How do we make sense of leaf-spinescence in Podocarpus?
Among conifers, leaf-spinescence occurs in certain species in the genera Picea (https://www.inaturalist.org/observations/104554252), Juniperus (https://www.inaturalist.org/observations/106243083), and Araucaria (https://www.inaturalist.org/observations/70805032).
It also occurs in a few species of Podocarpaceae, a family occurring mainly in the Southern Hemisphere.
There are 19 genera of podocarps (https://en.wikipedia.org/wiki/Podocarpaceae). The only leaf-spinescent species are several species of Podocarpus and Saxegothaea conspicua.
These can be categorised as follows:
- New Zealand: Podocarpus totara (tree), Podocarpus laetus (tree) and Podocarpus acutifolius (shrub),
- southeastern Australia: Podocarpus spinulosus (shrub), and
- South America: Podocarpus nubigenus (tree, surprisingly similar to totara var. totara of New Zealand), Podocarpus parlatorei (tree), Podocarpus glomeratus (tree), Podocarpus lambertii (tree), and Saxegothaea conspicua (tree).
The biogeographical picture that emerges:
Podocarpus is widespread on southern continents and across the equator to China, Japan and central America. However, it is leaf-spinescent only in South America and southeastern Australasia.
If leaf-spinescence is defensive against folivory, we might expect it to occur mainly:
- in shrubby species,
- at the sapling stage of tree species, and
- in regions with relatively intense folivory.
In podocarps, leaf-spinescence is not particularly associated with shrubby species, thus contradicting predictions.
There is scant information on how leaf-spinescence changes as plants grow from seedling through sapling to mature tree. However, at least in Podocarpus totara it does seem to be true that it is particularly associated with the sapling stage.
With respect to intensity of folivory, no clear trend is apparent.
Kevin Burns has hypothesised that, in Podocarpus in New Zealand, leaf-spinescence shows adaptation to extinct moa (https://www.tandfonline.com/doi/full/10.1080/0028825X.2015.1130727 and https://www.tandfonline.com/doi/full/10.1080/0028825X.2014.997254).
In southern Africa, folivory is particularly intense, and the species of podocarps include both shrubs (Podocarpus elongatus and P. henkelii) and trees (particularly Podocarpus latifolius). Contrary to predictions, no species is leaf-spinescent and P. latifolius is virtually ignored by mammalian folivores - including the savannah elephant (Loxodonta africana) - even at the sapling stage (see https://www.inaturalist.org/journal/milewski/61212-plants-eaten-by-the-savannah-elephant-in-the-cape-floristic-region-part-3#).
Southwestern Western Australia is exceptionally rich in leaf-spinescent plants (particularly shrubs), yet Podocarpus is not leaf-spinescent.
The only podocarp occurring here, Podocarpus drouynianus (https://www.inaturalist.org/taxa/135685-Podocarpus-drouynianus), is specialised as an understorey shrub - never growing taller than 3 meters - in combustion-prone forests of eucalypts. It regenerates from the roots, and fruits only after wildfire. One of its common names hints that its foliage is actually fire-promoting owing to its ‘resin’ content, and if so this is remarkable in a genus generally associated with wildfire-free environments.
The lack of leaf-spinescence in P. drouynianus suggests that podocarps do not follow the ecological patterns set by other leaf-spinescent plants in general.
The explanation suggested by Burns for leaf-spinescence in New Zealand does not seem to apply in South America, where there have been no counterparts for moa.
So it seems that we need to rethink, from scratch, why certain podocarps are leaf-spinescent and why these species mirror each other, approximately, in New Zealand and South America.
The following illustrations of the leaf-spinescent species of podocarps are arranged in alphabetical order.
Podocarpus acutifolius (New Zealand)
https://www.inaturalist.org/observations/104436880
https://www.inaturalist.org/observations/67766856
https://www.inaturalist.org/observations/62269937
https://www.inaturalist.org/observations/6279224
Podocarpus glomeratus (South America)
https://www.inaturalist.org/observations/95208588
https://www.inaturalist.org/observations/75498405
https://www.inaturalist.org/observations/37229525
https://www.inaturalist.org/observations/27366627
Podocarpus laetus (New Zealand)
https://www.inaturalist.org/observations/103446927
https://www.inaturalist.org/observations/90619451
https://www.inaturalist.org/observations/63680565
https://www.inaturalist.org/observations/9923864
https://www.inaturalist.org/observations/8925998
Podocarpus lambertii (South America)
https://www.inaturalist.org/taxa/135630-Podocarpus-lambertii
https://www.inaturalist.org/observations/90216166
https://www.inaturalist.org/observations/86739482
Podocarpus nubigenus (South America)
https://www.inaturalist.org/observations/104370398
https://www.inaturalist.org/observations/49661024
https://www.inaturalist.org/observations/65211377
https://www.inaturalist.org/observations/104816804
Podocarpus parlatorei (South America)
https://www.inaturalist.org/observations/71176095
https://www.inaturalist.org/observations/100028119
https://www.inaturalist.org/observations/64997612
https://www.inaturalist.org/observations/52725210
Podocarpus totara (New Zealand)
https://www.inaturalist.org/observations/45255940
https://www.inaturalist.org/observations/45046558
https://www.inaturalist.org/observations/43190361
https://www.inaturalist.org/observations/42209780
https://www.inaturalist.org/observations/17817035
https://www.inaturalist.org/observations/18485022
https://www.inaturalist.org/observations/11849432
Podocarpus spinulosus (southeastern Australia)
https://www.inaturalist.org/observations/57202753
https://www.inaturalist.org/observations/57202753
https://www.inaturalist.org/observations/41965679
https://www.inaturalist.org/observations/84920489
Saxegothaea conspicua (South America)
https://www.inaturalist.org/taxa/135546-Saxegothaea-conspicua
https://www.inaturalist.org/observations/106251910
https://www.inaturalist.org/observations/105171891
https://www.inaturalist.org/observations/89915794
https://www.inaturalist.org/observations/38799536
Also possibly leaf-spinescent:
Podocarpus sprucei https://www.inaturalist.org/taxa/135718-Podocarpus-sprucei.
Comments
The tree Podocarpus laetus and the more shrubby P. acutifolius are certainly leaf-spinescent in the 'juvenile' foliage. Another species of this genus occurring in New Zealand, namely P. nivalis, is not leaf-spinescent and is essentially the New Zealand counterpart of Australian P. lawrencei.
https://meaningoftrees.com/2019/02/28/totara-podocarpus-totara/
Podocarpus elongatus is restricted to the Fynbos Biome of the southwestern tip of Africa, and is in some ways the African counterpart of Australian Podocarpus lawrencei. It can grow into a tree, but usually occurs as a shrub in marginally fire-prone situations (e.g. on screes) beyond the small patches of fire-free temperate rainforest that occur marginally to the Fynbos Biome. Climate, as much as fire, is responsible for ‘stunting’ P. lawrencei in Tasmania and the mountains of southeastern Australia. The ‘stunting’ factor for P. elongatus is unlikely to be the climate, which is mild. Podocarpus elongatus occurs mainly as a ‘fire-stunted’ form, albeit not nearly to the degree of specialisation seen in P. drouynianus in southwestern Western Australia. Despite being among the more ‘fire-tolerant’ and ‘juvenilised’ species of Podocarpus, and restricted to habitat unusually rich in large herbivores such as the eland (Taurotragus oryx), P. elongatus lacks any leaf-spinescence even in its ‘juvenile’ foliage.
Podocarpus spinulosus of southeastern Australia is not as specialised as P. drouynianus for the understorey of wildfire-prone eucalypt forests, and retains a greater ‘ambivalence’ as a ‘rainforest element gone marginal’. But P. spinulosus is the leaf-spinescenct one. Also in southeastern Australia, Podocarpus lawrencei is even more ‘stunted’ than P. spinulosus but not at all leaf-spinescent.
In genera such as Leptospermum, it is the non-aromatic spp. that tend to be the leaf-spinescent ones. Furthermore, in families such as Rutaceae, those smallish-leafed taxa occurring in fynbos and kwongan are never leaf-spinescent, possibly because this family is so consistenly aromatic. The general idea is that plants defend their foliage either with smelly secondary compounds or leaf-spines, but usually not both. I realise there are exceptions to this rule, but perhaps is is a rough rule of thumb. Some species of podocarps are reputed to have ‘oily’ wood; the fleshy, endozoochorous receptacles in some spp. taste resinous; and the foliage of P. drouynianus is reputed to be extremely flammable. So perhaps some spp. of Podocarpus have more aromatic leaves than others? If so, I predict that there is generally in inverse correlation between the concentration of anti-herbivore compounds and the degree of leaf-spinescence in the various spp. of Podocarpus, comparing spp. and holding juvenile vs adult foliage constant, and perhaps also comparing juvenile with adult foliage within a given species.
Araucaria is essentially a ‘rainforest’ element in eastern Australia, yet shows some of the most extreme leaf-spinescence in the flora of conifers (in the juvenile foliage of Araucaria cunninghamii and in both juvenile and adult foliage of A. bidwillii).
https://www.conifers.org/po/Podocarpus.php
http://tropical.theferns.info/viewtropical.php?id=Podocarpus+latifolius
https://link.springer.com/article/10.1023/A:1009767931817
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