Dinâmica da composição botânica numa pastagem natural sob efeito de diferentes ofertas de forragem / Dynamic botanical composition in rangeland under forage allowances

A intensidade de pastejo e suas alterações ao longo do tempo podem afetar a composição botânica e a estrutura do pasto. Este trabalho buscou avaliar o efeito de distintas ofertas de forragem (OF) sobre a dinâmica populacional de espécies de uma pastagem natural dos Campos Sulinos. As avaliações da composição botânica foram realizadas em 12 de janeiro e 17 de julho através do método BOTANAL. Os tratamentos constaram de diferentes OF (kg MS 100kg-1 de peso vivo dia-1): 8, 12 e 16 durante todo ano e diferentes combinações de sequências: 8 na primavera e 12 no resto do ano (8-12); 12 na primavera e 8 no resto do ano (12-8); 16 na primavera e 12 no resto do ano (16-12). O delineamento utilizado foi o de blocos casualizados com duas repetições. A oferta de forragem provocou alterações na composição botânica da vegetação. A manipulação da OF na primavera alterou a composição botânica da vegetação e apresentou maior impacto nas menores OF.

Grazing intensity manipulation can affect structure and botanical composition of the pasture. The objective of this research was to evaluate the effects of different forage allowances and different seasonal sequences of forage allowance (FA) on the species population dynamics in the rangelands of Campos Sulinos. The botanical composition evaluations were made in January 12th and July 17th, using BOTANAL method. Treatments were of different FA (kg DM 100kg-1 of live weight day-1): 8, 12, 16 all around the year; and different sequences of FA: 8 on spring and 12 at the rest of year (8-12); 12 on spring and 8 at the rest of year (12-8) and 16 on spring and 12 at the rest of year (16-12). The statistical design used was randomized blocks with two replications. The forage allowance led to changes in botanical and structural composition of vegetation. It can be inferred that the forage allowance variation on the vegetation had higher impact on smaller forage allowances.

Demographic study of two population outbreaks of Elasmoderus wagenknechti (Liebermann) (Orthoptera: Tristiridae) in the transitional desert of Chile

Under certain environmental conditions, several species of grasshoppers inhabiting rangeland areas (e.g., inter-valleys) of the Chilean transitional desert can irrupt demographically. One of them is Elasmoderus wagenknechti (Liebermann), an endemic species. We studied two outbreaks occurred in 1996 and 1999. The objectives of the research were to (i) estimate some demographic parameters associated with the aforementioned events, and (ii) compare between them the population parameters. The parameters we studied were density (ind/m²), sexual proportion, female fertility (i.e., eggs/female), and the relationship eggs/female versus female size. Density (ind/m²) ranged from 0.2 to 0.4 in 1996 and from 0.5 to 0.6 in 1999. ANOVA-tests registered differences in density among study sites and between years. In some of the study sites, females were more abundant than males (e.g., Lomas del Sauce). Females fertility ranged from 27 to 39 eggs per female, with significant differences among sites in 1996 but not in 1999. For females of 30-50 mm body size, a linear relationship was detected between egg number/female and body size. It is concluded that the E. wagenknechti outbreaks are site-specific in the study area and that the demographic parameters examined show a high variability among sites and between events.

Sob certas condições ambientais, diversas espécies de gafanhotos que habitam áreas de campos, como as áreas entre vales do deserto transicional do Chile, podem experimentar surtos populacionais. Uma dessas espécies é Elasmoderus wagenknechti (Liebermann), espécie endêmica. Nós estudamos dois surtos que ocorreram em 1996 e 1999. Os objetivos da pesquisa foram (1) estimar alguns parâmetros demográficos associados aos eventos acima indicados, e (2) comparar os parâmetros entre os eventos. Os parâmetros estudados foram densidade (indivíduos/m²), razão sexual, fertilidade da fêmea (i.e., ovos/fêmea), e a relação entre a fertilidade e o tamanho corporal da fêmea. O intervalo da densidade do evento foi estimado entre 0,2 e 0,4 indivíduos/m² em 1996, e entre 0,5 e 0,6 indivíduos /m² em 1999. Foram encontradas diferenças (análise de variância) na densidade entre os locais para o mesmo evento, e entre os eventos. As fêmeas foram mais numerosas que os machos em alguns locais estudados (e.g., Lomas del Sauce). Foi estimado um intervalo médio de 27-39 ovos/fêmea, existindo diferenças entre os locais no evento de 1996, mas não no evento de 1999. Para fêmeas de longitude corporal de 30-35 mm. foi detectada uma relação linear entre número de ovos/fêmea e tamanho corporal. Em conclusão, a magnitude do evento na área de estudo é específica do local; além do que para os parâmetros demográficos examinados existe uma acentuada variabilidade entre locais e entre os eventos.

Seed weight variation of wyoming sagebrush in Northern Nevada

Seed size is a crucial plant trait that may potentially affect not only immediate seedling success but also the subsequent generation. We examined variation in seed weight of Wyoming sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young), an excellent candidate species for rangeland restoration. The working hypothesis was that a major fraction of spatial and temporal variability in seed size (weight) of Wyoming sagebrush could be explained by variations in mean monthly temperatures and precipitation. Seed collection was conducted at Battle Mountain and Eden Valley sites in northern Nevada, USA, during November of 2002 and 2003. Frequency distributions of seed weight varied from leptokurtic to platykurtic, and from symmetry to skewness to the right for both sites and years. Mean seed weight varied by a factor of 1.4 between locations and years. Mean seed weight was greater (P<0.05) in 2003 than in 2002 at both sites. This can partially be attributed to 55% greater precipitation in 2003 than 2002, since mean monthly temperatures were similar (P>0.05) in all study situations. Simple linear regression showed that monthly precipitation (March to November) explained 85% of the total variation in mean seed weight ( P=0.079). Since the relationship between mean monthly temperature (June-November) and mean seed weight was not significant (r²=0.00, P=0.431), this emphasizes the importance of precipitation as an important determinant of mean seed weight. Our results suggest that the precipitation regime to which the mother plant is exposed can have a significant effect on sizes of seeds produced. Hence, seasonal changes in water availability would tend to alter size distributions of produced offspring.