Posted by: oikosasa | April 9, 2013

When plants help other plants…

Nice to see that nature provides other kinds of interactions than nasty predation, competetion and parasitism! Christian Schöb and coworkers have studied the importance of “nursing” plants – plants that fascilitate for other plants – in community ecology. Read their Early View paper “Direct and indirect interactions co-determine species composition in nurse plant systems”.

Here is a summary of their study:

Our motivation to build a framework based on observational data in order to disentangle direct nurse effects from indirect effects among beneficiary species was twofold:

1) In some very common nurse plant systems, such as alpine cushion plant communities, the removal of the nurse to eliminate the direct effect and unambiguously estimate interactions among beneficiary species is simply not feasible.

Schobe1

The physical removal of the nurse cushion Arenaria tetraquetra ssp. amabilis growing in the Sierra Nevada Mountains in Spain would destroy the beneficiary species growing within the cushion canopy; in particular because beneficiary species often root in the organic matter accrued under the cushion.

2) Even in nurse plant systems where the physical removal of the nurse is feasible, taking away aboveground nurse parts does not remove the permanent effects of the nurse, e.g. the effects on soil properties including texture, resources and microbial communities.

An intact Retama sphaerocarpa nurse plant system in the semi-arid lowland of southeastern Spain with the nurse shrub and its understory community of beneficiary species.

An intact Retama sphaerocarpa nurse plant system in the semi-arid lowland of southeastern Spain with the nurse shrub and its understory community of beneficiary species.

The removal of aboveground parts would not remove the whole effect the nurse shrub, such as accumulation of soil organic matter and its specific communities of soil bacteria and fungi, which all together influence beneficiary species even after the nurse plant has been removed.

Schobe3

In our article we propose a simple but powerful mathematical framework to take apart direct effects of the nurse on its associated community from effects of interactions among beneficiary species. Our results showed facilitative effects of the nurse on its beneficiary species whereas interactions among beneficiary species where mostly competitive. Both interactions contributed significantly to the composition of the beneficiary plant community even though the direct nurse effect was ca. five times stronger than the effect of the interactions among beneficiary species. Interestingly, these patterns where similar in the two nurse plant systems studied even though they differed considerably in abiotic conditions (high alpine vs. semi-arid lowland) and growth form of the nurse (cushion plant vs. shrub). Our data therefore indicate functional parallelism of different nurse plant systems and highlight the complexity of species interactions within plant communities.

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