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| One-year old Cork oak Quercus suber seedling. |
Around this time last year I posted a blog entry about my idea to fight invasive Acacia
Acacia longifolia in the forested parts of the São Jacinto dune reserve by sowing acorns, in particular those from
Quercus suber (the Cork oak; Sobreiro in Portuguese) and
Quercus robur (the Pedunculate oak) (
http://www.timvannus.blogspot.nl/2013/09/projecto-sobreiro-planting-some-seeds.html). Besides these oaks (as well as some other species of native deciduous trees) would reduce chances for invasive acacia to grow in the reserve, there were several other reasons for this activity:
- The presence of oaks will increase the diversity of the reserve considerably.
- These oaks will reduce the chances of (large) forest fires to occur.
- More variation in tree species in the reserve would make it a lot more attractive to the public.
This blog entry gives an update on “Projecto Sobreiro", which is supposed to do just that, and provides some recommendations for future years.
Last winter hundreds of additional acorns were planted in the reserve. Many of these had no problem germinating, which resulted in many new seedlings. The summer of 2014 was also relatively moist, so most of this new generation of seedlings survived the hottest months fairly well.
The reserve's own acorns
Before I started sowing acorns there in 2012, there were already about 15 - 20 oaks of various species growing in the reserve. These were all planted as young trees of a few years old. The oldest tree currently is probably around 20 years and about 5 meters in height. An employee of the reserve complained to me that it takes very long before these trees would reach a maturity. That’s true, on the reserve's sandy soil oaks can be expected to develop much slower than elsewhere, so this project takes time. Many of these trees will not grow up straight with a single trunk, but due to the low amount of nutrients they'll branch out lower to the ground, developing into more of a bushy shape (which is normal for oaks growing on sand dunes).
I figured it would take quite some time until the first trees would produce their first acorns. Unexpectedly, this year two of these oaks produced acorns themselves! This event coincides with what seems to be a mast year for oaks in the surrounding region.
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| Acorns on presumably a Pyrenean oak Quercus pyrenaica (or a
hybrid?) in the São Jacinto dune reserve. Note the undeveloped and empty
cupule in the back. |
These two trees produced a total of only about 10 – 15 acorns. It seemed that a number of seeds never developed properly, as there were numerous empty cupules visible. These apparently never contained acorns, possibly because these never had been pollinated(?). I suspect this happened due to the lack of other flowering oaks or oak pollen in or around the park (even though oaks can pollenite themselves). The nearest oaks grow about five kilometers away from the reserve.
Besides the two species of oaks (
Q. suber and
Q. robur), as in 2012, last winter I also planted seeds of another native deciduous tree species: the Sweet chestnut
Castanea sativa. There currently are at least 10 of these growing in the reserve. Most of these are doing well, especially around the more moist parts of the reserve.
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| A one-year old Sweet chestnut Castanea sativa seedling in the São Jacinto dune reserve. |
Drawback
Unfortunately, there’s not only good news. I noticed that some seedlings disappeared or got stepped on, presumably by people that were clearing acacia near the trails. Around the duck pond, the area that I considered relatively save from the influence of human activity, the situation was worse. There numerous seedlings were either trampled upon or these had disappeared under piles of acacia cuttings that had been dumped there by students that cleared the trails around the pond (even though these had been informed about the presence of precious seedlings...).
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| Two-year old Quercus suber seedling after being stepped upon, even though it was marked with a stick. |
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| Two-year old Castanea sativa seedling, saved from underneath a pile of acacia cuttings. |
Currently the large majority of seedlings grow along the main trails of the reserve. Some time ago an employee of the reserve told me it would be better to sow them further away from the trails. The reason for me not to do this (yet) has been the following. After having visited the reserve for a few years I noticed that acacia gets cleared over larger areas in different ways. Until last year a company got paid to do this with the use of some very heavy machinery that would have destroyed any oak seedling in the area.
Last year I noticed that a number of private individuals were allowed to enter the reserve with chainsaws to cut whole patches of acacia for firewood. They were supposed to leave native flora intact. I was rather shocked to see the result of this. Not only was there the constant sound of multiple chainsaws over a large area in the reserve, each individual burned a fire to get rid of the smaller cuttings, as well as some garbage it seemed (at inspection the ashes contained the remains of several plastic bottles, bags and other waste). Besides this it was rather predictable that their activity would open the layer of leaf litter and would create new possibilities for acacia seeds to germinate.
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| A forest patch after it was cleared from acacia by private people. Not a single oak seedling would have survived here. |
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| As expected, the following summer in the same area acacia seedlings are thriving. |
Additionally, I heard that some people were allowed to enter the reserve to collect pine needles, presumably to be put in stables to suit the needs of livestock. Some oak seedlings apparently were destroyed by them in the process. The head of the reserve believes that the São Jacinto dune reserve (the only fully protected nature reserve in the whole region of Aveiro!) is there for the people. Now what people are we talking about here?
Development of the Cork oaks
I also got informed that there are some concerns about whether the Cork oaks would eventually reach reasonable height. The few that have been previously planted there thus far remained small, giving the impression of a bonsai tree. Therefore it would not be of any use continuing sowing Cork oak acorns (and this project should change its name into "Projecto Carvalho"?).
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| Demonstration “bonsai” Cork oak |
However, I still believe that the Cork oak is of importance to the reserve. There are several reasons why the older ones thus far remained small. One of these would be the availability of nutrients. Oaks that were planted in sandy soil of which the layer of leaf litter is absent or consists primarily of moss or pine needles (which take a very long time to decompose) will have a hard time finding enough nutrients. Oaks growing in areas containing even a thin layer of leaf litter have better access to nutrients that have infiltrated in the soil.
Furthermore the amount of sunlight and drought influences the rate in which these trees grow. Another thing that counts is the amount of cover around these trees (competition for light). When planted out in the open the oaks have no need to grow up high in search for light. The bonsai shown above grows almost in full sunlight on bare dune sand with no existing leaf litter layer (just moss that dries up in summer) and no competitors around. Probably not the best place.
The circa 6 year old Cork oak shown below demonstrates that it is indeed possible for them to grow up normally. This tree grows on a very dry artificial sandy dune wall. I must admit that the key to its rapid success is the fact that it gets watered occasionally during summer with water from the duck pond. This water contains small particles of mud (containing also some organic matter) that infiltrate into the soil easily, providing nutrients and keeping the soil relatively moist. Besides this the tree grows up in the cover of other plants, forcing it to grow up in height.
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| Circa 6 year old Cork oak, watered. |
With this I am not saying that all oaks should be watered during summer,
but that the cork oaks will grow faster at locations with a decomposing
layer of (deciduous) leaf litter, out of the sun and amongst other
trees that will stimulate upward growth. I suspect that even amidst acacia stands
these would grow, when the layer of leaves is not too thick for
seedlings to develop.
I suspect that, when canopy cover and a leaf litter layer develops (and nutrients subsequently infiltrate), even the
‘bonsai’s’ growing elsewhere in the reserve over time will grow up to become
normal mature trees.
Re-open the open dune reserve!
As mentioned before there is more to see in the São Jacinto dune reserve than just trees and I’m not aiming to turn the whole reserve into oak woodland. It is regarded an open dune reserve and much of its natural values still occur in the open areas. After the fire that took place there in 1995, the acacia also rapidly took over parts of the open dune habitat. Although in the past much of it has been removed, currently efforts to eradicate acacia focus primarily on areas around the most popular trails in the forested parts of the reserve. Obviously, this is done not only to keep the trails accessible and leave something for the public to see, but also to give them the idea that the acacia is being kept under control.
Unfortunately, during recent years little or no effort has been put into eradicating acacia from the open dune areas anymore. At the same time there seems to be another problem arising there. The pine trees that were once planted (they’re just as naturally occurring there as the oaks) are expanding rapidly. Together with the acacia they are now covering large areas of what once were largely open dunes.
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Besides acacia, the open dune areas have rapidly become
overgrown with pine trees, both purposely planted (left) as well as by natural
regeneration (right). A loss of flora and fauna characteristic for the dune reserve.
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The aerial photo below (partly taken in 2011 and 2012) gives the impression that the São Jacinto dune reserve is still a largely open dune reserve. Closer inspection of this photo and visits to the reserve reveal that in the open dune areas Pine trees have been planted on a large scale. At the same time natural regeneration occurred and those young pine trees are now covering large areas that were previously open. Why it was recently decided to start new Pine plantation remains unknown to me. The reserve does not produce timber for the market and 'protection' of the dunes from wind erosion is at those locations not applicable. These pine trees are of little value to the reserve's wildlife. Underneath them there is nothing more than a thick layer of very slowly decomposing pine needles, leaving no place for any other native plants. This leads to not only loss of flora, but also of other wildlife, like lizards, insects and birds that are characteristic for the dune habitat.
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| This aerial photo gives the illusion of the São Jacinto dune
reserve is still containing open dune habitat. Closer inspection reveals that
most of the open areas are currently overgrown with either Spruce trees or
acacia (source: Google Earth). |
Recommendations
- Stop the reckless clearing of whole patches of acacia and do not allow private people to start clearing acacia for firewood the way that is currently taking place. This does more harm to the reserve than it does any good.
- Continue to clear acacia around the main trails by hand, but be aware of the oak seedlings growing there. In the future these will function as a buffer for the acacia.
- As long as the present oaks do not produce significant numbers of acorns themselves, continue to sow oaks in the forested parts of the reserve wherever this seems possible. Aim for a mixed forest of pine and oak.
- For diversity, add other tree species that are native to the area such as Sweet chestnut Castanea sativa, Prunus avium and Tilia sp.. Concentrate these around the moist parts of the reserve.
- Re-open parts of the overgrown dunes as soon as possible. The São Jacinto dune reserve should not become primarily a closed forest and should certainly not contain large areas of monotonous pine plantations. Locally, allow clear-cuttings, but leave some individual trees as well as groups, scattered over the open dune areas. Avoid cutting trees in the birds' breeding season (February - September).
