For plants, the sensing of light in the environment is as important as vision is for animals. Fluctuations in light can be crucial to competition and survival. One way plants sense light is through the phytochromes, a small family of diverse photochromic protein photoreceptors whose origins have been traced to the photosynthetic prokaryotes. During their evolution, the phytochromes have acquired sophisticated mechanisms to monitor light. Recent advances in understanding the molecular mechanisms of phytochromes and their significance to evolutionary biology make possible an interim synthesis of this rapidly advancing branch of photobiology.

Phytochromes and light signal perception by plants—an emerging synthesis

Forest trees display a perennial growth behavior characterized by a multiple-year delay in flowering and, in temperate regions, an annual cycling between growth and dormancy. We show here that the CO/FT regulatory module, which controls flowering time in response to variations in daylength in annual plants, controls flowering in aspen trees. Unexpectedly, however, it also controls the short-day–induced growth cessation and bud set occurring in the fall. This regulatory mechanism can explain the ecogenetic variation in a highly adaptive trait: the critical daylength for growth cessation displayed by aspen trees sampled across a latitudinal gradient spanning northern Europe.

https://science.sciencemag.org/content/sci/early/2006/05/04/science.1126038.full.pdf

CO/FT Regulatory Module Controls Timing of Flowering and Seasonal Growth Cessation in Trees

Improved monitoring of vegetation dynamics at very high latitudes: A new method using MODIS NDVI

Evolutionary responses are required for tree populations to be able to track climate change. Results of 250 years of common garden experiments show that most forest trees have evolved local adaptation, as evidenced by the adaptive differentiation of populations in quantitative traits, reflecting environmental conditions of population origins. On the basis of the patterns of quantitative variation for 19 adaptation-related traits studied in 59 tree species (mostly temperate and boreal species from the Northern hemisphere), we found that genetic differentiation between populations and clinal variation along environmental gradients were very common (respectively, 90% and 78% of cases). Thus, responding to climate change will likely require that the quantitative traits of populations again match their environments. We examine what kind of information is needed for evaluating the potential to respond, and what information is already available. We review the genetic models related to selection responses, and what is known currently about the genetic basis of the traits. We address special problems to be found at the range margins, and highlight the need for more modeling to understand specific issues at southern and northern margins. We need new common garden experiments for less known species. For extensively studied species, new experiments are needed outside the current ranges. Improving genomic information will allow better prediction of responses. Competitive and other interactions within species and interactions between species deserve more consideration. Despite the long generation times, the strong background in quantitative genetics and growing genomic resources make forest trees useful species for climate change research. The greatest adaptive response is expected when populations are large, have high genetic variability, selection is strong, and there is ecological opportunity for establishment of better adapted genotypes.

/pdf/potential-for-evolutionary-responses-to-climate-change-2jb03f187c.pdf

Potential for evolutionary responses to climate change – evidence from tree populations

1. High elevation treelines 1.1 The task 1.2 Previous works 2. Definitions and conventions 2.1 The life form 'tree' 2.2 Lines and transitions 2.3 Limitation, stress and disturbance 2.4 Altitude-related and other environmental drivers 2.5 Treeline nomenclature 3. Treeline patterns 3.1 Treeline taxa 3.2 The summit syndrome and other treeline depressions 3.3 Mass elevation effect 3.4 Treeline elevation 3.5 Time matters 3.6 Forest structure near treeline 4. Treeline climate 4.1 Specific aspects of treeline climatology 4.2 Criteria to define temperature regimes at treeline 4.3 Treeline temperatures in different bioclimatic regions 4.4 Seedbed and branch temperatures 4.5 Whole forest temperatures 5. Global mountain statistics based on treeline elevation 5.1 Mountain geostatistics 5.2 Elevational belts 5.3 Global treeline ecotones 6. Structure and stature of treeline trees 6.1 Foliage properties 6.2 Wood properties 6.3 Bark properties 6.4 Root traits 6.5 Tree stature 6.6 Dry matter allocation in treeline trees 7. Growth and development 7.1 Tree growth near treeline 7.2 Xylogenesis at treeline 7.3 Apical growth dynamics 7.4 Root growth 7.5 Phenology at treeline 8. Evolutionary adjustments to life at treeline 8.1 Phylogenetic selection 8.2 Genotypic responses of growth and development 8.3 Genotypic responses of physiological traits 9. Reproduction, early life stages and tree demography 9.1 Amount and quality of seeds at high elevation 9.2 Germination, seedling and sapling stage 9.3 Tree demography at treeline 10. Freezing and other forms of stress 10.1 Stress at treeline in a fitness context 10.2 Mechanisms and principles of freezing resistance 10.3 Freezing resistance in treeline trees 10.4 Other forms of stress at treeline 11. Water, nutrient and carbon relations 11.1 Tree water relations during the growing season 11.2 Nutrient relations 11.3 Carbon relations 12. Treeline formation - currently, in the past and in the future 12.1 Causes of current treelines 12.2 Treelines in the recent past 12.3. Treelines in the distant past (Holocene) 12.4 Future treelines References Taxonomic index Subject index

Alpine treelines : functional ecology of the global high elevation tree limits

Survival of temperate-zone tree species under the normal summer-winter cycle is dependent on proper timing of apical growth cessation and cold acclimatization. This timing is primarily based on the perception of daylength, and through evolution many tree species have developed photoperiodic ecotypes which are closely adapted to the local light conditions. The longest photoperiod inducing growth cessation, the critical photoperiod, is inherited as a quantitative character. The phytochrome pigment family is the probable receptor of daylength, but the exact role of phytochrome and the physiological basis for the different responses between photoperiodic ecotypes are not known. This report shows for the first time that over-expression of the oat phytochrome A gene (PHYA) in a tree significantly changes the critical daylength and effectively prevents cold acclimatization. While the critical daylength for elongation growth in the wild-type of hybrid aspen (Populus tremula × tremuloides) was approximately 15 h, transgenic lines with a strong expression of the oat PHYA gene did not stop growing even under a photoperiod of 6 h. Quantitative analysis of gibberellins (GA) as well as indole-3-acetic acid (IAA) revealed that levels of these were not down-regulated under short days in the transgenic plants expressing high levels of oat PHYA, as in the wild-type. These results indicate that photoperiodic responses in trees might be regulated by the amount of PHYA gene expressed in the plants, and that the amount of phytochrome A (phyA) affects the metabolism of GAs and IAA.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313x.1997.12061339.x

Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization

Detailed knowledge of temperature effects on the timing of dormancy development and bud burst will help evaluate the impacts of climate change on forest trees. We tested the effects of temperature applied during short-day treatment, duration of short-day treatment, duration of chilling and light regime applied during forcing on the timing of bud burst in 1- and 2-year-old seedlings of nine provenances of Norway spruce (Picea abies (L.) Karst.). High temperature during dormancy induction, little or no chilling and low temperature during forcing all delayed dormancy release but did not prevent bud burst or growth onset provided the seedlings were forced under long-day conditions. Without chilling, bud burst occurred in about 20% of seedlings kept in short days at 12 degrees C, indicating that young Norway spruce seedlings do not exhibit true bud dormancy. Chilling hastened bud burst and removed the long photoperiod requirement, but the effect of high temperature applied during dormancy induction was observed even after prolonged chilling. Extension of the short-day treatment from 4 to 8 or 12 weeks hastened bud burst. The effect of treatments applied during dormancy development was larger than that of provenance; in some cases no provenance effect was detected, but in 1-year-old seedlings, time to bud burst decreased linearly with increasing latitude of origin. Differences among provenances were complicated by different responses of some origins to light conditions under long-day forcing. In conclusion, timing of bud burst in Norway spruce seedlings is significantly affected by temperature during bud set, and these effects are modified by chilling and environmental conditions during forcing.

/pdf/climatic-control-of-bud-burst-in-young-seedlings-of-nine-128rnnsfn9.pdf

Climatic control of bud burst in young seedlings of nine provenances of Norway spruce.

The purpose of this study was to determine the influence of temperature applied during short day-induced budset on induction of dormancy in six ecotypes of Betula pubescens Ehrh. and two ecotypes of Betula pendula Roth. Seedlings were grown in a phytotron at constant temperatures of 9–21°C under a 12 h photoperiod (SD) during dormancy induction. Induction of dormancy was monitored by following bud flushing and shoot growth after transfer to long photoperiod conditions (24 h) at 18°C. Chilling requirement was studied in seedlings exposed to 10 weeks of SD. In both species induction of bud dormancy developed most rapidly at 15–18°C, and both 9–12°C and 21°C delayed the induction of dormancy. Raising the temperature (from 9 to 21°C) applied during induction of dormancy significantly increased the chilling requirement. These responses were noted for all ecotypes tested, but in general the northern ecotypes entered dormancy more quickly than the southern ones. No such trend was recorded for chilling requiremen...

Effect of Temperature on the Induction of Bud Dormancy in Ecotypes of Betula pubescens and Betula pendula

Summary
• The aim was to elucidate the effects of elevated winter temperatures on the dehardening process of mountain birch (Betula pubescens ssp. czerepanovii) ecotypes and to evaluate their susceptibility to frost damage under warming climate conditions.
• Ecotypes from 60 to 71° N latitudes and 20–750 m altitudes were grown in northern Norway (70° N) and subjected to simulation of the photoperiod in southern Norway (60° N) by artificial illumination from September onwards. In November, the seedlings were transported to the south (60° N) to overwinter at ambient or 4°C above ambient temperatures. Frost hardiness and lipid peroxidation were determined during January–April.
• The higher winter temperature accelerated dehardening, and there were significant differences between the ecotypes. Among tree individuals of southern origin, the alpine ecotype exhibited the most rapid rate of dehardening, whereas the oceanic type showed the slowest rate. Lipid peroxidation supported the above findings.
• Since temperature elevation was unequal for the ecotypes with respect to climatic change, the frost hardiness results were normalized to obtain an equal +4°C temperature rise. The risk of frost injury seemed to be lowest in the northernmost ecotypes under a temperature elevation of +4°C, obviously due to their adaptation to a wider temperature range.

Dehardening of mountain birch (Betula pubescens ssp. czerepanovii) ecotypes at elevated winter temperatures

Temperature and light are the two main environmental factors influencing plant growth and development. Most basic metabolic reactions in plants are strictly dependent on temperature. Light is the ultimate source of energy for plants and, in addition, the morpho-genic development of plants is strongly affected by light conditions. For trees growing at the northern, marginal areas of their range, temperature is the main limiting factor. Low temperature during the growing season affects metabolism, photosynthesis, and respiration. Studies with herbaceous plant species strongly suggest that resource acquisition is generally not a limiting factor for plants growing under arctic and alpine conditions. In their review on plant growth under cold conditions, Korner and Larcher (1988) concluded that developmental processes, especially mitosis, play a significant role in adaptation to low temperature.

Jarle Nilsen

Papers

Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization

Climatic control of bud burst in young seedlings of nine provenances of Norway spruce.

Effect of Temperature on the Induction of Bud Dormancy in Ecotypes of Betula pubescens and Betula pendula

Dehardening of mountain birch (Betula pubescens ssp. czerepanovii) ecotypes at elevated winter temperatures

Growth and Development of Northern Forest Trees as Affected by Temperature and Light