Despite the ongoing conversion of pure spruce stands to mixed stands, the spruce (Picea abies) remains by far the most common and economically most important tree species in Germany. It is however considered to have only limited resistance to extended periods of drought.

Problem: climate change and unfavourable structures

Given the fact that climate researchers are predicting ever more frequent and longer dry periods, the cultivation of spruce is likely to be associated with higher risks in future. Extensive losses in spruce stands can be expected to occur. It may seem obvious to change the tree species, but this can only be applied to certain parts of the spruce area and must also be restricted initially to stands that are due to be restocked in the foreseeable future. The question is therefore how best to implement silvicultural measures to influence young spruce stands so that, despite the predicted climatic conditions, they are able to remain healthy until maturity, or at least remain vital for several decades until they can be transformed into mixed stands by advance planting.

An ongoing experiment is investigating whether predictable consequences of climate change can be mitigated by silvicultural measures, and whether it is possible to gain time for the conversion of stands that are at risk. Thinning measures are the only immediate measures possible. But can the thinning of young spruce stands make the selected beneficiary trees more tolerant of summer droughts?

Set-up of the experiment

In the described stand (see box), an experimental area of 50 m x 75 m has been marked out and divided into six squares measuring 25 m x 25 m. A measurement area of 10 m x 10 m has been established within these squares to exclude the possibility of edge effects. On each measurement area there are between 26 and 30 selected trees (corresponding to between 416 and 480 selected trees per hectare). All selected trees have been fitted with sensors that provide information on the transpiration-driven flow of water in the trunk and thus the water consumption of the crown (Fig. 1). 60 soil moisture sensors have also been installed at different depths in the soil. The precipitation in the stand is continuously recorded via channels.

Trees close to the selected trees were removed using motor manual methods, and those around the edge of the area were removed using a harvester. On each of two measurement areas

  • the stand density has been maintained (control area).
  • one or two competitor trees have been removed per selected tree (conventional selective thinning) or
  • all trees apart from the selected trees have been removed (very heavy thinning intensity).

The starting basal area (44 m2/ha on average) was reduced by 35 percent by the conventional thinning, and by 70 percent by the heavy thinning.

Higher stand precipitation and higher growth in selected trees at first

Initially, what was to be expected occurred: stand precipitation rose in the first two years. In comparison with the unthinned area, rainfall on the conventionally thinned stands increased in the year after the thinning by 11 %, and on the severely thinned areas by as much as 39 %. Supplemented by the lower overall level of transpiration, the soil humidity rose at all depths of the soil in the thinned areas above the level in the control areas (Fig. 2).

The comparison of the transpiration of the selected trees with that of the entire stand is interesting (Fig. 3). The selected trees in the heavily thinned areas transpired considerably more than the selected trees on the other measurement areas. The opposite is true if we consider the transpiration of the entire stand. The unthinned stand has by far the highest water consumption.

The persistently high transpiration of the selected trees in the areas subject to heavy thinning is explained by the greater quantity of water from precipitation available to each individual tree and solar radiation after the removal of competitor trees. As a consequence, the basal area increment of the selected trees in the heavily thinned areas was considerably higher in the first two years after the thinning (2010). They grew more than twice as much as the trees on the unthinned areas. The basal area increment of the trees on the conventionally thinned areas was also one and a half times higher. The growth increment of the selected trees was not however sufficient to compensate for the loss in production of the basal area of the stand. This difference in growth increment will however be reduced in years to come.

Many unresolved questions

As yet, no general recommendations can be issued for application in forest operations. It is not clear whether the trees that "get used" to having more water and light after the thinning will experience problems when the crown canopy closes again or how quickly these problems will arise. It also remains to be seen what effect the ground vegetation that develops explosively on the thinned areas will have. It is also unclear how the fine root biomass, as yet barely existent, will develop. The deaths of selected trees caused by droughts may have more severe consequences than if the risk is spread over several trees. As things stand, it can however be assumed that heavy thinning generally takes the pressure off the water budget of the remaining trees.