrary to expectations, lateral buds of species regarded as pyrophytes at maturity displayed some of the highest values of bud percent mortality when heated at two heat flux densities generated in a laboratory. Their responses may be related to clustering of their lateral buds. Testing of additional species using these methods in a laboratory setting, and perhaps additional methodologies in the field, is warranted.
... While hard seed coats may protect seeds from heating (Wiggers et al., 2017), they may also depend on fire to facilitate germination (Keeley & Fotheringham, 2000). Thus, it is unclear whether greater fire intensity due to grass invasions affects seedling emergence and resprouting in pyrogenic ecosystems sufficiently to establish a grass-fire cycle. ...
... We found that seedling emergence and resprouting of species, except for cogongrass, M. repens and S. nutans, decreased as fuel loads and soil heating increased, but sensitivity varied among species. The responses of species to soil heating can be due to differences in seed moisture, size or coat hardness (Tangney et al., 2019;Wiggers et al., 2017) and, in our study, may have been affected by scarification treatments. For example, soaking the seeds of P. elliottii, P. palustris, Chamaecrista fasciculata and L. leucocephala in water could have increased their moisture content and increased the likelihood of lethality due to soil heating (Tangney et al., 2019). ...
... Alternatively, species with harder seed coats are less sensitive to soil heating than species with softer seed coats (Wiggers et al., 2017), which may explain why seedling emergence of invasive Mimosa pigra, ...
Non‐native invasive grasses are driving intense fires across the globe but the impacts of native versus invader‐fuelled fires on community assemblages are poorly understood. By increasing fire intensity, grass invasions might increase below‐ground mortality of heat‐sensitive seeds and buds, thereby shifting community composition. We compared fuel loads in native and non‐native invasive (cogongrass, Imperata cylindrica ) plant‐dominated areas of pine savannas in Florida. Then, we conducted a field experiment to examine how fuel loads and native and invasive fuel types affected soil heating and seedling emergence or resprouting of native and invasive plant species. Average fuel loads in invaded communities were 152% greater than that in native communities. Soil heating, including heating duration >60°C, maximum temperature and heat flux >60°C, increased, and seedling emergence and resprouting decreased with greater fuel loads; these relationships were similar across the overlapping range of native and invasive fuel loads. However, longer durations of soil heating at the higher average fuel loads of invaded communities resulted in 23% lower predicted probability of seedling emergence compared to average fuel loads of native communities. Invasive cogongrass resprouting was not affected by fuel loads, indicating that cogongrass tolerates the intense fires it generates. In contrast, seedling emergence and resprouting of most other species was reduced by greater fuel loads. Synthesis and applications . By increasing fuel loading and soil heating, grass invasions may alter post‐fire community assemblages and facilitate invasive grass dominance at the expense of native species via an invasion‐fire cycle. Fuel loads can be used to predict soil heating duration and depth, and these data, combined with information on species tolerances to heating, can be used to forecast the impacts of invasions on post‐fire community composition. To maintain fire regimes that promote native communities and resist invader dominance, it is critical to manage invasive species that increase fuel loads.
... We chose 60 °C as the lowest temperature because it is commonly associated with the initial point of tissue necrosis (Stephan et al. 2010). The range of temperature and duration combinations is similar to that used in other published studies (e.g., Escudero et al. 1999;Wiggers et al. 2017) and expected to be representative of the conditions created by prescribed burns in the region (e.g., Lilly et al. 2012;Dey and Hartman 2005). These heat exposure treatments were applied by placing shortleaf pine seeds in a Yamato DKN-602C (Yamato Scientific American Inc., Santa Clara CA, USA) oven for each specific combination of temperature and time. ...
... Although other plant species are stimulated by exposure to heat (Wiggers et al. 2017;Herranz et al. 1998) and heat in combination with smoke (Singh and Raizada 2010), pine seed germination has generally been shown to be suppressed by heating. Seeds of seven Spanish pine species, Aleppo pine (Pinus halepensis), stone pine (P. ...
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Background Shortleaf pine is a fire-adapted tree species, and prescribed fire is commonly used to increase its regeneration success, improve wildlife habitat, and reach conservation objectives associated with open forest ecosystems. We studied direct effects of heat and smoke on shortleaf pine germination in a greenhouse study and effects of season of burning on the number of new germinants in a field study. Improved understanding of fire effects on shortleaf pine seed and regeneration success can help refine burn prescriptions to better meet specific management objectives. Results Temperatures ≥ 120 °C eliminated germination of shortleaf pine seeds in a greenhouse trial, and exposure of seeds to 60 °C resulted in no reduction in germination compared to the unheated control regardless of duration of exposure. At 80 °C, duration of heat exposure mattered, with exposure for 10 min reducing germination compared to unheated controls. Smoke exposure had no effect on germination. A field experiment showed that fall burns (prior to seedfall) resulted in greater initial germinant counts than early spring burns (after seedfall but before germination) or unburned controls, which both resulted in greater initial germinant counts than late spring burns (after germination). Conclusions Season of prescribed burning can affect the success of shortleaf pine germination. Late spring burning resulted in high mortality of young germinants. Burning in early spring likely resulted in direct damage to some seeds due to heating but may have also had indirect benefit by exposing mineral soil. Fall burning, before the dispersal of shortleaf pine seed, yielded the highest germinant count and is recommended if improving natural regeneration from seed is the primary objective.
... Many plant species of fire-prone ecosystems produce seeds that are adapted to resist high temperatures as a result of occasional fires [37][38][39][40]. Previous studies suggest that high temperatures can break seed dormancy, favoring the germination of fire-adapted species [37,[41][42][43][44]. ...
... The vegetation of the Cerrado has its own characteristics, which probably arose in response to local stressors, such as fire and herbivory [39,50]. As such, many plants in the Brazilian Cerrado produce seeds with adaptations to resist high temperatures resulting from wild fires [40] or seeds with nourishing appendages that favor dispersal and recruitment [51,52]. Furthermore, leaf cutting ants belonging to Atta and Acromyrmex genera are the principal defoliating agents of Cerrado vegetation [53]. ...
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Both biotic and abiotic environmental filters drive the occurrence, distribution, and persistence of plant species. Amongst drivers that influence the distribution of plants in harsh environments, seed predation and temperature are particularly important in habitats that are prone to fire. In this study, we highlight the combined effects of predation and high temperature simulating fire to understand its effects on the germination percentage and germination speed of the fire prone species Copaifera oblongifolia. Groups of seeds attacked by the beetles Rhinochenus brevicollis and Apion sp., seeds manipulated by the ant Atta laevigata, and seeds left intact were put to germinate in controlled environments. To evaluate the effects of abiotic filters, seeds with intact elaiosomes and seeds with elaiosomes removed by the ant Atta laevigata were exposed to temperatures of 27, 60, 100, and 200 °C. The results showed that only 2.8% of the seeds attacked by R. brevicollis germinated. Seeds attacked by Apion sp. germinated faster, followed by seeds with their elaiosomes removed and seeds with intact elaiosomes. Seeds attacked by Apion sp. had the lowest germination percentage. The temperature of 200 °C killed seed embryos, whereas seeds exposed to 100 °C took longer to germinate than seeds exposed to other temperatures. Our results reveal that fire intensity and seed damage are important drivers of seed germination of C. oblongifolia.
... The effect of certain physical seed properties on thermotolerance differs between plants, with higher seed size, mass, coat thickness and hardness correlating with enhanced heat tolerance in some species, but not others (Bell and Williams, 1998;Wiggers et al., 2017;Daibes et al., 2019). Although the Cactaceae studied here are relatively small-seeded species, we checked whether interspecific differences in seed weight showed an association with thermotolerance. ...
... In addition, seed coat hardness may potentially be a biophysical mechanism underlying differences in dry seed thermotolerance between species with thin coats. Most of the research involving seed hardness to date has focused on fire tolerance of species with (proportions of) water-impermeable seed coats (physical dormancy) (Daibes et al., 2019), with differences in hardness assessed through scarification (Wiggers et al., 2017). For example, Daibes et al. (2019) found that the presence of physical dormancy helped to avoid seed mortality in small-seeded legume species of a tropical savanna under the hottest treatment tested (200 • C for 1 m). ...
Extreme dry seed thermotolerance is an important trait in the context of plant adaptation to desert environments, yet the mechanisms underlying interspecific differences in seed extremophily remain little understood. Seeds of a selection of desert Cactaceae species (Leucostele litoralis, Leucostele skottsbergii, Eulychnia breviflora, Eriosyce paucicostata and Ferocactus wislizeni) were subjected to comparative biophysical and molecular analyses prior to (mature dry seeds) and post (imbibing seeds) heat treatment (103 °C). Biophysical properties of mature dry seeds, such as seed coat hardness and thermal transitions, were characterized through puncture force resistance and differential scanning calorimetry, respectively. In addition, the protein composition of dry seeds was compared by 2-D electrophoresis and de novo sequencing. Finally, oxidative DNA damage repair and gene expression during seed imbibition were analysed via ELISA (8-oxo-dG levels) and comparative transcriptomics. Results revealed that enhanced dry seed thermotolerance could not be explained by seed coat hardness, seed thermal transitions or (ROS-induced oxidative) DNA damage repair. Comparative proteomic analysis showed that the majority of overlapping proteins were more abundant in relatively heat sensitive species, suggesting that higher basal levels of (stress) proteins do not underly improved heat resilience in Cactaceae. However, comparative transcriptomics during imbibition following heat stress detected consistent expression patterns of heat- and drying-responsive genes that correlated with dry seed thermotolerance. These patterns included upregulation of positive (HSP101/CLPB1, MGE2, MBF1c, HSFB2A, RPN1a, RPT2a, DEGP14/PARK13, XPO1A, MAIGO2/MAG2, CAMTA1, ADH1) and downregulation of negative (RCF3, VOZ1, FAD7, ZAT12, RNP1) heat and drought tolerance regulators. Thus, our comparative analyses of extremophile Cactaceae species suggest that higher dry seed thermotolerance is realised through enhanced recovery from heat stress during imbibition, which is a critical stage in the plant life cycle.
... Under field conditions, increased temperature fluctuations partially alleviated physical dormancy in Mimosa leiocephala but showed little to no effect on seeds of other Cerrado legume shrubs (Daibes et al., 2017). Other studies have shown that high temperatures mostly alleviate physical dormancy under wet rather than dry conditions in legumes from fire-prone ecosystems (Van Klinken et al., 2006;Wiggers et al., 2017). In Brazil, dormancy break in one forest legume Senna multijuga was mediated by high incubation temperatures, becoming more "sensitive" during soil storage (Rodrigues-Junior et al., 2018). ...
South American savannas are an ancient, open, and species-rich ecosystem, currently threatened by numerous anthropogenic impacts, including human-driven climate change. In this chapter, we synthesize available evidence on how climate change likely will affect regeneration from seeds, focusing on the Brazilian Cerrado, a Neotropical mosaic of vegetation types. We conducted a literature survey to evaluate the main environmental drivers (fire, temperature, drought) affecting regeneration of plants from seeds in a changing climate. Unlike Mediterranean ecosystems, germination of most Cerrado species is not stimulated by fire-related cues, but heat-tolerant propagules would be selected under increasing fire frequency scenarios. Emergence from soil seed banks is closely related to the distinct rainy season, but seed drought tolerance and seed bank recruitment are not well studied. We propose a list of key research areas that need to be addressed to increase our predictive power on the effects of climate change on regeneration of plants from seeds in tropical savannas.
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Fire is a natural phenomenon that is very frequent in ecosystems of the Mediterranean basin. Cistus ladanifer and C. salvifolius ( Cistaceae family) are evergreen shrubs and components of plant communities found in this region. The objective of this study was to evaluate the effect of fire on the germination of C. ladanifer and C. salvifolius seeds. For this purpose, seeds of both species were subjected to different heat-shock treatments simulating natural fire conditions (50–175°C for 1 and 5 min). To elucidate the ecological significance of the effects of fire, viability and different traits informative about the dynamics of the germination process, such as final germination percentage (FGP), first day of germination (FDG), last day of germination (LDG), time spread of germination (TSG) and coefficient of velocity of germination (CVG), were determined. Seeds of both species exhibited fire-related behaviour, as they showed seed heat resistance and heat-stimulated germination. Germination in control, untreated seeds was very low, with 10 and 0% FGP for C. ladanifer and C. salvifolius , respectively. Thermal treatments significantly increased germination in both cases, with optimal results achieved at 100–125°C for 1 min and 75–100°C for 5 min. Under these conditions, greater germination percentages were obtained in C. salvifolius (92–95%) than in C. ladanifer (64–68%). Temperature proved to have a greater influence on seed viability and germination than the accumulated heat dose. The pattern of seed germination varied depending on the species and the heat shock treatments. In general, C. ladanifer exhibited earlier and faster germination. These characteristics, along with the high number of seeds produced per plant, make C. ladanifer a more competitive species for colonizing empty spaces after fire.
Fire has, does, and will shape forest structure, composition, and biodiversity. In this book, we introduce the driving forces, historical patterns, and future management challenges of fire in forested ecoregions across the continental USA. Climate warming and decades of fire suppression or exclusion have altered historical fire regimes and threaten diversity of fire-adapted forest vegetation into the future. Historical fire regimes ranged from frequent, low-severity fires in some ecosystems to infrequent, high-severity fires in others. They were driven by interactions among climate, drought cycles, topography and soils, fuel type and accumulation, and ignition frequencies by lightning; and increasingly by humans as Native American populations expanded in many ecoregions. Fires burned across large landscapes in ecosystems where fuels were continuous, such as pine-savanna ecosystems of the Southeastern Coastal Plain or ponderosa pine forests of the Southwest. Today, decades of fire exclusion have led to divergent outcomes: succession toward forests less apt to burn (mesophication), or more frequent or higher-severity wildfires. Management of fire toward future forests will require careful definition of desired future and reference conditions, establishing priorities, and working across agency boundaries to implement prescriptions.
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Modeling silvicultural practices after natural disturbance, with a particular focus on the use of fire and small canopy openings, may be particularly appropriate in longleaf pine (Pinus palustris Mill.) woodlands managed for multiple age classes and over long time scales. However, information about the effects of litter accumulation and fire temperatures on longleaf seedlings is inconsistent. This study examined the effects of season of burn, pine litter loading, and subsequent fire intensity on survival and growth of longleaf seedlings. For both fire seasons, mortality increased over time and was highest for the smallest grass stage seedlings and in the high litter treatment. Litter levels affected fi re intensity but had relatively minor effects on subsequent growth of surviving seedlings, but season of burn did affect seedling mortality. The grass and herbaceous fuels of the low litter treatment did not burn during either season, indicating the importance of pine needles for fuel.
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The California chaparral community has a rich flora of species with different mechanisms for cuing germination to postfire conditions. Heat shock triggers germination of certain species but has no stimulatory effect on a great many other postfire species that are chemically stimulated by combustion products. Previous reports have shown that charred wood will induce germination, and here we report that smoke also induces germination in these same species. Smoke is highly effective, often inducing 100% germination in deeply dormant seed populations with 0% control germination. Smoke induces germination both directly and indirectly by aqueous or gaseous transfer from soil to seeds. Neither nitrate nor ammonium ions were effective in stimulating germination of smoke-stimulated species, nor were most of the quantitatively important gases generated by biomass smoke. Nitrogen dioxide, however, was very effective at inducing germination in Caulanthus heterophyllus (Brassicaceae), Emmenanthe penduliflora (Hydrophyllaceae), Phacelia grandiflora (Hydrophyllaceae), and Silene multinervia (Caryophyllaceae). Three species, Dendromecon rigida (Papaveraceae), Dicentra chrysantha, and Trichostema lanatum (Lamiaceae), failed to germinate unless smoke treatment was coupled with prior treatment of 1 yr soil storage. Smoke-stimulated germination was found in 25 chaparral species, representing 11 families, none of which were families known for heat-shock-stimulated germination. Seeds of smoke-stimulated species have many analogous characteristics that separate them from most heat-shock-stimulated seeds, including: (1) outer seed coats that are highly textured, (2) a poorly developed outer cuticle, (3) absence of a dense palisade tissue in the seed coat, and (4) a subdermal membrane that is semipermeable, allowing water passage but blocking entry of large (molecular mass > 500) solutes. Tentative evidence suggests that permeability characteristics of this subdermal layer are altered by smoke. While the mechanism behind smoke-induced germination is not known, it appears that smoke may be involved in overcoming different blocks to germination in different species. For example, in Emmenanthe penduliflora, NO2 in smoke was sufficient to induce germination, and most forms of physical or chemical scarification also induced germination. For Romneya coulteri, NO2 alone failed to induce germination, and scarified seeds required addition of gibberellic acid. In Dicentra chrysantha, none of these treatments, nor smoke alone, induced germination, but germination was triggered by a combination of soil burial followed by smoke treatment. Smoke-stimulated species differed substantially in the duration of smoke exposure required to induce germination, and this was inversely correlated with tolerance to smoke exposure. We suggest that such differences in response may affect postfire community structure.
This chapter describes S functions for tree-based modeling. Tree-based models provide an alternative to linear and additive models for regression problems and to linear logistic and additive logistic models for classification problems. The models are fitted by binary recursive partitioning whereby a dataset is successively split into increasingly homogeneous subsets until it is infeasible to continue. The implementation described in this chapter consists of a number of functions for growing, displaying, and interacting with tree-based models. This approach to tree-based models is consistent with the data-analytic approach to other models, and consists primarily of fits, residual analyses, and interactive graphical inspection.
The methodology used to construct tree structured rules is the focus of this monograph. Unlike many other statistical procedures, which moved from pencil and paper to calculators, this text's use of trees was unthinkable before computers. Both the practical and theoretical sides have been developed in the authors' study of tree methods. Classification and Regression Trees reflects these two sides, covering the use of trees as a data analysis method, and in a more mathematical framework, proving some of their fundamental properties.
Wildland fire radiant energy emission is one of the only measurements of combustion that can be made at high temporal and spatial resolutions. Furthermore, spatially and temporally explicit measurements are critical for making inferences about ecological fire effects. Although the correlation between fire frequency and plant biological diversity in frequently burned coniferous forests is well documented, the ecological mechanisms explaining this relationship remains elusive. Uncovering these mechanisms will require highly resolved, spatially explicit fire data (Loudermilk et al. 2012). Here, we describe our efforts at connecting spatial variability in fuels to fire energy release and fire effects using fine scale (1 cm²) longwave infrared (LWIR) thermal imagery. We expected that the observed variability in fire radiative energy release driven by canopy-derived fuels could be the causal mechanism driving plant mortality, an important component of community dynamics. Analysis of fire radiant energy released in several experimental burns documented a close connection among patterns of fire intensity and plant mortality. Our results also confirmed the significance of cones in driving fine-scale spatial variability of fire intensity. Spatially and temporally resolved data from these techniques show promise to effectively link the combustion environment with postfire processes, remote sensing at larger scales, and wildland fire modeling efforts. 2016
Tools for performing model selection and model averaging. Automated model selection through subsetting the maximum model, with optional constraints for model inclusion. Model parameter and prediction averaging based on model weights derived from information criteria (AICc and alike) or custom model weighting schemes. [Please do not request the full text - it is an R package. The up-to-date manual is available from CRAN].
This study examines the complex feedback mechanisms that regulate a positive relationship between species richness and productivity in a longleaf pine-wiregrass woodland. Across a natural soil moisture gradient spanning wet-mesic to xeric conditions, two large scale manipulations over a 10-yr period were used to determine how limiting resources and fire regulate plant species diversity and productivity at multiple scales. A fully factorial experiment was used to examine productivity and species richness responses to N and water additions. A separate experiment examined standing crop and richness responses to N addition in the presence and absence of fire. Specifically, these manipulations addressed the following questions: (1) How do N and water addition influence annual aboveground net primary productivity of the midstory/overstory and ground cover? (2) How do species richness responses to resource manipulations vary with scale and among functional groups of ground cover species? (3) How does standing crop (including overstory, understory/midstory, and ground cover components) differ between frequently burned and fire excluded plots after a decade without fire? (4) What is the role of fire in regulating species richness responses to N addition? This long- Term study across a soil moisture gradient provides empirical evidence that species richness and productivity in longleaf pine woodlands are strongly regulated by soil moisture. After a decade of treatment, there was an overall species richness decline with N addition, an increase in richness of some functional groups with irrigation, and a substantial decline in species richness with fire exclusion. Changes in species richness in response to treatments were scale-dependent, occurring primarily at small scales (≤10 m²). Further, with fire exclusion, standing crop of ground cover decreased with N addition and non-pine understory/midstory increased in wet-mesic sites. Non-pine understory/midstory standing crop increased in xeric sites with fire exclusion, but there was no influence of N addition. This study highlights the complexity of interactions among multiple limiting resources, frequent fire, and characteristics of dominant functional groups that link species richness and productivity.