Abstract
Syndromes are life history responses that are correlated to environmental regimes and are shared by a group of species (Stebbins, 1974). In the California chaparral there are two syndromes contrasted by the timing of seedling recruitment relative to wildfires. One syndrome, here called the fire-recruiter or refractory seed syndrome, includes species (both resprouting and non-resprouting) which share the feature that the timing of seedling establishment is specialized to the first rainy season after fire. Included are woody, suffrutescent and annual life forms but no geophytes have this syndrome. These species are linked by the characteristic that their seeds have a dormancy which is readily broken by environmental stimuli such as intense heat shock or chemicals leached from charred wood. Such seeds are referred to as “refractory” and dormancy, in some cases, is due to seed coat impermeability (such seeds are commonly called hardseeded), but in other cases the mechanism is unknown. Seeds of some may require cold stratification and/or light in addition to fire related stimuli. In the absence of fire related cues, a portion or all of a species’ seed pool remains dormant. Most have locally dispersed seeds that persist in the soil seed bank until the site burns. Dispersal of propagules is largely during spring and summer which facilitates the avoidance of flowering and fruiting during the summer and fall drought. Within a life form (e.g., shrub, suffrutescent, etc.), the seeds of these species have less mass than those of species with non-refractory seeds and this possibly reflects the environmental favorableness of the postfire environment for seedling establishment. Regardless of when fire occurs, germination is normally delayed until late winter or early spring. In the absence of fire, or other disturbance, opportunities for population expansion are largely lacking for species with this syndrome.
The other syndrome, here called the fire-resister or non-refractory seed syndrome, includes species that are resilient to frequent fires (mostly by vegetative resprouting), but require fire-free periods for recruiting new seedlings. Included are shrubs, subshrubs, suffrutescents, lianas, geophytes and annuals. All are linked by the characteristic that their seeds germinate in the absence of cues related to wildfires. In many cases no form of seed dormancy is present and the seeds germinate soon after dispersal; consequently these species do not accumulate a persistent seed bank. Germination and seedling establishment is independent of fire and thus opportunities for population expansion are also independent of fire. The demographic pattern of seedling recruitment varies with the life form. For shrubs, seedling recruitment may be restricted to sites free of fire for periods of a hundred years or more. Recruitment appears to require relatively mesic conditions and this may account for the patchy distribution of these species within the matrix of relatively arid sites. Finding such sites has selected for propagules specialized for wind or animal dispersal; the majority are bird dispersed. These shrub species all disperse fruits in fall and winter and this may have been selected to take advantage of migratory birds as well as to time dispersal to the winter rains typical of the mediterranean-climate. Germination typically occurs within several weeks of the first fall or winter rains. Maturation of flowers and fruits during the summer and fall drought may account for the distribution of these species on more mesic sites. Seed mass of these species is large and this may have been selected to provide an advantage to seedlings establishing under the canopy of this dense shrub community.
Resumen
Síndromes son las respuestas de ciclos biológicos correlacionados con régimenes ambientales y compartidos por un grupo de especies (Stebbins, 1974). En el Chaparral de California se encuentran dos síndromes, los cuales contrastan por el tiempo requerido para el restablecimiento de retonos en relación con los incendios forestales. Uno de estos síndromes, aquí denominado “síndrome fuego-restablecedor” o “semilla refractaria,” incluye especies (tanto retoāntes como no retonantes) que comparten la característica de que el tiempo para el establecimiento de plántulas está especializado en la primera estación de lluvias después del incendio Incluídas se encuentran formas de vida anuales, sufrutescentes y lenosas, sin embargo ninguna hierba perenne presenta este síndrome. Estas especies están unidas por la característica de poseer semillas con una latencia fácilmente interrumpida por estímulos ambientales, tales como un intenso shock de calor o productos químicos lixiviados de madera carbonizada. A este tipo de semillas se les conoce como refractarias. En ocasiones, su latencia se debe a la impermeabilidad de la capa de la semilla. En otros casos, el mecanismo es desconocido. Además de los estímulos relacionados con el fuego, las semillas de algunas especies pueden requerir de una estratificación fría y/o de luz. En ausencia de indicadores relacionados con el fuego, una parte o todo el grupo de semillas de una especie permanece latente. La mayoría cuenta con semillas dispersadas localmente, las cuales permanecen en el banco de semillas en el suelo, hasta que el lugar sufre de un incendio. La dispersión de semillas ocurre principalmente durante la primavera y el verano, lo cual impide el florecimiento y producción de frutos durante la sequía del verano y otono. Dentro de una forma de vida, las semillas de estas especies tienen una masa menor a la de aquellas especies con semillas no refractarias, lo cual refleja posiblemente el favorecimiento ambiental del ambiente posterior a un incendio en cuanto al establecimiento de plántulas. Sin tomar en cuenta cuando ocurre el incendio, la germinación se retrasa normalmente hasta finales del invierno o principios de la primavera. En ausencia de incendios o cualquier otra alteración, las oportunidades de expansión de la población son casi nulas para las especies con este síndrome.
El otro síndrome, aquí denominado “resistidorde fuego” o “semilla no refractaria,” incluye especies resistentes a incendios fréquentes (por retono vegetativo en su mayoría), las cuales requieren, sin embargo, períodos libres de incendios para restablecer nuevas plántulas. Entre las especies incluídas se encuentran arbustos, subarbustos, sufrutescentes, lianas, hierbas perennes y anuales. Todas están unidas por la característica de tener semillas que germinan en ausencia de indicadores relacionados con incendios. En muchas ocasiones, las semillas no presentan latencia alguna y germinan poco después de su dispersión. Por consiguiente, estas especies no acumulan un banco de semillas persistente. La germinación y establecimiento de plántulas es independiente del fuego y, por lo tanto, las oportunidades de expansión para la población son asimismo independientes del fuego. El patrón demográfico de restablecimiento de plántulas varía de acuerdo a la forma de vida. En el caso de los arbustos, el restablecimiento de plántulas puede estar restringido a sitios libres de incendios por períodos de cien anos o más. El restablecimiento parece requérir condiciones relativamente mésicas, lo cual puede ser la causa de la distribución desigual de estas especies dentro de una matriz de sitios relativamente áridos. El encontrar estos sitios ha seleccionado en favor de las semillas especializadas en dispersión por viento o a través de animales. La mayoría son dispersadas por aves. Todas estas especies de arbustos dispersan sus frutos durante el otono e invierno, lo cual puede haber sido seleccionado para obtener ventaja de las aves migratorias, así como para programar la dispersión de acuerdo con las lluvias de invierno, típicas del clima mediterráneo. La germinacion ocurre típicamente por varias semanas durante las primeras lluvias de otono o invierno. La maduración de las flores y frutos durante la sequía del verano y otono puede explicar la distribución de estas especies en sitios mas mésicos. Las semillas de estas especies son de masa considerable, lo cual puede haber sido seleccionado para proporcionar una ventaja al establecimiento de plántulas bajo el follaje de esta densa comunidad de arbustos.
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Keeley, J.E. Seed germination and life history syndromes in the California chaparral. Bot. Rev 57, 81–116 (1991). https://doi.org/10.1007/BF02858766
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DOI: https://doi.org/10.1007/BF02858766