Plant facilitation under drought and grazing stress
In our research project we look at how plants can work together to survive in very stressful environments. Much empirical work already showed that plants can facilitate each other’s survival in grazed or very dry environments. Big shrubs can for example protect young plants against grazers, thereby increasing survival of young plants. Also, big shrubs might provide shade to decrease drought stress for young neighbouring saplings. In my PhD project we look at how facilitation between plants might disappear when both grazing stress and drought stress become very severe. We expect that with increasing stress facilitation firstly becomes more important, but that at very high stress positive interaction will disappear again. Pinpointing the stress level at which facilitation wanes is crucial to better understand future land degradation in arid ecosystems.
In the autumn of 2012 we have set up an experiment to test our hypotheses in the very dry region Murcia in South-eastern Spain. Here, we have built four big fences that we divided into four compartments. One compartment is highly grazed by goats, one is lowly grazed by goats, one is totally ungrazed and one compartment is only grazed by rabbits but not by goats. Building these fences was hard work, as in 3/4th of the compartments rabbit may not enter, so we had to dig in an extra fence to keep the rabbits out. Within the fences we planted over 1200 young saplings of an abundant shrub species that goats prefer to eat (Anthyllis cytisoides). The planted saplings are either planted alone or under the protective canopy of a different shrub species that is not eaten by goats (Artemisia barrelieri).
We built all the fences very close to the house of the shepherd, so that it would be very easy to get goats into our fences and mimic grazing events. But unluckily , after we had built the fences, the shepherd decided to move to another place with his herd and stay there. This place is only a few kilometres away from our fences, but still it’s quiet a logistic challenge to get the goats into our fences now. As you can see in the movie I have been transporting goats in my rental car. That was the only way to get the goats in these fences we built. We started with putting 6 goats in the car, but the shepherd eventually managed to get 15 goats in there. (the goats stayed very calm in the van, and I have not received additional cleaning costs from the rental company)
The results until now are promising, on the rabbit plots there is a clear contrast in sapling survival with higher survival when they have a neighbour. Also on the goat grazed plots the saplings seem to perform better when they have a neighbour. The next challenge in my PhD will be to link these outcomes to eco hydrological models to improve our ability to predict and prevent future land degradation in arid ecosystems. With such models we can improve predictions how ecosystem stability might change if grazing or drought stress further increases in the future.