The recent trend in climate has shown that UK temperatures are increasing. This varies from east to west and north to south, but in general summers are getting warmer and drier, and winters are getting wetter. It is thought that this trend is set to continue for the foreseeable future and that this will have an impact on the growth and quality of timber in the UK. Sitka spruce (scientific name Picea sitchensis) is one of the most widely planted and commercially important tree species in the UK, but our knowledge of its tree growth and wood properties is based on previous research and the climate of the past 40 – 80 years. The rotation time (from planting to harvest) for Sitka spruce in the UK is approximately 40 years so trees planted now will mature in the 2050s and 60s, when the climate could be quite different from today. This change in climate will likely affect the quality and quantity of the wood being produced. My research project aimed to predict the effect that changes in climate will have on Sitka spruce. I did this looking not only at growth
(increase in diameter of the tree trunk) but also at different properties of the wood and their susceptibility to any change in local climate that I was also monitoring. This information could then be used to help make decisions as to whether Sitka spruce is the best kind of tree to be planting now, at any specific site in the UK, to obtain the best quality wood in the future.
I measured the effect of seasonally changing weather on growth at two forest sites by the use of LVDT point dendrometers to record changes in the radius of the tree stems (basically a kind of electronic ruler fixed to 5 trees at each site). I compared the measurements to meteorological data (air temperature, relative humidity, and soil moisture) collected from the site and from local weather stations. I then compared these climate variables to the radial expansion measurements to investigate the interactive effects on temporal growth dynamics and the timing of radial expansion events. That’s a fancy way of saying that I looked at the times of year when the radial expansion (tree growth) started and stopped, when the tree growth changed from fast (producing the lighter colour earlywood part of a yearly growth ring) to slow (producing the darker colour latewood part of a yearly growth ring). I also looked at the daily fluctuations to see at which periods of the day the trees were growing.
The data collection from the site at Griffin Forest near Aberfeldy started in 2008 as part of a long-term project at that site. Measurements taken during 2008 and 2009 were used as part of a previous PhD study and continued as part of my own PhD from 2010. The second site was newly established at Harwood Forest in Northumberland, northern England. Competition between trees within a site can greatly influence trees’ growth and forest management practices (such as thinning – removal of small trees early on in the rotation) can have a big effect on competition for both soil moisture and sunlight. My analysis of tree growth at the Griffin site showed that there is a lot of variation in the total amount of growth of different trees, though the relative seasonal growth patterns remained the same.
At both sites, I discovered that the onset of growth at the beginning of the season corresponded to daily average temperature rising above 5°C, although radial expansion was seen to start when average temperatures were greater than 3°C perhaps as the trees start to take up water and the trunks swell up with sap.
The maximum growth in conifers like Sitka spruce, driven by photosynthetic carbon capture, occurs around the time of year when daylight hours are at a maximum. However the growth rate can also be affected by the amount of water available. In my research I found that a deficit of soil moisture during the peak growth period was what triggered the yearly decrease in the growth rate: the trees switching from producing large thin walled cells (earlywood) to producing cells with thicker walls (latewood). A soil moisture content of 30% seems to be the threshold below which the growth rate is affected; especially at the time of peak growth. If soil moisture did not get as low as 30% then the trees continued to produce earlywood for longer into the season. The cessation of growth (end of the growing period of the year) was similar in date throughout all the years I measured, and it happened before temperatures dropped below 5°C. This suggests that it is not temperature that controls when growth stops. My conclusion is that if the potential growing season is extended due to a warming climate then trees such as Sitka spruce might not take advantage of it.
Keywords: Sitka spruce, growth, dendrometers, climate, soil moisture
Authors
Steven Adams
Edinburgh Napier University, United Kingdom
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