Understanding the mechanisms that underlie a conifer invasion – aka the Black Pine Project

Author: Paul Caplat

This summary describes the Black Pine research project that was initially started by Yvonne Buckley in 2003 and took-off for good in winter 2009. The contents should be understood by most; if it appears too complex, silly or French, please contact the author, Paul Caplat on paul.caplat@gmail.com

Background

Everybody is familiar with the notion of “weeds”. At home, this word is used to define small plants that grow where they’re not wanted, usually your garden. In science, weeds are plants that have been introduced in new places (for instance, from Europe to Australia) and have negative effects on the local environment (disturbing the ecosystem, threatening other species or causing harm to human populations). Surprisingly enough, under this definition some trees can be classified as weeds! An array of tree species have been introduced far from their home (being very useful to produce wood, paper, and incidentally money) that are now spreading in places where they were not supposed to go, replacing native species that are less competitive and disrupting ecosystem functions (1). Such a phenomenon is obvious in New Zealand where pine trees have been planted throughout the century, mostly to create tall hedgerows (shelterbelts) that protect livestock from the violent winds. In many cases they have “escaped” the plantations and threaten the already weak[i] local biodiversity. To understand what drives this invasion and eventually suggest management options, appears essential. Fortunately enough, this is precisely what we intend to do in the black pine project.

Mt Barker forest

Photo 1. Mt Barker Forest

Study site

The Mt Barker Forest study site (Photo 1) is dominated by one species, black pine (hence the name of the project), which has spread dramatically from a small shelterbelt, in the foothills of the Southern Alps in Canterbury region, on the South Island of New Zealand. As with any study site, Mt Barker forest has upsides and downsides. The upside is the complete dominance of black pine that allows this project to:

  1. study the system with a single-species approach;
  2. have a well-known history of the site;
  3. easily conduct a research project there because the land belongs to the University of Canterbury; and
  4. have on-site accommodation thanks to a little en-suite cabin (Photos 2 & 3).

The downside is that it’s cold, wet, and windy, and quite isolated (three hours drive from Christchurch if you are towing a 4m long trailer (Photo 4) – which happens quite a lot).

Distribution of snow

Photo 2. Outside of hut

Hut_kitchen

Photo 3. Palatial settings inside hut

Shaun trailer

Photo 4. Longer trailer, complete with PhD slave

Methods

One mechanism that has been identified as essential to explain the spatial dynamics of a plant is seed dispersal (how far and in what conditions a plant kicks its kids out the house). Don’t be surprised then, that this project focuses on dispersal. Black pine is wind dispersed: that means that to understand seed dispersal one has to understand how wind works. Oh, and most pines disperse their seeds in winter: that means that one has to do fieldwork in the cold, wet, and windy NZ winter conditions.

Now we have to get a bit more technical. To fully understand how different mechanisms play a role in pine spread, and how management options could help control the species’ expansion, we aim to build a population (matrix) model (based on one already published (2)) that re-create the different trajectories a pine can take (growing, reproducing, dying) and allow to see what happens at the level of a landscape. Although such models have been developed in many instances, this one requires a very good description of seed dispersal, and that was not readily available. As a consequence we decided to apply a recent model of dispersal (another type of modelling) that has been developed by Gabriel Katul and (amongst others) Ran Nathan, world-leader in the matter of seed dispersal, called WALD (3). WALD is complex, as it takes into account the physics of wind turbulence to compute how far a seed will be carried if it is released a one given location, but it sums up eventually to a quite simple equation that can be easily integrated in a population model.

Of course, many other mechanisms play a role in a population’s dynamics, which are to be integrated in the model: fecundity, growth rate, mortality. So to develop the model we needed to collect quite a large amount of variables on the field.

(i)      Wind data was collected with sonic anemometers (Photo 5 & 6) (that measure wind speed –very accurately by evaluating the time needed to propagate a sound)  placed on 15m high towers, at two locations (to account for heterogeneity)

(ii)    We also collected seeds at different locations and different heights to later support the model (Photo 7) (one sentence in this text, a HUGE amount of work in reality)

(iii)    Demographic data was measured in permanent plots placed along a gradient of tree density (number of pine trees per hectare) by members of the Fellowship of the Pine (Photo 8).

first mast

Photo 5. A mast with sonic anemometers

Paul_mast

Photo 6. The author setting up an anemometer

seedtraptower

Photo 7. A tower with seed traps

Fellowship

Photo 8. The Fellowship of the Pine

Results

The core of the data is under analysis. At this stage, we can offer a few graphic…More to come !

BP_results

References

(1)     Richardson, D.M. & Rejmanek, M. (2004) Conifers as invasive aliens: a global survey and predictive framework. Diversity and Distributions, 10, 321-331.

(2)     Buckley, Y.M., Brockerhoff, E., Langer, L., Ledgard, N., North, H. & Rees, M. (2005) Slowing down a pine invasion despite uncertainty in demography and dispersal. Journal of Applied Ecology, 42, 1020-1030.

(3)     Katul, G.G., Porporato, A., Nathan, R., Siqueira, M., Soons, M.B., Poggi, D., Horn, H.S. & Levin, S.A. (2005) Mechanistic analytical models for long-distance seed dispersal by wind. American Naturalist, 166, 368-381.


[i] Look at the natural history of New Zealand anywhere and you will find many cases of abrupt landscape transformation or eradication of native species.

All photos have been taken by Paul Caplat and can not be used without permission

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2 thoughts on “Understanding the mechanisms that underlie a conifer invasion – aka the Black Pine Project

  1. The study is flawed because black pine seeds are dispersed by wood nypmhs in NZ (it looks like the wind but wood nymphs Nymphaleum silvaticus carry them around and drop them, Bilbo tells me). You need a nymph behavioural ecologist.

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