“Projecto Sobreiro”: planting some seeds in the São Jacinto dune reserve

Cork oak Quercus suber sapling in the São Jacinto dune reserve

Reserva Natural das Dunas de São Jacinto (the São Jacinto dune reserve) is a 666 ha dry and sandy dune reserve, located on the long peninsula along the north-central coast of Portugal, in between the Atlantic Ocean and the northern part of the Ria de Aveiro (a large intertidal lagoon). It is one of the 30 areas which are officially under protection in the country (a Natura 2000 area, I believe). In the early 20th century, Acacia longifolia, a fast growing invasive leguminous tree originating from Australia, was introduced in order to contain the sandy soil and protect the peninsula from the ocean. After its introduction it formed extensive stands, particularly enhanced by fire events. The most recent fire took place during the summer of 1995, in which about 200 ha of the reserve burned down to the ground. During the years following this fire, A. longifolia completely took over the burned areas and beyond, resulting in a dense monotonous Acacia jungle and some mixed stands, covering around 350 ha. Before this fire took place São Jacinto contained a mosaic of sparse native vegetation including herbs and few shrubs and Pine tree Pinus pinaster(?) plantations. The understory of these plantations included a few scattered individuals of A. longifolia (according to Silva & Merchante, in Moreira et al. 2012).

A. longifolia jungle

Seven years after the fire a study was conducted aiming to evaluate the impacts of the A. longifolia invasion and several techniques for eradication have been implied until now. These have cost a lot of money and none of these appeared to have been successful. A. longifolia continues to be dominating the São Jacinto dune reserve. Eradication of A. longifolia, the most abundant of six invasive species that grow in the area, has been pointed out as being complex and it requires persistence and patience (and a lot of money that could have been used for other matters) (http://www.publico.pt/sociedade/noticia/reserva-natural-das-dunas-de-sao-jacinto-combate-crescimento-descontrolado-de-acacias-83352). I want to present a partial solution here: plant oak seeds.

Silva & Merchante noted introduction (sown or transplanted) of desirable species to accelerate recovery, as this has successfully been implied in some restoration projects (Hartman & McCarthy 2004). Development of A. longifolia requires sunlight, so it doesn’t develop well under a shady forest canopy. At least that’s the idea. Transplanting of saplings would in this case, according to Silva & Merchante, probably be most successful as these will have a height advantage over the invasive seedlings (Galatowitsch and Richardson 2005). Thus far at the reserve, saplings of primarily the Pine trees have been introduced and only few saplings of other (deciduous) species were planted, including a few oaks.

During the last two years I have visited the São Jacinto dune reserve regularly and have noticed the rapid development of A. longifolia. I also noticed that there is no place in the reserve where these Pine trees give enough shade preventing A. longifolia seeds from germinating (and there is a very extensive seed bank of A. longifolia present throughout the reserve). Pine trees may form dense stands when they’re young, but at some point, as their roots don’t go very deep into the soil, storm damage will create large gaps in the canopy, allowing A. longifolia to germinate. Oaks (and other deciduous trees) will root deeper and I am expecting storm damage to be not so much of an issue. This allows for a more closed forest canopy, preventing future germination of A. longifolia much more successfully.

Open Pine tree canopy allowing regeneration of A. longifolia

In the winter of 2012/2013 I collected a few hundred seeds elsewhere in the region of two species of oaks: Quercus suber (the Cork oak; Sobreiro in Portuguese) and Quercus robur (the Pedunculate oak). I planted these at two main locations in the reserve, where their development could easily be monitored and young trees will (hopefully) not easily get destroyed during the clearing of A. longifolia. At a recent inspection I was surprised by the results: probably well over 80% of the seeds had germinated, was alive and in good condition (now who would have guessed that, after such a dry summer!?). A number of saplings that were present, however, were dead. It was obvious that the lack of shade was the cause of this. Surprisingly, even the seeds that were planted amidst dens stands of A. longifolia had germinated and these appeared to be in good condition. I suspect that in the future, regardless of the dense cover of A. longifolia, most of these will grow and eventually develop into mature trees. A. longifolia does not grow very tall at this location and storm damage eventually creates enough open space and light for the oaks to further develop their crown. At least that is what I suspect will happen.

Quercus robur, one of the survivors

Quercus suber, sowed in full sunlight

Why oaks? Oaks do not naturally occur in the São Jacinto dune reserve!?

So what! Neither do the Pine trees. Both species of oaks are native to the region of Portugal and I suspect that due to the relatively isolated location of the peninsula, oak seeds have not managed to find their way to the relatively young reserve yet. Oak seeds can disperse up to several kilometers, especially when these are picked up by Jays Garrulus glandarius or Red squirrels Sciurus vulgaris and buried by them for winter supply. However, considering the distance of the reserve from the nearest oak (largely due to the popularity of extensive monospecific Eucalyptus plantations in the surrounding regions, allowing barely any other native deciduous tree species to grow in the region (there’s barely a single native deciduous forest remaining!)) natural arrival is not to be expected in the near future. Besides these oaks, several other deciduous tree species could be introduced that are native to the region, i.e. Castanea sativa, Fraxinus angustifolia and Tilia spp. (and possibly also another oak species; Quercus faginea (Portuguese oak)?). The oaks however, in particular the Cork oak, will probably come with the most advantages.

In my opinion there are numerous advantages to gain when introducing these species in the São Jacinto dune reserve by seeding them. To summarize:

• Oaks will over time (yes, it will take quite some time, I realize!) provide a more closed canopy, preventing A. longifolia to develop and also killing its long lasting seed bank. This will eventually be a lot cheaper than the manual eradication of this species.
• Introduction of oaks and other deciduous trees will increase chances for wildlife dramatically, as the current diversity of the wooded parts of the reserve are limited (primarily existing of Pine trees, about one native shrub that rarely produces berries due to the drought, and invasive Acacia).
• The presence of deciduous trees, in particular Quercus suber, will decrease chances of forest fires to occur and to spread (and knowing the Portuguese a little you can be sure that fires will continue to occur there in the future!).
• In summer, deciduous trees provide more shade, which will make a walk through the São Jacinto dune reserve a very pleasurable experience for visitors, especially on those ridiculously hot days, as I have experienced there regularly. Additionally, with an increase in wildlife diversity these woods would become a much more exciting place to be.
• Seeding is, compared to planting saplings, a much easier, more effective and cheaper way of introducing these species than planting saplings, as has been done up till now.

There is however one thing that I must point out: the São Jacinto reserve is a DUNE reserve and not a forest. Especially the open dune areas contain unique vegetation and I must therefore make clear that my idea is certainly not to turn the whole reserve into primarily closed woodland!

I therefore want to make the following recommendations for future management of the park:

• Decide what parts of the reserve are designated open dune areas and what parts can be turned in closed broad-leaved or mixed woodland. Of course, in the management of a natural forest it’s never good to create such strong boundaries. Concentrate eradication activities of A. longifolia in and around the more open dune areas and in a few locations inside the woodland areas (but these must be surrounded by oaks).
• Collect and plant as much oak seeds as possible annually from nearby areas, up to the point where the first saplings in the reserve are old enough to produce seeds themselves. Plant the seeds in shaded and partly shaded locations. Crawl through the A. longifolia jungle to plant them if necessary. The few oaks that are currently there are between 2 and 9 years old, I believe, and will start to produce a ‘workable’ number of seeds in about 10 - 15 years from now (a very rough estimation). From then on Jays will do the rest of the work for you.
• Stop the introduction of Pine tree saplings. The Pine trees regenerate successfully by themselves, but have proven to be unsuccessful in the battle against A. longifolia. Pine trees have some advantages (i.e. wood production) and these will probably manage to remain present in the wooded areas. Eventually a mixed broad-leaved forest will develop; interesting for humans as well as for wildlife.
• Sow a high density of oaks in a 50 – 100 meter wide boundary around the park as soon as possible. This will prevent, or at least reduce, the speed in which future fires will spread into the reserve. This boundary will also work as a buffer for A. longifolia seeds from around the reserve to enter. A. longifolia seeds do not disperse very far, usually up to several meters at the most.
• Try out some other native deciduous tree and shrub species that could grow in the wooded parts of the reserve, species that would survive the bare sandy soil and the drought. Try to grow them from seeds straight in the soil, not from saplings. Don’t go crazy over it, try to keep the reserve natural, but keep in mind that for a visitor a single Castanheira can be just as exciting as one hundred Pine trees.

Good luck!

Quercus suber, along the blue trail.

Catry FX, Rego F, Moreira F, Fernandes PM, Pausas, JG (2010). Post-fire tree mortality in mixed forests of central Portugal. Forest Ecology and Management 206, 1184-1192. (see: http://www.uv.es/jgpausas/papers/Catry-2010-FEM_Postfire-tree-mortality-Portugal.pdf )

Fidalgo, B., R. Salas, J. Gaspar and P. Morais (2009). Assessing biodiversity at landscape level starting from forest inventory data (see: http://skog.for.msu.edu/meeting/proc2/Fidalgo_Salas_Gaspar_Morais.pdf )

Galatowitsch, S.M., Richardson, D.M., 2005. Riparian scrub recovery after clearing of invasive alien trees in headwater streams of the Western Cape. Biological Conservation 122: 509 – 521

Hartman, K. M. and B. C. McCarthy. 2004. Restoration of a forest understory after the removal of an invasive shrub, Amur Honeysuckle (Lonicera maackii). Restoration Ecol 12: 154 – 165.

Marchante H, Marchante E, Freitas H (2003) Invasion of the Portuguese dune ecosystems by the exotic species Acacia longifolia (Andrews) Willd.: effects at the community level. In: Child LE, Brock JH, Brundu G et al (eds) Plant invasions: Ecological threats and management solutions. Backhuys, Leiden, pp 75 – 85

Marchante H, Marchante E, Buscardo E, Maia J, Freitas H (2004) Recovery potential of dune ecosystems invaded by an exotic Acacia species (Acacia longifolia). Weed Technol 18: 1427 – 1433

Moreira, F., Arianoutsou, M., Corona, P., De lasHeras, J., (Eds.) 2012. Post-fire management and restoration of European forests. Managing Forest Ecosystems, Vol. 24, Springer, Dordrecht (see: http://books.google.pt/books?id=GXWYBvfIP8sC&pg=PA243&lpg=PA243&dq=sao+jacinto+fire&source=bl&ots=4mNKFlQYjx&sig=NBascBFhVRKGyDFdeZP_SbEqeZg&hl=en&sa=X&ei=u20zUuqKDaKr7Ab_uIH4DQ&ved=0CHwQ6AEwCQ#v=onepage&q=sao%20jacinto%20fire&f=false)