Current issue: 54(1)
Under compilation: 54(2)
The investigation is divided into statistical and experimental sections, the latter of which were conducted in a Vaccinium type Scots pine (Pinus sylvestris L.) stand with the aim of elucidating the interdependence of soil humidity and meteorological factors. The moisture content of pine needles and moss and wooden cylinders placed on the soil or slightly over the ground was determined by weighing. The results showed that there is correlation between the moisture content of the wooden cylinders and the relative humidity of air during the days without rain. Correlation between moisture content of pine needles and moss with the air was slightly poorer.
In the statistical section, based on meteorological observations made in the geophysical observatory at Sodankylä in Northern Finland in 1920-1943, and forest fire statistics of the area, forest fire days and days without forest fires were divided into 40 temperature-humidity groups. Of the 391 forest fires observed in the area, the cause of the fire was known in 353 cases, and 69% of these were caused by lightning. A forest fire danger index was calculated using the data. A sharp increase in the burned area when the index exceeded the limit k=0.3 seem to be explained by thunderstorms, which are the most notable cause of forest fires in the Sodankylä area.
The forest fire index was calculated also at four meteorological stations in different parts of the country using weather observations in 1927-1936. It appears that the number of days in which the k>0.1 decreases when proceeding northwards, obviously because of the shortening of the summer. On the other hand, the number of days in which k>0.3 increases towards the north.
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Germlings and small tree seedlings are exposed to extreme conditions in the forest floor. In this study the influence of climatic factors to seeds and seedlings were studied experimentally, and an attempt was made to estimate the importance of various factors in several sowing experiments in Finland.
Seeds of Scots pine (Pinus sylvestris L.) were subjected to temperature variations which simulated those of exposed forest sites. The seeds lost some of their germinative capacity during the five-day treatments. Succulent seedlings died when subjected to immersion for 15 minutes at temperatures from 51.5 to 55 ºC. After a hardening pretreatments the seedlings tolerated 2-3 ºC higher temperatures. In artificial humus soil exposed to strong insolation for 15 minutes, temperatures in the range of 54-65 ºC proved to be critical for the seedlings. In natural conditions, also little lower temperatures may prove fatal. Exposure of succulent seedlings of Scots pine and Norway spruce (Picea abies (L.) Karst.) to insolation showed that most damage occurred on humus, quartz sand, and humus-sand mixture, due to rapid evaporation. Seeds of Scots pine, Norway spruce, Betula pendula and Betula pubescens tolerated poorly drought if germination had progressed to a 5–10 mm long radicle. Succulent seedlings tolerated 53-77 days long drought better in humus than in fine silty sand. Seedlings of Pinus sylvestris, Picea abies, Alnus incana and A. glutinosa tolerated cold variably. The developmental stage of the seedling affected cold resistance. Pine seeds sown in furrows germinated well after rain and the survival was high. Frost heaving, snail and insects caused some damages. Germination was lowest at the shallowest furrows. Sowing on natural surfaces gave poor results. Largest damages were caused by birds and ants.
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The model HYDRA, which simulates water flow in the branched tree architecture, is characterized. Empirical studies of the last decades give strong evidence for a close structure-function linkage in the case of tree water flow. Like stomatal regulation, spatial patterns of leaf specific conductivity can be regarded as a strategy counteracting conductivity losses, which may arise under drought. Branching-oriented water flow simulation may help to understand how damaging and compensating mechanisms interact within the hydraulic network of trees. Furthermore, a coupling of hydraulic to morphological modelling is a prerequisite if water flow shall be linked to other processes. Basic assumptions of the tree water flow model HYDRA are mass conservation, Darcy's law and the spatial homogeneity of capacitance and axial conductivity. Soil water potential is given as a one-sided border condition. Water flow is driven by transpiration. For unbranched regions these principles are condensed to a nonlinear diffusion equation, which serves as a continuous reference for the discrete method tailored to the specific features of the hydraulic network. The mathematical derivation and model tests indicate that the realization of the basic assumptions is reproducible and sufficiently exact. Moreover, structure and function are coupled in a flexible and computationally efficient manner. Thus, HYDRA may serve as a tool for the comparative study of different tree architectures in terms of hydraulic function.
The aim of the study was to compare the behaviour of three selected provenances of Eucalyptus microtheca F. Muell. that were likely to respond differently to drought. For this purpose, we studied the effects of vapour pressure deficit and soil water content on leaf water potential in an irrigated plantation in Bura, eastern Kenya.
An international provenance trial of Eucalyptus microtheca, established as a part of Finnida-supported Bura Forestry Research Project in eastern Kenya in 1984 was used as a plant material in the study. The eastern provenance showed generally the lowest leaf water potential on a daily basis. Statistically significant differences in the daily leaf water potential fluctuations were detected. The eastern provenance exhibited the greatest and the northern one the smallest values. The minimum daily leaf water potential of the provenances responded well to changes in gravimetric soil water content, the western provenance being the most sensitive one. The relationship of the observed results and annual rainfall distribution in the geographic regions of the studied provenances is discussed.
The effect of root exposure on the shoot and root development of Pinus sylvestris (L.) seedlings was studied at two soil temperatures. Roots of bare-rooted three-year-old seedlings were exposed to the temperature of 32°C at relative humidity of 50–40% for 85, 155 and 270 minutes which corresponds to accumulated water pressure deficit of 24, 47 and 91 mbar·h, respectively. Thereafter, seedlings were grown for 65 days at the soil temperatures of 12 and 23°C. Drought exposures inhibited new root initiation, delayed shoot elongation, and reduced shoot and needle growth. The stronger the exposure the larger the proportion of needles from the lower part of current shoot that remained undeveloped. Low soil temperature increased the effect of exposures so that needle elongation and initiation of new root tips of seedlings in cold soil with the longest exposure were inhibited totally. Root growth assessments made in warm soil may overestimate the acclimation potential of planted seedlings.
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The interactive effects of water stress and temperature on the CO2 response of photosynthesis was studied in Salix sp. cv. Aquatica using the closed IRGA system. A semi-empirical model was used to describe the CO2 response of photosynthesis. The interactive effect of water stress and temperature was divided into two components: the change in CO2 conductance and the change in the photosynthetic capacity. The CO2 conductance was not dependent on the temperature when the willow plant was well watered, but during water stress it decreased as the temperature increased. The photosynthetic capacity of the willow plant increased along with an increase in temperature when well-watered, but during water stress temperature had quite opposite effect.
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In the experiment Scots pine (Pinus sylvestris L.) seedlings were transplanted out in the field. The effect of the treatments on gas metabolism and daily height increment were examined. The seedlings were 5-year old Scots pine plants growing in clay pots, covered with plastic bags. Transpiration and photosynthetic rates were monitored with open IRGA measuring system for a few days before being subjected to the treatments and for one month after. In addition, the daily amounts of transpired water and daily height increments were measured. A model for the potential rate of each metabolic process was constructed.
Planting and additional exposure had a strong and rather permanent effect on the self-regulation of the processes. This effect is very similar to that caused by water deficit. Exposure makes the disturbance more pronounced. Transpiration of the transplanted seedlings decreased in a few days after planting to less than half of the potential value and that of the exposed ones decreased to a quarter of the potential value. The daily amounts of photosynthesis decreased to half of the potential value. There was no recovery in photosynthesis during the whole monitoring period of four weeks. There was a slight recovery in transpiration about five weeks after transplanting.
Thus, the treatment probably generated stress conditions throughout the whole growing period, which is characterized by strong self-regulation of photosynthesis and transpiration, thus causing an essential decrease in the total amount of CO2 fixed. The photosynthesis was depressed especially at elevated temperatures after planting, as during water deficit. Planting and additional exposure did not produce any detectable changes in the dependence of the growth rate on temperature or in the effect of self-regulation on height growth. On the other hand, the level of growth was decreased as a result of planting out.
The PDF includes a summary in Finnish.