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Jon E. Keeley

Publications and source records attributed to Jon E. Keeley.

At least 181 records · Page 10Linked to original sources
USGS1993

Smoke-induced flowering in the fire-lily Cyrtanthus ventricosus

Flowering of certain fynbos geophytes has long been noted to have an obligate dependence upon fire. One of these species, Cyrtanthus ventricosus (Jacq.) Willd., is shown here to be stimulated to flower by smoke. Ethylene, a gaseous component of smoke, did not stimulate flowering in this species.

South African Journal of Botany↗
USGS1992

Recruitment of seedlings and vegetative sprouts in unburned chaparral

Age structure of 12 stands of chaparral, unburned for 56—120 yr, was investigated. All shrubs produced discernible growth rings, and ring counts on stems from stands of known age, plus synchrony in annual growth ring width among species in the same stand, were taken as evidence that growth rings represented annual rings. Species that survive fire by vegetative regeneration from the root crown were capable, in the absence of fire, of continuously regenerating their canopy with basal sprouts. These multistemmed resprouting species had an uneven age structure, indicating continuous recruitment of new stems into the population. The predicted age distribution for resprouting shrub stem populations illustrated very different patterns for species within the same stand and for the same species in different stands. In most resprouting shrubs, <15% of the stems dated back to the first decade after fire, and in many species the majority of stems were <10 yr of age. This contrasts with obligate seeding shrubs that do not initiate new stems from the root crown, and for which, in undisturbed chaparral, 100% of the stems date back to the years immediately following the last fire. Seedling recruitment was abundant in seven of these old chaparral communities. Species with abundant seedling populations, ranging from 1000 to 37 500 individuals/ha, were Quercus dumosa, Q. wislizenii, Rhammus crocea, Prunus ilicifolia, Heteromeles arbutifolia and Cercocarpus betuloides. These species share the characteristic of regenerating after fire solely by resprouting, and seldom recruit seedlings during the first few decades after fire. Thus, it is concluded that these postfire obligate resprouters require long fire—free periods for successful reproduction and potential population expansion. For all but the last species, seedlings were most abundant in stands with a closed canopy and deep litter layer. Safe sites for establishment were not in gaps, although gaps may be important for successful recruitment of saplings into the adult population. For all sprouting species the actual age distribution of seedlings and saplings indicated that recruitment did not occur every year; mortality was high but most populations had some saplings that survived beyond the first decade. For all postfire seeding taxa, viz., Adenostoma, Arctostaphylos, and Ceanothus, no significant seeding recruitment was observed in these ancient communities. Seedlings of woodland tree species had colonized several of these old chaparral communities; however, even in stands a century old, successional replacement of chaparral was not imminent.

Ecology↗
USGS1991

Diurnal photosynthesis cycle in CAM and non-CAM seasonal pool aquatic macrophytes

Seasonal pools undergo marked diurnal changes in pH, free carbon dioxide, and oxygen levels. Previous studies showed that Isoetes howellii utilized crassulacean acid metabolism (CAM) photosynthesis as a means of assimilating carbon at night when ambient carbon dioxide levels are high. However, much of the pool flora is not CAM. We hypothesized that coexistence under extreme carbon—limiting conditions would select for other photosynthetic characteristics in these non—CAM species. Quantitative carbon uptake measurements wee made in the field on the CAM Isoetes howellii and the non—CAM Eleocharis acicularis, Downingia bella, and Plagiobothrys undulatus. Despite wide phylogenetic separation, thee species are structurally convergent in that they all produce aquatic stage with an isoetid growth form. They are functionally similar in that none appear to utilize bicarbonate, and all are carbon limited through much of the day, as indicated by a marked midday depression in carbon uptake that last through the afternoon. All four species were capable of carbon fixation at night, although only in Isoetes howellii was this accompanied by an overnight accumulation of acid. It was estimated that dark fixation in I. howelli contributed up to 40% of its total carbon gain under submerged conditions, but was <1% under aerial conditions. The role of dark fixation in the other three, non—CAM, species is unknown. Short—term (1 s) steady—term (1 s) steady—state 1 4 C tracer studies in the laboratory revealed that, under submerged conditions in the light, all species assimilated carbon into the C 3 photosynthetic product phosphoglycerate (PGA) and the C 4 photosynthetic products of malate, citrate, and aspartate. The proportion of label fixed into C 4 organic acids was greatest in E. acicularis. The ratio of RuBP carboxylase/PEP carboxylase was broadly similar in all species, ranging from 8.8 to 11.5. When the pools dried down and the plants became aerial, photosynthetic rates increased and a midday depression such as occurred under submergence was not observed. Carbon fixation in the dark became negligible, and the RuBP carboxylase/PEP carboxylase ratio increased. Seasonal changes in biomass showed I. howellii and E. acicularis dominated the pools early in the season, and the biomass of the other two species increased later when the pools dried. The latter species, D. bella and P. undulatus, also had higher rates of photosynthesis under aerial conditions.

Ecology↗
USGS1991

The relationship between stable oxygen and hydrogen isotope ratios of water in astomatal plants

Isotropic fractination of leaf water during transpiration is influenced by both equilibrium and kinetic factors. Previous workers have predicted that the influence of each factor varies depending upon the path of water loss,m whether centralized through stomata, or diffuse through the cuticle. We studied the relationship between the δD and δ 18 O values of lead and stem waters of laurel sumac, Rhus laurina (Nutt.) T. & G., and its parasite, dodder, Cuscuta subinclusa D. & H., growing in the field. Stomatal transpiration, associated with more stagnant boundary layers, predominates in R. laurina ; cuticular transpiration, associated with more turbulent boundary layers, is most important in the largely astomatal C. subinclusa . We also studied the diurnal variation in the δD and δ 18 O values of lead waters of two astomatal plants, Chiloschista lunifera (Rchb. F.) J.J.S. and Stylites andicola Amstutz, and two stomatal plants, Tillandsia balbisiana Schult. and Lilaeopsis schaffneriana (Schlecht.) C. & R., growing with them under the same conditions in the laboratory. Slopes, m, for the relation δD = m δ 18 O + b were significantly higher for stem waters in C. subinclusa that for leaf waters in R. laurina (1.77), consistent with the difference in the boundary layers through which water was lost in the two species. The magnitude of diurnal heavy isotope enrichment of tissue water was smaller in C. subinclusa than in R. laurina , which is also consistent with predictions concerning evapotranspiration through difference types of boundary layers. The slopes, m, in plant waters in the laboratory experiments, conducted at high humidity, were not different than those observed during evaporation of water from pans, regardless of plant anatomy. The observation suggests that cuticular transpiration is important in influencing isotopic fractionation of water only at low humidity. Our results indicate that the isotopic composition of water vapor released by plants in arid regions may be influenced by the relative proportions of stomatal versus cuticular transpiration.

Book chapter↗
USGS1990

Photosynthetic pathways in freshwater aquatic plants

Recent studies show that generalizations about photosynthetic pathways, derived from terrestrial plant studies, do not apply to aquatic plants. Crassulacean acid metabolism (CAM) photosynthesis is of selective value not only in arid environments, where it enhances water-use efficiency, but also in aquatic plants of oligotrophic waters, where it enhances competitive ability in carbon acquisition. C 4 photosynthesis is present in many aquatic species, but in these species it is not coupled with the specialized anatomy of terrestrial C 4 plants. The ratio of the stable carbon isotopes, 13 C/ 12 C, in the biomass of terrestrial plants is a marker of their photosynthetic pathway. In aquatic environments, additional resistances to carbon-isotope fractionation make this technique of limited use in detecting photosynthetic pathways.

Trends in Ecology and Evolution↗