Tatiana A. Shestakova is a postdoc at the Woodwell Climate Research Center in the USA. She is an ecologist interested in understanding the reaction of trees to environmental change. Here, Tatiana shares her recent work on evaluating time series analysis on tree growth to evaluate synchrony in spatially segregated forests.
Tatiana Shestakova extracting a tree-ring core of Pinus sylvestris near the southern edge of its distribution range (Seira, north-eastern Spain).
Institute. Woodwell Climate Research Center, USA
Academic life stage. Postdoctoral researcher
Major research themes. Dendroecology, Forest ecosystems, Global warming, Spatial synchrony, Remote sensing, Forest-based mitigation policies
Current study system. Trees play a major role in many terrestrial ecosystems, including those in tropical, temperate and boreal biomes. The wide geographic distribution of tree species coupled with their longevity makes them a primary source of long-term information on physiological and environmental processes across large biogeographical gradients. Notably, data from contemporary populations of trees can be extended by combining records from living and subfossil assemblages to provide continuous sequences at centennial or even millennial time scales. In a rapidly changing world, this wealth of information is very useful to retrospectively study the impacts of climate on forest ecosystems. In turn, our improved understanding of past tree reactions to variable environmental conditions is key to forecasting whether forest trees will be capable of evolving and adapting to global warming, which is fundamental to tackle the climate crisis.
Recent JBI paper. Shestakova, T. A., Camarero, J. J., & Voltas, J. (2021). Are global forests performing in sync? The need to account for spatiotemporal biases in tree-ring records. Journal of Biogeography, 48(11), 2961–2965. DOI: https://doi.org/10.1111/jbi.14249.
Motivation behind this paper. In ecology, synchrony refers to parallel changes in time-varying features of geographically disjunct populations. The question of whether global forests are performing in sync has lately engrossed the minds of scientists working in the field of forest ecology and climate change. Indeed, climate has changed markedly over the last decades, prompting an array of physiological reactions in tree species that could strengthen growth–climate relationships. For example, in dry environments, warming-induced drought stress often makes trees gradually limited by water shortage which overrides local drivers of growth (e.g., topography, soil nutrients), resulting in enhanced common tree-ring signals regionally (i.e., more synchronous growth). Although this idea is simple and therefore attractive, real world systems are much more complicated than that. Despite current evidence of increased growth synchrony across local and regional forests, a systematic rise in common variability of annual tree growth globally, driven solely by climate warming, is unlikely. While relevant patterns may emerge from global analyses, our work challenges simplistic approaches to evaluate synchrony patterns in tree rings. It stresses the need for a comprehensive assessment of tree growth variability at appropriate biogeographical scales (i.e., from local to continental) as an effective tool to interpret current forest dynamics.
Gúdar mountain range dominated by Pinus halepensis. This conifer is one of the most drought-tolerant of all pine species thriving at low altitudes in the Mediterranean region (Gúdar, eastern Spain).
Key methodologies. Measurements of secondary growth patterns in trees – the ‘tree rings’ – are increasingly used to study the impacts of global change on forest ecosystems. Nowadays, thousands of tree-ring records are collected from forested areas across the globe and are often freely available (e.g., the International Tree-Ring Data Bank), allowing large-scale assessments of spatiotemporal changes in tree growth synchrony. However, the increasing biogeographical complexity of tree-ring patterns at progressively large scales of analysis requires advanced statistical approaches, as opposed to local studies. Here, we compare several methods for assessing synchrony patterns in tree-ring chronologies, including simple and variance-based correlations, which provide metrics for characterizing the strength and nature of the signal shared among time series. In addition, we critically review the utility of the mean absolute correlation between pairs of chronologies, which has been proposed as a metric of synchrony.
Unexpected challenges. Despite the existence of global dendrochronological databases, tree-ring data remain a local product resulting from internal tree processes modulated by a tree’s immediate biotic and abiotic environment, species’ life histories, and their complex interactions. Thus, large-scale comparisons and predictions based on tree rings are challenging and introduce uncertainty that researchers need to consider. Another complication is the lack of environmental data with high spatial resolution and temporal continuity for evaluating exogenous growth drivers. In global studies, all this information requires critical examination, careful interpretation, and stimulating synthesis. While robust analyses facilitate our understanding of a phenomenon or process, methodologies that do not consider spatial and temporal biases in data might detract from knowledge. Also, dealing with large datasets is computationally demanding and methodologically challenging. Luckily, some recently developed R packages, like ‘DendroSync’ (Alday et al., 2018; Dendrochronologia 47, 17–22), allow unravelling common growth patterns in tree-ring networks in a relatively pain-free way, making such analyses accessible to a wide audience.
Major results. By comparing synchrony trends in tree growth using alternative approaches, our work demonstrates that a rise in growth synchrony estimated from absolute correlations mostly vanishes once the dataset is filtered from spurious associations. Such a seemingly steady increase in synchrony coincides with a recent acute decrease in tree-ring chronology length, the distance between sampling sites, and the number of available chronologies. Thus, our results highlight the absence of a global trend towards rising temporal coherence of tree growth. We further discuss the importance of a carefully performed and interpreted attribution analysis of synchronous tree growth dynamics considering exogenous factors. Particularly, we highlight two issues that require extra caution: an imperfect availability of spatially-explicit environmental data with sufficient resolution and the spurious regression problem that often arises in longitudinal studies. Ultimately, proper identification of environmental controls of forest dynamics is highly relevant to forecast potential ecological consequences of global change on forests and their implications for terrestrial carbon sinks.
Next steps for this research. The use of flawed methods and inappropriate data can affect the entirety of our knowledge of a phenomenon. Previous evidence collected so far at local and regional scales points towards disparate changes in growth synchrony driven by certain climatic constraints and related global change impacts occurring at each location. Thus, a major extension of this study could be to systematically assess growth synchrony and its temporal trends at appropriate spatiotemporal scales and summarize them globally, enabling ecologically meaningful interpretations of synchrony changes for their respective forests.
A forest stand of Larix gmelinii growing on continuous permafrost. It is the northernmost and most cold-resistant tree species on the planet (Eastern Siberia, Russia).
If you could study any organism on Earth, what would it be? I am fascinated by forest ecosystems, especially those composed by trees living at their range margins. Tree species growing under extreme conditions are particularly sensitive to global warming and can help us to understand forest vulnerability to a rapidly changing environment. While there is general agreement on tree growth constrained by water availability as being negatively impacted by climate change, the fate of cold-limited ecosystems is debatable. These forests often exhibit divergent tree responses to changing climate mediated by environmental heterogeneity, and also by species’ discrepancies and their adaptive capacity. In addition, global warming is most pronounced at high latitudes. I am, therefore, keen to comprehend the complexity of spatiotemporal dynamics of tree growth in these extreme systems.
Anything else to add? Dendrochronological field campaigns are time-consuming and labour-intensive work. Although collecting tree-ring samples is usually fun and joy, it may not be a trivial task at times. This is especially true in remote areas such as Siberia, where fieldwork is often associated with complex logistics, difficult working conditions, and no connection to the outer world for weeks. On the other hand, it is a rewarding experience, both personally (discovering unique landscapes) and professionally (gaining a grounded understanding of forest ecology and on-going ecosystem processes). As much as I enjoy sampling tree cores myself, my work would not have been possible without freely available tree-ring data, which scientists from all over the world deposit in global repositories such as the International Tree-Ring Data Bank (ITRDB). Thus, I would like to acknowledge the hard work of all the data contributors that inspire large-scale studies of forest ecosystems in a variety of research contexts. They undoubtedly play an increasingly important role in shaping our understanding of current patterns and trends of global forests.
Tree-ring sampling at the margin of the Mukhrino mire complex surrounded by mosquitoes (Western Siberia, Russia).