The analysis of short-term forecast models for oak forest damage by gypsy moth (Lymantria dispar L.).

Проведена предварительная верификация четырех моделей краткосрочного прогнозирования дефолиации дубрав непарным шелкопрядом путем их сравнительного анализа. Модели характеризуют два подхода в определении будущей дефолиации по числу зимующих яиц насекомого: а) оценка кормовой нормы (количества зеленой массы, уничтоженной средней гусеницей в условиях нормальной смертности); б) использование соотношения между потерями листвы и величиной плотности популяции. Эффективность прогнозирования зависит от точности определения количества зеленой массы дерева и насаждения и численности питающихся гусениц (варьирования смертности насекомых). В моделях использовались разные элементарные единицы учета для оценки плотности популяции (деревья разного возраста или стволы разного диаметра, побег текущего года, 100 г листвы). По установленным соотношениям между этими единицами модели были пересчитаны и приведены к единой шкале. В качестве предиктора использован возраст насаждений. Прогнозные оценки, полученные по разным моделям, относительно близки в молодняках и средневозрастных древостоях. При увеличении возраста растет и варьирование оценок. Необходимы дальнейшая верификация моделей и их корректировка, так как для производственного прогнозирования они достаточно надежны только при возрасте насаждений до 60 лет. Вероятно, требуется в большей степени учитывать и региональные различия в средней массе листьев дуба. Preliminary verification of 4 short-term forecast models with their comparative analysis is presented. The models specify two approaches to identify future defoliation by insects that hibernate as eggs: a) feeding rate assessment (foliage mass consumed by an average caterpillar in normal mortality conditions); b) application of a ratio between foliage loss and population density. Forecast efficiency depends on accuracy of tree and whole stand foliage mass assessment and feeding caterpillar population (insect mortality variation). Various units were applied in population density assessment (trees of various age or diameter, current year shoot, 100 g of foliage). The models were recalculated and reduced to a unified scale based on found ratios between these units. Forest age was used as a predictor. The forecast assessments derived in various models are relatively close for young and mid-aged stands. Variation of the assessments grows with stand age increase. Further verification and adjustment of the models is necessary since they are only reliable in forecast for stands younger than 60 years. Probably regional difference in oak foliage mean phytomass should be taken more into consideration.

A Comparison of Three Dry Matter Forage Production Methods Used in South Africa

A common method for determining forage production of rangelands is by clipping and weighing forage from quadrats with predetermined areas. This technique is however time consuming. Other techniques which require less time and labour include amongst others using the disk pasture meter or phytomass derived from the vegetation classification program PHYTOTAB, in conjunction with the Plant Number Scale, which is used to determine vegetation canopy cover. The phytomass determined using PHYTOTAB/Plant Number Scale and the disk pasture meter was compared to the phytomass obtained from the actual clipping and weighing of forage. Tests showed that there were indeed statistically significant differences between the mean phytomass values of the three techniques. Considerable variation was shown in the results of the disk pasture meter readings compared to the other two techniques. The phytomass values obtained using the disk pasture meter were significantly higher than the phytomass determined using both the PHYTOTAB/Plant Number Scale and the clipping and weighing techniques. Results further indicated a significant similarity in the phytomass determined using the PHYTOTAB/Plant Number Scale and the clipping and weighing technique. The results of this pilot study need further investigation.

Effects of rainfall and grazing on vegetation yield and cover of two arid rangelands in Kuwait

Increasing human pressure has presumably led to a decrease in the cover and herbage yield of Kuwaiti desert vegetation, but, to date, there has been little detailed study on such human impacts. A study of Rhanterium epapposum (local name arfaj) and Haloxylon salicornicum (local name remeth) steppe was therefore effected to determine the seasonal variation in above-ground phytomass and percentage cover, and to investigate differences between protected and adjacent grazed areas.An average seasonal precipitation of 90 mm supported a mean of 223 kg ha-1 in arfaj steppe in 1979–1989, whereas an average mean seasonal precipitation of 73 mm during 1983–1989 maintained a mean phytomass of 102 kg ha-1 in the remeth steppe. Annual forbs and perennial shrubs were the greatest producers of dry matter per kg of phytomass in the arfaj and remeth steppes, respectively. The seasonal production of dry matter was related directly to the seasonal precipitation in the arfaj steppe, whereas the remeth steppe did not show an obvious relationship to the precipitation. The plant cover was 83% and 70% less, and herbage production was 76% and 91% less in grazed areas than in protected areas in the arfaj and remeth steppes, respectively.