Ned Fetcher

PHOTOSYNTHETIC LIGHT ENVIRONMENTS IN A LOWLAND TROPICAL RAIN FOREST IN COSTA RICA

SUMMARY (1) Measurements of photosynthetic photon flux density (PPFD) in four sites within a lowland tropical rain forest were compared. The habitats investigated were a 0.5 ha clearing, a 400 m2 gap, a 200 m2 gap, and heavily shaded understorey. (2) Measurements were made during both wet and dry seasons under a variety of weather conditions. Quantum sensors were used to monitor continuously 10-min average PPFD over a 3-10 day period at each location. Daily average PPFD, total daily PPFD, and daily frequency distributions were analysed for two adjacent sensors per site on each sampling day. (3) Daily total PPFD in the understorey, 200 m2 gap, and 400 m2 gap were 1-2%, 9%, and 20-35%, respectively, of PPFD in the clearing. Daily total PPFD in the 400 m2 gap during the dry season was, on average, 2.4 times greater than in the 200 m2 gap, and 20-25 times those in the understorey. In the 200 m2 gap, daily total PPFD was nine times greater than in the understorey during the dry season. (4) In the clearing, PPFD was significantly different between seasons with 24% higher PPFD during the dry season. In the 400 m2 gap and understorey, PPFD was not significantly different between seasons. (5) The percentage of available PPFD reaching the understorey was highest on cloudy, overcast days and lowest on sunny days. No correlation was found between daily total PPFD in the 400 m2 gap centre and in the adjacent understorey measured on the same days. (6) In the clearing, a high proportion of 10-min averages were greater than 500 ,umol m-2 s-I; in the 400 m2 gap, a high proportion of 10-min averages were between 100,umol m-2 s-1 and 500 umol m-2 s-1; in the understorey over 70% of the 10-min averages were below 10,urmol m-2 s-1. The clearing exhibited the greatest diurnal variation in PPFD and the least day-to-day variation, whereas the understorey exhibited the least diurnal variation and the greatest day-to-day variation in PPFD.

Photosynthetic Responses of Tropical Forest Plants to Contrasting Light Environments

Across the complex matrix of microsites that compose tropical forests, light availability varies more dramatically than any other single plant resource. On a sunny day, instantaneous measurements of photosynthetically active radiation range over 3 orders of magnitude, from less than 10 µmol m-2 s-1 in closed-canopy understory of mature forests to well over 1000 µmol m-2 s-1 in exposed microsites of gaps and large clearings, or at the top of the forest canopy (Chazdon & Fetcher, 1984b; Figure 1.1). Among the environmental factors that influence plant growth and survival in tropical forests, light availability is likely to be the resource most frequently limiting growth, survival, and reproduction (Chazdon, 1988; Fetcher, Oberbauer & Chazdon, 1994). Photosynthetic utilization of light is therefore a major component of the regeneration responses of forest species within the larger context of forest dynamics and succession.

PRODUCTIVITY AND NUTRIENT CYCLING OF ALASKAN TUNDRA: ENHANCEMENT BY FLOWING SOIL WATER'

Patterns of biomass, productivity, and nutrient cycling were compared be- tween water-track and nontrack areas in Alaskan tussock tundra. Water tracks (channels of subsurface water drainage) sampled in Alaskan tundra were 2.4-fold more productive than adjacent nontrack areas due almost entirely to the 10-fold greater aboveground pro- duction of the sedge Eriophorum vaginatum L. in the track. Nitrogen and phosphorus cycled more rapidly in water tracks than in nontrack areas due to slightly warmer soil temperature, deeper thaw, higher soil phosphatase and protease activities, and more rapid nitrogen mineralization. The importance of E. vaginatum in stimulating productivity and nutrient cycling in the water track may result from its deep-rooting habit, which enabled it to exploit flowing subsurface water. Observed lateral subsurface soil water flow rates in the track (0.57 ? 0.09 cm/h) were 6 and 8 times as rapid as diffusion of phosphate and ammonium, respectively, in water and are of the right magnitude to explain the 10-fold enhancement of Eriophorum productivity in the track. Roots of E. vaginatum growing in the track had a high capacity to absorb ammonium and phosphate and thus could exploit nutrients brought to the root surface by flowing soil water. The low degree of mycorrhizal colonization of these roots makes E. vaginatum particularly sensitive to the rate of nutrient movement to the root surface. The enhanced productivity and nutrient cycling in the water track compared with adjacent tundra probably reflect increased nutrient flux to the root surface caused by flowing water in the water track.

Effects of light regime on the growth, leaf morphology, and water relations of seedlings of two species of tropical trees

An experiment was conducted with Heliocarpus appendiculatus, a pioneer or large gap species of tropical moist forest in Costa Rica, and Dipteryx panamensis, a small gap species. Seedlings were grown in full sun, partial (80%) shade, and full (98%) shade. After one month of growth they were switched between environments and grown for two more months.Growth in height of Heliocarpus was greatly affected by irradiance, being increased in response to full shade and decreased in full sun. Height of Dipteryx was unaffected by irradiance level. Survival of Heliocarpus seedlings was only 49% in full shade, whereas Dipteryx had 100% survival. Biomass of Heliocarpus was not significantly greater in full sun than in partial shade whereas it was for Dipteryx. The response of root: shoot ratio was similar for both species. They were lowest in full shade and highest in full sun. Heliocarpus exhibited greater changes in leaf thickness, specific leaf weight, and stomatal density than did Dipteryx. Stomatal conductance of both species was lower in full shade and full sun than in partial shade.The results of the experiment indicate that growth of Heliocarpus is more plastic than that of Dipteryx in response to changes in irradiance. Previous environment did not affect the response to the present environment in either species. Both species responded positively to increases in irradiance.

ECOSYSTEM DEVELOPMENT AND PLANT SUCCESSION ON LANDSLIDES IN THE CARIBBEAN

Landslides are common in mountainous regions of the Caribbean and are triggered by heavy rains and earthquakes, and often occur in association with human disturbances (e.g., roads). Spatially heterogeneous removal of both substrate and vegetation is responsible for a variety of patterns of ecosystem development and plant successional trajectories within Caribbean landslides. Soil nutrient pools in exposed mineral soils reach levels comparable to mature forest soils within 55 yr but soil organic matter recovers more slowly. Plant colonization of landslides depends on the availability of propagules and suitable sites for germination, soil stability, and the presence of residual or newly deposited soil organic matter and associated nutrients. Once initial colonization occurs, the rate and trajectory of plant succession on landslides is strongly affected by plant/plant interactions. We present two conceptual models of landslide succession that summarize the major processes and pathways of ecosystem development and plant succession on landslides. Additional work is needed to characterize interactions between spatially heterogeneous zones, controls over soil development, impacts of key plant species, and the role of animals on Caribbean landslides.

Light Environments of Tropical Forests

Measurements of photosynthetically active radiation (PAR) in tropical forests are reviewed and discussed. Many studies of the light environment in tropical forests have emphasized the importance of sunflecks to the total light energy reaching the forest floor. Much of the spatial variation in total daily PAR in understory environments has been attributed to localized sunfleck activity, but levels of diffuse radiation were also found to vary spatially. Values of percent transmission differs greatly among forests throughout the world, ranging from 0.4–3.8, depending on forest structure and weather conditions. Total daily photosynthetic photon flux density (PPFD) in understory habitats is often below 1.0 mol m-2 d-1, and measurements as low as 0.15 mol m-2 d-1 have been recorded.

LIFE HISTORIES OF TILLERS OF ERIOPHORUM VAGINATUM IN RELATION TO TUNDRA DISTURBANCE

SUMMARY (1) Life-tables were constructed for vegetative tillers of Eriophorum vaginatum from undisturbed and disturbed tussock tundra in Alaska. Life-tables were also constructed for tillers from tundra that had been fertilized with N, P and K. The life-tables were used to estimate population parameters and to construct constant coefficient matrix models of tiller demography. (2) Tiller survival was lower in disturbed than it was in undisturbed tundra, and daughter tillers were produced sooner. Addition of nutrients (as fertilizer) produced no change in tiller survival and in the age distribution of daughter tiller production in the first year following fertilization. The shapes of the survival curves were intermediate between Deevey Type I and Deevey Type II curves. (3) The generation time for tillers in undisturbed tundra was 5.3 years-about twice as long as in disturbed tundra-while the annual rate of increase of tillers did not differ greatly. Fertilization produced large increases in the annual rate of increase, but the generation time was shortened only slightly. (4) Reproductive values, defined as the expected number of daughter tillers from tillers of a given age relative to the expected number from a tiller aged 0, fell into one of three patterns: (i), decline from maximum value at age 0; (ii), increase from age 0 followed by a monotonic decline; (iii), increase from age 0 followed by a gradual, irregular decline and a plateau in the older age classes. The peak in reproductive value for patterns (ii) and (iii) occurred 2-4 years after the onset of tillering at age 1 or 2 years; this result was attributed to increases in age-specific tillering rate with tiller age. (5) Sensitivity analysis of the matrix models revealed that sensitivity of the annual rate of increase to changes in tillering rate and survival declines with age. The annual rate of increase was equally sensitive to changes in tillering rate and survival from age 0 to age 3. In the older age classes the annual rate of increase was more sensitive to changes in tillering rate than to changes in survival.

Natural disturbance and human land use as determinants of tropical forest dynamics: results from a forest simulator

Forests are often subject to multiple, compounded disturbances, representing both natural and human-induced processes. Predicting forest dynamics requires that we consider how these disturbances interact to affect species demography. Here we present results of an individual-based, spatially explicit forest simulator that we developed to analyze the compounded effects of hurricane disturbance and land use legacies on the dynamics of a subtropical forest. We used data from the 16-ha Luquillo Forest Dynamics Plot in Puerto Rico, together with a reconstruction of historical wind damage, to parameterize the simulator. We used the model to ask two questions. (1) What are the implications of variation in hurricane frequency and severity for the long-term dynamics of forest composition, diversity, and structure? Both storm severity and frequency had striking effects on forest dynamics, composition, and structure. The periodicity of disturbance also played an important role, with periods of high hurricane activity fostering the establishment of species that may become rare in the absence of severe storms and quiescent periods allowing these species to reach reproductive size. Species responses to hurricane disturbance could not be predicted from their life history attributes. However, species perceived to be primary forest species exhibited low temporal variation in abundance through the simulations. (2) How do hurricanes and legacies from human land use interact to determine community structure and composition? Our results suggest that, over time, regardless of the storm regime, land use legacies will become less apparent but will lead to a forest community that contains a mixture of secondary and primary forest species formerly dominant in areas of different land use. In the long term, hurricane disturbance generated two communities with slightly greater similarity than those not subject to storms. Thus, the inclusion of hurricane disturbance does not alter the prediction that land use legacies in this tropical forest will diminish over time. Our simulations also highlight the contingent effects of human legacies on subsequent community dynamics, including the response to hurricane disturbance, therefore supporting the notion that compounded disturbances can interact in ways that cannot be predicted by the study of single disturbances. The widespread importance of land use as a large-scale disturbance makes it imperative that it be addressed as a fundamental ecological process.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...

RESPONSES OF TROPICAL PLANTS TO NUTRIENTS AND LIGHT ON A LANDSLIDE IN PUERTO RICO

NED FETCHER, BRUCE L. HAINES,t ROBERTO A. CORDERO, D. JEAN LODGE,*? LAWRENCE R. WALKER,: DENNY S. FERNANDEZ and WILLIAM T. LAWRENCE*? Department of Biology, Box 23360, and * Terrestrial Ecology Division, Center for Energy and Environment Research, University of Puerto Rico, GPO Box 363682, University of Puerto Rico, San Juan, Puerto Rico, tBotany Department and Institute of Ecology, University of Georgia, Athens, GA 30602-7271 and tDepartment of Biological Sciences, University of Nevada, Las Vegas, NV 89154-4004, USA