Recovery of Vegetation Structure and Species Diversity after Shifting Cultivation in Northwestern Vietnam, with Special Reference to Commercially Valuable Tree Species

A fallow stand (FS) in northwestern Vietnam that was created by shifting cultivation 32 years earlier had 43% of the species number, 72% of the stem density, and 53% of the basal area when compared with nearby old-growth forest (OGF); however, the values for commercial species were lower at 35%, 67%, and 26%, respectively. In terms of species diversity, the Shannon index of OGF (3.4) was significantly higher than that of FS (2.6), while the differences were not significant in terms of Evenness and species-size class distribution. Both FS and OGF had similar patterns of stem diameter frequency distribution but the diameters were more diverse in OGF compared to FS according to the Shannon index. Fallow stand was characterized by only 2 canopy layers (lower than 10 m and 10–20 m) and was simpler in vertical structure than that of OGF which included an additional upper canopy layer higher than 20 m. Our results indicate that increasing stem density of commercial species is necessary and can be realized by artificial seeding, planting seedlings, and/or natural regeneration from remaining mother trees in the fields.

Sprouting, succession and tree species diversity in a South African coastal dune forest

Sprouting may play a significant role in maintenance of plant diversity where prevailing disturbance frequency and severity allows. When disturbance frequency and severity decrease, strong sprouters may be outcompeted. As a result, species composition and diversity may change. This study was carried out to investigate the relationship between sprouting, succession and species diversity in a coastal dune forest that currently suffers from low-severity, chronic disturbance due to sea winds and loose sand substrate. Historically, the site was occupied by shifting cultivators who left the site about 80 y ago. Data on trees that were at least 1.3 m tall from 42 sample plots measuring 20 × 20 m were used. The plots were ranked in order of advancement of succession using the first axis of Principal Components Analysis of forest structural variables. Regeneration pattern was examined using analysis of stem diameter frequency distribution. Abundance and regeneration of strong basal sprouters, incidence of basal sprouting and species diversity decreased with advancement of succession. Only a few species could regenerate under the canopy of late-successional sites. Basal sprouts decreased with advancement of succession whereas trunk sprouts increased. These results suggest that maintenance of high species diversity may need a level of disturbance that allows regeneration and maintenance of strong basal sprouters.

Validation of an ovarian biopsy method for monitoring oocyte development in the fat snook, Centropomus parallelus Poey, 1860 in captivity

The validation of an ovarian biopsy method for in vivo assessment of oocyte maturation in Centropomus parallelus was studied. Diameters of intra-ovarian oocytes siphoned with cannula were analyzed fresh and preserved with 1% formalin in 0.7% NaCl solution. Oocytes in different stages were present along the ovaries, up to the tertiary yolk globule stage, which had a unimodal diameter frequency distribution. The oocyte diameter means were not significantly different at four sites along the ovaries (P > 0.05). Samples obtained with cannula were representative of the ovary central portion, in vivo and in vitro samples of the seven females examined were not significantly different (P > 0.05). An estimate of the coefficient of variation corrected for bias (P < 0.05) for 8 repeated in vivo samples was 1.9 ± 0.6. The results demonstrated that for the species, the biopsy method was satisfactory, providing representative samples of the ovaries.

A finite mixture of two Weibull distributions for modeling the diameter distributions of rotated-sigmoid, uneven-aged stands

The rotated-sigmoid form is a characteristic of old-growth, uneven-aged forest stands caused by past disturbances such as cutting, fire, disease, and insect attacks. The diameter frequency distribution of the rotated-sigmoid form is bimodal with the second rounded peak in the midsized classes, rather than a smooth, steeply descending, monotonic curve. In this study a finite mixture of two Weibull distributions is used to describe the diameter distributions of the rotated-sigmoid, uneven-aged forest stands. Four example stands are selected to demonstrate model fitting and comparison. Compared with a single Weibull or negative exponential function, the finite finite mixture model is the only one that fits the diameter distributions well and produces root mean square error at least four times smaller than the other two. The results show that the finite mixture distribution is a better alternative method for modeling the diameter distribution of the rotated-sigmoid, uneven-aged forest stands.

Age and diameter structure of a managed uneven-aged oak forest

We studied the age and diameter structure on one section (259 ha) of a 63 000-ha privately owned forest in the Ozark Highlands of Missouri. The forest has been managed using a partial cutting strategy since 1954. Because a majority of the trees predate current management practices, the existing stand structure is a function of not only the current silvicultural system and the dynamics of this ecosystem but also the initial stand conditions. To determine age structure and evaluate the relationship of diameter and age, a random sample of 600 oaks [Formula: see text]4 cm DBH were collected from ten 0.4-ha plots. Based on the test of a binomial proportion, the oak populations on 7 of the 10 plots were deemed uneven aged, two were deemed two aged, and one was even aged. DBH accounted for 40 (red oaks) to 62% (white oaks) of the variation in tree age. Although the overall diameter frequency distribution of oaks formed a reverse-J shape, the age-frequency distribution approximated a normal (bell-shaped) distribution. We show how this apparent inconsistency between diameter and age distributions can be an artifact of a minimum sampling diameter. Such a truncation of the sampled population reduces the observed frequency of trees in the younger age-classes, which in turn results in a bell-shaped rather than a reverse-J-shaped age-frequency distribution. Thus, the lack of a reverse- J-shaped age distribution should not be interpreted as a failure to sustain regeneration in an uneven-aged stand.