The old European oak trees (Quercus robur) in the cemetery pose challenges for arborists. With most of them planted in the late 19th century, they are becoming vulnerable to rot, and to toppling over in storms.
Arborists say it’s difficult to identify why some of the oak trees around Hobson’s grave and the Anglican memorial have declined. There could be many factors – including the trees’ age, differences in water supply to their roots from the concrete coverings/drains, and most importantly, changes in the nature of the underlying water table.
Oaks can grow very quickly in New Zealand conditions – but these in the Symonds Street Cemetery haven’t done so, perhaps because of the poor soils (for them) in this location.
Symonds Street cemetery is located in an area of ‘sterile clay soils’, as identified in an important 1859 geological map by Ferdinand Hochstetter. These soil conditions probably accounted for the fact the ridgeline was then covered only in scrub, with the thicker bush confined to the watered gully.
Auckland Council arborist Simon Cook notes:
“Central Auckland oaks are not that large in comparison to some around New Zealand, nor at different sites in the Auckland region (look at one beauty in Mary Aitkin Reserve, Kohimarama) . They are not as big as many 150 year old oaks in Europe, especially some of those growing in southern England’s higher pH soils.
“Failures are complex also, being related to a raft of reasons eg. pathogens, water stress (too little/ too much -change), nutrient deficiencies (phosphate levels are very low in the central city volcanic soil), physical damage, decay as you say and others.
The oaks were planted to remind colonists of their northern hemisphere home countries, and to symbolize strength and continuity in their new land. The oaks were planted in a grid, alternating with conifers.
Looking back along the paths you can see most of the trees leaning down the slope. Trees have self seeded or been close planted and have grown at odd angles to avoid each others canopies. The first oaks to be grown in the cemetery were inter-planted with conifers to encourage them to grow straight and tall. These conifers were later removed. Some of the trees have massive branches out into the canopy space. Arborists regularly check the health of the trees and undertake pruning to prevent them become unbalanced and prone to falling over.
These oaks have been know to suffer from powdery mildew – a fungal disease that forms a white mat over the leaves in the springtime. This stops the trees from photosynthesising (turning sunlight into chemical energy, and food for themselves).
Fifty to 60 old oak trees were lost in Auckland in the early 2000s in this way.
The fungus that causes powdery mildew has a life cycle based on alternating conditions: it needs moist conditions for spores to germinate (produce or release reproductive bodies), then dry conditions to grow. Sometimes, Auckland weather delivers this in exactly the right sequence.
Since 2000 it's been known that a spray of simple bi-carbonate of soda can prevent this fungus occurring.
Other opportunists cause problems for the oaks: leaf miners (an insect larvae that eats leaves) and oak anthracnose (a fungal disease, shown in dark spots on the leaves) add to stress for the trees in certain environmental conditions.
Oaks can be self-grafting (unlike pōhutukawas).
Where branches rub up against each other, without too much movement, the tissue of the two branches can grow together again, to form this lattice-like structure. This can work to strengthen the tree.