Structure of a unique inland mangrove forest assemblage in fossil lagoons on the Caribbean Coast of Mexico

Scrub mangrove wetlands colonize the intertidal zone of fossil lagoons located in carbonate continental margins along the Yucatan Peninsula of Mexico. These unique ecological types were investigated in October, 1994, by locating transects in several mangrove forests along the Caribbean coast of the peninsula. Four species of mangrove occurred at these sites including Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, Conocarpus erecta. This is one of the first examples of a species rich scrub forest. The mangroves fell into three height categories: short scrub less than 1.5 m, tall scrub to 3.0 m, and basin forests between 4.5 and 6 m. Average height, diameter (dbh), basal area, and complexity index generally increased from short scrub to basin forests. Basal area, ranged from 0.16 m² ha^–1 in a short scrub forest intermixed with Cladium jamaicense to 12.9 m² ha^–1 in a basin forest. Density ranged from 1520 trees ha^–1 to over 25,000 trees ha^–1 in a short scrub forest dominated by R. mangle. The complexity index ranged from 0.01 to 8.3. Height, dbh, basal area, and complexity index were positively related. A number of trees were growing as sprouts from larger downed trunks, suggesting that hurricanes, such as Gilbert that occurred in 1988, are important in controlling the structure of these forests. These forests appear isolated from the sea, but are influenced by groundwater exchange occurring at the land-margin zone.     

Short-term effects of thinning and liming on forest soils of pitch pine and Japanese larch plantations in central Korea

The influences of thinning (50% of standing density) and liming (Ca+Mg, 2 Mg ha⁻¹) on soil chemical properties were investigated for 2 years (2001, 2002) in 40-year-old pitch pine (Pinus rigida Mill.) and 44-year-old Japanese larch (Larix leptolepis Gord.) plantations established on similar soils. In general, soil properties varied significantly among plantations and treatments. For both plantations, thinning significantly increased soil organic C (SOC) concentrations whereas there were no significant changes in soil pH and Ca and Mg concentrations. In addition, thinning increased total soil N and Na concentrations for the pitch pine plantation and available P concentration for the Japanese larch plantation in the second year after the treatment. Liming did not affect soil chemical characteristics for the pitch pine plantation except for Na concentration. However, for the Japanese larch plantation, liming significantly increased soil pH and K, Ca and Mg concentrations and decreased SOC and total soil N concentrations. For both plantations, soil Al concentration did not change after thinning and liming and decreased exponentially with increased pH values. The increases in SOC and total soil N concentrations after thinning were possibly due to increases in decomposition of organic matter and root death. Although differences were not statistically significant, soil available P concentration tended to increase at early stages of liming for both plantations. These results suggested that thinning and liming seemed to regulate soil chemical properties for pitch pine and Japanese larch plantations established on similar soils.     

Simulation of the effects of deer browsing on forest dynamics

I introduced forest dynamics to a deterministic herbivore-vegetation model to examine the effects of vertically stratified forest structure on the dynamics of the herbivore–vegetation system, the resilience and stable states of vegetation, and the interactions between deer populations and vegetation. I constructed a model based on data from field studies performed in Hokkaido, northern Japan. Three phases of state were identified for a given deer density: (1) understory vegetation is maintained with a equilibrium proportion of canopy gaps in the absence of deer; (2) if the equilibrium proportion of canopy gaps is greater than that in the equilibrium state in the absence of deer, the understory vegetation can be maintained; and (3) the understory vegetation cannot be maintained. At the boundary between phases 2 and 3, the herbivore population level had discontinuous effects on vegetation. When the deer density was held below the threshold, forest vegetation had resilience to recover to the equilibrium stable state at the given deer density, although the equilibrium canopy gap ratio and vegetation biomass differed with deer density. However, the forest vegetation–herbivore system could not be maintained in a stable state without artificial deer population management if food limitation was the only mechanism to keep the deer population at low levels. The deer population must be kept below the boundary between phases 1 and 2 to maintain the forest regeneration processes. The level cannot be determined by observing the deer population; careful observation of forest regeneration processes is required.     

Community dynamics of evergreen broadleaf forests in southwestern Japan. I. Wind damaged trees and canopy gaps in an evergreen oak forest

The rates of treefall and canopy opening in the evergreen oak forest in southwestern Japan were determined by studying the number and size distribution of overstory trees, wind damaged trees, and canopy gaps in a belt transect in the Kasugayama Forest Reserve in Nara City. Thirty three percent of the overstory trees wereCastanopsis cuspidata. The total area of canopy gaps was about 20% of the total land area in the study area. The ages of the gaps were determined by counting the annual rings of various kinds of trees growing in gaps. By comparing gap ages with meteorological data, it became evident that gap formation was mainly caused by strong typhoons. The mean time interval between strong typhoons visiting the forest reserve, 6.57 years, was determined by applying the MNY method to the meteorological data. The treefall rate and the mean area of canopy openings per year were 0.84 overstory trees/ha·year and 55.6 m²/ha·year, respectively. The mean residence time of the forest canopy was about 180 years.     

Regeneration of<Emphasis Type="Italic"> Rhizophora mangle</Emphasis> in a Caribbean mangrove forest: interacting effects of canopy disturbance and a stem-boring beetle

Current theory predicts that in low-density, seed-limited plant populations, seed predation will be more important than competition in determining the number of individuals that reach maturity. However, when plant density is high, competition for microsites suitable for establishment and growth is expected to have a relatively greater effect. This dichotomous perspective does not account for situations in which the risk of seed predation differs inside versus outside recruitment microsites. We report the results of a field experiment and sampling studies that demonstrate such an interaction between microsite quality and the risk of propagule predation in mangrove forests on the Caribbean coast of Panama, where it appears to play a key role in shaping the demography and dynamics of the mangrove, Rhizophora mangle. Rhizophora's water-borne propagules establish wherever they strand, but long-term sampling revealed that only those that do so in or near lightning-created canopy gaps survive and grow to maturity. These microsites afford better growth conditions than the surrounding understory and, as importantly, provide a refuge from predation by the scolytid beetle, Coccotrypes rhizophorae. This refuge effect was confirmed with a field experiment in which Rhizophora seedlings were planted at different positions relative to gap edges, from 5 m inside to 20 m outside the gap. Mortality due to beetle attack increased linearly from an average of 10% inside a gap to 72% at 20 m into the forest. The interaction between canopy disturbance and propagule predation may be having a large impact on the composition of our study forests. Being shade-tolerant, Rhizophora seedlings that escape or survive beetle attack can persist in the understory for years. However, the high rate of beetle-induced mortality effectively eliminates the contribution of advance regeneration by Rhizophora saplings to gap succession. This may explain why the shade-intolerant mangrove, Laguncularia racemosa, is able to co-dominate the canopy in low intertidal forests at our study sites.     

The effects of X-rays on the proliferation dynamics of cells in the imaginal wing disc ofDrosophila melanogaster

Summary We report on the size distribution of clones marked by mitotic recombination induced by several different doses of X-rays applied to 72 h oldDrosophila larvae. The results indicate that the radiation significantly reduces the number of cells which undergo normal proliferation in the imaginal wing disc. We estimate that 1000 r reduces by 40–60% the number of cells capable of making a normal contribution to the development of the adult wing. Part of this reduction is due to severe curtailment in the proliferative ability of cells which nevertheless remain capable of adult differentiation; this effect is possibly due to radiation-induced aneuploidy. Cytological evidence suggests that immediate cell death also occurs as a result of radiation doses as low as 100 r. The surviving cells are stimulated to undergo additional proliferation in response to the X-ray damage so that the result is the differentiation of a normal wing.