View from the top
Around 50 English oaks (Quercus robur) that are nearing 40 years of age are gathered here in this slope. Some acorns were collected in Stockholm 1975 and subsequently raised in Umeå when the arboretum was just started. Due to the difference in climate but above all else, damage sustained by grazing animals and competition from nearby trees have stunted their growth considerably. This have resulted in low growing, bushlike oaks with crooked forms for the majority of the trees here. In Sweden we have a word for these places were oaks develop into shrubs called “ekkrattskog”. This usually occurs in the Southern parts of the country where the oaks still regenerate naturally (they need some help up here) on meager soils. Thankfully some of the oaks have developed into trees and after some thinning done during the last years they are starting to grow well now.
The oaks (Quercus) is a huge genus with some 500 species, still it is only half of the worlds largest tree genus (Syzigium) which has over 1000 species. The majority of the oaks have their home in areas with distinct dry seasons and frequent wildfires, what we call a Mediterranean climate, so most of the oaks in Europe (around 14) are situated around the medditerranean sea. However there are two exceptions; the previoulsy mentioned English oak (Quercus robur) and the sessile oak (Quercus petraea), which both occur naturally growing in the south of Sweden.
The biggest oaktree in Umeå is the so called “residenseken” which was planted in 1932 by “governor” Gustaf Rosén near the city centre. We have estimated the age of the plants, by looking at old phtographs, to 10-15 years old at the time of planting. This would make this oaktree close to 100 years of age today. During this time the tree has reached a height of around 20m and has a DBH (diameter at brest height) of 90cm (almost 3m in circumference). The growth rate is certainly nothing to be ashamed of, even if you compare it to the conditions in the south of Sweden. If the tree is left alone for another 1000 years maybe it can reach the same size as “Kvilleken” which is situated in Småland in the south of Sweden. At around 1000 years old, this is both Swedens biggest and oldest tree (with the same stem), in fact this is the largest English oak (Quercus robur) in the world, with a cirumference exceeding 15m.
Our largest specimen of scots pine (Pinus sylvestris) also grows on this slope. This is a interesting tree not only because of it’s size and large branches but it also have what we call a fire scar. These are wounds from past wildfires in the area. Most pines exhibit adaptations which makes it possible for them to cope with regulary occuring wildfires and our native pine is no exception. Thick bark that protects the cambium (the main growth layer) from lethal temperatures is one of those adaptations. Sometimes the flames will continue to burn on one side of the stem for an extended period of time, this will eventually kill the cambium on the inside of the bark, leaving a triangle shaped scar from the ground and up. The living cambium surrounding the damage will eventually start to produce new wood which will somewhat cover it from the sides, and now we have a fire scar. Trees with thinner bark, Norway spruce (Picea abies) for example, have a much harder time during intense fires and the cambium usually gets damaged around the whole stem which of course is life threatening for the tree.
