Actually you may have to define 'sentient'.
From my friend Google :
Outsmarting a hungry giraffe Acacias are thorny trees with foliage that is bright green or bluish-green and has small blooms that may be creamy white,
www.areenaresort.com
Giraffes are herbivores, which means they eat plant material like leaves high in the trees, and they can eat up to 29 kilograms of acacia leaves and twigs daily. However the acacia trees have developed a few defensive mechanisms to ensure their survival from the hungry giraffes and animals that like to chew on their leaves and twigs.
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The acacia trees have a further defence system to the thorns, the release of tannins, which taste terrible to Giraffe, so they stop eating them. This system is so effective that trees in the vicinity react to the trees tannin release and they too start to release their own tannin which forces the giraffe to move on, or go up wind to trees that have not sensed the tannin being released by the neighbouring trees.
So when Giraffe start eating their leaves, they pump terrible tasting tannins into their leaves, and all the other surrounding acacia trees get a wiff of the tannins and start pumping tannins into their leaves so the giraffe will leave them alone - sounds pretty sentient to me !