Publisher Summary This chapter discusses a number of factors influencing the throughfall and stemflow quality and their variation. The effect of the canopy in altering the precipitation quality is also discussed. The annual nutrient return to the forest soil for the elements K, Na, and S is predominantly via throughfall and stemflow and little because of litterfall. Stemflow transfers only 5–20% of the total in precipitation-borne solutes, yet it is the major nutrient input to restricted areas of the forest floor. Because throughfall and stemflow are associated with precipitation events, the transport of nutrients contained in throughfall and stemflow depends on the magnitude, timing, and form of the precipitation. Thus, a reliable estimate of throughfall nutrient flux demands a good forest hydrological budget. The quantity and distribution of throughfall and stemflow depends on microscale features of canopy structure, such as crown density, closeness of the foliar elements, distance from the nearest bole, or open spaces in the canopy.

Throughfall and Stemflow in the Forest Nutrient Cycle

Coweeta is one of the oldest continuously operating laboratories of its type in the world. For the first time, a complete review and summary of more than 50 years study of the hydrological and ecological responses of baseline and managed Southern Appalachian hardwood forests at Coweeta is now supplied by this volume. The long-term research approach represents a continuum of theory, experimentation and application using watersheds as landscape units of investigation. Thus, the information encompasses a wide range of interpretations and interests. In addition to in-depth analyses of terrestrial and stream processes, the breadth of coverage includes historical perspectives and relevance of ecosystem science to management needs. In a broader sense, the Coweeta research effort is considered from a perspective of national and international forest hydrology and ecology programs.

Forest hydrology and ecology at Coweeta.

Percentage of evergreenness in 158 forest stands from seven community types in north-central Florida ranged from 0 to 100. In general, the evergreen species are more im- portant on dry sterile sites and the deciduous species on mesic fertile sites. Nine environ- mental variables or combinations of variables accounted for approximately 51% of the variation in stand evergreenness. The segregation of evergreen species to dry sterile sites may be related to the gradual return of nutrients to the soil or the. establishment of a more closed mineral cycle through (1) the leaching of nutrients from leaves and (2) year-round leaf fall. A study of 60 southern mixed hardwood forest stands from north-central Florida revealed that evergreenness ranged from 7% to 100% (Monk 1965). Preliminary analyses suggested that deciduous species were important on mesic fertile sites while evergreen species were more important on dry sterile sites. Since the initial study, data have been obtained from 98 additional stands repre- senting six different forest community types. An attempt was made to determine the relationships between stand evergreenness and certain environmental factors and to de- velop a hypothesis explaining a possible ecological sig- nificance of evergreenness.

An ecological significance of evergreenness

Water economy and thermal relations of plethodontid salamanders were studied in the laboratory and in the field. Laboratory measurements included behavioral responses in temperature and relative humidity gradients, rates of dehydration and dehydration at various relative humidities and soil—moisture levels, and determination of critical thermal maxima by rapid controlled heating to a definite endpoint. Salamanders were acclimated to a combination of three temperatures and two photoperiods: 5°C, 16 hr of light alternating with 8 hr of darkness (LD 16:8); 5°C, (LD 8:16); 15°C (LD 16:8); 15°C (LD 8:16); 25°C (LD 16:8); and 25°C (LD 8:16). Three series of experiments were conducted on 20 populations representing 14 species. Critical thermal maximum increased with an increase in acclimation temperature indicating that the salamander's heat resistance was readily altered by its previous thermal history. Salamanders selected a definite range of temperatures and did not merely avoid extremes in the thermal gradient. Thermal preferenda were relatively stable for each species and were not significantly affected by either acclimation temperature or photoperiod. Rate of dehydration was dependent upon body size, drying power of the air, and ambient temperature. Interspecific differences in dehydration rates appeared to be related in part to differences in size. Species composed of small individuals lost weight faster than species made up of large individuals. As the vapor pressure deficit increased the dehydration rate increased. Dehydration was more rapid at higher temperatures. Rehydration rate increased as the percentage of weight loss due to dehydration increased and was more rapid at higher temperatures. Salamanders absorbed water from soil when soil—moisture tension was as high as 2.8 atm at 25°C. All species absorbed water from unsaturated soil at similar rates that were dependent upon soil—moisture content. Salamanders in the humidity gradient responded positively to differences in the moisture content of the air, and all but one species were more than 70% successful in selecting the highest relative humidity available in the gradient. Interspecific differences were apparent, but were not always correlated with habitat preferences. Plethodon glutinosus did not show any specific adaptation in its thermal responses or water relations that would account for its widespread distribution in the eastern United States. Plethodon ouachitae and P. caddoensis apparently have survived in the Ouachita Mountains due to favorable microhabitats and their ability to burrow deep beneath talus—covered slopes during hot and dry summers. Surface activity of P. caddoensis is limited in the summer by hot and dry conditions in its microhabitat.

Role of Temperature and Water in the Ecology of Lungless Salamanders

Spiders were the most important entomophagous predators of a 137Cs-tagged Liriodendron tulipif era decomposer community. Radioisotope techniques were used to quantify the bio- energetic impact of spider predation on the forest-floor detritus food web. Temperature. dependent 137Cs elimination by spiders and 137Cs concentrations in field animals were used in intake-loss equations to estimate ingestion by the spider population. Radiocesium body burdens fluctuated seasonally in response to litterfall and seasonal differences in temperature and precipitation. Ingestion rates, assimilation efficiencies, and respiratory energy losses were used to estimate gross and net production of the forest spider population. Annual mean population density of spiders was 126 individuals/M2. Population-density peaks occurred in fall, early winter, and summer. Mean biomass for the year was 43 mg dry wt/m2; maxima occurred in early winter and spring. Spiders 1-10 mg in size accounted for 59% of total biomass; spiders 10 mg comprised the remaining 24% and 17% of the population, respectively. Predicted values for ingestion, assimilation, and respiration indicate that forest- floor spiders were active throughout a temperature range of 5 -20'C, with 13.30 the optimum for net production by spiders of all size classes. Spiders killed an equivalent of 2.08 kcal prey tissue/M2 per year and ingested 1.68 kcal/m2 per year. Respiratory energy loss equaled 1.23 kcal/m2 per year. Trophic level efficiencies calculated from seasonal energy-budget data were: (1) intake efficiency (prey killed/ingestion), 73-86%; (2) as- similation efficiency (assimilation/ingestion), 90-99%; (3) respiration efficiency (respiration/ ingestion), 58-96%; (4) respiration/assimilation, 63-106%; (5) ecological growth efficiency (net production/ingestion), -5-34%; and (6) tissue-growth efficiency (net production/as- similation), -4-37%.

Significance of spider predation in the energy dynamics of forest-floor arthropod communities

Experimental Sites 405 Inoculation of Trees ........ 406 Plant Sampling 406 Litter and Soil Sampling ....... 406 Determination of Radioactivity ..... 406 Soil Moisture 406 Tree Harvesting 407 Transfer of Cs-134 from Litter to Vegetation .... 407 RESULTS 407 Site Descriptions . 407 Soil Moisture 407 GAINS AND LOSSE.S OF Cs-134 DURING THE

John P. Witherspoon

Papers

Cycling of Cesium‐134 in White Oak Trees

Reactions of Anopheles quadrimaculatus Say to Moisture, Temperature, and Light