Editorial: Small-leaved Lime Tilia cordata, remarkable germination in 2015

Harry Green

After each issue of Worcestershire Record has been prepared, printed and posted, I vow to myself (and occasionally to others) that the next issue will be out on time matching the cover dates of April and November. However this ambition is difficult to fulfil. Other activities get in the way and a form of editorial block sets in.

So, in some respects, this is something of an apology and I will try to do better. However the delay of this issue does give an opportunity to alert readers to an unusual event. This year there is an amazing superabundance of seedlings of Small-leaved Lime Tilia cordata. In 40 years of interest in this tree I have never seen anything like it. In Shrawley Wood, Worcestershire’s nationally important Small-leaved Lime woodland, there are hundreds of thousands, if not millions of seedlings, as I write this in mid-June 2015. There are also reports of similar germination in the Wye Valley and southern England. In this respect 2015 is a very unusual year.

Why the excitement? The production of fertile seed by Small-leaved Lime is dependent on warm conditions when the tree is flowering in late June and early July. The pollen grains form clusters and pollination is mostly by insects including honey bees, bumblebees, hoverflies and many others. Wind-blown pollination is minimal. On reaching the style a pollen grain develops a tube which penetrates tissue for 4-5 mm to reach an ovule so that fertilization can occur. Pollen tube growth requires a temperature of 14-15°C and growth rate increases with temperature up to about 25°C when it stops. In Small-leaved Lime growth increases rapidly between 18-19°C and temperatures in this range, probably over several weeks, appear essential for sufficient pollen tube growth to enable fertilization and subsequent development of seed. When these conditions occur fertile seed production follows and if conditions are right in the following spring seedlings are produced. For many years Small-leaved Lime seed production has been haphazard usually only following a warm summer. Despite occasional seedling-years growth of new saplings is rare and most of the seedlings fail. They are attacked by small rodents, slugs and snails, and damaged by forestry activities (information from Pigott 2012).

Small saplings of Small-leaved Lime are rarely seen although some have been recorded in the years following warm good seed producing summers. They are shade-tolerant and can remain as small plants until a space is made by a tree falling or forestry letting in light which encourages them to grow. In this respect they are similar to but not as tolerant of shade as Beech Fagus sylvatica saplings.

There is a suggestion that climatic warming may be increasing the the frequency of production of fertile Small-leaved Lime seed and there is some evidence that flowering is now, on average, about five days earlier than 30-40 years ago (Pigott 2012).

It is worth looking out for seedlings this summer and I would be very interested to hear of any in places other than Shrawley Wood. Small-leaved Lime grows in many woods in west Worcestershire. Please take a photograph which can be emailed to me at zen130501@zen.co.uk. If anyone finds them it would also be interesting to follow their fate. Perhaps count the number of seedlings in a small area over time.

The seedlings of Shrawley Wood 2015 are shown in the pictures 01, 02, 03, 04.

Reference

Pigott, D. 2012. Lime-trees and Basswoods. Cambridge University Press.

Images

01. Small-leaved Lime seedlings Shrawley Wood 16th June 2015. Harry Green.

02. Small-leaved Lime seedlings Shrawley Wood 16th June 2015. Harry Green.

03. Small-leaved Lime seedlings Shrawley Wood 16th June 2015. Harry Green.

04. Small-leaved Lime seedlings Shrawley Wood 16th June 2015. Harry Green.