It's the land use not the fuels: fires and land development in southern California
No abstract available.
Real Estate Review↗
Geology topics
Philip W. Rundel
Publications and source records attributed to Philip W. Rundel.
Fire and the Miocene expansion of C4 grasslands
C 4 photosynthesis had a mid-Tertiary origin that was tied to declining atmospheric CO 2 , but C 4 -dominated grasslands did not appear until late Tertiary. According to the ‘CO 2 -threshold’ model, these C 4 grasslands owe their origin to a further late Miocene decline in CO 2 that gave C 4 grasses a photosynthetic advantage. This model is most appropriate for explaining replacement of C 3 grasslands by C 4 grasslands, however, fossil evidence shows C 4 grasslands replaced woodlands. An additional weakness in the threshold model is that recent estimates do not support a late Miocene drop in p CO 2 . We hypothesize that late Miocene climate changes created a fire climate capable of replacing woodlands with C 4 grasslands. Critical elements were seasonality that sustained high biomass production part of year, followed by a dry season that greatly reduced fuel moisture, coupled with a monsoon climate that generated abundant lightning-igniting fires. As woodlands became more open from burning, the high light conditions favoured C 4 grasses over C 3 grasses, and in a feedback process, the elevated productivity of C 4 grasses increased highly combustible fuel loads that further increased fire activity. This hypothesis is supported by paleosol data that indicate the late Miocene expansion of C 4 grasslands was the result of grassland expansion into more mesic environments and by charcoal sediment profiles that parallel the late Miocene expansion of C 4 grasslands. Many contemporary C 4 grasslands are fire dependent and are invaded by woodlands upon cessation of burning. Thus, we maintain that the factors driving the late Miocene expansion of C 4 were the same as those responsible for maintenance of C 4 grasslands today.
Ecology Letters↗
Evolution of CAM and C 4 carbon-concentrating mechanisms
Mechanisms for concentrating carbon around the Rubisco enzyme, which drives the carbon-reducing steps in photosynthesis, are widespread in plants; in vascular plants they are known as crassulacean acid metabolism (CAM) and C 4 photosynthesis. CAM is common in desert succulents, tropical epiphytes, and aquatic plants and is characterized by nighttime fixation of CO2. The proximal selective factor driving the evolution of this CO2-concentrating pathway is low daytime CO2, which results from the unusual reverse stomatal behavior of terrestrial CAM species or from patterns of ambient CO2 availability for aquatic CAM species. In terrestrials the ultimate selective factor is water stress that has selected for increased water use efficiency. In aquatics the ultimate selective factor is diel fluctuations in CO2 availability for palustrine species and extreme oligotrophic conditions for lacustrine species. C4 photosynthesis is based on similar biochemistry but carboxylation steps are spatially separated in the leaf rather than temporally as in CAM. This biochemical pathway is most commonly associated with a specialized leaf anatomy known as Kranz anatomy; however, there are exceptions. The ultimate selective factor driving the evolution of this pathway is excessively high photorespiration that inhibits normal C3 photosynthesis under high light and high temperature in both terrestrial and aquatic habitats. CAM is an ancient pathway that likely has been present since the Paleozoic era in aquatic species from shallow-water palustrine habitats. While atmospheric CO2 levels have undoubtedly affected the evolution of terrestrial plant carbon-concentrating mechanisms, there is reason to believe that past atmospheric changes have not played as important a selective role in the aquatic milieu since palustrine habitats today are not generally carbon sinks, and the selective factors driving aquatic CAM are autogenic. Terrestrial CAM, in contrast, is of increasing selective value under extreme water deficits, and undoubtedly, high Mesozoic CO2 levels reduced the amount of landscape perceived by plants as water limited. Late Tertiary and Quaternary reductions in atmospheric CO2, coupled with increasing seasonality, were probably times of substantial species radiation and ecological expansion for CAM plants. C4 photosynthesis occurs in only about half as many families as CAM, and three-fourths of C4 species are either grasses or sedges. Molecular phylogenies indicate C4 is a more recent innovation than CAM and that it originated in the mid-Tertiary, 20–30 Ma, although some data support an earlier origin. While the timing of the origin of C4 remains controversial, the nearly explosive increase in C4 species is clearly documented in the late Miocene, 4–7 Ma. Increasing seasonality has been widely suggested as an important climatic stimulus for this C4 expansion. Alternatively, based on models of photosynthetic quantum yield at different temperatures and CO2 concentration, it has been hypothesized that the late Miocene C4 expansion resulted from declining atmospheric CO2 levels. This model is most appropriate for explaining the transition from C3 grasslands to C4 grasslands but by itself may not be sufficient to explain the more likely scenario of a late Miocene transition from C3 woodland/ savanna to C4 grasslands. A largely unexplored hypothesis is that climatic changes in late Miocene altered disturbance regimes, in particular the incidence of fires, which today are often associated with maintenance of C4 grasslands. Oceanic charcoal sediments that appear to represent Aeolian deposits from continental wildfires follow a strikingly similar pattern of explosive increase in late Miocene. Climate, CO2, and disturbance are not mutually exclusive explanations and probably all acted in concert to promote the expansion of C4 grasslands. More recently, late Quaternary changes in CO2 may have been responsible for driving major changes in the landscape distribution of C4 species. The theory is sound; however, many of the studies cited in support of this model are open to alternative interpretations, and none has eliminated climatic factors as important selective agents. CAM and C4 evolution required coupling of biochemical pathways with structural changes in photosynthetic tissues, succulence in CAM and Kranz in C4. This was apparently accomplished by piecemeal evolution beginning with mechanisms for recapturing respiratory CO2, although this need not have been so in aquatic CAM species. It has been proposed that the extreme rarity of both pathways in the same plant results from biochemical and structural incompatibilities (Sage 2002). Equally important is the fact that the selective environments are quite different, with CAM evolution thriving on stressful sites inhospitable to C3 species whereas C4 evolution has selected for rapid growth capable of outcompeting associated C3 plants.
International Journal of Plant Sciences↗
A population model for a long-lived, resprouting chaparral shrub: Adenostoma fasciculatum
Extensive stands of Adenostoma fasciculatum H.&A. (chamise) in the chaparral of California are periodically rejuvenated by fire. A population model based on size-specific demographic characteristics (thinning and fire-caused mortality) was developed to generate probable age distributions within size classes and survivorship curves for typical stands. The model was modified to assess the long term effects of different mortality rates on age distributions. Under observed mean mortality rates (28.7%), model output suggests some shrubs can survive more than 23 fires. A 10% increase in mortality rate by size class slightly shortened the survivorship curve, while a 10% decrease in mortality rate by size class greatly elongated the curve. This approach may be applicable to other long-lived plant species with complex life histories.
Ecological Modelling↗
Quantitative variation and the ecological role of vulpinic acid and atranorin in thallus of Letharia vulpina
High pressure liquid chromatography was used to determine concentrations of vulpinic acid and atranorin in the thaullus of Letharia vulpina . Vulpinic acid concentration is lowest in the old basal branches of the thallus and increases toward the young branch tips, whereas the reverse is true for atranorin. The suggested role of vulpinic acid as an anti-herbivore defense compound is supported by both this distribution of the compound and our observations that vulpinic acid acts as a feeding deterrent to certain invertebrates. No significant difference in the content of atranorin or vulpinic acid was found in lichens from microhabitats of different sunlight intensities.
Biochemical Systematics and Ecology↗