Ute Sass-Klaassen

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

The importance of wood traits and hydraulic conductance for the performance and life history strategies of 42 rainforest tree species.

*In a comparative study of 42 rainforest tree species we examined relationships amongst wood traits, diameter growth and survival of large trees in the field, and shade tolerance and adult stature of the species. *The species show two orthogonal axes of trait variation: a primary axis related to the vessel size-number trade-off (reflecting investment in hydraulic conductance vs hydraulic safety) and a secondary axis related to investment in parenchyma vs fibres (storage vs strength). Across species, growth rate was positively related to vessel diameter and potential specific hydraulic conductance (K(p)), and negatively related to wood density. Survival rate was only positively related to wood density. *Light-demanding species were characterized by low wood and vessel density and wide vessels. Tall species were characterized by wide vessels with low density and large K(p). Hydraulic traits were more closely associated with adult stature than with light demand, possibly because tall canopy species experience more drought stress and face a higher cavitation risk. *Vessel traits affect growth and wood density affects growth and survival of large trees in the field. Vessel traits and wood density are therefore important components of the performance and life history strategies of tropical tree species.

Studying global change through investigation of the plastic responses of xylem anatomy in tree rings

Variability in xylem anatomy is of interest to plant scientists because of the role water transport plays in plant performance and survival. Insights into plant adjustments to changing environmental conditions have mainly been obtained through structural and functional comparative studies between taxa or within taxa on contrasting sites or along environmental gradients. Yet, a gap exists regarding the study of hydraulic adjustments in response to environmental changes over the lifetimes of plants. In trees, dated tree-ring series are often exploited to reconstruct dynamics in ecological conditions, and recent work in which wood-anatomical variables have been used in dendrochronology has produced promising results. Environmental signals identified in water-conducting cells carry novel information reflecting changes in regional conditions and are mostly related to short, sub-annual intervals. Although the idea of investigating environmental signals through wood anatomical time series goes back to the 1960s, it is only recently that low-cost computerized image-analysis systems have enabled increased scientific output in this field. We believe that the study of tree-ring anatomy is emerging as a promising approach in tree biology and climate change research, particularly if complemented by physiological and ecological studies. This contribution presents the rationale, the potential, and the methodological challenges of this innovative approach.

Hydraulic adjustment of Scots pine across Europe.

* The variability of branch-level hydraulic properties was assessed across 12 Scots pine populations covering a wide range of environmental conditions, including some of the southernmost populations of the species. The aims were to relate this variability to differences in climate, and to study the potential tradeoffs between traits. * Traits measured included wood density, radial growth, xylem anatomy, sapwood- and leaf-specific hydraulic conductivity (K(S) and K(L)), vulnerability to embolism, leaf-to-sapwood area ratio (A(L) : A(S)), needle carbon isotope discrimination (Delta13C) and nitrogen content, and specific leaf area. * Between-population variability was high for most of the hydraulic traits studied, but it was directly associated with climate dryness (defined as a combination of atmospheric moisture demand and availability) only for A(L) : A(S), K(L) and Delta13C. Shoot radial growth and A(L) : A(S) declined with stand development, which is consistent with a strategy to avoid exceedingly low water potentials as tree size increases. In addition, we did not find evidence at the intraspecific level of some associations between hydraulic traits that have been commonly reported across species. * The adjustment of Scots pines hydraulic system to local climatic conditions occurred primarily through modifications of A(L) : A(S) and direct stomatal control, whereas intraspecific variation in vulnerability to embolism and leaf physiology appears to be limited.

Controls on Coarse Wood Decay in Temperate Tree Species: Birth of the LOGLIFE Experiment

Dead wood provides a huge terrestrial carbon stock and a habitat to wide-ranging organisms during its decay. Our brief review highlights that, in order to understand environmental change impacts on these functions, we need to quantify the contributions of different interacting biotic and abiotic drivers to wood decomposition. LOGLIFE is a new long-term ‘common-garden’ experiment to disentangle the effects of species’ wood traits and site-related environmental drivers on wood decomposition dynamics and its associated diversity of microbial and invertebrate communities. This experiment is firmly rooted in pioneering experiments under the directorship of Terry Callaghan at Abisko Research Station, Sweden. LOGLIFE features two contrasting forest sites in the Netherlands, each hosting a similar set of coarse logs and branches of 10 tree species. LOGLIFE welcomes other researchers to test further questions concerning coarse wood decay that will also help to optimise forest management in view of carbon sequestration and biodiversity conservation.

JUNIPER FROM ETHIOPIA CONTAINS A LARGE-SCALE PRECIPITATION SIGNAL

Most semiarid regions are facing an increasing scarcity of woody vegetation due mainly to anthropogenic deforestation aggravated by climate changes. However, there is insufficient information to reconstruct past changes in climate and to evaluate the implications of future climate changes on the vegetation. Tree‐ring analysis is a powerful tool for studying tree age, population dynamics, growth behavior, and climate‐growth relationships among tropical tree species and for gaining information about the environmental forces driving growth change as well as for developing proxies for climate reconstruction. Wood anatomical and dendrochronological methods were used on Juniperus procera trees from two Ethiopian highland forests to check (i) whether tree‐ring series of juniper are cross‐datable and hence suitable for building tree‐ring chronologies, and if so, (ii) which climate factors mainly drive wood formation in juniper from this region. Visible growth layers of the juniper wood were shown to be annual rings. Tree‐ring sequences could be cross‐dated between trees growing at the same site and between trees growing at sites 350 km apart. Evidence was found that annual growth of junipers is mainly controlled by one climatic factor, precipitation. This strong precipitation influence proves the potential of African juniper chronologies for accurate climate reconstruction and points out the relevance of building a network of juniper chronologies across East Africa.

Dendrochronology in the dry tropics: the Ethiopian case

Dendrochronology is developing outside temperate and boreal regions. Over the past decade substantial progress has been made in Mediterranean and wet tropical regions. However, research in dry tropical regions, notably those of sub-Saharan Africa, has remained fragmentary. Here, we try to identify the unique challenges and opportunities of dendrochronology in the dry tropics. First, we briefly review the status of dendrochronology outside temperate and boreal regions with an emphasis on sub-Saharan Africa. Subsequently, we focus upon one of those areas where dendrochronology in the dry tropics is at the forefront of scientific advance: Ethiopia. A detailed review of tree ring studies in the lowlands and highlands highlights the complexity of ring formation and made us identify four major types of growth ring expression: anatomically not distinct rings, multiple rings per year, annual rings and multiple missing rings. This complex tree growth behaviour is associated with large-scale variations in precipitation regime (unimodal to multimodal) and relatively small-scale variations in tree sensitivity to water availability. Literature results are used to develop a scheme that can be used to predict differences in growth ring formation along gradients in these two factors. Because of the exceptional growth sensitivity of and the importance of local site conditions (topography, biological factors, etc.) for most trees sampled, those growing at the limits of their ecological amplitude are prone to possess multiple rings per year or multiple missing rings. In such circumstances, site selection should not always take place at the limits of the ecological amplitude of a species, but may sometimes have to be diverted to more mesic environments. Successful studies are now appearing, such as those reporting correlations between tree ring chronologies and Blue Nile river flows.

Towards a reconstruction of Blue Nile baseflow from Ethiopian tree rings

Most of the water in the River Nile originates from monsoonal rainfall over the Ethiopian Highlands. Despite warnings of future climate change, little is known about the historical variability in this supply, particularly at annual resolution. Development of tree-ring records in this region has been limited by the occurrence of bi- or multimodal rainfall regimes, causing the development of multiple growth rings that cannot be dated with confidence. In this study, we identified annual rings in 30 Juniperus procera trees from northwest Ethiopia by dendrochronology and AMS radiocarbon dating. Carbon isotope ratios (4 series) and ring widths (73 series) were measured.The carbon isotope series did not contain strong trends possibly attributable to increased anthropogenic atmospheric CO2 concentrations or the juvenile effect. Both carbon isotope values and ring widths were strongly correlated with Blue Nile baseflow, and from composite chronology indices (r=0.75, p < 0.01), a preliminary reconstruction of Blue Nile baseflow back to AD 1836 was developed. Subsample signal strength remained above 0.85 for most of the reconstruction. Uncertainty bands were relatively narrow and the reliability of the preliminary reconstruction was confirmed by correspondence with reported years of drought and famine. The preliminary reconstruction is characterized by an exceptional decline in baseflow during the late AD 1960s. Flows recovered during the late 1990s. Additional sampling is advised to increase replication, spatial coverage and length of the preliminary reconstruction.

A tree-centered approach to assess impacts of extreme climatic events on forests

A major task of our society is to manage forests in a way that their resources are preserved to meet future generation needs (Forest Europe et al., 2015). Current scenarios of climate change effects are making this task extremely challenging (Kirilenko and Sedjo, 2007). Climate shifts will impact forest vitality and affect goods and services forests provide, including carbon sequestration and climate change mitigation (IPCC, 2014). To guide sustainable forest management, forest researchers are asked to provide concrete answers about forest resilience in response to expected climatic trends, and extreme climatic events (Lindner et al., 2014). This is not an easy task, because responses of trees and forest ecosystems to environmental conditions are often non-linear and moreover vary on spatial and temporal scales (Smith, 2011; Anderegg et al., 2012; Reichstein et al., 2013). For instance, although drought is one of the most frequent and widespread climatic extremes affecting forests worldwide (e.g., Allen et al., 2010), the assessment of its impact on future forests is currently under intense debate. Mechanisms behind tree growth and mortality are complex (McDowell et al., 2008, 2011; Fatichi et al., 2014; Anderegg et al., 2015; Meir et al., 2015). Besides strength or frequency of external factors, such as extreme events, also the trees ability to resist and recover is relevant, which, in turn, is largely determined by intrinsic factors such as the trees life stage, life history, and genetic characteristics. In this paper, we advocate for a tree-centered approach. By providing an improved mechanistic understanding of physiological and growth responses of trees growing under various conditions we can define the trees capacity to respond to external stress factors. This concept can valuably contribute to the debate on how to shape future forests toward resilient forest ecosystems.

Reconstructing High Arctic growing season intensity from shoot length growth of a dwarf shrub

Annual shoot length of the circumarctic dwarf shrub Cassiope tetragona has proved to be a reliable proxy for past and ongoing climate change in the Arctic. This is based on its strong linear relationship with monthly climate parameters. Monthly means are, however, coarse units for prediction of growth in marginal regions with short growing seasons. An alternative to monthly averages are parameters that quantify the growing season length (GSL) and its intensity (growing degree-days; GDD5). GDD5 is defined as the cumulative daily mean temperature above 5°C. GSL is defined as the number of days on which the average temperature exceeds 5°C. The aims of this study were to test whether these parameters are a better predictor of growth than monthly means and to reconstruct past High Arctic growing season climate. Correlative analysis shows that GDD5 is a better predictor of annual shoot length growth than mean monthly temperatures and GSL, both at C. tetragona’s European northern and southern distribution limit, as well as at its assumed climatic optimum. Svalbard Airport GDD5 was reconstructed back to 1857. The reconstruction shares 61% of variance with the instrumental record. This opens the possibility to obtain an Arctic network of climate reconstructions with high temporal and spatial resolution through construction of C. tetragona shoot length chronologies.