Growth resistance and resilience of mixed silver fir and Norway spruce forests in central Europe: Contrasting responses to mild and severe droughts

Abstract

Extreme droughts are expected to increase in frequency and severity in many regions of the world, threatening multiple ecosystem services provided by forests. Effective strategies to adapt forests to such droughts require comprehensive information on the effects and importance of the factors influencing forest resistance and resilience. We used a unique combination of inventory and dendrochronological data from a long‐term (>30 years) silvicultural experiment in mixed silver fir and Norway spruce mountain forests along a temperature and precipitation gradient in southwestern Germany. We aimed at examining the mechanisms and forest stand characteristics underpinning the resistance and resilience to past mild and severe droughts. We found that (i) fir benefited from mild droughts and showed higher resistance (i.e., lower growth loss during drought) and resilience (i.e., faster return to pre‐drought growth levels) than spruce to all droughts; (ii) species identity determined mild drought responses while species interactions and management‐related factors strongly influenced the responses to severe droughts; (iii) intraspecific and interspecific interactions had contrasting effects on the two species, with spruce being less resistant to severe droughts when exposed to interaction with fir and beech; (iv) higher values of residual stand basal area following thinning were associated with lower resistance and resilience to severe droughts; and (v) larger trees were resilient to mild drought events but highly vulnerable to severe droughts. Our study provides an analytical approach for examining the effects of different factors on individual tree‐ and stand‐level drought response. The forests investigated here were to a certain extent resilient to mild droughts, and even benefited from such conditions, but were strongly affected by severe droughts. Lastly, negative effects of severe droughts can be reduced through modifying species composition, tree size distribution and stand density in mixed silver fir‐Norway spruce forests.

The Past Matters: Previous Management Strategies Modulate Current Growth and Drought Responses of Norway Spruce (Picea abies H. Karst.)

“…Its comparatively high susceptibility to drought has already been highlighted in several studies across Central Europe, supporting the general assumption that spruce is inferior to predicted climatic changes [109][110][111][112]. This is particularly evident in stronger growth reductions, respectively lower drought resistance, in comparison to other species [19,28] and was confirmed for different elevations [113] and drought levels [114]. In the aftermath of the 2003 drought, our analysis showed that most trees barely recovered or even continued to decline in growth.…”

“…In addition, Giuggiola et al [42] recommended reducing stand density for isohydric tree species, such as N. spruce, to improve the water balance when trees occur near their ecological limit. Such positive effects of lower stand densities on drought response were observed at both the tree [46,47,114] and the stand level [18,136].…”

“…Further, comparisons between the same indices, using detrended and raw basal area increments, revealed high correlations (r ≥ 0.9). The same was observed in a similar study, where the detrended tree-ring series were also excluded from further analysis [114]. Before and after each identified drought, there was at least a two-year span with normal or wet climatic conditions, considered as favorable for growth.…”

Assessing Tree Drought Resistance and Climate-Growth Relationships under Different Tree Age Classes in a Pinus nigra Arn. ssp. salzmannii Forest

“…This relatively higher stand density could make these forest stands sensitive to future droughts and warming events [66]. Commercial thinning operations could reduce the competition between trees and could increase resilience to upcoming drought events [5,66,78,81,82].…”

“…Climate observations at a global scale indicate an increased frequency and severity of drought events [1], though uncertainties remain concerning the responses of tree species to extreme droughts [2]. Tree growth responses to droughts depend on tree and stand characteristics [3][4][5]. Although several studies have investigated the potential impacts of droughts in drought-prone forests (e.g., [6][7][8]), we still need research focusing on identifying the morphological characteristics of vulnerable individuals across different species to accurately prepare and understand drought responses in forests [9].…”