Recrudescence of foliage, with particular reference to acacias

@arthur_chapman @abedggood @mattbarrett @iancastle @jeremygilmore @alan_dandie @tonyrebelo

Many lineages of woody plants have ’juvenile foliage' different from the foliage of adult/mature individuals. This is an example of heteroblasty (https://en.wikipedia.org/wiki/Heteroblasty_(botany)).

 However, there may be two different phenomena conflated here.

The first is true juvenile foliage, seen in seedlings and, in some cases, also saplings. The second is what I would prefer to call ‘recrudescent foliage’, which can appear even in old plants when they are damaged.
 
An example of recrudescence (https://www.merriam-webster.com/dictionary/recrudescence) is needle-leaf foliage in Cupressaceae (https://crataegus.com/2015/07/01/juvenile-growth-on-junipers-cut-leave-alone/ and https://www.kusamurabonsai.org/articles/juvenile-and-mature-juniper-foliage/#:~:text=Juniperus%20procumbens%20%27Nama%27%20is%20a,take%20care%20of%20a%20bonsai.).

Members of this family are well-known to be able to live for centuries. However, if a mature plant - with its mature foliage of adpressed, scale-like leaves - is damaged, then the regrowth can be composed of needle-like leaves, in some cases foliar-spinescent. Since the plant is so old, does it not seem misleading to use the term ‘juvenile’ to describe this foliage?
 
The genus Acacia (https://www.inaturalist.org/observations?taxon_id=47452) is also worth considering, as follows.
 
Acacia contains many spp. bearing phyllodes (https://en.wikipedia.org/wiki/Phyllode) instead of leaves. All of the phyllodinous spp. have juvenile foliage in the strict sense (i.e. in seedlings, https://www.sciencedirect.com/science/article/abs/pii/S0015379617301270 and https://upload.wikimedia.org/wikipedia/commons/6/68/Acacia_facsiculifera_seedling.jpg).

However, a noteworthy feature of this genus is that few spp. have recrudescent foliage, in sharp contrast to ecologically related lineages with convergent foliage, such as eucalypts (https://en.wikipedia.org/wiki/Eucalypt).

What I call recrudescent foliage is one of the most consistent and ecologically important features of eucalypts (https://www.castlemaineflora.org.au/pic/e/eucal/jleaf/jleaf.htm and https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1926.tb06691.x and https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/0012-9658%281999%29080%5B1944%3AJFATSO%5D2.0.CO%3B2).

However, the genus Acacia, which coexists with eucalypts in most habitats of eucalypts, seems generally devoid of recrudescent foliage even in the spp. that grow into fairly tall trees. So, in Acacia, the well-known juvenile foliage is something quite different from the sort of foliage usually called juvenile foliage in the case of eucalypts.
 
One species of Acacia, namely A. melanoxylon, is known to possess recrudescent foliage. However, this species is also rather extreme in its genus in being a particularly tall tree, associated mainly with fire-free situations, growing even within rainforests in its natural state, and being widely planted for its timber.
 
Even In a genus of acacias such as Vachellia (https://en.wikipedia.org/wiki/Vachellia), there is a difference among juvenile (seedling) foliage, recrudescent foliage, and mature foliage.

The mature foliage is virtually non-spinescent (https://www.inaturalist.org/observations/61102835 and https://www.inaturalist.org/observations/11165998), where it outgrows the reach of even giraffes. This applies also to unpruned saplings in gardens, that have been protected from mammalian herbivores. However, the recrudescent foliage is extremely spinescent, the spines being non-photosynthetic stipules (https://www.shutterstock.com/de/image-photo/vachellia-xanthophloea-acacia-tree-close-thorns-1550994515 and https://www.inaturalist.org/observations/122965582).

The spines, although not green, are technically part of the foliage in a way not true for nodal-spinescent plants. This is because the stipule is technically part of the leaf - in the broad sense of leaf including both the hyperphyll (https://en.wiktionary.org/wiki/hyperphyll) and the hypophyll (https://en.wiktionary.org/wiki/hypophyll).

So, the leaf-stipule complex in Vachellia, which consists of a bipinnately compound leaf and its spiny stipule, is a kind of homologue for what is usually called a ‘juvenile’ leaf on a mature eucalypt that has been damaged.
 
It may seem far-fetched to compare the leaf-stipule complex in Vachellia with the ‘juvenile’ leaf of a eucalypt. However, I would point out that so-called juvenile leaves in eucalypts (which I prefer to call recrudescent leaves) are more than just differently-shaped from adult/mature leaves on the same individual plant. The differences lie not just in the shape and size of the leaf-blade, but also in the presence/absence of a petiole, the arrangement about the axil (i.e. whether opposite or alternate), the surface-texture, and the chemical defences. In many eucalypts, the so-called juvenile foliage is so distinctive that it would seem to belong to a different species from the adult/mature foliage.

There seems to be analogy with this in Vachellia. However, a difference is that it is mainly the stipular part of the hyperphyll-hypophyll complex that is subject to change in the phenomenon of foliar recrudescence.
 
Some spp. of phylllodinous Acacia possess stipular spines. So, I wonder whether these spp., like Vachellia, can actually outgrow the stipular spines when the shrub/small tree becomes too tall to be defoliated by its main mammalian herbivores. No student of Acacia seems to have had this search-image in mind, possibly because of the vagueness implicit in the usual term ‘juvenile foliage’.

Given the current use of the term ‘juvenile foliage’, it may not have occurred to any botanist to ask

• why do most spp. of Acacia, beyond the seedling stage, lack ‘juvenile’ foliage even if damaged? or
• is the stipular spine in Acacia functionally part of the foliage, or part of the stem system?

Bipinnate leaves occur in the seedlings of – as far as I know – all of the hundreds of phyllodinous spp. in the genus Acacia. When an adult individual Is damaged, most spp. do not revert to the divided leaf of the seedling. I have found this puzzling, because

• many eucalypts have an extremely well-developed pattern of recrudescence when damaged, and
• the phyllodes of some spp. of Acacia are convergent with the adult leaves of eucalypts.

One possible explanation for this overall difference between Acacia and eucalypts invokes the following differences between the genera in other aspects of their ecology and life history.

Firstly, most spp. of Acacia are shorter at maturity (being tall shrubs or short trees) than most spp. of eucalypts. Even mallee eucalypts are substantial relative to coexisting Acacia spp., owing to the development of lignotubers in the eucalypts with no counterparts among even those spp. of Acacia (e.g. A. rostellifera, http://worldwidewattle.com/speciesgallery/rostellifera.php) that profusely sucker (https://en.wikipedia.org/wiki/Basal_shoot).

Secondly, Acacia tends to be short-lived despite having dense wood. In many species, senescence sets in after only a few decades of life. This combination of limited body size and limited lifespan, together with a capacity to fix atmospheric nitrogen, may mean that there is comparatively little advantage in developing a particular form of foliage during vegetative regeneration. Acacia tends to respond to wildfire by dying and recovering germinatively.
 
It may shed light on these questions to examine some of the few spp. of Acacia that do possess recrudescent foliage.

One is A. melanoxylon, as mentioned above. The other is Acacia adunca (https://en.wikipedia.org/wiki/Acacia_adunca and http://worldwidewattle.com/speciesgallery/adunca.php and https://www.anbg.gov.au/gnp/gnp9/acacia-adunca.html and https://www.anbg.gov.au/gnp/gnp9/acacia-adunca.html), which is a tall shrub or short tree (maximum height 7 m) restricted to a small area around the border between New South Wales and Queensland. Acacia adunca is cultivated extensively in New Zealand, where it is known as Golden Glory.
 
Holiday (1974), page 8, states of A. adunca: “the feathery true leaves persist for some time on young trees and return when the tree is cut.” This seems to indicate both that the juvenile (seedling) foliage persists into the sapling stage, and that what I call recrudescence occurs in this species, and perhaps also its closest relatives within the genus Acacia, viz.

• A. betchei (https://en.wikipedia.org/wiki/Acacia_betchei), and
• A. linearifolia (https://en.wikipedia.org/wiki/Acacia_linearifolia).

Given that ‘juvenile’ poorly describes the post-seedling heteroblasty discussed above, can readers improve on my suggested new term of ‘recrudescent’ for the foliage in question?

Publicado el junio 25, 2022 05:47 MAÑANA por milewski