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Forests are ecologically important ecosystems, for example, they absorb CO2 from the
atmosphere, mitigate climate change, and constitute habitats for the majority of terrestrial
flora and fauna. Currently, due to increasing human pressure, forest ecosystems are
increasingly subjected to changing environmental conditions, which may alter forest growth
to varying degrees. However, how exactly different tree species will respond to climate
change remains uncertain and requires further comprehensive studies performed at different
spatial scales and using various tree-ring parameters.
This dissertation aims to advance the knowledge about tree-ring densitometry and
tree responses to climate variability and extremes at different spatial scales, using various
tree species. More specifically, the following aims are pursued: (i) to obtain and compare
wood density data using different techniques, and to assess variability among laboratories
(Chapter I). (ii) To investigate microsite effects on local and regional Scots pine (Pinus
sylvestris L.) responses to climate variability (Chapter II) and extremes (Chapter III),
using ring width (RW) and latewood blue intensity (LBI) parameters. (iii) To give a general
site- and regional-scales overview of Scots pine, pedunculate oak (Quercus robur L.), and
European beach (Fagus sylvatica L.) RW responses to climate variability (Chapter IV). (iv)
To discuss the challenges which may result from compiling tree ring records from different
(micro)sites into large-scale networks. The study area comprises nine coastal dune sites, each
represented by two contrasting microsites: dune ridge and bottom (Chapters II and III), and
310 different sites within the south Baltic Sea lowlands (Chapter IV).
The dissertation confirms that sample processing and wood density measuring are
very important steps, which, if not performed carefully, may result in biases in growth trends,
climate-growth responses, and climate reconstructions. The performed experiment proved
that the mean levels of different wood density-related parameters are never comparable due
to different measurement resolutions between various techniques and laboratories. Further,
the study revealed substantial biases using data measured from rings of varying width due
to resolution issues, where resolution itself and wood density are lowered for narrow rings
compared to wide rings (Chapter I).
The (micro)site-specific investigation showed that, depending on the species,
different climate variables (temperature, precipitation, or drought) constitute important
factors driving tree growth across investigated locations (Chapters II and IV). However,
there is evidence that the strength and/or direction of climate-growth responses differ(s)
between microsite types (Chapter II) and across sites (Chapter IV). Moreover, climategrowth
responses are non-stationary over time regardless of the tree species and tree-ring
parameter used in the analysis (Chapters II and IV). There are also differences in RW and
LBI responses to extreme events at dune ridge and bottom microsites (Chapter III).
The regional-scale investigations revealed that climate-growth responses (strength
and non-stationarity) are quite similar to those observed at the local scale. However,
compiling RW or LBI measurements into regional networks to study tree responses to
extreme events led to weakened signals (Chapter III).
The findings presented in Chapters II and IV suggest that the strength, direction,
and non-stationary responses are very likely caused by several climatic and non-climatic
factors. The mild climate in the south Baltic Sea region presumably does not constitute a
leading limiting growth factor, especially for Scots pine, whose distribution extends from
southern to northern Europe. Thus, the observed climate-growth responses are usually of
weak to moderate strength. In contrast, for other species reaching their distribution limit at
the Baltic coast, the climatic signal can be very strong. However, the observed findings also
result from the effects of microsite conditions, and potentially other factors (e.g.,
management, stand dynamic), which all together alter the physiological response of the tree
at a local scale. Although climate at the south Baltic Sea coast is mild, extreme climate events
may occur and affect tree growth. As demonstrated (Chapter III), extreme climate events
affected tree growth across dune sites, however, to varying degrees. The prominent
differences in tree responses to extreme climate events were significant at the local scale but
averaged out at the regional scale. This is very likely associated with observed microsite
differences, where each microsite experiences different drivers and dynamics of extreme
growth reductions.
This dissertation helped to demonstrate that integrating local tree-ring records into
regional networks involves a series of challenges, which arise at different stages of research.
In fact, not all possible challenges have been discussed in this dissertation. However, it can
be summarized that several steps performed first at the local scale are very important for the
quality and certainty of climate-growth responses, tracking tree recovery after extreme
events, and potential climate reconstructions at the larger scale. Among them, identification
of microsite conditions, sample preparation, and measurement, examination of growth
patterns and trends, and identification of a common limiting growth factor are very
important. Otherwise, the compilation of various tree-ring data into a single dataset could
lead to over- or underestimation of the results and biased interpretations.
A massive shift in agricultural practices over the past decades, to support exceptionally high yields and productivities involving intensive agriculture, have led to unsustainable agriculture practices across the globe. Sustenance of such high yields and productivities demand high use of organic and industrial fertilizers. This acts as a negative pressure on the environment. Excessive use of fertilizers leads to nutrient surplus in the fields, which, as a part of catchment runoff, flows into the water bodies as diffuse pollution. These nutrients through rivers are eventually passed into seas. High nutrients ending up into water bodies cause eutrophication. The situation is worsened when such unsustainable agricultural activities are carried out on drained peatlands. As a result, the nutrients that were not part of the nutrient cycle in the landscape for years begin to leach out due to mineralization of peatlands, thereby putting an additional load of nutrients on the environment, that was already under the negative impact of nutrient surplus. In view of the above, a small lowland catchment of the Ryck river in northeast Germany was assessed for its nitrogen losses from agricultural lands through empirical modelling. Initial empirical modelling resulted in an average annual total nitrogen loss of 14.7 kg ha−1 year−1. After a comparative analysis of these results with procured data, the empirical equation was modified to suit the catchment, yielding more accurate results. The study showed that 75.6% of peatlands in the catchment are under agricultural use. Subsequently, a proposal was made for potential wetland buffer zones in the Ryck catchment. Altogether, 13 peatland sites across 8 sub-catchments were recommended for mitigation of high nutrient runoff. In the end, nutrient efficiency of proposed WBZs in one of the sub-catchments of Ryck has been discussed. The results show that (i) the modified empirical equation can act as a key tool in application-based future strategies for nitrogen reduction in the Ryck catchment, (ii) restoration of peatlands and introduction of WBZs can help in mitigating the nutrient runoff for improved water quality of Ryck, and subsequently (ii) contribute to efficient reduction of riverine loads of nutrients into the Baltic Sea.
Abstract
River estuaries are characterized by mixing processes between freshwater discharge and marine water masses. Since the first are depleted in heavier stable isotopes compared with the marine realm, estuaries often show a linear correlation between salinity and water stable isotopes (δ18O and δ2H values). In this study, we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity‐isotope correlation in spring, but ‐in two of the three studied transects‐ hyperbolic and partially reverse correlations in summers. We conclude that additional hydrological processes partially overprint the two‐phase mixing correlation during summers: aside from the isotope seasonality of the riverine inflows, the shallow inner lagoons in the studied estuaries are influenced by evaporation processes. In contrast the estuarine outflow regions are under impact of significant salinity and isotope fluctuations of the Baltic Sea. Deciphering those processes is crucial for the understanding of water isotope and salinity dynamics. This is also of relevance in context of ecological studies, for example, when interpreting oxygen and hydrogen isotope data in aquatic organisms that depend on ambient estuarine waters.
Abstract
The role of future forests in global biogeochemical cycles will depend on how different tree species respond to climate. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of common tree species. We constructed a new network of 310 tree‐ring width chronologies from three common tree species (Quercus robur, Pinus sylvestris and Fagus sylvatica) collected for different ecological, management and climate purposes in the south Baltic Sea region at the border of three bioclimatic zones (temperate continental, oceanic, southern boreal). The major climate factors (temperature, precipitation, drought) affecting tree growth at monthly and seasonal scales were identified. Our analysis documents that 20th century Scots pine and deciduous species growth is generally controlled by different climate parameters, and that summer moisture availability is increasingly important for the growth of deciduous species examined. We report changes in the influence of winter climate variables over the last decades, where a decreasing influence of late winter temperature on deciduous tree growth and an increasing influence of winter temperature on Scots pine growth was found. By comparing climate–growth responses for the 1943–1972 and 1973–2002 periods and characterizing site‐level growth response stability, a descriptive application of spatial segregation analysis distinguished sites with stable responses to dominant climate parameters (northeast of the study region), and sites that collectively showed unstable responses to winter climate (southeast of the study region). The findings presented here highlight the temporally unstable and nonuniform responses of tree growth to climate variability, and that there are geographical coherent regions where these changes are similar. Considering continued climate change in the future, our results provide important regional perspectives on recent broad‐scale climate–growth relationships for trees across the temperate to boreal forest transition around the south Baltic Sea.