Wintertime photosynthesis and water uptake in a boreal forest

Warm air in combination with frozen soil is a major cause of wintertime drought damage in evergreen plants in subalpine and boreal environments. We analyzed diurnal tree stem diameter variation (SDV), which reflects soil water uptake, canopy-level water vapor flux (Fw), stand photosynthesis (Ps), photosynthetically active radiation (PAR), soil and air temperatures (Ts and T air, respectively) and soil liquid water content (theta) to determine under what conditions photosynthesis is possible in wintertime and how crucial water uptake from soil is for photosynthesis. Measurements were made under field conditions in a Scots pine forest in southern Finland during winter 2002-2003. We found four wintertime periods when there was measurable Ps and SDV, the latter always starting 2-7 days after photosynthesis and both usually ending on the same day. Stand photosynthesis began when T air reached 3-4 degrees C and ended when T air dropped below -7 degrees C. The trees appeared to rely on stored stem water first and started taking up water from the soil a few days later, when the transpirational demand became strong enough. The more difficult it was to access soil water because of low Ts or low theta, the longer the trees used water stored in their stems. Even partial stem freezing did not prevent photosynthesis or soil water uptake.     

Numbers of autotrophic nitrifiers and nitrification in fertilized forest soil

The most probable number (MPN) method was used to estimate how numbers of autotrophic nitrifiers in Myrtillus-type and Calluna-type pine forest soils in southern Finland were affected by seven different fertilization treatments. No NH⁺₄ oxidizers and only a few hundred NO₂^?1 oxidizers g^?1 of soil were found in unfertilized organic (O) horizons. Ammonium nitrate and nitroform (ureaformaldehyde) had hardly any effect on the nitrifiers. Urea, alone or applied together with apatite + biotite or with apatite + biotite + micronutrients, increased numbers of NH₄⁺ and NO₂^? oxidizers. Wood ash, alone or with apatite, also had a stimulative effect. The effects of the stimulative fertilizers were less in the A₂ horizon than in the O horizon. The MPN counts were considerably affected by the duration of incubation: counts of NH₄⁺ oxidizers kept increasing for at least 8 weeks and counts of NO₂^? oxidizers for at least 15 weeks. These MPN counts were compared with earlier results from incubation experiments on the same soils to find out how they reflect changes in soil nitrification after fertilization.     

The time series of flowering and leaf bud burst of boreal trees (1846–2005) support the direct temperature observations of climatic warming

The timing of spring phenological events of boreal trees, such as flowering and leaf bud burst, is controlled to a great extent by the ambient air temperature, and these events are already showing an advancement that can be attributed to climatic warming. In this paper we utilised this phenomenon to verify direct observations of climatic warming. We constructed eight phenological time series using observations covering 160 years of the leaf bud burst of two species and of the flowering of six species of native deciduous trees growing in Finland. To eliminate the effect of urbanization on local temperature, we rejected the observations made in densely populated areas. When analyzing the trends in the timing of leaf bud burst and flowering we found an advancement of these phenological events ranging from 3.3 to 11.0 days per century. We then converted the observed phenological trends into temperature trends by using thermal time-type models that link the timing of bud burst and flowering to the temperature conditions during bud development. Warming estimates derived from the phenological trends suggested that the mean spring temperature increase in Finland has been 1.8 °C per century, which is very close to the value of 1.5 °C per century indicated by long-term temperature records.     

Effects of dormancy and environmental factors on timing of bud burst in Betula pendula

We tested three theories predicting the timing of bud burst in mature birch (Betula pendula Roth) trees utilizing a 60-year phenological time series together with meteorological temperature observations. Predictions of the timing of bud burst based on light conditions in addition to temperature were more accurate than predictions based on dormancy development and temperature (prediction standard error of 2.4 days versus 4.3 days). The signal from light conditions, represented by fixed calendar date, determined the start of bud ontogenesis rather than dormancy release. We suggest that models developed to predict the timing of bud burst be utilized in the analysis of plant responses to climate change and of climate change itself.     

Effects of Natural Chelating Agents on the Solubility of Some Physiologically Important Mineral Elements in Oat Bran and Oat Flakes

The solubility of mineral elements from oat bran and flake samples was studied by a method using equilibrium dialysis after enzymatic digestion of starch and proteins. The effects of six potential chelating agents common in food were tested on the solubility of mineral elements. The minerals studied were calcium, magnesium, iron, manganese, zinc and potassium, and the chelating agents were citric, lactic, malic, and ascorbic acids, glucose and xylitol. The mineral elements were tightly bound to the dietary fiber of the samples. Bran fiber bound even the zinc and calcium contributed through the enzymes used. Adding citric, malic, or lactic acids increased the solubility of the mineral elements studied, except for potassium which was easily dialyzable as such. Iron was insoluble in all situations. Citric acid was the most efficient chelating agent in solubilizing the mineral elements. The effect of malic and lactic acids on the solubility of minerals was small. No effect was found with glucose, ascorbic acid, and xylitol. Thus, the intestinal availability of mineral elements may be affected by dietary hydroxy acids such as citric and malic acids in high dietary fiber diets.