Dendrochronology Glen Affric, 2014

Scot 2k - Dendrochronology, Highlands, 2014

Scot2K – a dendrochronological project the Scottish Highlands (NERC (NE/K003097/1)) that aims to push back the high resolution climate record of the Scottish Highlands back 2000 years.  It has both local (UK) importance and will also contribute to the wider climate record of Northern Europe and Northern Hemisphere.Growth of Scots Pine trees in the Scottish Highlands is predominantly controlled by growing season temperatures. Using parameters measured from tree-rings (i.e. ring-width and density) it is possible to derive estimates of past summer temperatures. However, the mean age of most pine trees in the Cairngorms is around ~225 years. Therefore, to derive climate estimates prior to ~1750, information must be gleaned from older preserved woody material.

Where to find this material?  Well thankfully, Scotland is not without its lochs and it is into these that many of the older trees have either fallen or have been felled.  Finding these trees and then sampling them was the goal of a recent trip to Glen Affric. The team, under its climate warrior leader, Dr Rob Wilson, had previously scouted out (swimming with snorkels)  some of the small lochs. Others were known from the Trees for Life project (http://www.treesforlife.org.uk/).

The UK team, including individuals from Sweden and Switzerland (Dr Richard Bates, Dr Cheryl Wood , Stacey-Anne Averill, Dr Mark Neal, Dr Björn Gunnarson, Dr Neil Loader, Dr Daniel Nievergelt, Dr Coralie Mills) arrived in Glen Affric armed with dry suits, saws, corers and a new remote-control survey boat.  The boat, built by students at Aberystwyth University Computer Science Department contained a 900kHz sidescan sonar and GPS logging to small laptop in a hull that was sufficiently small  to be back-packed into some of the more remote lochs.  This was considerably easier than trying to get either of our previous survey vessels, the Zego Boat or Minty (see previous blogs for both) onto the water.  Why use sonar? Well if you have ever swam in an upland loch you would know how murky the peat-ladened waters are that restrict visibility to a few centimetres.

Processing of the sidescan sonar data was using Chesapeake SonarWiz and ultimately data display within GIS.  Targets were identified and the dive team went to work.  Many of the sites contain vast numbers of trees but many are in too deep water or are too far from the shore for recovery (usually an operation using wire cables and winches attached to living trees on the loch shore).  Many of both living and dead tree root structures were visible on the sidescan records as were very curious sinuous features that we have so far been unable to identify due to the murky water.  These are likely branches.

At the end of the week the team had found and sampled over 150 trees – a highly successful hunting trip.  Now for all the analysis though and a hope that the material will contain sufficiently old records to push back the dendro record.To date, 661 sub-fossil pine samples have been collected from lakes in the NW Cairngorms: Loch Gamnha (215), Loch an Eilein (293), Green Loch, Ryvoan (67) and multiple lakes in Abernethy (86) - about a third of which have been dated using radiocarbon dating or tree-ring (dendrochronology) methods. However, dendrochronological dating of the samples has been a challenge. It has become clear that the impact of human disturbance (related to tree felling) has had a profound influence on the growth of the trees which decouples tree-growth from climate. Although these human based disturbances impact on the potential climatic information that can be gleaned from these samples, an unplanned outcome of this work will be the detailing of the human timber extraction/land-use history (likely a part of the Highland Clearances story) of the regions sampled through the Scottish Pine Project (i.e. the native pine woodlands). A network of over 50 pine woodland sites have now been sampled across Scotland (http://www.st-andrews.ac.uk/~rjsw/ScottishPine/). This is an exciting outcome as it will allow a new appreciation of the impact of logging on pine woodlands and their resultant recovery over multiple centuries.

Tree-ring based reconstructions of past summer temperatures are still in development. Through Milos Rydval's Carnegie Trust funded PhD project, Rob’s team have spent a substantial amount of time developing the new Blue Intensity (BI – a proxy of lignin content and cell wall thickness of the latewood) parameter (Rydval et al. 2014; Wilson et al. 2014). Utilising both RW and BI data from Abernethy, Green Loch, Loch an Eilein and Loch Gamnha we can already produce a well calibrated (55% of the July/August temperature variance explained) temperature reconstruction back to AD 1460 (see Figure).