Hakea sericea vs Hakea decurrens ssp. physocarpa

In July 2019, when I started learning about invasive alien plants in Portugal, I learned that one of them was Hakea sericea. [1] I had a faint idea of having seen a few in Tróia (near Setúbal) in my childhood, but no recent memory of them. Then, one year ago, I heard of their presence in Lousã... and a few months later I finally met them: a frightful invader indeed, with all those needles. I believed it was easy to identify, just having to differenciate it from a small pine tree.

According to [1], this is how to identify Hakea sericea:

  • Shrub or small tree(#) up to 4 m, with an irregular canopy.
  • Leaves: evergreen, in the shape of a needle of 0,5-1,5 mm(#) diameter, very robust, of 4-8 cm(#), extremely sharp, dark green to greyish-green.
  • Flowers: white, bland, arranged in axillary fascicles of 1-7 flowers.
  • Fruits: woody follicles, dark brown, with a patent crest and beak, having two black, winged seeds.
  • Flowering: January to April.

(This description might be wrong: the features indicated with (#) actually seem match Hakea decurrens [5] more closely than Hakea sericea [4].)

About a week ago, some fellow users of iNat refused to identify as Hakea sericea a plant that looked just like one to me, claiming that it could be Hakea decurrens ssp. physocarpa instead. [3] Apparently the spiky Hakea species observed in Portugal had been mistakenly identified as Hakea sericea for years. In that case, hundreds of observations at https://www.inaturalist.org/ and https://www.invasoras.pt/ would have the wrong ID.

Leaving the ID at genus level is not a good option: Hakea sericea or Hakea decurrens, whatever it is, is very different from the other Hakea species observed in Portugal (Hakea salicifolia) and probably has different environmental and economical impacts: for example, Hakea sericea or Hakea decurrens is much harder to remove than Hakea salicifolia, because of its needles. Moreover, leaving the ID at genus level will prevent the observations from reaching Research Grade, eventually causing those observations to be discarded by researchers. This issue has to be settled: we need to decide whether it is Hakea sericea or Hakea decurrens ssp. physocarpa and then identify all observations correctly.

I will now try to contrast the descriptions of both species, from a couple of sources, and compare them with my own observations. In the following, HS refers to Hakea sericea, HDp refers to Hakea decurrens ssp. physocarpa, (+) indicates features that I believe to have seen and (-) indicates features that I believe not to have seen in the spiky Hakea plants around Lousã, (?) indicates features that I have not yet confirmed, (!!) highlights features that are described differently in different sources.

  • Overall aspect:
    "Sericea appears much denser(+) than decurrens." [10]
    ~-~
    HS: spreading bushy(+) shrub 1–3 m(+) high [4]
    ~-~
    HD: spreading shrub to small tree 0.3–5(-) m high [5]
    HDp: small tree or shrub, 0.8–5(-) m tall [8]

  • Young branches / branchlets (!!)
    HS: young branches white(?)-pubescent(+)(?), glabrescent(+) [4]
    HS: branchlets persistently woolly(-) tomentose(-) [7]
    ~-~
    HD: new growth glabrous to sparsely or densely(-) hairy [5]
    HD: branchlets quickly glabrescent(+) or persistently and densely tomentose [7]
    HDp: branchlets sparsely to densely appressed-sericeous, quickly glabrescent or persistent to flowering [8]

  • Lignotuber
    HS: not lignotuberous [6]
    ~-~
    HD: lignotuber present(-?) [5]
    HD: lignotuberous [6]

  • Leaves (!!)
    HS: leaves ± at right angles(+?) to stem [4]
    HS: leaves spreading widely(+) to narrowly(+) angled to stem, flexible(-?) or rigid [6]
    ~-~
    HD: leaves spreading widely from stem [5]
    HD: leaves spreading widely from stem, rigid [6]
    HDp: leaves widely spreading, grooved below to varying extents, 1.5–8 cm long, 0.7–1.6 mm wide, rapidly glabrescent; apex porrect, with mucro 1–3.5(+) mm long [8]

  • Mucro
    HS: 1-3mm(+) long [4]
    ~-~
    HD: 1mm long [5]

  • Rachis
    HS: usually simple(+?), mostly 3–8(+?) mm long [4]
    ~-~
    HD: knob-like(-?), [up?] to 3 mm long [5]
    HDp: rachis simple, 0.5–2.8 mm long, with tomentose or appressed white and/or ferruginous hairs, extending onto pedicels [8]

  • Pedicels (!!)
    HS: sparsely(+?) white-pubescent(+) [4]
    HS: pedicel villous(-) to hirsute(-) [6]
    HS: pedicels villous(-), hairs white [7]
    ~-~
    HD: pubescent(+) [5]
    HD: appressed(+) pubescent(+) [6]
    HD: tomentose or appressed(+) pubescent(+), hairs white(+) and/or rust-brown [7]
    HDp: pedicels 1.2–4.8 mm long [8]

  • Perianth
    HS: 4–5 mm long, white, glabrous [4]
    HS: perianth (-)2.5–4.7 mm long [7]
    HS: perianth white [6]
    HS: flowers white, pink in bud(+) [7]
    ~-~
    HD: 4–7(+) mm long, white or sometimes tinged pink(+), glabrous [5]
    HD: 4.2–7.2(+) mm long [7]
    HD: perianth white or pink(+) [6]
    HDp: perianth 4.2–7.2(+) mm long, glabrous [8]
    HDp: white to pink flowers [8]

  • Pistil / gynoecium
    HS: (-)(4.5–) 5–7 (–7.5) mm long [6]
    HS: gynoecium (-)4–7.5 mm long [7]
    ~-~
    HD: 8.5–12(+) mm long [6]
    HD: gynoecium 9–12(+) mm long [7]
    HDp: pistil 8.5–12.2 mm long [8]

  • Follicle
    "Decurrens has smoother(+) fruit than sericea which is tubercular to deeply wrinkled." [10]
    "Fruit on decurrens is narrower(-) and has a prominent beak(-) with two horns but are often eroded. Sericea is broadly ovoid(+) and horns are often obscure." [10]
    ~-~
    HS: ± globose(+) to ovoid, 25–30 mm long, 20–25 mm wide, deeply wrinkled(-), beak 3–4(+) mm long, ± smooth(-) [4]
    ~-~
    HD: ± ovoid, 18–35 mm long, 14–36 mm wide, covered in discrete warts(+); beak prominent(-); horns present(+) [5]
    HDp: fruit 2.1–3.2 cm long, 1.3–2.5 cm wide, finely or coarsely tuberculate(+), obliquely ovate to broadly ovate(-) [8]
    HDp: beak small(+) to moderately large, sparsely pustulate or smooth [8]
    HDp: horns 1–5 mm long [8]

  • Distribution
    HS: (...) on the coast(-) and adjacent ranges [4]
    ~-~
    HD: (...) often grows in sandy or rocky(+?) situations [5]
    HDp: found in eucalypt forest, damp heath or dry scrubland in hilly areas in sand, clay, granite, basalt or sandstone, from sea-level to 300 m [8]
    HDp: (...) also in Portugal [9]

Even after reading all this, I still can't reach a final conclusion: the plants that I see have some features that resemble Hakea sericea, while other features resemble Hakea decurrens ssp. physocarpa. The authors of [11] also aknowledge the ambiguity between the Hakea sericea and Hakea decurrens but choose to mention only Hakea sericea in the title and throughout the paper. Likewise, I guess I'll continue identifying the plants as Hakea sericea, adding the field "Identification confidence" with value "moderate" and the tag "migh be: Hakea decurrens ssp. physocarpa" to those observations.

GLOSSARY:

  • Adventitious: produced in an unpredictable or unusual position [2]
  • Appressed: pressed closely but not fused, e.g. leaves against a stem [2]
  • apex: the tip; the point furthest from the point of attachment [2]
  • Bland: mild, smooth, gentle
  • Bud: a compact growth on a plant that develops into a leaf, flower, or shoot
  • Bushy: growing thickly
  • Axillary: coming from the leaf's axil, i.e. the insertion of the leaf [2]
  • Carpel: the basic female reproductive organ in angiosperms, either consisting of a single sporophyll or a single locule of a compound ovary, with a style and a stigma [2]
  • Crest: a "ridge"
  • Fascicle: a cluster of flowers [2]
  • Follicle: a dry fruit formed from one carpel, splitting along a single suture... [2]
  • Gynoecium: the collective term for all of the carpels of a single flower [2]
  • Hirsute: bearing coarse, rough, longish hairs
  • Lignotuber: a woody swelling of the stem below or just above the ground; contains adventitious buds from which new shoots can develop, e.g. after fire [2]
  • Matted: (especially of hair, wool, or fur) tangled into a thick mass
  • Mucro: a sharp, short point, generally at the tip of a leaf [2]
  • Pedicel: the stalk of a flower [2]
  • Perianth: the collective term for the calyx and corolla of a flower (generally used when the two are too similar to be easily distinguishable) [2]
  • Pistil: 1. a single carpel when the carpels are free / 2. a group of carpels when the carpels are united by the fusion of their walls [2]
  • Porrect: extended forwards
  • Pubescent: covered with short, soft hairs, especially erect hairs [2]
  • Rachis: the axis of an inflorescence [2]
  • Shaggy: having a covering resembling rough, long, thick hair
  • Tomentum: a dense covering of short, matted hairs [2]
  • Tubercle: a small wart-like outgrowth or protuberance of tissue [2]
  • Tuberculate: covered in tubercles
  • Villous: abounding in or covered with long, soft, straight hairs; shaggy with soft hairs [2]
  • Wart: verruga in Portuguese
  • Woolly: very densely covered with long, more or less matted or intertwined hairs, resembling a sheep's wool [2]
  • Wrinkled: having wrinkles or slight folds

REFERENCES:
[1] https://invasoras.pt/en/invasive-plant/hakea-sericea
[2] https://en.wikipedia.org/wiki/Glossary_of_botanical_terms
[3] https://www.inaturalist.org/observations/58692770
[4] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Hakea~sericea
[5] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Hakea~decurrens
[6] https://vicflora.rbg.vic.gov.au/flora/key/2411
[7] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Hakea
[8] http://www.flora.sa.gov.au/efsa/lucid/Hakea/key/Australian%20Hakea%20species/Media/Html/Hakea_decurrens_ssp._physocarpa.htm
[9] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=in&name=Hakea~decurrens+subsp.~physocarpa
[10] https://www.inaturalist.org/observations/59023578#activity_comment_5413253
@belmontmargie: another person, with a lot of plant knowledge, says decurrens has smoother fruit than sericea which is tubercular to deeply wrinkled. Sericea appears much denser than decurrens. leaves in decurrens are widely spreading as in your photo. Fruit on decurrens is narrower and has a prominent beak with two horns but are often eroded. Sericea is broadly ovoid and horns are often obscure.
[11] https://www.researchgate.net/publication/341452607_Pest_Risk_Analysis_for_Hakea_sericea

Posted by mferreira mferreira, September 10, 2020 12:03

Observations

Photos / Sounds

What

Pincushion Trees (Genus Hakea)

Observer

mferreira

Date

August 17, 2020 06:41 PM WEST

Description

A planta identificada é a que se vê na base da imagem.

Photos / Sounds

What

Pincushion Trees (Genus Hakea)

Observer

mferreira

Date

August 17, 2020 06:44 PM WEST

Description

A planta identificada é a que está a crescer junto à rede.

Photos / Sounds

What

Pincushion Trees (Genus Hakea)

Observer

mferreira

Date

August 17, 2020 06:46 PM WEST

Description

A planta identificada é a que se vê na margem esquerda da imagem.

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