On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes

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Abstract

An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. These possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt and related changes in seasonality, and differential growth/climate relationships inferred for maximum, minimum and mean temperatures. Another possible cause of the divergence described briefly herein is ‘global dimming’, a phenomenon that has appeared, in recent decades, to decrease the amount of solar radiation available for photosynthesis and plant growth on a large scale. It is theorized that the dimming phenomenon should have a relatively greater impact on tree growth at higher northern latitudes, consistent with what has been observed from the tree-ring record. Additional potential causes include “end effects” and other methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations.

Introduction

Tree rings are a critically important proxy for reconstructing the high resolution climate of the past millennium and are the dominant data type in most large scale hemispheric reconstructions [e.g. Mann et al., 1999, Esper et al., 2002, D'Arrigo et al., 2006]. The statistical calibration and verification of tree-ring based reconstructions have made the science of dendrochronology perhaps the most rigorous of those available in this regard. Such records are invaluable for placing recent climatic changes in a long-term context, which can aid considerably in the detection of anthropogenic change.

A number of recent tree-ring studies have addressed the ‘divergence problem’ in northern forests. It is defined herein as the tendency for tree growth at some previously temperature-limited northern sites to demonstrate a weakening in mean temperature response in recent decades, with the divergence being expressed as a loss in climate sensitivity and/or a divergence in trend (Jacoby and D'Arrigo, 1995, Briffa et al., 1998a, Briffa et al., 1998b, Vaganov et al., 1999, Barber et al., 2000, Briffa, 2000, Jacoby et al., 2000, Wilson and Luckman, 2003, Briffa et al., 2004, D'Arrigo et al., 2004a, Wilmking et al., 2004, Wilmking et al., 2005, Driscoll et al., 2005, Büntgen et al., 2006a). Divergence-related studies have investigated what appears to be a widespread shift in the ecophysiology of tree growth response to climate, at least for many sites within the higher latitudes of the Northern Hemisphere (Briffa et al., 1998a, Briffa et al., 1998b). This problem is rather distinct from the forest decline issue identified at many temperate sites beginning in the 1960s, which was determined to be caused by a stress syndrome partly linked to air pollution (e.g., Cook et al., 1987, Wilson and Elling, 2004; E. Cook, TRL-LDEO, pers. comm.).

Herein we provide an overview of key studies published on the divergence problem to date and describe their varying assessments of the nature, spatial extent and possible causes of shifts in tree growth sensitivity identified in tree-ring data over the recent period. Despite the considerable efforts documented thus far to understand the divergence phenomenon, there is still substantial uncertainty regarding its possible causes. This uncertainty is largely due to the fact that there are a variety of potential environmental forcing factors, both climatic and non-climatic, natural and anthropogenic, that have covaried with each other over the twentieth century and which could potentially impact radial growth in the manner that has been observed. Possible explanations for the divergence which have been proposed by various researchers are reviewed herein, along with discussion of some of the complexities involved in evaluating this problem. Significant implications of the divergence problem are also reviewed, including impacts of this phenomenon on the ability to reconstruct large-scale temperatures from tree rings, and to directly place recent anthropogenic changes in a long-term context with prior natural variations. We also introduce the first attempt to purposely develop a “divergence-free” Northern Hemisphere temperature reconstruction up to 2000 (Wilson et al., in review).

The paper is organized as follows: Section 1 provides an introduction to the divergence problem, Section 2 presents an overview of published case studies which describe evidence of divergence on varying spatial scales, Section 3 addresses the implications of the divergence on the generation of large-scale temperature reconstructions using tree rings, Section 4 addresses possible causes of the divergence, and Section 5 provides the discussion and conclusions.

Section snippets

Northwestern North America

Declines in temperature sensitivity (divergence effects) have been described in several local to regional scale studies, the first of which was published only a decade ago in the mid 1990s. A number of these studies focused on tree-ring sites in Alaska and vicinity. For example, Jacoby and D'Arrigo (1995; and see Taubes, 1995), in the first study to note this problem, observed that ring width and maximum latewood density chronologies from elevational and latitudinal treeline white spruce (Picea

Reconstructions

The divergence problem has important consequences for the utilization of tree-ring records from temperature-limited boreal sites in hemispheric-scale proxy temperature reconstructions (Jones et al., 1998, Mann et al., 1999, Briffa, 2000, Briffa et al., 2001, Esper et al., 2002, Cook et al., 2004a, Moberg et al., 2005, D'Arrigo et al., 2006, Hegerl et al., 2006). The principal difficulty is that the divergence disallows the direct calibration of tree growth indices with instrumental temperature

Possible causes of tree growth divergence

We have discussed a number of factors that can potentially impact climate sensitivity on local to regional scales and cause or simulate divergence effects (e.g., Moisture stress: Jacoby and D'Arrigo, 1995, Barber et al., 2000, Lloyd and Fastie, 2002, Büntgen et al., 2006a; Complex non-linear or threshold responses: Vaganov et al., 1999, D'Arrigo et al., 2004a; Local pollution: Wilson and Elling, 2004, Yonenobu and Eckstein, 2006, Differential response to maximum and minimum temperatures: Wilson

Discussion and conclusions

We have presented an overview of the currently available literature regarding the divergence problem observed in tree rings over recent decades. This phenomenon has been described on a range of spatial scales, and appears to be largely confined to northern forests (e.g., Briffa et al., 1998a, Cook et al., 2004a). However, the relative scarcity of ring width and density records from the lower mid latitudes, tropics and Southern Hemisphere precludes making definitive conclusions about the spatial

Acknowledgements

This research was funded by the National Science Foundation's Earth System History and EROC programs, and NOAA's Climate Change and Data Detection Program. It was also partially funded by the Office of Science (BER), US Dept of Energy, through the Western Regional Center of the National Institute for Global Environmental Change under Cooperative Agreement No. DE-FC03-90ER61010 and DE-FC02-03ER63613. Additional funding for R. Wilson was obtained from the European Union Millennium Project (Grant

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