Ivy growing on trees – is there a preference?

Ivy (Hedera helix), which is classed as a woody liana, can be both good or bad for a tree, depending upon the setting. For example, in urban locations where trees exist as solitary specimens for largely amenity purposes then ivy may be an issue, because its presence increases the wind sail of a tree and also detracts from its amenity value (one cannot appreciate the tree’s structure so readily). Conversely, in a setting where a tree is retained for ecological value, or it simply sits amongst a group (copse, woodland, or forest) of trees, then ivy may be hugely beneficial. The love that ivy receives is therefore relative.

This pine (Pinus sp.) was so laden with ivy in its crown that it got blown over in the wind.

Looking beyond the mere pros and cons of ivy however, what drives its colonisation? Does ivy discriminate between trees as to which it will ascend and thus colonise extensively (in time), or does it simply colonise whatever host it has in its immediate grasp? In a recent study from 2013 undertaken in the Siro Negri forest in Italy, such questions are answered.

Before looking at the results from the study however, it is important to understand a bit more about the Siro Negri forest. Located in northern Italy, it is a small remnant (9ha) of alluvial forest once more abundant across the landscape. A habitat considered ideal for ivy (climatically), tree species include field maple (Acer campestre), hornbeam (Carpinus betulus), hawthorn (Crataegus monogyna), white poplar (Populus alba), black poplar (Populus nigra), oak (Quercus robur), false acacia (Robinia pseudoacacia), and elm (Ulmus minor). Many trees are over 100 years of age, and there has been no significant disturbance since 1970.

Back to the study, the authors first identified four plots of 250-320 square metres in 2005 and 2009, and within these plots measured the DBH all trees. Those with a DBH of over 7.5cm were tagged. In 2011 (I don’t know why there was such a wait), the authors returned and undertook an assessment of all ivy stems growing upon the tagged trees that had a DBH of greater than 0.5cm. For all tagged trees, they were assigned one of five categories relating to the colonisation extent of ivy upon their structure: (0) ivy not present / below 1.3m on the trunk; (1) ivy present upon lower half of trunk; (2) ivy extending up into second half of trunk but not yet into the crown; (3) ivy within lower crown’s principal branching structure, and (4) ivy across entire crown.

In terms of further data gathered, on top of the DBH measurements undertaken in 2005-2009, radial cores at a 50cm height were taken from each tree to estimate age in 2011. If age was not evident as ring growth was asymmetrical or decay was present, the core was discarded. Similarly, radial cores from 50 randomly-selected ivy stems (43 once some were discarded) were taken at a 50cm height, again in 2011. The number of ivy stems present on each tree was also measured.

Moving on to the all-important results, the authors found that there were a greater number of ivy stems of 0.5cm-1cm in diameter than there were of diameter between 1cm-2.5cm, and 2.5cm+ (657, 345, and 225 stems, respectively). 52% of tree surveyed were host to more than one ivy stem, though only 17% of trees had ivy growing throughout their crown (category 4).The ivy stem of most significant size was 14cm in diameter, and had 53 growth rings. However, an older ivy stem of 69 years was found, albeit with a stem diameter of just 9.5cm.

In addition to this, not only was it was found that areas where tree density was lower and trunk diameter was greater that ivy stems greater than 0.5cm were more abundant, but that tree age was also associated with increased ivy colonisation (which is expected, considering older age usually means a greater trunk diameter and, at least in woodlands, lower tree density per hectare). Therefore, isolated oaks (Querus robur) of more significant age and size were observed to be host to a greater abundance of ivy stems and overall extent of ivy than the smaller and denser field maples (Acer campestre) and hawthorns (Crataegus monogyna). Other tree species sat in between these two ‘extremes’. However, tree age and size did not appear to majorly influence growth rate of ivy stems; asides from if the tree trunk had no extra space for the ivy stems to grow into. In this sense, the growth rate of ivy across trees of all age and sizes (and also species) was not found to be markedly different.

In this image, ivy can be seen to have colonised upon the trunks quite significantly, though not yet into the crowns of the trees. Source: Weed Wise.

Perhaps these results are not all too surprising, given the longer a tree has been in existence the longer that ivy has the potential to grow up alongside and into the tree’s crown. Granted, a younger tree with a greater trunk diameter than an older tree would, as suggested by the authors, still very likely support better ivy growth. The ‘ascension’ strategy of ivy may be also an important factor in why larger trees are more extensively colonised, however. Because ivy will attach itself to its host via adhesive adventitious roots, larger trees with rough bark (such as Quercus robur) can support ivy far more successfully than a younger tree with smooth bark. In fact, given bark typically roughens quite extensively with age (excluding species such as Fagus sylvatica), it can perhaps also be suggested that older trees are more abale to support significant ivy growth by mere virtue of older age bringing with it the rougher bark.

Looking at the crown of a tree, the fact ivy was observed to colonise large oaks of a healthy age is actually quite interesting. Oaks usually have dense foliage crowns, which would mean any constituent ivy is less able to obtain the light it needs to photosynthesise (which is identified, by the authors, as a reason for why ivy will prefer to grow into the crowns of tall trees with smaller leaves and generally lighter foliage crowns). Therefore, we can potentially conclude three things: (1) ivy will ‘rank’ large trunks with rough bark as more important than lighter crowns, (2) the oaks in the study had thin crowns, or (3 – as suggested by the authors) that isolated large trees, even if they have dense crowns, can still provide prferable light conditions (the adverse impact in negligible) for ivy.

A more isolated tree also means that its trunk is exposed to more light, which is also beneficial for ivy. This may explain why isolated trunks had more ivy stems than similar trunks in denser stands. Crossing over into urban areas therefore (this study was done in woodland), we can begin to understand why exposed, older, and larger trees (particularly oaks, though interestingly also hawthorns) can become so laden with ivy.

Having understood the above, such a sight should perhaps now be wholly understandable. Source: Cronodon.

In light (pun intended) of the above therefore, hopefully we can begin to understand what drives ivy colonisation. From looking around on Google Scholar, further articles exist on lianas (and herbaceous vines) of other species and their growth rate / extent upon trees, though it largely appears to be an unexplored topic. Perhaps scope exists for additional research, and particularly for ivy.

Source: Castagneri, D., Garbarino, M., & Nola, P., (2013) Host preference and growth patterns of ivy (Hedera helix L.) in a temperate alluvial forest. Plant Ecology. 214 (1). p1-9.

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Ivy growing on trees – is there a preference?

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