It 'very important to try to get the maximum compatibility of its compliance with the tree held sails. For a beginner this will mean keeping an eye on the mating shaft length extends longer than the paametro sailing as luff or mast (depending on brand). Beware of sellers: there are trees for all the prices without having to take the 100% carbon. Let's look to the materials available (especially in view of the used)
Fiber is considered "the Alberaccio" battle but to start is fine. Surely it is not a long-term investment because there are now trees, low-carbon (20-40%) at attractive prices and when you buy the first of 6.5 sq m sail, you can not put a tree whatever.
Aluminum is no longer a good investment because the trees low carbon cost the same amount, and last longer (see maintenance).
Carbon will be the de facto choice when you start to adjust the sails according to the conditions. Initially you will not have this requirement but the carbon is the only material currently available
Advanced

It 'very important to match the right tree for each sailing, a stiffness and in some cases a length too far from those required on the wing, can seriously affect performance. For example, if your tree is too rigid will happen to the tensioned tantissimo downhaul before seeing a minimum easing in leech. Similarly a difference on the length can cause discomfort such as the small sails with varied top, where you can then mount the trees are much longer in his pocket, the installation of a tree much larger than the size required will cause a tightening of 'equipment. Same thing happens when we use a tree too short sail on the largest lengthening the maximum extension. So look for trees that have long stiffness MCS compatible with your sails.