Severe Dry Season Research Articles

Climatic Periodicity, Phenology, and Cambium Activity in Tropical Dry Forest Trees

The seasonal time course of vegetative phenology and cambium growth is compared for tree species from Central America and Asia growing in tropical climates with a long, severe dry season. Although the inhibition of plant growth by water stress is weH established, responses to seasonal drought vary widely among such trees, and their annual development is not weH synchronized by climatic seasonality. In deciduous trees growing at microsites with low soil moisture storage, phenology and cambium growth are weH correlated with each other and with seasonal rainfall, and most trees have distinct annual rings. Phenology and cambium growth are progressively uncoupled from climatic seasonality in brevideciduous and evergreen trees growing at microsites with large soil water reserves which buffer trees against seasonal drought and thus may prevent the formation of distinct annual rings. There is some experimental evidence conceming the control of growth initiation in apical meristems and the cambium, but little is known about the mechanisms which arrest growth and deterrnine qualitative changes in organ development and cambium cell differentiation.

Fruit removal and postdispersal survivorship in the tropical dry forest shrub Erythroxylum havanense: ecological and evolutionary implications.

We studied the relationship between the removal rate and the spatiotemporal availability of ripe fruits of the tropical deciduous shrub Erythroxylum havanense in western Mexico. We also evaluated the effects of dispersal on seed survival during the first stages of establishment. Fast and early dispersal should be favored in E. havanense, since propagules have more time to grow and accumulate resources before the beginning of the severe dry season. In general, high rates of fruit removal imply faster and earlier dispersal. Thus, plants producing large crops should benefit from high removal rates, which will increase the probability of successful establishment by their progeny. To characterize both individual and population fruiting patterns, we made daily counts of fruits on 51 plants arranged in six clumps of different sizes. The daily number of fruits removed per plant was higher for plants with larger initial crop sizes and larger numbers of ripe fruits on a given day, but decreased as clump size increased. Additionally, we monitored postdispersal survival and germination in an experiment manipulating seed density, distance from adult plants, and seed predation. Early establishment was independent of density or distance, and vertebrate seed predation was the main agent of seed mortality. Our results indicate that the critical variable with respect to fruit removal is the number of fruits a plant produces, large plants having higher dispersal rates. Large plants are also more likely to have more seeds escaping postdispersal seed predation.

Mortality Rates of 205 Neotropical Tree and Shrub Species and the Impact of a Severe Drought

Mortality rates of 205 tree and shrub species were estimated during two intervals, 1982—1985 and 1985—1990, in two size classes, 1—10 and ≥ 10 cm in diameter, in a 50—ha census plot in tropical moist forest on Barro Colorado Island in Panama. The severe dry season of 1983 was the focus of the study, since prior observations had demonstrated that it caused mortality in the forest. Here we document that forest—wide mortality was ≈ 3%/yr during the drought interval but only 2%/yr during the period afterwards, and that excess mortality during the first interval amounted to 2% of stems in the larger size class and 1% in the smaller. Overall, just under 70% of all species had higher mortality during the first census interval, but not all species were equally affected. Canopy trees had significantly higher mean mortality rates during 1982—1985 than during 1985—1990, but treelets and shrubs showed no or slight differences. This was counter to our prediction that species with short root systems would suffer more from a long drought. Shrubs did, however, have higher mortality rates than trees and treelets during both census intervals. We also evaluated mortality rates for subgroups of species that specialized on different microhabitats in the forest. As we predicted, colonist species (those associated with light gaps) had higher mortality rates than generalist species, 7—10%/yr compared to 2—4%/yr, but only in the smaller size class. Unexpectedly, colonizers had similar mortality rates as non—colonizers in the larger size class. Gap colonizers and generalist species were similarly affected by the drought–both had elevated mortality during 1982—1985. Species whose distributions were associated with moister soils (on the slopes around the island's plateau or in a swamp in the midst of the 50—ha plot) also had elevated mortality during the drought period, but no more so than generalist species. This was counter to our prediction that species from moist microhabitats would suffer more during an extended drought than generalists. Understory treelets that were slope specialists had higher mortality than generalists during both census intervals, but not large trees that were slope specialists. Our conclusions emphasize diversity as well as pattern. Every trend we illustrated had well—documented exceptions: large trees with lower mortality during the drought period, for example. Clearly, accurate predictions about how tropical forests will respond to climatic perturbations will require much detailed information from many species.

Water status and development of tropical trees during seasonal drought

Bud break, shoot growth and flowering of trees involve cell expansion, known to be inhibited by moderate water deficits. In apparent contradiction to physiological theory, many trees flower or exchange leaves during the 6 month-long, severe dry season in the tropical dry forest of Guanacaste, Costa Rica. To explore this paradox, changes in tree water status during the dry season were monitored in numerous trees. Water potential of stem tissues (Ψstem) was obtained by a modification of the pressure chamber technique, in which xylem tension was released by cutting defoliated branch samples at both ends. During the early dry season twigs bearing old, senescent leaves generally had a low leaf water potential (Ψleaf), while Ψstem varied with water availability. At dry sites, Ψstem was very low in hardwood trees ( −0.2 MPa) in lightwood trees storing water with osmotic potentials between −0.8 and −2.1 MPa. At moist sites trees bearing old leaves rehydrated during drought; their Ψstem increased from low values (<−3 MPa) to near saturation, resulting in differences of 3–4 MPa between Ψstem and Ψleaf. Indirect evidence indicates that rehydration resulted from osmotic adjustment of stem tissues and improved water availability due to extension of roots into moist subsoil layers. In confirmation of physiological theory, elimination of xylem tension by leaf shedding and establishment of a high solute content and high Ψstem were prerequisites for flowering and bud break during drought.

Drought acclimation among tropical forest shrubs (Psychotria, Rubiaceae).

Mechanisms of dry-season drought resistance were evaluated for five evergreen shrubs (Psychotria, Rubiaceae) which occur syntopically in tropical moist forest in central Panama. Rooting depths, leaf conductance, tissue osmotic potentials and elasticity, and the timing of leaf production were evaluated. From wet to dry season, tissue osmotic potentials declined and moduli of elasticity increased in four and five species, respectively. Irrigation only affected osmotic adjustment by P. furcata. The other seasonal changes in leaf tissue properties represented ontogenetic change. Nevertheless, they made an important contribution to dry-season turgor maintenance. Small between-year differences in dry season rainfall had large effects on plant water status. In 1986, 51 mm of rain fell between 1 January and 31 March, and pre-dawn turgor potentials averaged <0.1 MPa for all five Psychotria species in March (Wright 1991). In 1989, 111 mm of rain fell in the same period, pre-dawn turgor potentials averaged from 0.75 to 1.0 MPa for three of the species in April, and only P. chagrensis lost turgor. The relation between leaf production and drought differed among species. P. limonensis was buffered against drought by the lowest dry-season conductances and the deepest roots (averaging 244% deeper than its congeners) and was the only species to produce large numbers of leaves in the dry season. P. chagrensis was most susceptible to drought, and leaf production ceased as turgor loss developed. For the other species, water stress during severe dry seasons may select against dry-season leaf production.

Nutrient content of bulk precipitation in south-central Java, Indonesia

AbstractThe chemical composition of bulk precipitation was studied for a period totalling 18 months between 1975 and 1978 at a high rainfall site in central Java. The following elements were determined: calcium, magnesium, sodium, potassium, nitrate-nitrogen, chlorine, phosphate-phosphorus, silicon, aluminium, iron, manganese and pH. Ammoniacal nitrogen, total nitrogen and sulphate-sulphur were determined, during four weeks in May 1983. Concentrations of all elements rose dramatically during the exceptionally severe dry season of 1977 which may have caused accession rates for some nutrients to be overestimated (i.e. compared to a year with ‘normal’ rainfall) by about 10%. Results are explained in terms of prevailing wind directions and proximity to, or absence of, particular solute sources, such as the ocean, volcanoes or eroding lands.Most elements exhibited intermediate concentrations when compared with data for other humid tropical locations; sodium and perhaps nitrogen were on the low side and pH on the high side of the spectrum. Annual accession rates at the study site were considerable by pan-tropical standards, mainly as a result of high rainfall totals. Comparing accession rates with 25-year-average nutrient immobilization rates in stemwood of local plantations ofPinus merkusiiJungh. et de Vr. andAgathis dammaraWarb. showed atmospheric inputs of magnesium and potassium to be high enough (theoretically) to account for losses of these elements in stemwood removal. Corresponding figures for nitrogen were 90% (Pinus) and 50% (Agathis), and for calcium and phosphorus 60 and 40% respectively (both species).

Growth Rings, Increment and Age of Trees in Inundation Forests, Savannas and a Mountain Forest in the Neotropics

Investigations on growth zones of tropical trees were carried out and published since the beginning of our century.In tropical regions with severe annual dry seasons or inundation phases trees form annual rings. This is demonstrated for trees from Central Amazonian inundation forests and the Gran Sabana in Venezuela using a combination of several dendrochronological methods (wood anatomy, radiocarbon dating, ring width measurements).The occurrence of annual rings allows the determination of age and growth rate of trees. The growth rate depends on the growth strategy of the species and the growth conditions. At a given site growth rate shows a weak negative correlation with the specific gravity of the wood of trees from the upper story. Several reported ring width patterns are explained by the vegetation history of different forest stands.

Patterns of foraging and range use by three species of neotropical primates

This paper describes the diet and range use patterns of the three species of primates in Santa Rosa National Park, Costa Rica (Ateles geoffroyi, Alouatta palliata, andCebus capucinus) and examines the variation in these variables as they relate to seasonal changes and concomitant changes in food availability. These three primate species were studied over a four-year period for a total of 24 months in the field. Santa Rosa National Park is in an area that experiences a long severe dry season in which little if any rain falls and the majority of the non-riparian trees lose their leaves. However, even though the three species were very flexible in terms of diet and range use, the behavioural variability did not correspond to changes in food availability or season.

Adaptations of the reed frog Hyperolius viridiflavus (Amphibia, Anura, Hyperoliidae) to its arid environment : IV. Ecological significance of water economy with comments on thermoregulation and energy allocation.

After breeding African savanna dwelling reed-frogs of the "superspecies" Hyperolius viridiflavus face a severe dry season. The frogs withstand the adverse abiotic conditions in exposed positions, clinging to dry vegetation. Only juveniles (300-700 mg) are able to adjust water economy and metabolism to a prolonged dry season. Wet season frogs attain low levels of evaporative water loss (EWL) within 6-8 days after incipient water shortage. This time course is mainly determined by the animal's ability to lower metabolism and activity level to the minimum demands of a dry season. Barriers against diffusion of water which most probably are built up by the stratum corneum and/or the overlying film of dried mucus seem not to be essentially modified during adjustment to dry season conditions. Changeover to dry season physiology is greatly accelerated through preconditioning frogs to water shortage. AdultHyperoliusare unable to reduce activity and metabolism as fast and effectively as juveniles. Most probably these are the main reasons for their very restricted survival capability under dry season conditions; the generally poor energy reserves after the breeding period may further shorten their survival time. At the critical thermal maximum (CTM) Hyperolius uses skin gland secretions for evaporative cooling. Acclimation effects and regulation of evaporative cooling within some 1/10° C help to employ limited water reserves very economically. Dry adapted, dehydrated frogs take up water, whenever available, via specialized skin areas. Rate of uptake is high and is mainly determined by the actual stage of dehydration. The onset of the water-balance-response is also affected by preconditioning. Survival time of small (<500 mg) estivating Hyperolius is limited by their water reserves.Due to their unfavourable surface to volume ratio they loserelatively more water by evaporation than larger conspecifics. Therefore, smaller specimens should allocate energy preferably to growth, until reducing EWL so far to survive the average periods between the rare precipitations. In larger frogs (>500 mg) the amount of stored energy determines maximal survival time. When a critical size is reached in postmetamorphic growth, a change in energy allocation from body growth to energy storage would improve prospects of survival and should therefore be expected. Species specific differences in regard to EWL and CTM indicate a strong correlation between physiological properties and ecological demands.

Problems of Protection Against Piping and Surface Erosion in Central Tunisia

The formation of quasi-tubular voids in the subsoil, known as ‘piping’, is an important form of subsoil erosion that arises when substantial volumes of subsurface lateral throughflow pass through dispersible subsoils. Amongst the environmental factors that favour the development of piping are high contents of expansible clay minerals, a severe dry season, and convexities in the slope profile. The first two of these are characteristic of considerable areas in Central Tunisia.In addition to natural slope-convexities, further topographic loci for pipe initiation and development are provided by the artificial convexities introduced into the slope profile by the construction of conservation banks. The earthworks are designed as protection against surface erosion processes, which prevail for most of the year in this part of Tunisia. However, it appears that the banking is particularly vulnerable to piping, which is causing considerable disruption. Australian experience indicates that banking should be used very sparingly on soils which are at all prone to piping, and that care is needed in both the alignment and construction of such banks as are deemed unavoidable.

Strategies for improving the nutritional status of small ruminants in tropical ecosystems

Research on goat and sheep nutrition and feeding systems has been initiated in five tropical or Mediterranean countries, by United States and host-country institutions which are participants in the Small Ruminant Collaborative Research Support Program. The major objective is to increase small ruminant productivity by better utilizing locally available feedstuffs and to more closely meet the nutritional needs of animals during the critical phases of their production cycle. West Java, Indonesia, is a humid tropical site with high rainfall and intensive agriculture, with small ruminants raised in nearly total confinement. Village feed resources include native grasses, tree legumes, and foliage from food crops. Energy may be the most limiting nutrient, due to seasonal feed availability and intake limitation when tropical grasses form a large proportion of the diet. Western Kenya is a wet-dry tropical site with a long and a short rainy season. The feasibility of intensifying dual purpose goat production within mixed crop-livestock systems is under study. Solutions to severe dry season feed limitations are being evaluated. Sweet potato vines and legumes such as pigeon peas have been demonstrated to be excellent in nutritional value, with high intakes by lactating does. Intercropped or ratooned sorghum also has promise. Northeast Brazil is representative of the semiarid tropics with a short, variable rainy season and prolonged dry season during which all production parameters for goats and hair sheep are unsatisfactory. The native “caatinga” brushland is the major feed resource. Dry season confinement of lambs has resulted in adequate growth (90 g/day or more) when fed maized crop residue plus supplements. Weanling kids fed the same ration have lower intake and gains of 20 to 40 g/day. Sorghum hay has been fed to lactating dairy goats. The fourth site, Morocco, has a Mediterranean climate with good rainfall in spring, prolonged drought in summer and fall, and cold limitation in winter (especially at higher elevations). Wheat stubble and other crop residues are under study for their nutritional contribution and supplementation requirements in summer, fall and winter. Also, studies are underway on productivity responses of native sheep in extensive or semi-intensive systems, as well as the prolific D'man breed in total confinement. The fifth site is Peru, where Andean highland areas are limited by cold year-round and extended drought for part of the year. Grazing systems for sheep and alpacas are under study at three Andean locations. At a coastal site, which receives sporadic rain (1 to 3 months, not every year), improved feeding systems for seminomadic herds of dual purpose goats are under study.

Establishment and growth of different plantain propagation materials

AbstractFive different types of plantain (Musa spp. AAB) planting materials—early sword suckers, late sword suckers, peepers, maidens and bits—were established at the beginning of the rainy season and grown for 9 months, over the first subsequent dry season. Their pre‐floral vegetative growth during the 9 month period was then studied. The late sword suckers made most growth and had the best survival rate over the dry season followed by maidens and early sword suckers. Peepers and bits were the least successful materials and are not recommended as propagation materials in areas with long and severe dry seasons.

Seed Size as a Correlate of Temporal and Spatial Patterns of Seed Fall in a Neotropical Forest

Seed fall in a Brazilian subtropical lower montane, moist forest was measured over a year using 120 traps that had a total collecting surface of 174 M2. Twenty-two thousand seeds from 227 plant species were collected. Wind-dispersed species released seeds mainly during the late dry season and early wet season. Among nonwind-dispersed species, large-seeded species matured seeds primarily during the wet season. Small-seeded species were much less seasonal. This difference in seasonality may stem from a correlation between seed size and mode of animal dispersal. Arrival probability per collection period, approximated as the percentage of traps hit by free seeds of each species, was regressed against several independent variables. For nonwind-dispersed species, the logarithm of seed area best predicted arrival probability of free seeds, being inversely related to it. Existence of this inverse relationship has often been supposed but only now demonstrated. THE REPRODUCTIVE BIOLOGY of any seed-plant population is believed to reflect evolutionary compromise among various conflicting aspects of reproduction (Harper et al. 1970, Stebbins 1971). Of these, seed size is fundamental because it apparently influences many other aspects of reproduction. This paper examines relationships in a tropical forest between seed size and two other reproductive parameters: the temporal and spatial extent of seed fall. Numerous workers have assumed that small seeds are more widely dispersed than large ones (e.g., Harper et al. 1970), yet substantial data to test this idea have been lacking. Because of the great temporal and spatial spans over which seed dispersal occurs, the dispersal process at the community level is likely to remain intractable to direct study. Instead of attempting to measure dispersal, I measured arrival probabilities of free seeds at fixed locations. Arrival probability is clearly a crucial component of dispersibility. Seed size has also been hypothesized to influence the temporal pattern of seed fall in tropical forests. Smythe (1970) reported fruiting of largeseeded species in a Panamanian forest to be synchronous, whereas small-seeded species were less seasonal in fruiting time. I sought to determine whether such temporal patterns occur in another neotropical forest. STUDY SITE AND METHODS The study was conducted in a virgin evergreen forest at 800-900 m elevation in Reserva Biologica Nova Lombardia, Municipio de Santa Tereza, Espirito Santo, Brazil, at latitude 19? 53' S. The site is surrounded by several square kilometers of old-growth forest, although some agricultural clearings exist less than 2 km away. Jackson (1978) provides climatic data for the site. Reserva Nova Lombardia experiences a moderate temperature differential between winter (June-August) and summer (December-February) and a long but not severe dry season (April-September). In the Holdridge Life Zone System (Holdridge 1967) the vegetation is classified as Subtropical Lower Montane Moist Forest. As measured by frequency along line transects, the commonest trees are Manilkara elata (Sapotaceae), Lucumza laurifolia (Sapotaceae), Campomazesia warmingiana (Myrtaceae), Guarea petiolulata (Meliaceae), Emimotuni zitens (Icacinaceae), Didymnopanax longipetiolatuxm (Araliaceae), Nectandra puberula (Lauraceae), Psidiuni macrospermnuni (Myrtaceae), Ocotea declinata (Lauraceae), Andira stipulacea (Leguminosae), Clusia grandiflora (Guttiferae), Terminalia ijanuariensis (Combretaceae), Leandra alpestris (Melastomaceae), Sideroxylon gardnerianum (Sapotaceae), Cybianzthus cunieifolius (Myrsinaceae), Ocotea nmacropoda (Lauraceae), Copaifera langsdorffi (Leguminosae), and Ficus doliaria (Guttiferae) (A. Ruschi, pers. comm.). Falling seeds and fruits were sampled with 120 horizontally placed rectangles of green, plastic window screen. Each corner of a screen was secured with plastic cord to a vertical bamboo stake at one meter above the ground. Screens measured 1.4x1.2 m when flat (area 1.68 Mi2), but trap sag reduced the average collecting surface per trap to 1.44 m2?. The mesh in the screen was 1.2 mm square. The seed traps were set in two straight lines approximately 200 m apart. Sixty traps comprised each transect, with 6 m between adjacent traps. One transect began at midslope on a hill (836 m elevaBIOTROPICA 13(2): 121-13

Canopy conductance of estate tea in Kenya

The energy balance of estate-grown tea in Kenya was measured during eighteen days in a wet season and two days at the end of a severe dry season. Canopy conductance values g c were calculated from the measurements, and were shown to accord with corresponding estimates g s from a diffusion porometer. For dry canopy conditions during the wet season a simple model has been derived that expresses g c as a function of net irradiance R n and saturation vapour pressure deficit D. Net irradiance correlated strongly with shortwave irradiance S (which was not continuously measured in this study), and the model indicated, for this wet season data, that R n (or S) was the major factor determining the diurnal changes of g c, but that D did, independently of R n, explain a significant portion of the total variance. In the dry season, g c was on average some 40% less than in the wet season; the reduction was attributed primarily to seasonal differences in D and, to a much lesser extent, in leaf area. The data suggested that conductance per unit leaf area was little affected by soil moisture deficits as large even as 37 cm, behaviour that can be explained by the tea's deep rooting (to 6 m and more). In the wet season, the canopy conductance of a wet canopy was found to be 12% higher than for a dry canopy experiencing the same R n and D.

Predation on Swollen-Thorn Acacia Ants by White-faced Monkeys, Cebus capucinus

White-faced monkeys, Cebus capucinus, intensively preyed on the ant Pseudomyrrmex belti, that obligatorily occupies and protects swollen-thorn acacias, primarily Acacia collinsii, at the study site. The Cebus obtained ants and probably ant larvae from acacias by ripping off branches and opening thorns with their teeth. This action often resulted in extensive physical destruction of the acacia, loss of the ant colony, and subsequent death of the plant. Implications of this predation and other interactions between monkeys and myrmecophytes are discussed. IN THE MUTUALISTIC ASSOCIATION between obligate acacia ants (several species of the genus Pseudomyrmex) and swollen-thorn acacias (several species of the genus Acacia) of Central America, the plant provides the ants with food and domicile, while the ant protects the plant from herbivores (primarily insects) by stinging and from invading vegetation by biting it off (Janzen 1966, 1967, 1974). Although some arthropods, lizards, birds, and mammals are known to eat swollen-thorn acacia plant parts, extensive damage of ant-occupied swollen-thorn acacias from these animals is very infrequent. Evidence strongly suggests that large browsing mammals avoid occupied acacias because of the aggressive ants. Similarly, acacia ants and their larvae are seldom eaten from the swollen-thorn acacia by birds and arthropods. No mammal has been reported to intentionally eat acacia ants (Janzen 1969b). During a study of the ecology and behavior of the white-faced monkey, Cebus capucinus, in the tropical dry forest of northwest Costa Rica, Cebus interactions with ant-occupied swollen-thorn acacias provided an apparent exception to the effectiveness of this mutualistic association. These interactions and other observations of interactions between monkeys (Cebus and Ate/es geoffroyi) and myrmecophytes during the study are described in this paper. METHODS AND STUDY AREA From October 1971 to August 1973 observations were made intermittently on the three monkey species, Cebus capucinus, Ateles geoffroyi, and Alouatta palliata, of Santa Rosa National Park in Guanacaste Province, northwest Costa Rica. A more intensive study of Cebus ecology, using the focal animal technique (Altmann 1974), was conducted one week each month from January to August 1973 on a troop of 18 to 21 animals. Focal animals were under observation for about 25-35 hours each study week (except for only 13 hours in the study week in June) with observations generally spanning all hours of the day for a given week. Santa Rosa National Park is located approximately 50 km north-northwest off Liberia, Guanacaste Province. Most of the park is on a plateau of about 280 m, where a mosaic of forests and man-created savannahs occurs. The Cebus study area was located in deciduous, secondary forest averaging 10-15 m tall, the predominant forest type in the plateau area. Annual rainfall averages approximately 1750 mm, and almost all of it falls between mid-May and midDecember. Deciduous trees lose their leaves during the severe dry season. Swollen-thorn acacias (primarily Acacia collinsii) are common in these deciduous

Effect of Irrigation and Nitrogen on the Dry Matter and Crude Protein Yields of ‘Pangola’ Digitgrass1

AbstractIn Trinidad, West Indies, a dairy industry is being developed on Piarco fine sand/fine sandy loam soil, classified as Aquoxic Tropudults. The soil is physically and chemically poor. The cattle obtain most of their food supply by grazing ‘Pangola’ digitgrass (Digitaria decumbens Stent.). There is an urgent need to increase grass production on this soil type. Inadequate water supply in the dry season limits grass production. Previous fertilizer trials with Pangola digitgrass on this soil type have indicated that N fertilizer is likely to give better yield response than P, K, or Ca. This trial was therefore conducted to determine the irrigation‐N management for wet and dry season production of Pangola digitgrass on this soil at adequate levels of P and K.The dry matter and crude protein yield responses of the grass to irrigation and N were studied in the field on established pastures. A split plot design was used with irrigation levels as the main plots and N levels as subplots. There were three levels of irrigation (irrigation back to field capacity at soil water deficits of 1.25, 2.50, and 3.50 cm per 30 cm of soil) and a control (no irrigation); and three levels of N, 224, 448, and 672 kg/ha.In the moderately dry year of 1972 in Trinidad, irrigation had little effect on the dry matter yields or the percentage or total crude protein of Pangola but in the severe dry season of 1973, dry matter yield was essentially doubled even when the soil was allowed to dry to a maximum water deficit of 3.75 cm per 30 cm.In the dry season of 1972, dry matter and crude protein yields increased with an increase in N fertilization from 224 kg to 672 kg N/ha/year but this was reversed in the wet season. The total annual dry matter production was best at 224 kg N/ha and irrigation back to field capacity at 1.25 cm deficit.The best irrigation‐N management for Pangola depended on the season of the year as well as on the severity of the dry season. In the moderately dry season, N substituted for irrigation and 672 kg/ha of N without irrigation was best. In a severe dry season, 224 kg N/ha/year and irrigation at the 1.25 or 2.50 cm deficit gave the best yields; and in the wet season, the management should be 224 kg N/ha and irrigation at the 1.25 cm deficit. The fertilizers were applied as split applications; N was applied after every 6‐week harvest and phosphate and potash, after every third harvest.

Tropical Agroecosystems

Tropical Agroecosystems

Regulation of Numbers in an Island Population of the California Vole, A Problem in Community Dynamics

A population of California voles, Microtus californicus, living on Brooks Island in San Francisco Bay was studied for 13 years. During the first 2 years following its establishment, the performance of the population was significantly different from that in the subsequent 11 years. The 11—year pattern that developed is characterized by: (1) annual peaks in abundance; (2) a delay of up to 5 months between the return of wet season conditions and population growth; and (3) an alternating pattern of "high" and "low" winter densities. Reproduction shows a strongly seasonal pattern, beginning 1—2 months after the start of the rainy season and ending with the desiccation of the vegetation in June. In contrast to this, reproduction started promptly with the autumn rains during the initial 2—year colonization phase, and limited summer reproduction occurred as well. Within the breeding season, litter size and number of corpora lutea produced per ovulation episode show marked seasonal changes which appear to be independent of annual variations in demographic detail and condition of the mice. Males are more vagile than females, although both show a reduction of movements during the main breeding period. The incidence of wounding was positively correlated with reproductive condition in males, but not in females. Mortality schedules were studied by means of population age structure, recruitment rates, and sex ratios. Apparent aging ceases for almost 5 months from about 1 month after the dry season begins to 60 days after it ends. This period is associated with heavy mortality, particularly among old males and young females. Consequently, late fall and early winter sex ratios favor males. In general, however, females survive longer than males, and during the breeding season the sex ratio progressively favors females. Recruits first appear 1—2 months into the wet season, but initially their numbers are very few and are insufficient to reverse population declines. Rapid population growth in late spring is largely dependent on reproduction by these early recruits. Physiological condition was measured by study of body—weight changes, particularly ratios of body weight to body length, ectoparasite loads, including scabies mite infections (mange), and molting patterns. The body—weight data reveal that wet season condition, especially in spring, is much better than dry season. Surprisingly, males begin to recover during the winter period whereas females continue their decline. Moreover, the whole pattern of weight losses is more severe in poor survival summers than in the alternate ones. A flea—mite index reveals a strong seasonal pattern which seems to be induced by vole demography; during the colonization period, ectoparasites were barely noticeable. It took 4 years for scabies to reach significant levels of infestation. Highest incidence generally occurs in winter, and an outbreak occurred in the seventh winter; subsequently, the mange mites returned to endemic levels. Molt activity showed two strongly marked seasonal peaks, one in the late spring—early summer and one in the autumn. Males molt at a slower rate than females; female molt is strongly inhibited by reproduction. The autumn molt begins before the wet season arrives suggesting a photoperiodic cue. During colonization, molts were completed more quickly than afterwards. The quasi 2—year cycle of regularly recurring peaks every year and alternating high and low winter densities which came to characterize the Brooks Island vole population is considered to be the result of a regulation process in which a multiplicity of factors interact to achieve regulation. Both density—unresponsive and responsive factors are involved: unresponsive–rainfall pattern, photoperiod and/or gonadotrophic potency of diet, heavy autumn dews, and possibly winter sex ratios favoring males; responsive–dry season resource levels, long—term physiological damages, and ectoparasite load, including scabies infections. The first two factors in each category are probably the essential ingredients. During the colonization phase, regulation involved the additional factors of emigration and possibly intraspecific strife of some sort. Finally, a model is proposed to explain the longer term cycles in abundance presumably characterizing mainland populations. It is suggested that a 2—year cycle (peak every other year) would be generated without the involvement of mammalian predation. Such a cycle would result from the action of all the essential factors proposed for Brooks Island, plus a supposedly more severe dry season and much more extensive emigration. Longer term cycles probably require the further addition of mammalian predators to this complex. Thus, changes in numbers in this species can be explained most satisfactorily in terms of the interacting effects of at least six variables.

Sweep Samples of Tropical Foliage Insects: Effects of Seasons, Vegetation Types, Elevation, Time of Day, and Insularity

Detailed data (numbers of species, numbers of individuals, measures of species diversity, evenness, dry weight) are presented on the adult beetles, adult bugs, or all arthropods, in sweep samples from Costa Rican secondary vegetation and primary forest understory during night and day, over a 3,340—m elevational transect, during the wet and dry seasons, and from Caribbean Island secondary vegetation. Adult bugs were found to be reduced much more severely than adult beetles in moving from secondary vegetation to primary forest understory. The number of species and individuals of insects is severely reduced in this transition, though the reduction in number of species is probably an artifact of inadequate samples from the forest understory. There is a strong movement of insects into moist refugia during the dry season, and a strong reduction in numbers of species and individuals during the dry season in areas with a severe dry season. In areas with a very mild dry season, the numbers and species of insects appear to rise during the dry season. There is very low overlap in insect species composition between secondary vegetation and the adjacent forest understory. The numbers of insects and species above intermediate elevations show a general decrease, and intermediate elevations appear to have the highest insect density. This is believed due to a higher harvestable productivity from the plant community at intermediate elevations owing to lowered plant maintenance costs on cool nights.The insect community changes little from day to night, except during the dry season in areas that have a severe dry season. Here there is a dramatic increase in individuals and numbers of species at night, in dry sites with nearby moist refugia. It appears that a high proportion of the insect community passes the dry season as active adults in reproductive diapause. The islands have dramatically reduced numbers of individuals as well as species of insects in both the wet and dry seasons. Part of the large insectivorous lizard community supported by these depauperate islands may be due to the lizards turning vegetarian. In order to digest vegetable matter, lizards probably have to bask in the sun; this exposes them to predators, but lizard predators are absent from the islands.

Predation on Scheelea Palm Seeds by Bruchid Beetles: Seed Density and Distance from the Parent Palm

In a test of the following hypotheses, the first was rejected and the second accepted. Rejected: (1) The farther a freshly cleaned [pile of] palm nut[s] (endocarp) is placed from a fruiting parent, the lower will be the probability of the nut[s'] being found by a bruchid. Accepted: (2) The larger the pile of freshly cleaned nuts at a site where there is no parent the more likely that it will be found by bruchids. The study was carried out in a palm forest in Guanacaste Province, Costa Rica, from March until July 1971. In addition to testing the above hypotheses, the study generated natural history data on bruchid oviposition behavior, oviposition intensity, survivorship of the larvae, and developmental rate.