Peter G. Murphy

EARLY PLANT SUCCESSION AFTER CUTTING AND BURNING IN THE UPPER RIO NEGRO REGION OF THE AMAZON BASIN

(1) Changes in vegetation structure, species composition, and species dominance were studied over the first twenty-two months of succession after the cutting and burning of several mature tierra firme (i.e. non-flooded) forest sites in south Venezuela in the upper Rio Negro region of the Amazon Basin. To explain the observed succession, additional field investigations were undertaken. (2) Four months after burning the density of plants was less than 1 m-2. The density of successional woody plants, forbs, and grasses increased rapidly from the fourth to the tenth month of succession. There was little change in plant density between the tenth and twenty-second month, but a large change in vegetation height occurred during this time. By the twenty-second month a loose canopy of Cecropia spp. was present at 5 m height. (3) Many of the forest tree species sprouted from the stump when cut, but burning killed sprouts and significantly reduced the size of the seed bank. Nevertheless, the first successional woody colonizers established from seed which survived the burn. The first forb and grass colonizers established from seeds dispersed onto the site after the burn. (4) The common successional woody species had bator bird-dispersed seeds. Many forbs and grasses had wind-dispersed seeds. (5) Population densities were low during the first months of succession because of the low density of propagules and because the low rainfall during this time was not favourable for seedling germination or establishment. The rapid increase in plant density after about the fourth month occurred because the early colonizers had then begun to produce, and disperse locally, large numbers of seeds. (6) There were six distinct microhabitat types present on the sites after burning. Most species tested showed distinct microhabitat preferences.

Seasonally Dry Tropical Forests: Dry forests of Central America and the Caribbean

Introduction Holdridge (1947, 1967) developed a bioclimatic classification system by which the worlds terrestrial biota may be categorized into approximately 120 life zones, each distinguished by climatic parameters that coincide with particular vegetational characteristics. Approximately 68 life zones are in the tropics and subtropics, of which 30 are dominated by forest of various types. Lugo, Schmidt & Brown (1981) estimated that 28 tropical and subtropical forested life zones are represented in Central America and the Caribbean, and 13 are found on the islands of the Caribbean. Despite this diversity, approximately half of the vegetation of Central America and the Caribbean is within the dry forest life zone (sensu Holdridge, 1967). Dry forests are not infrequently referred to as deciduous forests, but the degree of deciduousness varies greatly (see below). Not all dry forests are conspicuously deciduous, and not all deciduous forests are dry forest. By Holdridges criteria, tropical and subtropical dry forests are found in frost-free areas where mean annual biotemperature (a special calculation that reduces the effects of extreme temperatures) is above 17 °C, annual rainfall ranges from 250 to 2000 mm, and the ratio of potential evapotranspiration to precipitation is greater than one, to a maximum value of two. By these criteria, 49% (8.2 × 10 5 km 2 ) of the vegetation of Central America and the Caribbean is considered dry forest (Brown & Lugo, 1980). Africa has the most dry forest (16.5 × 10 6 km 2 ; 73% of the continents vegetation); worldwide, about 42% of all intratropical vegetation is dry forest. Global patterns in dry forest distribution and overall ecological characteristics relative to wetter tropical and subtropical forest ecosystems were reviewed by Murphy & Lugo (1986a).

Structure and biomass of a subtropical dry forest in Puerto Rico

Guanica Forest, with seasonal rainfall averaging 860 mm annually, is among the driest of tropical or subtropical forests studied to date. It is composed of over 12,000 live tree stems per hectare, only 2.3 and 12 percent of which exceed 10 cm DBH or 5 m in height, respectively. Of all stems greater than 2.5 cm DBH, 57 percent are stump or root sprouts, attributable to forest cutting 50 years earlier. The dry winter months induce maximum deciduousness and are reflected in a 50 percent reduction in leaf area index, from approximately 4.3 to 2.1. Although less in magnitude, leaf fall was also observed in the moderately dry midsummer months. Relative to wetter forests, tree species richness and total community biomass is low. Approximately 50 percent of the total live-plant biomass of 89.9 t/ha occurs below ground, a higher proportion than for any other comparable forest measured thus far. IN RECENT YEARS, tropical forests have received unprecedented ecological attention. Interest in these ecosystems has been stimulated, in part, by the alarming rate at which they are being modified or completely destroyed. But interest in them is also attributable to their vast stores of carbon and the potential effects of their disruption on the worlds carbon balance. Most studies in the tropics have focused on forests growing in humid climates even though they account for a relatively small portion of the forested tropical landscape. Of the total global extent of tropical forest, Brown and Lugo (1982) estimated that about 25 percent is tropical and subtropical wet and rain forest and 33 percent tropical or subtropical moist forest. The remaining 42 percent is tropical or subtropical dry forest (sensu Holdridge 1967). Unlike humid forest, tropical and subtropical dry forest has been very little studied, particularly with respect to taxonomic composition, stand structure, biomass, primary productivity, and rates of carbon turnover. It is, therefore, difficult at the present time to evaluate the significance of tropical and subtropical dry or seasonal forest relative to the global carbon cycle. Additionally, data regarding biomass and related characteristics (e.g., growth and primary productivity) would be useful in assessing the resource potential of dry forest and in furthering our understanding of forest function, especially with respect to climate and seasonality. In 1981 we initiated a comprehensive, long-term study of structure, primary productivity, and plant succession in a subtropical dry forest in southwestern Puerto Rico. This paper reports on the taxonomic composition (woody plants), structure, and biomass of the forest.

Forest structure and productivity in Puerto Rico's Luquillo Mountains.

Net aboveground primary production (NPP) at 725 m in elevation in the colorado forest (montane rain forest, sensu Beard; lower montane wet forest, sensu Holdridge) of the Luquillo Mountains of Puerto Rico averaged 7.60 t/ha yr on two 0.4-ha permanent plots and was determined by summing aboveground biomass accrual (0.59 t/ha yr), litterfall (6.80 t/ha yr), and herbivory (0.21 t/ha yr). The total leaf area index (trees, epiphytes, and ground vegetation) was 4.95 m2/i2, the aboveground woody volume and biomass, 220 m3/ha and 130 t/ha, respectively, and standing herbivory and the herbivory rate, 5.1 and 4.0 percent/yr, respectively. The mean standing crop of litter was 680 g/m2, with a turnover rate of 0.78 times per year. Of the structural features studied over a 600-m elevational gradient in the Luquillo Mountains, the number of trees per hectare, basal area, and soil organic matter increased with elevation; whereas, the specific leaf area, canopy height, range of tree diameters, forest volume and biomass, leaf area index, and species richness declined. Of the dynamic features studied, ingrowth and mortality of trees, tree growth (diameter, volume, and biomass growth), litterfall, loose litter, standing herbivory and herbivory rates, litter turnover, aboveground woody NPP, and total NPP all declined with an increase in aboveground elevation. The structural and floristic impoverishment of forests and changes in forest dynamics with ascent in the Luquillo Mountains reflect cooler temperatures, fog, and heavier rainfalls that interact to produce saturated soils and retard the mineralization of organic matter.

Disturbance versus edge effects in sugar-maple/beech forest fragments

Abstract Edge-to-interior gradients of woody vegetation were examined on northern and southern aspects of two sugar-maple/beech forest fragments that differed in cultural history. In site 1, a remnant old-growth fragment, the importance of sugar maple and beech increased from edge to interior on the southern aspect, while that of less-shade-tolerant species decreased. Differences in total stem densities also differentiated edge from interior, with highest canopy densities and lowest ground-layer densities near the edge. Compositional and structural gradients were further reflected in ordinations depicting a vegetation gradient from the southern edge to the interior. A similarity was apparent between the extreme edge positions on the northern aspect and the mid-to interior-gradient positions on the southern aspect. An assumed microclimatic gradient was invoked to explain the vegetation patterns. In site 2, a formerly grazed second-growth fragment, differences in composition and structure between edge and interior forest did not occur as a gradient. Retardation in edge-forest development was attributed to the disturbance history of the fragment. The recognition of differences in edge-forest development relative to disturbance history may be requisite for accurate determination of edge effects in disturbed landscapes.

Nutrient dynamics of a Puerto Rican subtropical dry forest

Se estudio la distribution de los nutrientes N, P y K en suelos y vegetacion y su mobilidad en la caida y descomposicion de hojarasca en rodales maduros y sucesionales del bosque subtropical seco en Guanica, Puerto Rico. Los suelos almacenan altas cantidades de N (9100 kg/ha), P (1820 kg/ha), y K (7460). Altas cantidades de Ca extractable (> 4000 mg/g) y alto pH (> 7–8) explican porque solamente el 1.3 y 25% del total de P y K, respectivamente eran extractables. El almacenaje total de N, P y K en el ecosistema fue 10,300, 1900 y 7700 kg/ha, respectivamente, del cual la vegetacion almacenaba el 10, 2 y 3%, respectivamente. La caida de hojarasca retorno el 26, 18 y 180%/ano del N, P y K almacenado en la hojarasca del bosque. Los arboles retranslocaron alrededor del 30 y 65% del N y P requerido para satisfacer la produccion primaria neta sobre tierra e inmobilizaron el P en las raices muertas. La descomposicion lenta de hojas (7.3 anos para el 95% de la descomposicion libero K mas rapidamente que masa, P tan rapidamente como masa y ceniza y N mas lentamente que masa. La eficiencia de uso de P por la caida de hojarasca fue alta al compararse con otros bosques tropicales, mientras que de N y K fue similar a la de otros bosques tropicales y templados. La tala y recrecimiento de vegetacion resulto en diferencias en la concentration de nutrientes en la hojarasca y en la eficiencia de uso de nutrientes de la vegetacion sucesional.

The Dry Forests of Puerto Rico’s South Coast

The subtropical dry forests of Puerto Rico are found mainly on the island’s southwest coast in the rain shadow of the central mountain system. Studies of the 4000-ha Guanica Forest have provided basic information on ecology and management potential. Even on the more favorable sites, the dry forests are smaller in stature and biomass, lower in biodiversity, lower in productivity, and more seasonally pulsed in tree growth, reproductive cycles, and organic matter turnover than forests in areas of higher and less seasonal rainfall. The limestone-based soils are primarily mollisols, characterized by high organic matter, low bulk density, and high pH. Soil nutrient contents are relatively high, but less than 2% of the P is in a readily available form. Nevertheless, because of the proportionately small nutrient inventory in biomass, it appears that the soil nutrient pool could sustain several rotations of aboveground biomass removal without depleting reserves to a level that would immediately jeopardize forest productivity. Relatively rapid recovery of a cut-over dry forest occurs if stump and root sprouts are allowed to survive. Because of sprouting, most plant species are reestablished within 3 years, and aboveground biomass accumulates to almost half of precut levels within 13 years. Chronically disturbed sites, however, are much slower to recover. On the basis of the available ecological data, we make recommendations concerning the conservation, utilization, and management of dry forests and offer suggestions concerning research needs.

A comparison of productivities and energy values between slash and burn agriculture and secondary succession in the upper Rio Negro region of the Amazon Basin

Abstract An analysis of the energy inputs and outputs for conuco agriculture (i.e., shifting cultivation) and a comparison of productivity between conuco crops and successional vegetation was undertaken near the town of San Carlos de Rio Negro in southern Venezuela. The main crop, yuca ( Manihot esculenta , Crantz), yielded (m.t. = metric tonnes) 4.31 m.t. ha −1 wet weight in year one; production declined to 2.81 m.t. ha −1 in the second year. The infertile soils of the region are the major cause of these low yields. Although relatively unproductive, conuco agriculture is much more energy efficient than mechanized agriculture. The ratio of energy output, in the form of processed food, to energy input in the form of human labor, averaged 13.9:1 over two cropping periods. Slightly more energy was devoted to field activities than to processing activities. The conuco crops were more productive than the adjacent succession site vegetation (532 vs. 109 g m −2 dry weight) during the first year after cutting and burning; but in the second year the successional vegetation was more than twice as productive as the conuco crops (1446 vs. 529 g m −2 dry weight). This difference in second year production was attributable to greater resource allocation to leaves and longer leaf retention time for the successional vegetation.

Post-logging tree mortality and major branch losses in Kibale Forest, Uganda

Abstract This study was undertaken to establish whether selective felling and the existence of large forest gaps influence the dynamics of tree and branch falls. Censuses of fallen trees and large branch falls in uncut, lightly cut and heavily cut forest tracts were conducted for 20 months. Dead-standing trees in the study compartments were enumerated only three times at 6-month intervals. In lightly cut forest, the densities of live and dead tree and branch falls were similar but monthly tree snaps were more than uproots. However, for heavily cut forest, total tree falls (snap + uproot) involved more live than dead trees. There were more live than dead tree snaps and the rate of tree uproots was low. in contrast to the above, the density of tree snaps and uproots in uncut mature forest was similar. For total tree falls and live tree or large branch falls, the heavily selectively cut forest exhibited higher rates than uncut and lightly cut forest tracts. The annual rates of live tree falls ha −1 were 1.30 for lightly cut, 3.30 for heavily cut and 1.74 for uncut mature forest. It appears that heavy logging increases tree mortality rates through falls where more live tree snaps than uproots are involved.

Long-term Ecological Consequences of Anthropogenic Disturbance on Saginaw Bay Coastal Wet Meadow Vegetation

Natural disturbance related to fluctuating water levels governs the structure and composition of Saginaw Bay coastal wet meadow vegetation. European settlement introduced anthropogenic disturbance to the Saginaw Bay coastal zone, but the impact of anthropogenic disturbance on the vegetation is not known. We compared vegetation and abiotic data from six reference and six disturbed sites to examine anthropogenic impacts on Saginaw Bay coastal wet meadow vegetation. Reference sites were the least disturbed wet meadows available, whereas disturbed sites had suffered various anthropogenic impacts 10–30 years before the study started. We observed no significant difference in mean aboveground plot biomass, stem density, litter depth, hummock height and elevation, or mean site species richness, Shannon-Wiener diversity, coefficient of conservatism, and floristic quality between reference and disturbed sites. The importance values of only two of 15 major species differed significantly between reference and disturbed sites. Native wet meadow species re-vegetated previously disturbed sites, suggesting that Saginaw Bay coastal wet meadow vegetation is resilient once released from at least some forms of anthropogenic disturbance. Purple loosestrife (Lythrum salicaria L.) and several agricultural weeds occurred only in disturbed sites, suggesting a potentially useful metric for monitoring the health of Great Lakes coastal wet meadow vegetation. Additional studies are needed to gain insight into the probable revegetation trajectories of recently disturbed coastal wet meadows and to determine the form and utility of a “weedy plants” metric for classifying the condition of these wetlands.