Picea abies and climate change – does increasing thinning intensity prevent drought stress?
by Timo Gebhardt
Date of Examination:2017-02-03
Date of issue:2017-10-11
Advisor:Prof. Dr. Christian Ammer
Referee:Prof. Dr. Christian Ammer
Referee:Prof. Dr. Thorsten E. Grams
Referee:Prof. Dr. Dirk Hölscher
Files in this item
EnglishThe predicted increase in frequency of intense and prolonged drought events challenges forest management. Centuries of forest management has led to Norway spruce (Picea abies [L.] Karst.) monocultures outside its natural range. In Germany, Norway spruce covers ca. 25% of the total forest area and represents the economically most import tree species. Thinning as a silvicultural measure has been discussed to reduce drought risk in young Norway spruce stands. The aim of the present study is to investigate the potential and the limits of increasing thinning intensity to mitigate drought by reducing intraspecific competition and increasing the availability of soil water. A two-tier approach (i.e. retrospectively and in-situ measurements) was applied to examine differences in tree response to severe drought within increasing thinning intensities and the effects of such thinning intensities on the water balance. To investigate tree response to severe drought events radial growth increments in combination with stable carbon and oxygen isotopes in the early- and latewood per year ring were measured retrospectively at two long-term thinning experiments in southern Germany. The initial thinning with increasing intensities was conducted in 1974 within a ca. 27-year-old Norway spruce stand. Annual radial growth and stable isotope analyses were used to unveil differences in physiological performance and radial growth between trees within increasing thinning intensities to the exceptional drought events 1976 and 2003. To address changes in the stand water balance modified by increasing thinning intensity and therefore to investigate the potential to mitigate drought, a thinning experiment was established in 2008 in a 26-year-old un-thinned Norway spruce monoculture. Prior to the thinning treatment at the beginning of 2009 about 430 target trees per hectare were selected. On the experimental site 3 thinning intensities were conducted: A “not-thinned” control (NT) with a stand basal area of 42 m² ha-1, a moderate thinning (MT) with a reduction of the stand basal area by 43 % (thinning from above and fostering the target trees by removing 1- 2 competitors) and a heavy thinning (HT) with a reduction of 67% (removal of all trees except the target trees). Besides the variables of the water balance, like open field precipitation, throughfall, soil water content and stand transpiration, structural changes (i.e. fine root biomass, ground vegetation) altered by the increasing thinning intensities were continuously assessed on the research plots. The retrospective approach showed, that the drought induced decline in radial growth during drought years was reduced by intense thinning on the short-term but increased on the medium to long-term. The higher resistance in radial growth on the short-term was attributed to increased soil water availability, but this advantage is likely to be reduced over time. After the drought events, the recovery was increased on the heavy-thinned plots from a likely increase of available soil water content in recently thinned stands and additionally structural adaptions of the target trees fostered by repeated intense thinning interventions (i.e. higher foliage area and fine root biomass) on the medium to long-term. The experimental approach showed, that the removal of basal area and therefore leaf area reduced transpiration on stand-level and an increased throughfall within the first years following the thinning intervention. But the thinning effect on stand transpiration and on Interception was not proportional to the reduction in basal area. The thinning led to an increased transpiration on tree-level of the remaining trees. Nevertheless, thinning increased the growth-related water use efficiency on tree- and stand level. Furthermore, over the whole study period the basal area increment was enhanced by thinning, also during the drought year. Furthermore, both thinning intensities allowed for a reduced time span below critical soil water content up to 5-7 years. However, with increasing time-lag after the thinning intervention the potential of thinning to reduce the time span decreased. Furthermore, the fast establishment of vital ground vegetation on the heavy-thinned plots added an additional water loss via evapotranspiration. The additional transpiration of the ground vegetation diminished the differences between MT and HT in stand transpiration within 2 years. Similar to the retrospective approach the radial growth decline in the target trees with increasing thinning intensity tended to be higher during the drought year in 2015 (7 years after initial thinning). This may partly be attributed to an intensified reduction of the canopy conductance compared to the pre-drought year(s) due to the open position of the target trees. Nevertheless, on HT the vital ground vegetation enhanced the competition for soil water. Moreover, the belowground competition of the ground vegetation also suppressed the recovery of fine root biomass on the heavy thinning plots compared to the more moderate thinning on the medium-term. The reduced fine root biomass compared to the increased leaf area on tree-level of HT and the competition for water may intensify drought stress for the target trees during the drought period and decrease the above mentioned favourable thinning effect on the recovery from the drought for HT on the medium-term. In summary it can be concluded, that thinnings have the potential to mitigate drought during and to improve the recovery from drought. However, it has also been shown, that the benefits of a thinning intervention on the water balance are temporary and heavy thinnings may foster ground vegetation, enhancing competition for water and belowground fine root recovery. Furthermore, if the thinning intensity is too low (by fostering not enough future crop trees) and therefore insufficient removal of basal area on stand level, the thinning intervention may increase the evaporative demand and insolation of the future crop tree without improving the water balance of the whole stand. Nevertheless, if the thinning intensity is too heavy the emerging ground vegetation may diminish the benefit of increased thinning intensity on the water balance compared to a more moderate thinning intervention without ground vegetation. The intense moderate thinning on the experimental site (removal of about 40 % basal area, fostering ca. 400 target trees, by removing 1-2 competitors), but with a relative short thinning frequency for the first interventions of about 5 years seems to be a viable option to mitigate drought during and in combination with fostering leaf and fine root biomass increment of the target trees, to improve the recovery from drought. This may be particularly important for Norway spruce endangered by secondary pests. Therefore, the present study suggests that frequent and intense thinning interventions while preventing the establishment of a vital ground vegetation seems to be the most promising forest measure to mitigate drought in young Norway spruce stands without increasing risks and without losing sight of economic needs.
Keywords: drought stress; thinning; Norway spruce; water balance; stable isotopes