The influence of land use and fallow period on the properties of two calcareous soils in the humid tropics of southern Mexico

Calcareous soils are common in the humid tropics of Mexico, but little is known about their productivity and resistance to changes in land use. The influence of land use and duration of fallow on physical and chemical properties were investigated in (a) a loamy Rendzina (at Site A) rich in organic matter and limited in depth by calcareous gravel, and (b) a deeper Calcaric Phaeozem (at Site B) with a more clayey texture throughout. Plots at different successional stages within farmers' shifting cultivation systems were selected on both soils: cropland/young fallow (zero years fallow length), shrub-fallow and tree-fallow at Site A, and cropland/young fallow and forest at Site B. Changes in soil properties suggested a process of recovery with fallow length, in which organic matter, total N and CEC increased with time. The increase in organic matter was remarkably large, on average 0.5–0.6% per year. The Rendzina contained more organic matter than the Calcaric Phaeozem, probably because of its greater carbonate content, which is presumed to protect organic matter against decomposition. The differences in soil properties were mostly greater over time than between treatments.     

Comparing bird community responses to forest fragmentation in two lowland Central American reserves

Habitat loss and fragmentation reduce diversity of tropical bird communities, but the predictability of how communities in fragments disintegrate over time remains unclear. We compared bird community changes of two lowland forest reserves, La Selva Biological Station (LSBS), Costa Rica and Barro Colorado Island (BCI), Panama, both approximately the same size (1500 ha) and at similar latitude (9-10 N) in Central America. Both reserves are losing bird species despite LSBS’s connection to an existing large park (incomplete isolation) and BCI’s favorable location within a largely forested landscape. We examined characteristics of guilds and species lost from the sites to determine whether patterns were similar, and thus predictable. Some of the same guilds declined at both reserves, particularly insectivores and ground/understory nesters. At LSBS mixed-species flock participants, forest species, and burrow-nesters also declined or became extirpated disproportionately. At BCI edge species became extirpated. Body mass was a poor predictor of species and guild loss at both sites, except for carnivores at La Selva. Thus, fragmentation appears to influence some guilds more than others, but which species decline or disappear in tropical forest fragments is also influenced by site-specific factors, mostly yet to be determined. We need to understand such idiosyncratic effects of fragmentation better, rather than rely on one-size-fits-all management plans to conserve bird communities in tropical forest fragments.     

Understory development in young Cordia alliodora plantations

The biomass and composition of theunderstory plant community associated with Cordiaalliodora planted at 2 m × 2 m spacing arecharacterized for the first (1992) and fifth (1996)years after stand establishment. The study plantationscomprise three replicates each of C. alliodoramonocultures and C. alliodora interplanted withperennial monocots (polycultures), part of anexperiment located at the La Selva Biological Stationin northern Costa Rica. In the first year, there wasno significant difference in the mean abovegroundunderstory biomass between the monocultures of C.alliodora (7975 kg/ha) and the polycultures of treesand monocots (5427 kg/ha). After 5 years, theunderstory biomass in the monocultures increased(16,918 kg/ha), while in the polycultures it decreased(116 kg/ha). In contrast to the first year, therefore, by the fifth year the understory biomass differed significantly between the two types of plantations (p = 0.005, R² = 0.78, LSD = 5200 kg).In the first year, 109 plant species were present inthe understory of the C. alliodora monoculturesand polycultures. The dominant species in both themonocultures and polycultures was the monocot Tripogandra serrulata; Panicum polygonatum wasranked second. After 5 years, 20 species representing15 families were present in the monocultures. Thedominant species was Paspalum conjugatum,followed by Tripogandra serrulata. Only two newspecies were recruited, and both were woody shrubs:Nectandra membranacea and Vernonia patens.     

四季草莓赛娃与一季草莓丰香光合特性的比较

利用自动光合测定系统,在人工气候室内研究了CO2浓度、光照强度以及温度与四季草莓"赛娃"、一季草莓"丰香"光合特性的关系。结果表明:在CO2浓度处于正常浓度(350μmol/mol)或稍偏高浓度(500μmol/mol)时,丰香的光合作用略强于赛娃;当CO2浓度低于700μmol/mol时,赛娃的净光合速率与丰香的相差不大,但之后随着CO2浓度的增加,赛娃呈下降趋势,而丰香仍继续持上升状态,说明丰香的光合作用相比赛娃更适于高CO2浓度;在不同光照强度下,丰香的CO2饱和点明显高于赛娃,适当增加CO2浓度,可大幅提高丰香的产量;在低温(5℃)和高温(30℃)条件下两草莓品种均能光合作用,但赛娃叶片的净光合速率明显高于丰香,表明赛娃对温度的适应性强于丰香。     

Effects of history of use on secondary forest regeneration in the Upper Parana Atlantic Forest (Misiones, Argentina)

The Upper Parana Atlantic Forest, as with other areas, has been modified due to anthropogenic activities, generating a mosaic of agricultural fields and forests of varying ages and levels of conservation. The objective of this study was to identify structural and floristic patterns in secondary forests generated in abandoned fields with varying historic uses. Changes in vegetation were surveyed using chronosequences from 2 to 70 years. Stands with different land use histories in three areas of northern Misiones, Argentina, were considered. Adults (DBH ≥ 10 cm), saplings (3 ≥ DBH < 10 cm), seedlings (DBH < 3 cm and height ≥ 50 cm) and non-arboreous vegetation (herbaceous plants and shrubs) were surveyed. Data was analyzed using multivariate techniques (TWINSPAN, Detrended Correspondence Analysis, Cluster Analysis, and Kruskal–Wallis test). In the floristic analysis, two main forest groups were identified: one made up of primary forest and oldest secondary forest (>20 years), and another of the youngest secondary forests (<20 years). Basal area and density of individuals (seedlings, saplings, and adults) rapidly increased during the first two decades, and subsequently the adult category reached values comparable to those of mature forests (32 m²/ha and 850 individuals/ha). During the early years of succession, the forests showed floristic and structural differences related to history of use previous to abandonment; abandoned annual crops showed the greatest densities of saplings and seedlings, while abandoned pastures presented only individuals in the seedling category, and at very low densities. In stands analyzed since the beginning of succession, species of several ecological groups were present (pioneer, initial secondary, late secondary), while climax species appeared only in the seedling category, and in abandoned pastures no species of the later groups were recorded. Our results suggest that differences recorded during the early years of regeneration in sites with varying historic uses may be generated mainly by remnant vegetation at the time of abandonment of the fields (grasses and remnant monoculture plants). After 20 years of succession, it was no longer possible to differentiate the forests structurally or floristically, regardless of age or historic use of each stand. Due to their rapid regeneration capacity, the forests studied play an important role from the start of the successional process in heterogeneous anthropogenic landscapes such as that of Misiones, and contribute to maintaining biodiversity while providing numerous environmental services.