Most epiphytic species have no access to the soil of the forest floor and simply live off the water and nutrients stored in small pockets of canopy soil. Canopy soil is the organic matter that has gathered in branch forks or nest epiphytes.
Canopy soil usually exists in relatively small volumes and therefore does not store a lot of water. Even after rain events, this resource quickly dries out.
What does this mean for epiphytes? Presumably, it means that they need to be able to cope with frequent drought and to use water during the short time that it is present. This topic was part of my MSc research:
Canopy soil usually exists in relatively small volumes and therefore does not store a lot of water. Even after rain events, this resource quickly dries out.
What does this mean for epiphytes? Presumably, it means that they need to be able to cope with frequent drought and to use water during the short time that it is present. This topic was part of my MSc research:
Griselinia lucida seedlings lined up for my MSc experiments. Photo: C. Kirby.
I investigated the response of the epiphytic shrub puka (Griselinia lucida) to drought, along side its (mainly) ground-dwelling cousin kapuka, (Griselinia littoralis).
I found that puka and kapuka can tolerate a low level of drought stress but their key response to extended drought was to effectively shut-down. The graph below shows the stomatal conductance (y axis, high conductance = high photosynthesis, low conductance = low photosynthesis) as water potential (x axis) dropped. The filled circles are results from puka and the open symbols are kapuka.
I found that puka and kapuka can tolerate a low level of drought stress but their key response to extended drought was to effectively shut-down. The graph below shows the stomatal conductance (y axis, high conductance = high photosynthesis, low conductance = low photosynthesis) as water potential (x axis) dropped. The filled circles are results from puka and the open symbols are kapuka.
What this indicates is a shut-down of function as the soil dried out, this reduces the risk of complete desiccation and death because water is not being lost through photosynthesis.
The shut-down is a useful way to avoid damage but the key is to be able to re-activate normal function when water becomes available (after rain or fog). Griselinia lucida (puka) showed a very quick response after 70 days in drought. The following photos show the same plant on day one (A) after rewatering, day two (B) and day three (C).
The shut-down is a useful way to avoid damage but the key is to be able to re-activate normal function when water becomes available (after rain or fog). Griselinia lucida (puka) showed a very quick response after 70 days in drought. The following photos show the same plant on day one (A) after rewatering, day two (B) and day three (C).
A Griselinia lucida plant that has been watered after 70 days of drought on. A: 1 day after watering, B: 2 days after watering, C: 3 days after watering. Photo: C. Kirby.
To summarise, epiphytes grow in canopy habitats because they can cope with drought conditions and quickly respond to water when it becomes available.
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