Juniperus communis, a widespread example of foliar spinescence in the Northern Hemisphere

(writing in progress)
 
Several genera of coniferous plants are foliar-spinescent, such as

• Juniperus (https://www.inaturalist.org/observations?place_id=any&taxon_id=47574&view=species),
• Picea (https://www.inaturalist.org/observations?place_id=any&taxon_id=63573&view=species),
• Podocarpus (https://www.inaturalist.org/observations?place_id=any&taxon_id=85284&view=species), and
• Araucaria (https://www.inaturalist.org/observations?taxon_id=53718).

In most of the foliar-spinescent spp., only the ‘juvenile’ (including recrudescent) foliage is spinescent.
 
Juniperus communis (https://www.inaturalist.org/taxa/58725-Juniperus-communis and https://www.conifers.org/cu/Juniperus_communis.php) is a particularly strong example of foliar spinescence in conifers because

• it is extremely widespread, being described as the most widespread sp. of woody plant on Earth,
• it never attains adult/mature foliage even when it reaches its maximum height of up to 16 m, and
• the spinescent leaves can be conspicuous in a way that suggests aposematic colouration (warning colouration) directed at large herbivores.

Please see https://en.wikipedia.org/wiki/Juniperus_communis .
 
In North America and Eurasia alike, J. communis inhabits mainly the boreal ecosystem (https://en.wikipedia.org/wiki/Boreal_ecosystem), which is the biome of coniferous forest on permafrost. It ranges right up to the Arctic Circle (https://en.wikipedia.org/wiki/Arctic_Circle), where trees give way to tundra (https://en.wikipedia.org/wiki/Tundra#) and J. communis itself adopts a nearly prostrate growth-form contrasting with the nearly cypress-like shape adopted in milder climates.

In Europe it extends into the mediterranean climate (reaching as far south as Greece, Sicily, and southern Spain) to a degree not seen in North America. Juniperus communis does occur in California, but only in small areas of the Sierra Nevada (https://en.wikipedia.org/wiki/Sierra_Nevada), a mountain range too high to conform strictly with the mediterranean-type climate.
 
In several photos of J. communis, the leaves appear conspicuously pale, which has puzzled me. The pale surface seems to be on the underside (ventral surface) of the leaf, which is flanked by the rolled-under, green margins of the leaf (https://www.arbolapp.es/en/species/info/juniperus-communis/).

It is not odd for semi-sclerophyllous small-leafed plants in mediterranean-type climates to have leaves with rolled edges, or pale-coloured undersides, features which are usually explained in terms of drought-tolerance and protection of the stomata in the interests of conserving water in transpiration.

However, what is odd is that in several photos the pale surface seems to be uppermost on many of the leaves, indicating that the leaves have been twisted somewhat at the petiole, turning them in a sense ‘upside down’ (https://es.123rf.com/photo_109513477_leaves-of-juniper-juniperus-communis.html and https://www.agefotostock.com/age/en/details-photo/common-juniper-ground-juniper-juniperus-communis-twig-with-leaves-germany/BWI-BS331761).

Since the leaves are prickly (https://en.wikipedia.org/wiki/Juniper#/media/File:Juniper_needles.jpg and https://www.alamy.com/stock-photo-common-juniper-juniperus-communis-with-ripe-and-unripe-berry-shaped-85995011.html?imageid=10BCA4F5-A69F-4DC6-A1F3-6D12670DF40C&p=245512&pn=1&searchId=3cbf2f655c2f750afbd620385a909f13&searchtype=0), it is tempting to think that this sort of display of dark/pale contrast (in which the leaves look like pale needles against a dark background) is a form of warning colouration to browsing herbivores, such as deer, which would forage on the plants.
 
The following (https://en.wikipedia.org/wiki/Juniperus_communis#/media/File:Juniperus_communis_range_map.png) illustrates the distribution of J. communis. The species spans Eurasia and North America above certain latitudes and altitudes.

Nowhere does this species reach the latitudes of kwongan (https://en.wikipedia.org/wiki/Kwongan) in Australia, which (except possibly for Tasmania, https://en.wikipedia.org/wiki/Tasmania) is warmer in winter than any habitat of J. communis.
 
The following shows that J. communis can attain a growth-form similar to that of a typical cypress (https://www.botanikfoto.com/en/details/image-photo-common-juniper-juniperus-communis-558123.php and https://3dbaza.com/juniperus-communis-11832m-109413). However, as I understand it the species retains its ‘juvenile’, spinescent foliage even when its crown attains this mature form, above the height of even the moose (Alces alces, https://en.wikipedia.org/wiki/Moose).

The following shows the nearly prostrate growth-form of J. communis in cold and windy climates, at high latitudes and/or altitudes. I do not know if the leaves are more spinescent in this growth-form than in that shown above.

The following shows J. communis in vegetation otherwise dominated by the erica Calluna vulgaris (https://en.wikipedia.org/wiki/Calluna). This sort of vegetation is a ‘heathland’ comparable with kwongan although growing in a colder climate and being far simpler floristically.
 
The following again shows the coexistence of J. communis with a heathy stratum, the conifer adopting a cypress-like growth-form but apparently retaining ‘pungent’-tipped leaves right to the top of its crown, which in this instance I estimate to be perhaps 5 m high.
 
The following two photos show J. communis in a shrubby growth-form: https://www.alamy.com/stock-photo-alpine-juniper-juniperus-communis-var-alpina-growing-between-rocks-17413802.html?imageid=E624C926-9F6D-42A2-8421-A12BACDF0937&p=56319&pn=1&searchId=3cbf2f655c2f750afbd620385a909f13&searchtype=0 and https://www.alamy.com/stock-photo-common-juniper-juniperus-communis-26690581.html?imageid=5F691C5D-D99D-41C2-B403-7F0E17A20D27&p=9998&pn=1&searchId=3cbf2f655c2f750afbd620385a909f13&searchtype=0.

The following three photos show the conspicuous paleness of the ‘ventral’ surfaces of the spinescent leaves, which are here for some reason turned upwards instead of being invisible in ventral orientation as expected. I suggest that this functions as warning colouration.

(writing in progress)

Publicado el julio 1, 2022 09:04 TARDE por milewski