Risto Häkkinen

Development and growth of primordial shoots in Norway spruce buds before visible bud burst in relation to time and temperature in the field.

The timing of bud development in ecodormancy is critical for trees in boreal and temperate regions with seasonally alternating climates. The development of vegetative buds and the growth of primordial shoots (the primordial shoot ratio) in Norway spruce were followed by the naked eye and at stereo and light microscopic levels in fresh-cut and fixed buds obtained by regular field samplings during the spring of 2007, 2008 and 2009. Buds were collected from 15 randomly selected trees (all 16 years old in 2007) of one southern Finnish half-sib family. The air temperature was recorded hourly throughout the observation period. In 2008 and 2009, initial events in the buds, seen as accumulation of lipid droplets in the cortex area, started in mid-March and were depleted in late April, simultaneously with the early development of vascular tissue and primordial needles. In mid-April 2007, however, the development of the buds was at least 10 days ahead as a result of warm spells in March and early April. Variation in the timing of different developmental phases within and among the sample trees was negligible. There was no clear one-to-one correspondence between the externally visible and the internal development of the buds. The dependence of the primordial shoot ratio on different types of temperature sum was studied by means of regression analysis. High coefficients of determination (R(2) ≈ 95%) were attained with several combinations of the starting time (beginning of the year/vernal equinox), the threshold value (from -3 to +5 °C), and the time step (hour/day) used in the temperature summation, i.e., the prediction power of the primordial shoot ratio models turned out to be high, but the parameter estimate values were not unambiguous. According to our results, temperature sums describe the growth of the primordial shoot inside the bud before bud burst. Thus, the results provide a realistic interpretation for the present phenological models of bud development that are based on temperature sums and external observations of bud burst only, and they also provide new tools for improving the models.

Bud burst in Norway spruce (Picea abies): preliminary evidence for age-specific rest patterns

This study examines the effect of chilling and photoperiod on rest completion and bud burst in Norway spruce [Picea abies (L.) Karst.] using twigs from both 15-year-old (“young trees”) and 56-year-old (“old trees”) trees. The material was transferred between September and May from outdoors to experimental forcing conditions with four different photoperiods. The bud burst percentage of the twigs from young trees generally increased in all photoperiods until the end of the year. After that it decreased until vernal equinox (March 20) and then increased steeply towards spring. This new observation of transient rest completion during autumn suggests that young trees have (I) a transient time window during late autumn when ontogenetic development is possible, and (ii) a secondary rest culminating approximately at the time of vernal equinox. In twigs from old trees the transient rest completion was much weaker as the bud burst percentages generally remained under 20 during autumn and winter. At vernal equinox there were no burst buds in twigs from the old trees in any photoperiod and after that the bud burst percentage increased basically in the same manner as in the young trees. The bud burst percentage of the twigs from young trees was generally higher as the photoperiod increased. However, no evidence for absolute long photoperiod requirement of rest completion was observed.

Anatomy and morphology in developing vegetative buds on detached Norway spruce branches in controlled conditions before bud burst

We studied the light and stereomicroscopic structure of developing vegetative buds from a 16-year-old Norway spruce [Picea abies (L.) Karst.] of southern Finnish origin in relation to temperature sum and to externally visible changes in the buds before and during bud burst in forcing conditions. Branches were collected on 17 January and transferred to the greenhouse where they were first subjected to preforcing conditions (darkness, +4 degrees C) for 7 days and then to the forcing conditions (day length 12 h, +20 degrees C). Buds were sampled 20 times between 17 January and 13 February. Air temperature was recorded hourly throughout the study period. The first microscopic change was a temporary increase in the size and number of lipid droplets before the onset of temperature sum (T > or = +5 degrees C) accumulation. From the 4th to the 9th day under the forcing conditions, tracheids started to develop from the base up to the top of the bud. This was closely synchronized with an observed morphological change in the shape of needle tip from rounded to pointed ones. Development from the first visible change in the bud scales on the 12th forcing day to bud burst took 9 days when the temperature sum was 313 d.d. The temperature sums in our experiment overestimated the requirements of temperature sum for bud development phases measured in the field. Bud development could be divided into four structural phases. The first two phases, i.e., morphological changes in the primary needles, occurred without any externally visible changes in the buds. Thus, these phases have a potential for testing and improving the phenological models, which, up to now, have mainly been based on the bud burst observation by the naked eye.

Reliability of temperature signal in various climate indicators from northern Europe

We collected relevant observational and measured annual-resolution time series dealing with climate in northern Europe, focusing in Finland. We analysed these series for the reliability of their temperature signal at annual and seasonal resolutions. Importantly, we analysed all of the indicators within the same statistical framework, which allows for their meaningful comparison. In this framework, we employed a cross-validation procedure designed to reduce the adverse effects of estimation bias that may inflate the reliability of various temperature indicators, especially when several indicators are used in a multiple regression model. In our data sets, timing of phenological observations and ice break-up were connected with spring, tree ring characteristics (width, density, carbon isotopic composition) with summer and ice formation with autumn temperatures. Baltic Sea ice extent and the duration of ice cover in different watercourses were good indicators of winter temperatures. Using combinations of various temperature indicator series resulted in reliable temperature signals for each of the four seasons, as well as a reliable annual temperature signal. The results hence demonstrated that we can obtain reliable temperature information over different seasons, using a careful selection of indicators, combining the results with regression analysis, and by determining the reliability of the obtained indicator.

Processes in Living Structures

Cells are the basic functional units in forest ecosystems. Plants have strong cell wall, formed by cellulose and lignin. Cell membrane isolates the cell from its surroundings, starch acts as storage and enzymes enable synthesis of new compounds. Membrane pumps allow penetration of cell membrane and pigments capture of light energy. We call enzymes, membrane pumps and pigments as functional substances. The biochemical regulation system changes the concentrations and activities of the functional substances: In summer, metabolism is very active, but in winter, vegetation is dormant and tolerates low temperatures. The action of the biochemical regulation system generates emergent regularities in the functional substances, called the state of the functional substances. The effect of environmental factors on metabolism is built in the complex chain of enzymes, membrane pumps and pigments, acting in each metabolic task. The process-specific state of functional substances and the environmental factors determine the rate of each metabolic process. Microbes have dominating role in the soil. Together with soil fauna, microbes break down macromolecules with extracellular enzymes to small molecules that can penetrate the microbial cell membrane through membrane pumps. The microbial metabolism utilises the small carbon-rich molecules for the energy needs, growth and synthesis of the extracellular enzymes.