Plant of the Week – 12th October – Lime or Linden (Tilia)

Tilia (Lime or Linden) has heart-shaped leaves and hermaphroditic flowers that attract pollinating insects; each flower stalk has a tongue-like bract. This is Tilia americana, photographed near Ithaca, New York State © Chris Jeffree

Tilia, vernacularly Lime or Linden, is a genus of trees, five species of which are found in the British Isles.  The name ‘Lime’ is a strange one, as this genus is nothing to do with Citrus, the genus of limes and lemons. It seems that ‘Linden’ is a better name, and the one used by W Turner “the Father of English Botany” who wrote in 1562 “Lind Tree…groweth very plentuously in Essekes in a parke within two mile from Colichester”. That appears to be the very first botanical record of Tilia (Pearman 2017); however Linden makes an earlier appearance in the epic Anglo-Saxon poem Beowulf (author unknown, date around 970)1. Most of world follows Beowulf’s example and uses the name Linden, not Lime.

Tilia x europaea at Limetree Walk, near Tyninghame, East Lothian, Scotland. Note the numerous shoots at the base of the trunk. Photo: John Grace.

Three species are native in Britain: T. cordata Mill, Small-leaved lime; T. platyphyllos Scop, Large-leaved lime; and their hybrid T. × europaea L2 Common Lime. Beware: the size of the leaf is not a good way to discriminate large-leaved from small-leaved lime, as leaf sizes overlap considerably.

There are other Tilia species found elsewhere and appearing in Britain as neophytes or garden specimens. Bean (1980) reports as many as 19 in his Trees and Shrubs Hardy in the British Isles. However we need mention just the two most frequent: T. tomentosa Moench., Silver Lime and T. × euchlora K.Koch, Caucasian Lime.

The three natives and two neophytes are extensively planted as ornamental trees in parks and by roadsides: T. tomentosa and T. × euchlora are almost wholly planted, and I shall have little to say about them.  The most valuable source of information about Tilia in the British Isles is Pigott (1989, 1991) and the series of papers by Pigott and Huntley (1979, 1980, 1981).

At the large scale, the distribution of Tilia is circumboreal, covering almost all of Europe south of the Arctic and all of China and Mongolia with a belt across the United States.  The British Isles are at the edge of its range, which is why their distribution relies on human assistance here.  Most species are warmth-loving, which is perhaps why T. cordata is not part of the vegetational history of Scotland. Its natural range has, for a very long time, extended as far north as Cumbria and County Durham3. However, planted trees will thrive considerably further north (but rarely set seed). There are many in the warmer parts of Scotland, some very old. I discovered that Scottish Water is planning to replace elderly conifers at its head office in Edinburgh with trees that include several T. cordata.

Global distribution of the three most often-seen species. T. cordata (upper) T. platyphyllos (centre) and the hybrid between them, T. x europaea (lower). Source: GBIF Secretariat (2019). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2020-10-06.

My interest in the genus is that Tilia cordata was one of the pioneer species in the British Isles after the last ice age, at one time formed extensive forests in England and Wales (but not in Scotland), and is still a characteristic species in ancient forests in parts of England and Wales.  It is very much part of the vegetational history of the British Isles (Birks 1989, Brewer et al 2017). 

Taxonomy and distributionTilia has long been recognised as closely related to Malvaceae, but was given its own family, Tiliaceae, which it shared with two smaller genera.  Heywood et al. (2007) recognised that there was a case, on molecular-genetic grounds, for merging Tiliaceae with Malvaceae, but decided to keep ten families separate within the core Malvales.  However, in Stace (2019) Tilia is merged with Malvaceae, leaving only Malvaceae, Thymelaeaceae and Cistaceae in the British Malvales.  Tilia is distinguished from other British Malvaceae by being trees, by having five separate groups of fused stamens, and fruit an indehiscent nut characteristically fused to a persistent bracteole, which acts as a wing in wind dispersal.  Malvaceae is a family of about 115 genera and 2000 species, with a world-wide distribution except in the Arctic. 

The lectotype specimen of Tilia, and therefore of the former Tiliaceae, is in the Linnaean Herbarium in London.  Rather surprisingly, Pigott and Sell (1995) found that this hallowed material is T. × europaea L. cv ‘Pallida’, the oldest and most common cultivar of the hybrid. Through the wonders of the internet, you may view Linnaeus’s actual pressed specimen 679.1 at:

http://linnean-online.org/view/collection/linnean=5Fherbarium/Tilia.html.

It seems unsatisfactory to me that the representative of a genus and family should be a hybrid and cultivar, but the ‘types’ of botany, like the laws of the ancient Medes and Persians, cannot be altered, so we would seem to be stuck with it!

Tilia x europaea in a mixed deciduous woodland, October 2020, Aldouran Glen, Dumfries and Galloway. Photo: John Grace.

Best keys. It can be quite challenging to identify the species. The differences between the five species in the British Isles are laid out in the appendix below, mostly from Stace (2019). The best source is probably the article by Pigott, written as a contribution to the BSBI’s Plant Crib (Rich and Jermy 1998).

Etymology. Latin tilia is cognate to Greek πτελέᾱ, ptelea, ‘elm tree’, τιλίαι, tiliai, ‘black poplar’, ultimately from a Proto-Indo-European word ptel-ei̯ā with a meaning of ‘broad’, perhaps ‘broad-leaved’ or similar.  The name ‘lime’, possibly a corruption of ‘line’ originally from ‘lind’, has been in use for centuries and includes any species of Tilia.

The specific epithet cordata is Latin for ‘heart-shaped’, referring to the leaves, platyphyllos is Latin for ‘with broad leaves’, tomentosa is modern Latin for ‘hairy’, ‘covered in hairs’, and euchlora is Greek for ‘greenish’.  Europaea of course means ‘European’.

Status, habitat and ecology.  T. cordata is a woodland species, being a component of oak/hazel woods (National Vegetation Type W8) and  damp oakwoods (W10).   It is incombustible, so forest fires in Tilia woodlands are very rare.  It shades more heavily than many other trees, and therefore tends to suppress shade-intolerant ground flora.  It tolerates poor soils but grows best in fertile conditions. T. x europaea is widely planted but as a native it is now restricted to cliffs and screes in limestone districts, notably in the Derbyshire dales. T. platyphyllos occurs with T. cordata in post-glacial deposits from Cambridgeshire and so is certainly native.

Most of the limes seen in Scotland are T. x europaea planted in avenues. These avenues date back to the 17th and 18th centuries. Typically, these old trees develop large amounts of shoots from the base of the stem (‘epicormic shoots’), and the stem is highly fluted. A good example is at Limetree Walk near Tyninghame in East Lothian, illustrated at the head of this blog. T. x europaea can sometimes be found in semi-natural deciduous woodland, as shown in the photo above taken at a delightful spot in SW Scotland – most probably it was planted by a previous land-owner.

In the European system for classifying vegetation types, lime is a constituent of the Tilio-Acerion forests of slopes, screes and ravines. In Scotland there are some of these forest types, but lime is nearly always missing and replaced by elms, see: https://sac.jncc.gov.uk/habitat/H9180/.

Limes may grow tall (40 m) and reach an old age. Maiden trees may attain 300 years but after coppicing the trees may live much longer; no-one knows how long, but 2000 or even 3000 years has been suggested. An excellent source of information on ancient limes is the Woodland Trust: https://ati.woodlandtrust.org.uk/how-to-record/species-guides/lime/.

Sexual reproduction. Tilia flowers are hermaphroditic. Cross-pollination occurs, mainly by bees seeking the nectar.  Its nectar has been supposed to be lethal to bumble bees, see for example Donath, 1989, but Fossen et al., Koch and Stevenson 2017 and Lande et al. 2019 think not: they found nothing in the nectar that is toxic to bumble bees, and think that the mortality is due to exhaustion.  Seeds are only fertile if produced in exceptionally hot summers (the British Isles are at the north-western limit of its range) (Piggot I), and then germinate in 3rd year after shedding (Rackham 2015).  It is therefore possible that global warming will lead to an increase of Tilia seedlings.

Asexual reproduction and spread.  Lime is capable of pollarding (including self-pollarding) and layering (Branches producing adventitious roots if they bend down to ground level, as Rubus fruiticosus does).  Limes are very easily blown down by comparison with other trees, but, if so, the fallen trunks readily produce adventitious roots and vertical shoots, which are shade-tolerant, so the plant is likely to survive.

Symbioses and parasites.  Tilia is ectomycorrhizal, with a wide range of fungal partners (Rackham 2015).  Lime trees may be susceptible to fungal disease, which can cause root rot and bleeding cankers. Trees can also suffer infestations of aphids, sap-sucking insects and gall mites, including the nail gall, and are occasionally affected by wilt, which can be fatal.

Ancient History and History in the British Isles. An extinct species named Tilia circularis (Chaney) comb. nov. about 39 My BP has been found in Oregon (Manchester, 1994).  There are fossils of T. platyphyllos in Turkey dating to early Pliocene (5.4 my BP) (Kasaplıgil 1975).  Tilia cordata has existed in the British Isles since at least 7500 BP (Birks 1989). Paleobotanical analysis of tree pollen preserved in peat deposits demonstrates that T. cordata was present as a woodland tree in the southern Lake District c 3100 B.C. (Piggott IV) Oak, alder and lime replaced pine and birch c.8000BC.

Uses and Economic significance.  The wood of lime, known as basswood in the USA and Linden in the UK, is light coloured and straight grained with a smooth uniform texture. Because of its light colour, even grain and ease of working, lime wood has been used to manufacture a wide range of objects, and has been extensively used for artistic carving, such as those by Grinling Gibbons, though, because it is rather soft, not for woodcut printing. If you fancy a bit of carving, small quantities of this wood may be delivered to your door via Amazon.uk. It was often used for making cuckoo clocks.  It is used for musical instruments because it does not warp.  There are enthusiastic proponents for Tilia tea, which is said to have health-promoting qualities, and Linden honey, like heather honey in Britain, is prized throughout Europe.

Cultural significance. Tilia cordata is the national tree of the Czech and Slovak republics, and its leaf is a national symbol of Slovenia.

Footnotes.

1In Seamus Heaney’s translation of Beowulf, we read “The warriors’ protector, prince of the hall-troop, ordered a marvellous all-iron shield from his smithy works. He well knew that linden boards would let him down and timber burn”. The Linden boards are presumably their wooden shields.

2The hybrid is sometimes called Tilia x vulgaris. The hybrid is highly variable and some have the status of cultivar in the arboricultural world. Bean (1980) lists 11 cultivars of T. europaea.

3Pigott & Huntley (1981) discovered that the growth of pollen tubes of T. cordata was too slow at the lower summer temperatures prevailing in the northern half of Britain.

Appendix: identification

T. tomentosa:              Tree to 33m.  Young twigs densely hairy.  Leaves 6-11cm (4-13cm Wikipedia), petiole 2.5-5cm (2.5-4cm Wikipedia), longest in cv ‘Petiolaris’, the commonest cultivar. hairless above, densely white tomentose below.  Flowers in pendant cymes, 7-11cm long, of 6-10 creamy-white flowers.

T. platyphyllos:        Narrowly-domed tree to 41m.  Young twigs hairy. Leaves. 6-11cm, petiole 2.5-5cm, dark green above and below, hairy esp.om underside veins.  The small, fragrant, yellowish-white flowers are arranged in drooping, cymose clusters in groups of 1-5(7).  Some specimens in central Europe are hundreds of years old, the oldest (in Slovakia) is 700yr.  There are fossils in Turkey dating to early Pliocene (5.4 my BP).  It is rated RR as a ‘wild’ species by Stace 2019, but is frequently planted.

T. cordata:                  Tree to 37m.  Young twigs glabrous.  Leaves are mostly 3-8cm, dark green and heart-shaped; petioles 2-5cm, glabrous and, according to the Woodland Trust but not other sources, with rusty-red hairs between vein joints.  Spreading cymes with 5-11(15) flowers appear in early summer.  The flowers are held ± erect, are creamy-white and have a rich, heavy scent which attracts many insects, particularly bees. They are followed by the conspicuous winged fruits.

T. × europaea:            Tree to 46m.  Young twigs glabrous. the flowers are held beneath the bract. Leaves mostly 6-10cm, petioles 2.5-5cm, with tufts of white hairs between vein joints.  Cymes of 7-9 flowers pendant among the leaves.  The flowers are fragrant, and pollinated by bees. The fruit is a dry nut-like drupe 8 mm (0.3 in) diameter, downy and faintly ribbed.  It is frequently attacked by honeydew-secreting aphids.  It is rated RRR as a ‘wild’ species by Stace 2019, but is frequently planted.

T. × euchlora            Tree to 22m, twigs ± glabrous, leaves 5-8cm, petioles 4-5cm, cymes pendant with 5-7 flowers.

References:

Bean, W.J. (1980). Trees and Shrubs Hardy in the British Isles, vol. IV. John Murray, publisher.

Birks, H.J.B. (1989). Holocene isochrone maps and patterns of tree-spreading in the British Isles.  Journal of Biogeography 16(6) 503-540.  DOI: 10.2307/2845208 https://www.jstor.org/stable/2845208

Brewer, S., Giesecke, T., Davis, B.A.S., Finsinger, W., Wolters, S. and Binney, H. (2017).  Late-glacial and Holocene European pollen data.  Journal of Maps 13(2) 921-928. https://doi.org/10.1080/17445647.2016.1197613

Donath, H. (1989).  Erhebliche Verluste bei Hummeln und anderen blütenbesuchenden Insekten durch ausländische Lindenarten (Tilia tomentosa Moench, Tilia euchlora C. Koch) [Considerable losses in bumble bees and other flower-visiting insects due to exotic species of lime (Tilia tomentosa Moench, Tilia euchlora C. Koch)] Archiv für Naturschutz und Landschaftsforschung 29(2) 117-120. (In German)

Fossen, T., Holmelid, B. and OlavØvstedal, D. (2019) Bumblebee death associated with Tilia × europaea L.  Biochemical Systematics and Ecology 82, 16-23 https://doi.org/10.1016/j.bse.2018.11.001

Kasaplıgil, B. (1975).  Pliocene Flora of Güvem village near Ankara, Turkey, Abstracts of the Papers Presented at the XII International Botanical Congress, Akademika Nauk SSSR, 1 115, Leningrad.

Koch, H. and Stevenson, P.C. (2017) Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa).  Royal Society Biology Letters 13(9).  No page nos. given.  DOI: https://doi.org/10.1098/rsbl.2017.0484

Lande, C., Rao, S., Morré, J.T., Galindo, G., Kirby, J., Reardon, P.N., Bobe, G. and Stevens, J.F. (2019).  Linden (Tilia cordata) associated bumble bee mortality: Metabolomic analysis of nectar and bee muscle.  Plos One https://doi.org/10.1371/journal.pone.0218406

Manchester, S.R. (1994). Inflorescence bracts of fossil and extant Tilia in North America, Europe, and Asia: patterns of morphologic divergence and biogeographic history.  American Journal of Botany 81(9) 1176-1185.

Pearman, D. (2017). The Discovery of the Native Flora of Britain & Ireland. BSBI.

Pigott, C. D. (1989). Factors controlling the distribution of Tilia cordata Mill. at the northern limits of its geographical range. IV. Estimated age of trees.  New Phytologist 112 117-121.

Pigott, C. D. (1991). Biological flora of the British Isles Tilia cordata (Miller) (T. europaea L.pro parte, T. parvifolia Ehrh. Ex Hoffm., T. sylvestris Desf., T. foemina folio minore Bauhin). Journal of Ecology 79 1147-1207.

Pigott, C. D. and Huntley, J. P. (1978). Factors controlling the distribution of Tilia cordata at the northern limits of its geographical range. I. Distribution in North-West England. New Phytologist 81 429-441.

Pigott, C. D. and Huntley, J. P. (1980). Factors controlling the distribution of Tilia cordata at the northern limits of its geographical range. II. History in North-West England. New Phytologist 84 145-164.

Pigott, C.D. and Huntley, J. P. (1981). Factors controlling the distribution of Tilia cordata Mill. at the northern limits of its geographical range. III. Nature and cause of seed sterility. New Phytologist 87 817-839.

Pigott, C.D. and Sell, P.D. (1995).  Nomenclature of the European Species of Tilia. I. Tilia europaea L.  Kew Bulletin 50(1) 135-139. DOI: 10.2307/4114618   https://www.jstor.org/stable/4114618

Rackham, O. (2015). Woodlands.  William Collins (Harper Collins), London.  ISBN 978-0-00-815691-6.

Rich, T.C.G. & Jermy, A.C. (1998). Plant Crib. BSBI.

Stace, C. (2019).   New Flora of the British Isles, 4th edition.  C & M Floristics. 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: