Legacies of pre-industrial land use can bias modern tree-ring climate calibrations

Abstract

In Scandinavia, dendrochronological reconstructions of past climate have mostly been based on tree-ring data from forests in which there has been, supposedly, very little or no human impact. However, human land use in sub-alpine forests has a substantially longer history and more profound effects on the forest ecosystems than previously acknowledged. Therefore, to assess human influence on tree-ring patterns over the last 500 yr, we have analyzed tree-ring patterns using trees from 2 abandoned Sami settlements and a reference site with no human impactall situated in the Tjeggelvas Nature Reserve in north-west Sweden. The hypothesis was that landuse legacies have affected tree-ring patterns, and in turn, the resulting palaeoclimate in ferences that have been made from these patterns. Our results show that climate signals are strongest at the reference site and weakest at one of the settlement sites. From the 1940s to the present, tree growth at this settlement site has been significantly lower than at the reference site. Lower tree growth at old settlements may have resulted from rapid changes in the traditional land use, or following the abrupt change when the settlements were abandoned. Without site-specific knowledge of past land use, there is a high risk of accidently sampling trees that have been affected by human-induced disturbances in the past. This may create bias in the climate signals inferred from such trees, and hence bias the outcome of climate reconstructions. We therefore recommend sampling several separate sites in study areas to improve the robustness of inferences.KEY WORDS: Scots pine · Tree-rings · Maximum latewood density · Climate reconstruction · Human land use · Vegetation history Resale or republication not permitted without written consent of the publisherClim Res 53: [63][64][65][66][67][68][69][70][71][72][73][74][75][76] 2012 (TRW) is primarily correlated with mid-summer (July) temperature, while the maximum latewood density (MXD) is strongly associated with temperatures over the full length of the growing season (McCarroll et al. 2003, Grudd 2008, Gunnarson et al. 2011. Peak response for the growing season is found with temperatures from August and is related to development of latewood cells. While TRW may provide information on other climate and environmental influences, such as precipitation or duration of snow cover, MXD provides stronger and more robust signals of the warm-season temperatures and is thus a more powerful proxy for summer temperatures than TRW.Collecting tree-ring data from forests at high altitudes or latitudes is also preferable since they are often located in inaccessible areas, where direct anthropogenic disturbance, which can affect treegrowth patterns, has been (and still is) limited (Sano et al. 2009). In northern Fennoscandia, large numbers of dendrochronological analyses have been performed to improve reconstructions of past climate using tree-ring data from Scots pine Pinus sylvestris L. trees growing in remote sub-alpine forests (Briffa et ...

Scientific Merits and Analytical Challenges of Tree‐Ring Densitometry

“…While previous research has indicated potential challenges in smaller comparisons (Clauson & Wilson, ; De Ridder et al, ; Gunnarson et al, ; Helama et al, ; Ivkovich & Koshy, ; Mannes et al, ; Melvin et al, ; Park & Telewski, ), our results firmly consolidate that the mxd measurement in practice continues to be inherently dependent on measurement idiosyncrasies despite the fact that the anatomical principle of mxd having been defined (Vaganov et al, ). This awareness requires that when building mxd chronologies for climate reconstructions aiming to preserve multi‐centennial variability (e.g., Cook et al, ), data combined from different laboratories or techniques must at a minimum be scaled to a common mean and standard deviation prior to amalgamation (Esper et al, ; McCarroll et al, ; Melvin et al, ; Zhang et al, ).…”

“…The Itrax multiscanner by Cox Analytical Systems (Sweden) was mainly, but not exclusively, developed for wood samples-the functionality permits the analysis of speleothems and other small flat samples and additionally offers information about chemical composition of the samples when equipped with an X-ray fluorescence detector (Hevia et al, 2018;Scharnweber et al, 2016). Available since 2004, an increasing number of laboratories (at present 15) have used the Itrax multiscanner to produce density data for a number of dendroecological and/or dendroclimatological studies (e.g., Björklund et al, 2015;Björklund et al, 2013;Cameron et al, 2015;Duan & Zhang, 2014;Gunnarson et al, 2011;Gunnarson et al, 2012;Helama et al, 2014;Liang et al, 2016;Linderholm et al, 2015;Melvin et al, 2013;McCarroll et al, 2013;Xing et al, 2014;Zhang et al, 2015Zhang et al, , 2016. Many of these studies combine new or updated measurements with previously published MXD data acquired by other analytical techniques such as the Walesch technique.…”

Long-term drought sensitivity of trees in second-growth forests in a humid region

“…Disturbance frequency in nonclassical forests, however, could be a less surmountable limitation. Land-use legacies could degrade the climatic signal in surviving trees (Gunnarson et al 2012). We see some indication of this here.…”

Spatial reconstruction of Scottish summer temperatures from tree rings

“…The impact of disturbance related to human activity on forest ecosystems is not uncommon and has been reported for example in Scandinavia even in areas considered undisturbed (Josefsson et al , ) and has been shown to affect the climate signal in RW chronologies from such locations (Gunnarson et al , ). Rydval et al () identified the presence (and succeeded in reducing the influence) of disturbance‐related growth release trends in several Scots pine RW chronologies which were attributed to a history of extensive woodland timber extraction over several centuries in Scotland.…”