Appendix 9 Plant flash

How can we increase wildlife diversity?

Article published in Natuurpunt's Plant Flash 20 April 2021

Pollarded willow rows are ubiquitous in the Flemish landscape. Despite their great visual and historical value, opportunities in terms of biodiversity have been missed in the past because the willow legs usually come from the same mother tree. In this post, willow enthusiast Pol Meert explains how and why it is best to pay more attention to biodiversity when planting willow rows.

If you look closely, you will see that pollarded willows from the same row often look suspiciously similar. Logical, as willow legs are usually taken from the same parent tree. Add several generations back, and there is a chance that all trees in a given area are genetically identical. What looks like a row of many trees to the observer is actually just ... one tree to nature. In this case, it is a clone, because species and genus are the same for all specimens. Although many pollarded trees also give a lot of tree volume, genetic uniformity poses some problems.

1.  The presence of only one willow species excludes many organisms bound to other willows. Bees and bumblebees need pollen for their development, which they will not find in an area with a female willow clone. In turn, certain insects feed in or on female willow clones, so you will hardly find them in a male plantation. 

2.  Clones show the same susceptibility or resistance to certain diseases, fungi and other parasites. Suppose the trees present are well resistant to aphid infections, there is little for aphid-eaters to eat there. If resistance to a particular disease is lacking then not one will survive the disease. 

3.  Phenology runs concurrently, so everything converges in a time span of just a few days in leaf, in bloom, in fruit, ... which is detrimental to insects. You can compare this to a city where all shops are closed at the same time. 

Man adds to this by planting and maintaining largely at the same time. Consequently, all trees are the same age and are cut at the same height and at the same time. And it sometimes takes years before those pollarded willows return to catkins. Therefore, whole populations of animals are regularly forced to move. If they get to their new destination at all ... 

The ideal pollarded willow row

Imagine a pollard willow row where all the trees are genetically different: several species each with their loyal followers, male and female specimens grow interchangeably and their flowering periods are spread over several months. Then vary the frequency and height of pollarding and you create a real hotspot for biodiversity.

Pollarded willow = fetish? 

Who said that pollarded trees always have to be willows? Numerous other trees such as black poplar, ash, sweet chestnut, black alder, Spanish beech, lime, large-leaved lime, pedunculate oak, hornbeam, ... also lend themselves perfectly to pollarding.  

As cultural and ecological landscape elements, the existing pollarded willows should better not be replaced too abruptly by another planting. They are best maintained properly where relevant. Planting new lengthy rows of the same willow clones for pollarding has little natural value. By the way, pollarding willows is very intensive, so think before you start. Even when modern, expensive equipment is used, pollarding remains an arduous task involving time, money and CO2 emissions. 

Plea for a low-bush willow hedge

A diverse low-bush willow hedge is more ecological and easier to maintain. It can consist of the following species and hybrids: S. aurita, S. cinerea, S. cinerea x S. aurita, S. caprea x S. cinerea, S. atrocinerea, S. eleagnos, S. Americana, S. gmelinii var. gewone kletters, S. myrsinifolia, S. purpurea, S. triandra x S. viminalis, S. udensis ‘Sekka’, S. viminalis, S. caprea x S. aurita, S. aurita x S. atrocinerea, S. aurita x S. viminalis, S. cinerea x S. atrocinerea, S. caprea x S. atrocinerea, S. purpurea x S. cinerea, S. cinerea x S. repens. Plenty to choose from, then, and preferably both genera are planted. Unfortunately, not everything is always available. These willow hedges can be interrupted here and there by a standard willow or another tree species that is allowed to grow: Pay attention to the planting distances for willow hedges. There are weak and strong growers, choose the distances so that the weak ones are not competed away. 

Pollarded willows anyway? 

Willow taxa that may (and do) qualify for pollarding are: S. alba, S. alba var. caerulea, S. caprea, S. euxina, S. x fragilis with var. furcana – vitellina, S. triandra, S. pentandra, S. pentandra x S. euxina, S. triandra x S. euxina, S. caprea x S. viminalis, S. cinerea x S. viminalis, S. gmelinii var Kattekletters, S. daphnoides. Also (native x non-indigenous) S. acutifolia, S. apennina, S. caprea x S. daphnoides, S. caprea x S. aegyptiaca.

Want to read more about the diversity of willows in Belgium? 

Willow specialist Arnout Zwaenepoel prepared a paper on Willow diversity in Belgium and the Netherlands in which traditional basket weaving played a key role. 

https://www.dendrologie.be/sites/default/files/Downloads/190525_SalixDiversityBelgiumThe%20Netherlands_Zwaenepoel.pdf

To name wild willows (wild and feral), A. Zwaenepoel prepared a key that you can find here. https://www.natuurpunt.be/sites/default/files/images/inline/wilgensleutel_2018.pdf

 Finally, there is "De Wilgenzoeker" by Pol Meert, which repeats Zwaenepoel's key and adds info on all the willows you can find here (including garden species). You can always contact wilgen@natuurpunt.be with questions about willows. 

Text & photos: Pol Meert, willow enthusiast 

Read more about willows: https://www.natuurpunt.be/pagina/wilgen  (in Dutch)

Willows and catkins

The name 'katjes'

According to the Oxford English Dictionary, the word katje/catkin entered English in 1578 when Henry Lyte (1529-1607) coined it in his translation of Dodoens' New Herbal as a translation of the Dutch "katteken" (kitten) used for the downy inflorescences of willows and other plants. ( Dodoens, 1578 )

Rembert Dodoens, a renowned botanist, was born in Mechelen (1517).

Willow kitten

A willow kitten is a collection of willow flowers. Depending on the willow species, the number of flowers varies. An average of 100 flowers per willow kitten is assumed. A kitten of a forest willow has a length of 3.5 cm to 4 cm. A flower is about 8 mm long.

Colour: most female catkins vary from grey-green to green. In male catkins, the colour range is wider: from lemon yellow to egg yellow and sometimes brick red. There are even black catkins: Salix gracilistyla 'Melanostachys'.

Why does a willow produce catkins?

It is important to know that a willow is either female or male. So there are male catkins and female catkins. Never together on the same tree, with a few exceptions. But these, as usual, confirm the rule.

Since the willow wants to produce seeds, pollination must take place. Even if willows are close together, some help is needed to get the male pollen to the female pistil.

Help can come from the wind but mostly from insects anyway. Willow pollen is usually sticky and this sticks to insects which then transfer it.

Why are insects attracted to willow catkins? 

Pollen is an important food source for many insect larvae: proteins, fats, minerals, vitamins. And the nectar provided by the catkins also means: sugars, vitamins, minerals, flavourings.

Male catkins provide both pollen and nectar, the female only nectar.

The nectar is produced in small glands located at the base of each flower. Such a nectar gland is tiny (usually 0.5 mm) but a real source. Depending on the willow species, flower heads have either 1 or 2 nectar glands.

Every year?

Catkins emerge every year. But they are already formed in the previous year in the catkins on fresh young twigs. Usually, they can already be recognised by the somewhat thicker buds. The buds from which leaves emerge are thinner.

Often, but again depending on the species, the first buds, starting at the end of the twig, are leaf buds followed by a number of thicker catkins.

Flowering times

Interesting both for willows and insects (and for beekeepers), is the fact that flowering time is spread over a period of time. So that there is a whole time to feast on. The prerequisite, however, is that a diversity of willows is present. A pollard willow row consisting of 30 clones of the same species provides a food bomb for 14 days, but after that the song is over. Moreover, with female clones, there is no pollen. So rather a diversity of willows.

That diversity, on the other hand, also causes willow hybrids. A willow easily crosses with a grey willow. Which in turn results in a slightly different flowering time for those catkins.

Flowering times can also differ according to climatic conditions and regions. In Flanders, the forest willows are usually finished flowering, while in the Ardennes the catkins have yet to bud.

 After flowering

The female catkins that were lucky enough to be pollinated by the male pollen then form box fruits from which fuzzies emerge with a tiny (0.1 mm) seed inside. And then it takes help from the wind to make that fluff-with-seed land in a suitable place.

Depending on the species, the willow seed has a few hours to a few days to germinate. Here, too, there are some exceptions such as the bay willow.

A suitable place is moist bare soil. Moisture and sufficient light are crucial. Banks of canals, streams, rivers and ponds where the water level fluctuates are the preferred sites for willow seedlings. 

-----

quote

"In Flanders, willows, especially the short-cat species, are home to no fewer than eight specialised bee species and are used by almost all wild bees that fly in spring, both for pollen and nectar. On the online database waarnemingen.be, no fewer than 62 species of wild bees (incl. bumblebees) were thus reported foraging on flowering willows."

From Natuurfocus jaargang 21 nr. 2 - juni 2022

-----

"From the analysis (Kennedy & Southwood 1984 study), willows and native oaks emerged as the most species-rich tree species, with 450 and 423 phytophagous insect species per genus, respectively. The exercise was later repeated in Germany (Brändle & Brandl 2001). 

In continental Europe, the species are even higher, but the ranking of tree species remains almost the same: 728 species for willow Salix sp. and 699 species for oak Qercus sp."

Natuurfocus jaargang 21 nr. 2 - juni 2022

---------

'Attraction of Bees to Native and Introduced Willows (Salix spp.)'   Gabrielle Grandstaff, Yulia A. Kuzovkina, Ana Legrand

 https://www.mdpi.com/1999-4907/14/5/959

"The data obtained during this investigation suggested that bees did not have a preference for either native or introduced willows, and the origin of willows did not affect bee abundance or assemblages at the subfamily levels."

-------------

Determination catkins  White willow - Crack willow

info  https://de.wikipedia.org/wiki/Prim%C3%A4rblatt#cite_note-Wagenitz-1

key  https://www.zobodat.at/pdf/AZBG_27_0001-0170.pdf

Male catkins

Trees (rarely shrubs), about 3-30 m high; catkins earlier than leaves; primary leaves ± lanceolate

 Bract flower outer surface at top and usually long shaggy to base, rounded at top; twigs glabrous, clay-coloured, shiny, brittle at the

base, broadly branched; buds externally strongly convex, glabrous; primary leaves completely glabrous underneath, only silky twisted.

Catkins about ½ cm stalked, 3-5 cm long, 4-5x as long as thick; filaments hairy at base;

  > S. fragilis (meant euxina)

Bract flower mostly as in fragilis, rounded at top; twigs glabrous, only the bud axils (?) hairy, reddish-brown, brittle or tough, broadly to sharply branched; buds outside ± domed, sparsely hairy or glabrous; primary leaves spreading to loosely silky hairy at base.

Catkins similar to fragilis

  > S. x rubens

Bract flower petal bare on the outside at the top, curly-ruffled only at the base, pointed at the top; twigs hairy towards the top just like the buds, in wild forms olive brown to dark reddish brown, tough; buds flat on the outside, usually adjacent hairy; primary leaves loosely to densely silky hairy underneath.

Catkins about 1 cm stemmed, 3-7 cm long, 5-7x as long as thick; filaments at base hairy.

 >  S. alba

 Additional info: bract flower 2-coloured in fragilis and x rubens; in alba 1 colour (yellow)

Female catkins

Catkins V 2 - W 2 /- 3 cm long stalked, not or slightly pendulous, always sloping after pollination, slender, 3-4/-8X as long as thick; style up to 1/2 mm long; primary leaves with intact margin, not scented.

Bract flower petal long shaggy hairy on the outside at the top and usually to the base, rounded apex; twigs glabrous, clay-coloured, shiny, brittle at the base, broadly branched; buds on the outside strongly convex, glabrous; primary leaves completely glabrous on the underside, only twigged; fruiting buds 0.3/-1 mm long stalked. Catkins 0.5-1--3 cm long stalked, 3-4-8 cm long, 3-4/-8x as long as thick.    

  > S. fragilis

Bract flower mostly as in fragilis, obtuse; twigs glabrous, only the axils of buds hairy, reddish brown, brittle or tough, sharply to broadly branched; buds exterior ± domed, sparsely hairy or glabrous; primary leaves spreading to loosely silky hairy underneath; fruiting buds 0.2-0.5/-1 mm long petiolate. catkins similar to fragilis.

 > S. x rubens

Bract flower outer surface bare at top, curly-ruffled hairy only at base, pointed; twigs hairy at tips like buds, forming olive-brown to dark reddish-brown in the wild, tough, sharply branched; buds flat on outer surface, usually hairy; primary leaves loosely to densely silky hairy underneath; fruiting buds sessile or pedunculate 0.2/-0.3 mm long. Catkins 0.5-1--2 cm long stalked, 3-4/-5 cm lg, 4-8x as long as thick.     

  > S. alba