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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
Introduction
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
e-mail: marion_wiggers@fws.gov
J. K. Hiers
Tall Timbers Research Station, Tallahassee, FL 32312,
USA
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
123
Plant Ecol (2017) 218:151–171
DOI 10.1007/s11258-016-0674-x
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Germination can occur under a great variety of environmental conditions because species maximise establishment and survival of new sexually produced individuals in different abiotic and biotic contexts (Baskin & Baskin 2014). Fire may stimulate, have no effect or inhibit germination in different species (Keeley & Fotheringham 2000;Pauchard et al. 2008;Contreras et al. 2011;Jaureguiberry & D ıaz 2015;Wiggers et al. 2017). Accordingly, considering that early lifecycle stages are crucial for invasion success (Hobbs & Huenneke 1992, but see Flores-Moreno & Moles 2013), germination response to fire in fire-prone ecosystems may have direct implications on the capacity of species to spread in the invaded system, particularly if seed dispersal occurs in the fire season. ...
... To examine our data with regression techniques, we converted discrete time and temperature combinations into a 'heat index' that summarises the accumulated heat received by seeds in each treatment (see Paula & Pausas 2008;Wiggers et al. 2017). This heat index (H) was calculated by multiplying the treatment temperature (°C) by the natural logarithm of the exposure time (min) plus one. ...
... Germination response to heat-shocks is frequently used to this end (e.g. Hanley et al. 2003;Jaureguiberry & D ıaz 2015;Wiggers et al. 2017). However, in the wild, fires are far more complex and the combination of several fire surrogates (e.g. ...
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en It has been frequently recognised that there is a positive feedback between plant invasion and fire underlying invasion success in fire‐prone ecosystems. Accordingly, the response of woody alien species germination to fire may have direct implications on their invasiveness in those ecosystems, particularly when fruit ripening occurs in the fire season. Here, we experimentally evaluated the germination response of some of the main woody invaders of the Chaco Serrano dry woodlands (Gleditsia triacanthos, Cotoneaster glaucophyllus, Ligustrum lucidum, Pyracantha angustifolia and Melia azedarach), which fruit in the seasons of highest fire frequency. Seeds were subjected to heat‐shock treatments that simulated a range of heat intensities, and the species were classified according to their germination response as heat sensitive, tolerant or stimulated. Since Gleditsia triacanthos has indehiscent fruits that fall from the plant and might be exposed to flames, its germination response was also assessed of seeds exposed to fruit burning. Germination responses to heat varied among the invasive species. G. triacanthos seeds experienced increased germination under very low and low heat indexes; it was therefore classified as heat stimulated. The other four species showed no change in germination under very low heat indexes and were therefore considered heat tolerant. However, all species were sensitive to high heat as indicated by their significant decline in germination. G. triacanthos would have limited capacity to recruit from seeds following flaming combustion of its fruits. The prevalence of heat‐tolerant rather than heat‐stimulated germination responses suggests that the occurrence of frequent and seasonal fires in this subtropical savanna system might delay rather than boost the expansion of these invasive species in the system. Yet, the presence of heat‐stimulated germination in one of the studied species warns against generalisation, even within the same ecosystem, and further supports the idiosyncratic nature of invasion success. Abstract in Spanish is available with online material. Resumen es Ha sido frecuentemente reconocida la existencia de una retroalimentación positiva entre las invasiones de plantas y el fuego, determinando el éxito de invasión en ecosistemas propensos al fuego. En concordancia, la respuesta de germinación al fuego de especies leñosas invasoras, puede tener implicancias directas en su invasividad en esos ecosistemas, sobre todo si la fructificación ocurre en la temporada de fuegos. En esta investigación, se evaluó experimentalmente la respuesta de germinación de algunas de las principales especies leñosas invasoras de los bosques xerófilos del Chaco Serrano (Gleditsia triacanthos, Cotoneaster glaucophyllus, Ligustrum lucidum, Pyracantha angustifolia y Melia azedarach) que fructifican en la temporada de mayor frecuencia de fuego. Las semillas fueron expuestas a tratamientos de golpe de calor que simulaban un rango de intensidad de calor, siendo clasificadas como sensibles, tolerantes o estimuladas de acuerdo a su respuesta de germinación al calor. Debido a que Gleditsia triacanthos posee frutos indehiscentes que caen de la planta, pudiendo estar expuestos a las llamas de un fuego, también se evaluó la respuesta de germinación de semillas expuestas a la quema directa del fruto. La respuesta de germinación al calor fue variada entre las especies invasoras. Las semillas de G. triacanthos experimentaron un incremento en su germinación ante índices bajos y muy bajos de calor; por lo que fueron clasificadas como estimuladas por calor. Las otras cuatro especies no mostraron cambios en su germinación antes índices muy bajos de calor por lo que fueron consideradas como tolerantes al calor. Sin embargo, todas las especies fueron sensibles a calor intenso como indica la disminución significativa de su germinación. G. triacanthos tendría capacidad limitada para reclutar desde semillas a partir de la combustión de sus frutos. La prevalencia de tolerancia más que de estimulación como respuesta de germinación al calor sugiere que la ocurrencia de fuegos frecuentes y estacionales en este sistema de sabana subtropical puede retrasar más que impulsar la expansión de estas especies invasoras en el sistema. Sin embargo, la presencia en una de las especies estudiadas de germinación estimulada por calor nos advierte acerca de la generalización inclusive dentro del mismo ecosistema, apoyando la naturaleza idiosincrática del éxito de invasión.
... 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, ...
Article
Non‐native invasive grasses are driving intense fires across the globe but the impacts of native versus invader‐fueled fires on community assemblages are poorly understood. By increasing fire intensity, grass invasions might increase belowground 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 in native communities. Soil heating, including heating duration >60 °C, maximum temperature, and heat flux >60 ºC, 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 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.
... 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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... 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). ...
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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.
... Hence, the mechanisms underpinning PY break in the Cerrado might differ from those in Mediterranean-like ecosystems, depending upon a broad of environmental combinations yet underexplored in the literature. Within Fabaceae, dormant seeds have been reported to show PY break related to wet heat (Van Klinken et al., 2006;Wiggers et al., 2017), or to increased sensitivity after storage in the soil seed banks, thus spreading germination through time (Liyanage and Ooi, 2017;Rodrigues-Junior et al., 2018). Likewise, wet temperature fluctuation may break PY in Sapindaceae, dislodging the water gap (Turner et al., 2009) or PY alleviation can required a combination of more than one temperature treatment (Van Assche et al., 2003). ...
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... Plant species or communities may be classified along a spectrum of fire-tolerance to fire-intolerance, and those that are fire-adapted have specific traits related to avoidance, tolerance, or regeneration (Lavorel and Garnier, 2002). For example, traits associated with fire-adapted species include heat-tolerant seeds or fire-stimulated seed germination (Paula and Pausas, 2008;Wiggers et al., 2017), thick bark (Pausas, 2015), or vigorous re-sprouting capability . In contrast, fire intolerant species lack these traits but may have other traits, such as shade tolerance or rapid growth, with potential to https://doi.org/10.1016/j.foreco.2020.118163 ...
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... However, potential interaction effects of fire and climate on stimulating germination could be expected, as laboratory studies have indicated that moist heat could promote germination more than dry heat in some species (i.e. hard-seeded legumes) (Wiggers et al., 2017). Consequently, to study the influence of fire regimes on plant traits under different environmental conditions will contribute to better understanding the potential impacts of fire on vegetation as well as to identifying which traits are key for the resilience of the community. ...
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... Most of these species were associated with patches that have high tree density, basal area, and canopy cover and have been observed occurring in similar conditions in other pine savanna systems [53]. Seeds of some species in Fabaceae have exhibited high heat tolerance during fires [58] and have fire-stimulated seed germination [59]. These conditions correspond to those in mid-age patches where species in Fabaceae were preferentially found. ...
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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.
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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
Code
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].
Article
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.