On Tuesday morning 6th of October we left our offices at Stockholm University and headed towards the transitional region called Limes Norrlandicus, which is the border in central Sweden that roughly divides mixed deciduous forests (south) and coniferous forests (north).
Our first stop was a deciduous mixed forest south of Limes Norrlandicus. Here we looked at all the broad-leaved tree species we could find, including oak (Quercus robur), maple (Acer platanoides), lime (Tilia cordata), elm (Ulmus glabra) and, ash (Fraxinus excelsior). These species have their distributional limit in this region and Jacqueline is investigating their range dynamics result of climate warming.
Broad-leaved forest. In the first picture we see a huge oak (Quercus robur).
We took a closer look at the soil type occurring in these forests and discussed threats to deciduous trees, such as the Dutch elm disease and the ash die back – both caused by Ascomycete fungi. We also discovered autumn-colored leaves of aspen that still had a green part left, which we learned is caused by a leaf-mining larva (from the moth Ectoedemia argyropeza) that blocks the flow of chlorophyll! Of course, we also had a look at some bryophytes living in this forest…
The group learning more about soil type; Caroline beautifully reproduced what she saw in the field: a leaf-mining larva prevents parts of the aspen tree leave from losing chlorophyll; Irena and Ditte taking a look at mosses.
We continued north of Limes Norrlandicus to explore the coniferous forests. Here we quickly got shocked by the number of dead Spruce trees. Spruce mortality has been evident in many parts of Europe and is caused by the extreme drought in 2018 combined with Bark beetles: bark beetles took advantage of the susceptible trees after the drought, which became lethal for many trees (maybe even half of the forest where we were!). We took a closer look at these trees and elaborated on the effects this die-back could have on microclimatic conditions. We ended this stop with -the closest you can get to- walking on water: the bog! Here we looked at the Sphagnum-species that build up these environments.
Dead spruce species, Bark beetle's traces and the wetland.
To continue on the topic of tree mortality, we moved on to the “Västmanland Wildfire”, where 15000 hectares of forest area was burned down in 2014. Even the thick humus layer had burned, leaving just bare rocks behind. This disturbance was of course detrimental to organisms at that time, but opened opportunities for other species that thrive after fire (or even need fire to germinate, e.g. Geranium bohemicum). Furthermore, the pine and spruce trees that survive fires will become more resistant to future fires (and fungal/insect attacks) and their “fire scars” can be used to date back past fires in the area. Controversially, this largely disturbed area is expected to become the largest deciduous forest in central Sweden in a few decades, which will likely facilitate significant amounts of biodiversity in the future!
The 2014 "Västmanland wildfire" place.
From left to right: Kristoffer Hylander, Irena Koelemeijer (PhD), Caroline Greiser (PhD), Biruk Nurihun (PhD), Ditte Christianssen (PhD), Jacqueline Lima (Postdoc), and
Beyene Hailu (PhD).