The sexual frustration of the isolated tree

Habitat clearance is threatening global biodiversity. In many places, isolated trees are all that remains of once extensive forest. So what happens to these trees and how can they survive in their new, highly modified environment without becoming the ‘living dead’?

Habitat clearance is arguably the major issue threatening the worlds biodiversity, and is a key driver of extinction. Yet in landscapes that have been cleared for agriculture, isolated trees remain at the margins of cultivated areas (e.g. road verges), in small pockets that are difficult to access (e.g. steep slopes) or are deliberately left to prevent erosion issues (e.g. along watercourses), or for future harvest during tough times (e.g. in the case of high value timber trees). In many regions of the world, these isolated trees may be all that is left of a once extensive forest ecosystem that covered the area. So what happens to trees surviving in these small fragments. can they survive and eek out an existence in their new, highly modified environment? Are they any use for future conservation efforts? Or are they, in the words of the well-known tropical ecologist, Dan Janzen, just the ‘living dead’?

The problem with being isolated

So what are the problems facing such isolated trees? Well imagine that suddenly most of the houses in your hometown, except your house and those of your neighbours, were torn down and replaced by forest.

This is the analogous, but inverse, situation for a small patch of trees left standing after the forest around them has been cut down and planted with crops or tarmacked over to build a city.

The world’s most isolated tree, an acacia in Nigeria, used for decades as a marker by Tuareg traders and caravans, was recently knocked down by a drunk truck driver who thought it was a mirage

Anyway, back to your street now in the middle of the forest. Assuming you can get food and water from the forest, initially this experience might be quite peaceful and tranquil. However as with many things in life, sooner or later it’s going to come down to sex. When you lived in the town, a Saturday night out presented many potential partners to rub shoulders with. But isolated in the forest with only your neighbours for company, your market for partners is much, much slimmer. You may not fancy any of your choices, your neighbours may be your parents, siblings or cousins, or your only choice may be someone with a life threatening disease, which is heritable and would be passed onto your children. There may not even be anyone else of reproductive age left in your street, reducing your future parenting options to zero.

And so these are a similar range of problems facing isolated trees.

Problems for future generations

Following population genetic theory, we can make several predictions for the future genetic health of isolated trees. The first is that the genetic diversity of trees inhabiting small isolated fragments will decrease over time, as the pool and diversity of individuals decreases. The second prediction is that individuals inhabiting small isolated patches will become more genetically different from each other over time due to a reduction in the exchange of genes via pollen and seed, a process termed genetic drift. It is this gene exchange, or gene flow, that maintains the genetic connection between individuals and populations. The third prediction is that individuals will become more inbred over time as individuals are forced to mate with related individuals. Remember your cousin living up the street? The key problem with inbreeding is that it can increase the frequency of some maladaptive characters – remember your neighbour with that genetic condition? This reduction in vigour of populations due to inbreeding is known as inbreeding depression. These three forces, loss of genetic diversity, increasing genetic differences and increasing inbreeding, can also interact synergistically to drive the populations inhabiting such genetic ghettos towards an extinction vortex.

Does the evidence follow the theory?

So are these predictions borne out by studies? Well, interestingly, not really. A number of features of trees can actually help them survive life in isolation. The first of these characteristics is the ability to maintain an extensive gene flow network, even in highly fragmented landscapes. Habitat clearance actually facilitates the wind to blow small pollinators and seeds greater distances across landscapes. The second factor is that trees are long-lived, which allows multiple overlapping generations to coexist, and helps slow the loss of genetic diversity. Third, some trees have what’s called a flexible mating system, and so are  able to pollinate themselves or receive pollen from others, depending on what is available, and so can produce seed even if there are no compatible mates in a landscape.

So isolated trees are fine – right?

Not quite – these characteristics don’t make trees completely resilient to a life of isolation. Rather impacts vary by species, and are more nuanced than simply losing genetic diversity. Some of the key consequences have now been identified in a recent special issue on the topic published in the journal Heredity. These recent findings emphasise that in focusing on adult populations, previous studies have been looking in the wrong place. Instead, we need to focus attention on the quality of seed produced by isolated trees. When we look closely these seed are producing the next generation of trees which are less vigorous due to a combination of lower numbers of potential fathers in the landscape and inbreeding problems.

However the specific landscape context (for example, the scale of isolation and land-use types) and nature of impact (for example, habitat fragmentation versus selective logging) are also important variables influencing the severity of response.

What does this all mean for the future?

These findings are important for deriving advice on how we source seed for ecological restoration. Seeds of isolated trees tend to be quality of lower quality, have poorer germination, grow less vigorously and react poorly to stressful conditions (e.g. droughts, heat waves). Our advice for restoration practitioners is that isolated trees should not be heavily relied on as a revegetation seed sources.

A continued focus on the scientific issues of forest fragmentation genetics is likely to be fruitful and deliver real progress for the management of trees in impacted landscapes. In particular, a more in depth examination of the link between gene flow dynamics and seed quality is likely to yield important insights into the mechanisms driving the changes we observe.

But what is the future prognosis for these isolated trees? We have already heard recently that large and isolated trees are being lost from the world’s ecosystems. The problem is that these trees may be the only remnants of once extensive forests. As such these trees may harbour unique genetic variation that may be useful for the future resilience of populations (e.g. disease resistance), but the seed they produce is suboptimal and not ideal for restoration programs. It is therefore an urgent priority that these genetic resources are preserved before they are all gone. Practices that build habitat restoration around these isolated fragments (e.g. widening road side verges or expanding isolated stands of trees) are required. Without such measures these trees, and the future generations they produce, may not quite be the living dead, but without intervention will certainly remain the very very sick.

By Andrew Lowe and Martin Breed 

References & further reading

Bacles CFE, Jump AS (2010). Taking a tree’s perspective on forest fragmentation genetics. Trends Plant Sci 16: 13–18.

Breed MF, Ottewell KM, Gardner MG, Marklund MHK, Dormontt EE, Lowe AJ (2015a). Mating patterns and pollinator mobility are critical traits in forest fragmentation genetics. Heredity (Edinb) 115: 108–114.

Breed MF, Ottewell K, Gardner MG, Lowe AJ (2011). Clarifying climate change adaptation responses for scattered trees in modified landscapes. J Appl Ecol 48: 637–641.

Lowe AJ, Cavers S, Boshier D, Breed MF, Hollingsworth P (2015) The resilience of forest fragmentation genetics – no longer a paradox – we were just looking in the wrong place. Heredity 115: 97-99. doi:10.1038/hdy.2015.40

Lowe AJ, Boshier D, Ward M, Bacles CFE, Navarro C (2005). Genetic resource loss following habitat fragmentation and degradation; reconciling predicted theory with empirical evidence. Heredity 95: 255–273.

Petit RJ, Hampe A (2006). Some evolutionary consequences of being a tree. Annu Rev Ecol Evol Syst 37: 187–214.

Ward M, Dick CW, Gribel R, Lemes M, Caron H, Lowe AJ (2005). To inbreed, or not to inbreed: a review of mating systems and pollen dispersal variance in neotropical trees. Heredity 95: 246–254.