For the past two years, I have walked through Cave Gulch and across a patch of chaparral nearly every day on my way to school. I almost always take the same route, and I generally pay attention to my surroundings (a blooming Trillium here, a fresh scat there), so I was surprised last week to find a tree that I had never before seen.
I was first drawn to the tree by its puffy, white flowers, but what really piqued my curiosity were the leaves. Unlike other local foliage, these simple leaves were asymmetrical and had nearly parallel veins. I tried to identify the tree using several keys, but ultimately I had to fall back on a higher authority. I sent a photo of the leaves and flowers to my lab-mate Mike “The Manzanita Maniac” Vasey who clarified the problem in short order.
In its homeland of eastern Australia and Tasmania, my mystery tree is commonly referred to as a wattle, but in the U.S. people know it as Australian blackwood (Acacia melanoxylon). As an Acacia, this plant is a member of the world’s largest tree genus; Acacia contains over 1200 species and is found on every continent except Antarctica. California has its own native Acacia in the southern deserts (A. greggii), but blackwood is a relatively recent arrival, presumably brought to California for forestry or landscaping.
Mike also pointed out to me that the “leaves” I was looking at are actually phyllodes, or flattened leaf stems, modified to perform the functions of the real leaves that are nowhere to be seen. The Acacias that I am more familiar with in Central America generally have compound leaves with veins branching from central midribs on each leaflet. Apparently blackwood does too, but only as a young seedling.
Why trade your leaves for phyllodes? In dry climates, plants must maintain a tricky balance between photosynthesizing and conserving water. In order to turn sunlight into energy, plants exchange water vapor for carbon dioxide through tiny pores called stomata. Plant tissue with more stomata can do more photosynthesis, but it also loses more moisture. Because phyllodes are modified leaf stems, they have fewer stomata than normal leaves, so they lose less water during photosynthesis – a nice adaptation if you live on the world’s driest continent.