Kenji Hata

Complex Interrelationships Among Aboveground Biomass, Soil Chemical Properties, and Events Caused by Feral Goats and Their Eradication in a Grassland Ecosystem of an Island

This study examined the recovery, via biotic and abiotic pathways, of a grassland ecosystem after eradication of introduced exotic goats. We used path analyses to evaluate the relative strength of relationships among aboveground biomass, soil chemical properties (carbon, nitrogen, and phosphorus content; soil acidity), presence of nesting seabirds after goat eradication, extent of vegetation degraded by goats before their eradication, plant species composition after removal of goats, and topography. Models including the same variables with different paths were constructed using the Bayesian estimation method, and the best-fit models were constructed by comparing deviance information criterion values. Results of the path analyses demonstrated that vegetation degradation and soil erosion prior to goat eradication increased soil exchangeable acidity, which resulted in limitation of aboveground biomass. Seabird nesting after goat eradication increased the quantity of soil nutrients, possibly through inputs of feces, eggshells, and dead chicks or adults. The increase in nutrients was affected indirectly, via seabird nesting, by topography and vegetation type after goat eradication. The direct and indirect relationships demonstrated by our results suggest the existence of complex interrelationships during recovery of ecosystem function after eradication of exotic mammals.

Vegetation changes between 1978, 1991 and 2003 in the Nakoudojima island that had been disturbed by feral goats

Changes in vegetation of the Nakoudojima island between 1978, 1991 and 2003 were quantified with aerial photographs. The island was divided into 9200 plots (10 m × 10 m) on the aerial photographs and the plots were categorized into forest, grassland, bare ground or others. Percentages of the grassland increased 66.3% to 69.0% and those of the bare ground increased from 7.0% to 15.5% for 23 years. Some grasslands became bare ground between 1978 and 1991, which resulted in a decrease in the area of the grassland and an increase in the area of the bare ground during this period. On the other hand, some bare ground changed to grasslands between 1991 and 2003, which meant an increase of grasslands. These changes in vegetation would be due mainly to grazing and trampling, and their termination caused by feral goats. Percentages of the forest decreased from 16.2% to 6.0% for 23 years. Many forests changed into grasslands or bare ground even after the eradication of feral goats, which suggests that canopy trees in the forest died by natural disturbances in addition to the lack of seedlings by the grazing of feral goats.

A 19-Year Study of the Dynamics of an Invasive Alien Tree, Bischofia javanica, on a Subtropical Oceanic Island

ABSTRACT A 19-yr study of the dynamics of an invasive alien species, Bischofia javanica Blume, in a secondary forest was conducted in the Bonin Islands, Japan. The study was begun in 1984 when another alien species, Pinus luchuensis Mayer, had begun to die because of infection by a pine nematode as well as typhoon damage in 1983. Diameters at breast height (DBHs) of all trees in a 20 by 20 m plot and heights of all saplings (<1.3 m, ≥0.3 m in height) were measured almost every 3 yr. The total basal area of P. luchuensis decreased over time, and all trees had fallen over by 1998. The total basal area of B. javanica increased more than 10-fold over 19 yr without changes in tree or sapling density. Up to 1990, growth rates of trees of B. javanica were higher than those of two native canopy trees (Pouteria obovata and Machilus kobu), but a third native canopy tree (Schima mertensiana) had growth rates comparable with those of B. javanica. After 1990, there were few differences between growth rates of B. javanica and native species. However, mortality and recruitment of B. javanica were lower than those of native species of canopy trees during the survey period. The higher growth rate, lower mortality, and lower recruitment led to a shift from a skewed size distribution of the individuals of B. javanica toward a more bell-shaped size distribution. Our results suggest that regeneration and maintenance of B. javanica populations in the secondary forests depend on canopy gaps occasionally created by disturbances.

Increases in soil water content after the mortality of non-native trees in oceanic island forest ecosystems are due to reduced water loss during dry periods

The control of dominant, non-native trees can alter the water balance of soils in forest ecosystems via hydrological processes, which results in changes in soil water environments. To test this idea, we evaluated the effects of the mortality of an invasive tree, Casuarina equisetifolia Forst., on the water content of surface soils on the Ogasawara Islands, subtropical islands in the northwestern Pacific Ocean, using a manipulative herbicide experiment. Temporal changes in volumetric water content of surface soils at 6 cm depth at sites where all trees of C. equisetifolia were killed by herbicide were compared with those of adjacent control sites before and after their mortality with consideration of the amount of precipitation. In addition, the rate of decrease in the soil water content during dry periods and the rate of increase in the soil water content during rainfall periods were compared between herbicide and control sites. Soil water content at sites treated with herbicide was significantly higher after treatment than soil water content at control sites during the same period. Differences between initial and minimum values of soil water content at the herbicide sites during the drying events were significantly lower than the corresponding differences in the control quadrats. During rainfall periods, both initial and maximum values of soil water contents in the herbicided quadrats were higher, and differences between the maximum and initial values did not differ between the herbicided and control quadrats. Our results indicated that the mortality of non-native trees from forest ecosystems increased water content of surface soils, due primarily to a slower rate of decrease in soil water content during dry periods.

Higher Soil Water Availability after Removal of a Dominant, Nonnative Tree (Casuarina equisetifolia Forst.) from a Subtropical Forest

Abstract: The eradication of dominant, nonnative trees can alter soil conditions in forest ecosystems by changing the forests water balance. To test this idea, we measured water in surface soil in forests dominated by an invasive nonnative tree (Casuarina equisetifolia Forst.) on Nishijima Island, a subtropical island in the Ogasawara Islands group, northwestern Pacific Ocean. The volumetric water content of surface soils at sites where all trees of C. equisetifolia were killed by herbicide was compared with adjacent invaded control sites, and effects of time since tree removal (up to 3 yr) were assessed. Dry weights of accumulated litter, total canopy openness of the forest canopy, maximum heights, and total plant cover of herbaceous vegetation were also compared between treatment and control plots. Volumetric soil water content in quadrats where C. equisetifolia trees were removed (removal area) was significantly higher than at control quadrats. We found that the effect of C. equisetifolia removal on soil water content slightly decreased 3 yr after removal. In addition, the dry weights of accumulated litter in removal quadrats decreased with time after removal, and maximum heights and total plant cover increased. We observed higher values of total canopy openness at removal sites regardless of time after removal. These results suggested that the death of these dominant, nonnative trees could increase the available water in soils of invaded forests, and that increases may be related to changes in the water balance of the ecosystem, which can in turn affect restoration of forest ecosystems.

Fine-scale distribution of aboveground biomass of herbaceous vegetation and soil nutrients on an oceanic island after goat eradication are correlated with grazing damage and seabird nesting

We tested whether vegetation recovery on an oceanic island after the eradication of feral goats is related to disturbances by goats and recovery of seabird nesting in an island. To test, we investigated relationships among the aboveground biomass of herbaceous vegetation, nutrient concentrations in the soil after goat eradication, and vegetation degradation before goat eradication and seabird nesting after the eradication. The investigation was conducted on the island of Nakoudojima, a subtropical island situated in the northwestern Pacific Ocean with consideration of fine-scale variation of topography and structure of current vegetation. The aboveground biomass of herbaceous vegetation was correlated with the presence of vegetation degradation before goat eradication but not with seabird nesting after eradication. Concentrations of carbon and nitrogen in the soil were correlated with both the presence of vegetation degradation and seabird nesting. The concentration of available phosphorus in the soil was correlated with nesting seabirds but not with vegetation degradation. The presence of nesting seabirds was correlated with differences in dominant species in the vegetation after goat eradication and with topography. Our results suggested that fine-scale heterogeneities of the aboveground biomass of herbaceous vegetation and the concentrations of nutrients in the soil after goat eradication in an island could be related to vegetation degradation and subsequent erosion of surface soil caused by goats and seabird nesting after the goat eradication.

Seedlings of a native shrub can establish under forests dominated by an alien tree, Casuarina equisetifolia, on subtropical oceanic islands.

We investigated the effects of litter accumulation by an alien tree, Casuarina equisetifolia, on the germination and seedling establishment of Rhaphiolepis wrightiana, a shrub native to the Ogasawara (Bonin) Islands, in the northwestern Pacific Ocean, in a field experiment. We compared the emergence of seedlings in forests dominated by C. equisetifolia with that in native forests, with and without litter removal. More than 75% of seedlings emerged during a year except at the C. equisetifolia sites with litter removal (approximately 45%). Thus, seeds of R. wrightiana can germinate under C. equisetifolia if they are dispersed on the ground.

Establishment of early-stage planted seedlings of a native woody species under a closed canopy of invasive Casuarina equisetifolia in the subtropical oceanic Ogasawara Islands

ABSTRACT Extirpation of invasive plants does not always result in successful restoration of native plant communities because it can alter ecosystem function or promote further incursion of other invasive plants. To test whether seedlings of native plants can become established without extirpating invasive plants, we evaluated the survival and growth of seedlings of a native tree species, Schima mertensiana, planted under closed forest dominated by an invasive species, Casuarina equisetifolia in the Ogasawara Islands, an oceanic archipelago in the western Pacific Ocean. The initial establishment of S. mertensiana is inhibited by the highly accumulated litter of C. equisetifolia; therefore, few seedlings and saplings of S. mertensiana were observed in the forest. We planted seedlings of three initial size classes, and their size parameters (diameter at ground level, height, and number of leaves) were monitored for 69 months. More than 75% of the planted seedlings survived during the study period. There were no significant differences in the number of surviving seedlings among the initial size classes. Steady growth of seedlings was observed regardless of initial size class. Our results indicate that planted S. mertensiana seedlings can survive and grow in forests without removing C. equisetifolia, and provide a case study that shows that planted seedlings of a native woody species can establish under a closed canopy dominated by an invasive woody species, at least for several years.