Regeneration success of European beech and other temperate tree species under increased drought and nitrogen deposition in forests across Germany

Hochschulschrift

Abstract: Background
Declines in the growth and vitality of forest ecosystems due to climate change, with longer and
more severe droughts, have been reported in temperate climates. In particular, European beech,
the most naturally widespread tree species in Central Europe, has suffered severe growth
declines attributed to climate change reported in recent studies. Another major concern for
forest ecosystems today is the process of eutrophication, which leads to increased nitrogen (N)
availability due to increased deposition in regions with extensive agriculture or industry.
Although attempts have been made to reduce N deposition in Central Europe, deposition levels
are still well above known thresholds and are affecting the stability and vitality of forest
ecosystems. For temperate tree species, increased N availability has been found to affect
drought resistance and recovery, and thus increased N deposition may interfere with expected
tree species growth, morphological, and physiological adaptations to long-term and short-term
drought conditions. In this dissertation, these different manifestations in the effects of N
deposition on trees were studied and synthesized. The focus is specifically on tree regeneration
because it is highly dependent on the upper soil layers for water uptake, which dry out first
during drought, and because small changes in morphological and physiological adaptations
become critical more quickly. In addition, European beech is compared to more droughttolerant species such as sessile oak, Douglas fir, and silver fir in their growth and vitality
response, as well as morphologically and physiologically, to contribute to the development of
forest ecosystems that are resilient to both climate change and N deposition.
Overall aim
The overall goal of this thesis was to identify and quantify the interaction effect of drought
and/or low mean annual precipitation with N deposition on productivity, vitality, morphological
and physiological aspects of temperate tree regeneration and to compare European beech,
sessile oak, Douglas fir and silver fir with respect to this interaction effect.
Research objectives
The main research objectives to evaluate the responses of tree regeneration along a climatic and
N deposition gradient were:
(1) To assess changes in growth and regeneration density
(2) To identify changes in biomass allocation, fine root morphology and mycorrhization
(3) To investigate whether nutrition is altered and nutrient imbalances are caused
(4) To test whether intrinsic water use efficiency changes as a proxy for water demand
(5) To examine which source of N uptake is being used as the level of N deposition increases
Study sites and sampling design
In total, 55 forest stands of mature European beech with vital tree regeneration below 2m in
height in the understory were selected along a N deposition (10-31 kg N ha-1 a-1) and climatic
gradient across Germany. At or near each European beech stand, at least one of the more
drought tolerant species (sessile oak, Douglas-fir, and silver fir) was also present in natural
regeneration for comparison. Only a smaller gradient of N deposition could be sampled for
sessile oak (10-21 kg N ha-1 a-1). For each species and stand, 3 plots were established to record
shoot length growth, regeneration density, estimates of herb layer and canopy cover, and basal
area and height of adult trees. At each plot, the depth of the organic layer was measured and
mixed soil samples (depth: 0-10, 30-50 cm) were collected and analyzed for texture, field
capacity, pH, cation exchange capacity, and C and N content. For each of the four species, 9
trees smaller than 70 cm were harvested from a subset of forest stands covering the extreme
conditions of drought and N deposition along the gradients studied. Harvested trees were used
to determine foliar nutrient concentrations, stable isotope ratios (δ13C and δ15N), belowground
and aboveground biomass, as well as fine root morphology and mycorrhization rate using the
functional classification approach to root analysis. Data on climatic conditions and N deposition
were obtained from modeled raster data.
Results and discussion
The species studied differed in their responses to N deposition and drought or low precipitation
throughout the study. Nitrogen deposition reduced shoot length growth in European beech and
in interaction with spring drought regeneration density of sessile oak, but increased shoot length
growth and regeneration density of silver fir, while Douglas fir was more affected by drought
conditions (objective 1). Reduced shoot length growth of European beech with increasing N
availability was most likely related to the simultaneously observed reduced biomass investment
belowground compared to aboveground, combined with reduced root tip frequency and
mycorrhization rate on sites with both elevated N deposition and reduced precipitation
(objective 2), which reduced the water uptake capacity of young beech trees. The observed
reductions in belowground biomass in European beech due to increased N deposition were
consistent with optimal partitioning theory. In parallel, N deposition decreased intrinsic water
use efficiency for beech and all other tree species studied (objective 4) due to its stimulating
role in the process of stomatal opening, which increased water demand and thus counteracted the reduced belowground exploration in beech (objective 2). Especially in beech, intrinsic water
use efficiency was more tightly controlled by N deposition than by changes in temperature or
precipitation regime, and especially reactive N in foliage was responsible for the process of
stomatal opening (objective 4), underlining the importance of N deposition as a stress factor.
Sessile oak, Douglas fir, and silver fir did not show changes in morphological adaptations with
increasing N deposition, which would indicate reduced drought tolerance (objective 2). Douglas
fir and silver fir even increased root elongation under increased N deposition, which may have
contributed to the observed increased shoot length growth and higher regeneration density with
increasing deposition in silver fir by facilitating the subsequent increase in water demand
(objectives 1, 2). Another reason for the observed growth stimulation of silver fir by additional
N supply may be its lower uptake of ammonium during drought, which was the main source of
N deposition at the study sites, so that growth of silver fir may still be N-limited on drier sites
with increased deposition.
In beech, Douglas fir and silver fir, foliar N content increased with increasing N deposition and
foliar P content decreased (N/P ratio increased) under the interaction of increasing N deposition
and decreasing mean annual precipitation, but nutrition of other elements like K, Ca, Mg and
Fe was not reduced (objective 3). The higher N/P ratio was attributed to decreased P mobility
and availability in dry soils together with increased N deposition, but did not result in decreased
shoot length increment or stomatal conductance, thus not indicating a deficiency. In European
beech, higher foliar N/P ratios with increased N deposition and reduced precipitation may also
be related to the observed reduced root tip frequency and mycorrhization rate under the same
conditions, as they play a key role in nutrient uptake (objective 2, 3).
The increase in stomatal conductance with increasing N deposition was mediated by
temperature in all species, implying higher N uptake under warmer conditions, as intrinsic water
use efficiency decreased at warmer sites with high N deposition and foliar N content with
temperature (objective 4). In the broadleaved species beech and oak, increased N uptake with
increased N deposition resulted from a positive feedback mechanism of increased reactive N
availability on N mineralization and from their better decomposable leaf litter (objective 5). In
contrast, the uptake of reactive N from deposition was more important for the foliar N content
of conifers with increasing N deposition due to the higher canopy surface area for interception
of deposition, and especially in silver fir, reactive N uptake continuously contributed to
increased needle N under increased N deposition (objective 5).
Conclusion
European beech showed the most negative responses to elevated N deposition with and without
interaction with drought or reduced precipitation, implying that atmospheric N deposition
should be further reduced to maintain healthy regenerating forests of European beech. Because
all species studied responded differently to changes in N deposition, precipitation, temperature,
and drought, patterns of growth, morphology, and physiology remain species-specific and need
to be elucidated for mature forest stands as well. For silver fir and sessile oak, more research is
needed to further clarify why growth and density are facilitated under elevated N deposition for
silver fir, or if and at what level of elevated N deposition sessile oak responds negatively.
Juvenile Douglas fir trees showed only minor morphological or physiological changes due to
increasing N deposition that could lead to increased drought susceptibility, and were strongly
affected by water deficit itself, underlining the drought sensitivity of this tree species observed
in Europe

Angaben zum Objekt

• Standort: Deutsche Nationalbibliothek Frankfurt am Main
• Umfang: Online-Ressource
• Sprache: Englisch
• Anmerkungen: Universität Freiburg, Dissertation, 2024

Klassifikation und Themen

• Schlagwort: Waldökosystem ; Anthropogene Klimaänderung ; Stickstoffbelastung ; Waldbau ; Rotbuche ; Anthropogene Klimaänderung ; Stickstoffbelastung ; Waldbau ; Rotbuche ; Mitteleuropa ; Europa

Beteiligte, Orts- und Zeitangaben

• Ereignis: Veröffentlichung
• (wo): Freiburg
• (wer): Universität
• (wann): 2025
• Urheber: Dietrich, Viktoria
• Beteiligte Personen und Organisationen: Hauck, Markus ; Lang, Friederike ; Professur für Angewandte Vegetationsökologie ; Albert-Ludwigs-Universität Freiburg. Fakultät für Umwelt und Natürliche Ressourcen

Weitere Informationen

• DOI: 10.6094/UNIFR/262776
• URN: urn:nbn:de:bsz:25-freidok-2627768
• Rechteinformation: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Letzte Aktualisierung: 15.08.2025, 07:30 MESZ