Branching and branch mortality
Warm early winters (November-December) after the growing season may have led to higher branch mortality rates. In years with high branch mortality, hourly temperatures were in general more frequently above freezing point in November-December than in years with low branch mortality (Fig. S9). Such winter warming events can cause reduced snow cover leaving the plants vulnerable to subsequent freezing damage and dieback , as observed for C. tetragona in Svalbard . Moreover, winter warming often coincides with rain-on-snow events , the frequency of which has increased in Greenland in recent decades . Such events can lead to encasement of plants in ground-ice, which increases branch mortality in C. tetragona , as has been shown experimentally in Svalbard . In the same icing-experiment, it was found that shoot length of surviving branches increases after icing treatment. This may offer an additional or alternative explanation for the positive influence of winter temperatures on C. tetragona observed here.
Warm springs (Mays) prior to the growing season may have resulted in increased branch mortality, following a similar mechanism. Spring warming may lead to an advanced start of the growing season. This can leave plants exposed to late frost, which can result in damage to soft tissues and reduced growth, as was observed the evergreen shrubsEmpetrum nigrum ssp. hermaphroditum , at a near tree-line site in the Central Norwegian Scandes and Rhododendron ferrugineum at a low-alpine site in the French Alps .
Branching occurred more often after years with cool summers than after years with warm summers. There was also a negative correlation between shoot length growth in the previous year and branch initiation frequency (r=-0.21, p<0.05). Hence, when shoot length growth in the previous growing season is below average, perhaps due to a cool summer,C. tetragona shrubs tend to invest more in new branches to increase or maintain its photosynthetic capacity. observed an increase in the number of branches in C. tetragona shrubs in response to experimental shading. Similarly, the shrubs seem to form more side-branches after recent stress, such as damage to branch tips (own observation). Dead or damaged C. tetragona shoot tips with a cluster of new branches are also a remarkable feature of shrubs exposed to experimental icing . Likewise, browsing by ptarmigan and moose leads to increased production of vegetative shoots in the deciduous tall shrubSalix alaxensis in northern Alaska . The capacity of making new branches, sometimes, though rarely, on decades-old C. tetragonastem segments, may be related to their presence at the basal part of annual shoot increments , i.e. near WMS, which consist of green meristem tissue. C. tetragona may thus be well-adapted to and capable of recovery from branch dieback caused by winter warming events, as its primordia are often protected under old leaves at lower stem positions.