Tropical Forests Of Central America Research Articles

The disturbance of forest ecosystems: the ecological basis for conservative management

The extensive literature on natural disturbance in forests is reviewed in terms of the hypotheses: (1) that disturbance is a major force moulding the development, structure and function of forests; and (b) that management of forests for all their benefits can be controlled so that the effects can be contained within those which result from natural disturbance. The causal factors of natural disturbance are both endogenous and exogenous; there are major difficulties in the formal characterization of disturbance and of recovery after disturbance. As to the latter, the acceptance of classical generalizations of the nature of succession has led to particular difficulties in the assessment and interpretation of recovery. Tree fall, which creates gaps, is fundamental to the development of many forests, and has been most intensively studied in tropical forests of Central America and the Amazon and in temperate forests of North America. Tree fall is part of autogenic change; mechanisms of gap-filling and subsequent growth and species composition vary widely with forest type and geography. Disturbance by wind is particularly difficult to characterize. Wind varies along a continuum; the blow-down of an individual tree may be mostly due to autogenic processes of ageing and decay, whereas catastrophic hurricanes and cyclones may be defined as wholly exogenous. Nevertheless, the resilience in terms of species diversity of tropical forests following catastrophic disturbance by hurricane is remarkable. A number of studies support the view that the tropical forest in hurricane-prone areas is not a stable steady-state ecosystem but rather that heterogeneity is maintained by catastrophe. The ability to regenerate by suckers and the coincidence of regenerative space and gregarious flowering are important components of the response of rainforest following disturbance. For much of the world, ‘fire is the dominant fact of forest history’. As examples, fire and its effects are reviewed for the northern boreal forests, oak-pine forests and north-western sub-alpine forests of North America. The effect of fire on species composition varies with intensity and frequency. That, together with the popular view of fire as unnatural and therefore unacceptable, places great demands on management of forests for all of their benefits, including national parks and reserves. These difficulties also affect management of other ecosystems, such as Mediterranean-type shrublands and heathlands where species diversity, productivity and cycles of regeneration and degradation are governed by fire as a natural disturbance. Shifting agriculture is a traditional form of agriculture used by at least 240 million people in the humid tropics. Shifting agriculture, together with wind, lightning and fire, is an exogenous disturbance which has little effect on soil fertility and on structure and composition of the rainforest which re-establishes after abandonment. As the intensity of disturbance of rainforest increases, resilience of the forest decreases and the current problems of extensive clearing for improved pasture and of uncontrolled logging are resulting in degraded ecosystems. Regeneration follows the often extensive death of trees caused by outbreaks of insects in many coniferous forests of northern America. This disturbance by herbivory halts increasing stagnation (as measured by decreasing rates of ecosystem production and nutrient cycling) and reinitiates succession. Other disturbances to forests occur through damage from ice-storms, snow avalanches, erosional and earthquake landslides, and volcanic activity; the development of Nothofagus forests in Chile and New Zealand is determined by such catastrophic mass movements. An extensive literature supports the hypothesis that natural disturbance is fundamental to the development of structure and function of forest ecosystems. It follows that our management of natural forest should be based on an ecological understanding of the processes of natural disturbance. Whether or not we want to do this, and the extent to which we want to derive all of the benefits from the forest, including timber, depends on social attitudes. Whereas humanism may treat conservation as the wise husbanding of forests in the interests of social traditions and harmony, animism may give nature unalienable rights. The conclusion from this review is that the ecological framework of natural disturbance and the knowledge of its component processes and effects provides the basis on which we can manage our forests as a renewable resource which can be utilized so that the forests ‘retain their diversity and richness for mankind's continuing benefit’. Nowhere is this management more desperately needed than for the protection of the world's tropical forests, its peoples and their cultures.

Contrasting leaf phenotypes control seasonal variation in water loss in a tropical forest shrub.

Psychotria marginata, a shrub common to humid tropical forests in Central America, produces leaves in two seasonal flushes with contrasting morphology and physiology. Leaf production is bimodal with a major peak at the beginning of the annual wet season and a secondary peak at the end of the wet season. Together these peaks account for 75-87% of annual leaf production. Leaves produced just prior to the dry season have higher specific mass and, during drought, have lower stomatal conductances and higher water-use efficiencies. Plants irrigated during two consecutive dry seasons continued to produce leaves with these morphological and physiological differences, indicating that this feature has been strongly canalized by some factor other than water availability in this highly predictable yet seasonal habitat. The bimodal leaf production results in acclimation to changing conditions through co-occurring leaves that lack the physiological plasticity usually associated with acclimation. Heterophylly of this form suggests that the moderate drought of the dry season has been a significant selective factor for understory plants.

Nitrogen dynamics and growth of seedlings of an N-fixing tree (Gliricidia sepium (Jacq.) Walp.) exposed to elevated atmospheric carbon dioxide.

Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO2 and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO2 partial pressures of 350 and 650 μbar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO2 when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO2 enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO2 enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO2. In seedlings supplied with soil N, elevated CO2 increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf δ15N). While CO2 enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO2 partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO2 will depend greatly on plant and soil nutrient status.

THE REPRODUCTIVE BIOLOGY OF CUPANIA GUATEMALENSIS RADLK. (SAPINDACEAE).

Intraspecific floral polymorphism is of widespread occurrence in flowering plants. Perfect, staminate, and pistillate flowers may be found on the same or different plants in diverse combinations as exemplified by monoecy, dioecy, androdioecy, gynodioecy, polygamy and the like; in addition, within perfect-flowered species, polymorphism may exist in the form of distyly and tristyly. It is generally presumed that floral polymorphisms increase the probability of outcrossing. The level of outcrossing, however, has not been measured in any zoophilous species with polymorphic flowers except for some heterostylous species (Ornduff, 1971, 1975). Floral polymorphism, by introducing spatial variation in floral rewards, also influences the foraging behavior of pollinators. The foraging behavior of pollinators, in turn, is likely to influence not only gene flow but also the evolution of floral polymorphism itself. Yet it is somewhat surprising that the effect of floral polymorphism on the foraging behavior of pollinators has not so far been examined. This paper describes the reproductive biology of a monoecious species, Cupania guatemalensis Radlk. (Sapindaceae), a small tree found in lowland tropical forests of Central America. It examines the pattern of spatial and temporal variation in the relative frequencies of staminate and pistillate flowers at the individual and population levels; differences in floral reward of staminate and pistillate flowers; differences in the foraging behavior and visitation rates of pollinators on staminate and pistillate flowers; and the effect of flowering behavior and floral polymorphism on the foraging behavior of pollinators and the degree of outcrossing. In addition, the floral phenology of some other members of the Sapindaceae is interpreted in the light of findings reported here. The investigations are part of a wider study of the breeding systems of lowland tropical forest trees (Bawa, 1974; Bawa and Opler, 1975).

Phenology and Floral Biology of Gustavia superba (Lecythidaceae) in Central Panama

Leaf fall, leaf production, flower and fruit production, and floral biology of Gustavia superba (Kunth) Berg were studied for a year in the tropical moist forest of central Panama. Although leaves fall throughout the year, peak fall occurs in the first month of the wet season. Leaf production on any branch is possible throughout the year, but within the population studied two pronounced peaks were observed, one in the late wet-early dry season and the other after the first rains of the wet season. Flowering began about one month after the rains ceased and continued throughout the dry season into the first month of the following wet season. Fruits reached maturity at the end of the dry season and into the early wet season. The severity of the dry season of 1974-1975 may have altered the normal flowering pattern and reduced fruit production in this species. Observations revealed that the flowers are diurnal, without odor anid nectar, and are visited by bees for their pollen. A limited number of crosses suggest that this species is self-incompatible. SEVERAL PAPERS IN RECENT YEARS have emphasized the peak in flowering of trees in the seasonally dry forests of Central America in the dry season (Fournier and Salas 1966, Janzen 1967, Croat 1969, Frankie et al. 1974). Not only do more tree species bloom during this time of the year but those that do bloom tend to have larger, more spectacular flowers. In central Panama the large, bright-yellow flowers of Tabebuia guayacan (Seem.) Hemsl., T. ochracea (Cham.) Standl. subsp. neochrysantha (A. Gentry), A. Gen,try, Cespedesia macrophylla Seem., and Cochlospermum vitifolium (Willd.) Spreng., the golden-yellow flowers of Cassia moschata Benth., the duller yellow flowers olf Byrsonima crassifolia (L.) H.B.K., the pink flowers of Tazbebiaz rosea (Bertol.) DC., and the red ones of several species of Erythfina are among the more spectacular indicators of the dry season. The number of species with maturing fruit is also greater during the dry season (Janzen 1967, Croat 1969, Smythe 1970, Frankie et al. 1974). Smythe (1970) has demonstrated a pronounced peak of large animal-dispersed seeds (over 1.5 cm in greatest diameter) at the end of the dry season into the early wet season. In addition to flowering and fruiting, certain vegetative phenomena coincide with the dry season in the seasonal tropical forests of Central America. Frankie et al. (1974) and Daubenmire (1972) have shown a distinct dry-season peak in leaf fall for a number of species of trees studied in Guanacaste Province, Costa Rica. Production of new leaves at the beginning of the dry season has been demonstrated in several species by Rockwood (1975, table 8) and toward the end of the dry season and into the rainy season by Daubenmire (1972) and Frankie et al. (1974). Even though certain phenological patterns predominate in moist tropical forest with seasonal rainfall, there are, nonetheless, many deviations from these patterns. For example, most deciduous tree species lose their leaves in the dry season, but some species like Jacquinia pungens A. Gray (Janzen 1970) and Cordia alliodora (Ruiz & Pavon) Cham. (T. Croat, pers. colmm.) lose their leaves in the wet season. The large animal-dispersed seeds of Spondias mombin L. and S. radlkoferi J. Donn. Sm. ripen and fall in the midto late-wet season in contrast to the peak for this fruit type at the end of the dry season and early wet season (Smythe 1970, Croat 1974). Because of these and other exceptions to general phenological patterns, phenological studies of individual species of seasonal tropical forest are needed.