Artur Jankowski , Tomasz P. Wyka, Roma Żytkowiak, Bengt Nihlgård, Peter B. Reich, Jacek Oleksyn
Short growing season, low temperature and nutrient shortages have caused many plants in subarctic regions to adapt a slow, conservative life style. For example, few subarctic woody plants can afford to replace their leaves annually as is common in the temperate zone. Moreover, several conifers whose distribution extends across temperate and subarctic zones show a spectacular extension of longevity of their needles at northerly sites. For example, while in Central Europe individual needles of Scots pine are typically maintained on a tree for 2-3 years, in Northern Scandinavia they may last for 7 years or longer. We supposed that such increase in needle longevity requires modifications of internal needle structure that will facilitate survival under recurring stressful conditions.
We sampled needles of Scots pine along a nearly 2000-km line from Poland to Northern Sweden, representing a range of mean and extreme temperatures. Through microscopic examination of needle tissues we found that long living needles from the north, although quite short, are more densely packed with building materials and have thicker and more sturdily built outer protective cells, compared to needles form the south. Such characteristics are suggestive of greater mechanical durability of northern needles. Secondly, needles of northern pines contain narrower water-conducting cells, a feature that should make them more resistant to blockage by air bubbles. Since air blockages are caused by frost, such adaptation was to be expected in needles that may be exposed to temperatures below -40°C. Thirdly, needles from the north have especially large resin ducts. Resin ducts are structures containing a mix of terpenoid chemicals that make needles unpalatable to a variety of herbivores. Such enhanced protective mechanisms may be an asset under growth limiting conditions, where needles cannot be quickly replaced.
Our study shows that within a single tree species one may observe a gradual transition in life style and leaf structure: from short-lived, relatively delicate needles at warm, productive sites in the temperate zone to tougher, well protected and resistant needles at the northern reaches of its distribution range. Such variability is a product of past evolutionary processes and provides an insight into what it takes to colonize cold environments.
Image caption: Collecting pine needles for anatomical study, Northern Sweden (photo taken by T. Wyka)