Touch of the Shadow Sun (Su): A Shadow Sun ninja strikes a careful balance between light and dark, good and evil. Although your heart is dedicated to good, you know that the shadow of evil will always be a part of you. This realization, along with careful control of your ki energy, allows you to wield the energy of both the dark and light aspects of your being. This understanding and mastery allows you to deal damage with one attack, then turn and heal an ally with the reverse of that harmful energy. With a melee touch attack made as a standard action, you can deal negative energy damage equal to your base unarmed damage + your Wis modifier. Note that undead are healed by this attack, since it uses negative energy. In the round after you use this ability, you can touch a creature as a standard action and heal an amount of damage equal to the damage you dealt with your negative energy touch on the previous round. If the target of this healing touch makes no effort to prevent you from touching him, you can touch the creature as a swift action. You cannot use both aspects of this ability on the same round, nor can you use the negative energy touch again in a round after you have already successfully used it. After you channel negative energy into a creature, you convert that energy into a positive energy charge that prevents you from using the negative energy aspect of this ability until the positive energy has been discharged.
I look in vain to see where the negative energy touch has to be upon a creature, or anything specific. So, RAW, I would have to say yes, it would be legal. As to how much damage it would deal, how many HP does a tree have?
"How many HP does a tree have ?" D20SRD : The trunk of a typical tree has AC 4, hardness 5, and 150 hp. Medium and dense forests have massive trees as well. These trees take up an entire square and provide cover to anyone behind them. They have AC 3, hardness 5, and 600 hp. – Trajan
Thank you Trajan!
Also, blight specifically says that it damages plants because that is the only thing that it damages.
In response to your second question,
"This also leads to the question that if a divine caster can do burst area damage with these energies, could not a savvy tracker find a cleric by following the patches of growth in the ground (or areas of blight, accordingly)?"
For the instance you state above, I don't see why it would have to be a savvy tracker. "Hey, Bubba, look at this trail of dead stuff.....Lets see where it goes".
"One creature," "Creature touched," or "Creatures within x feet," as most do.
However, sense this effect only hits one specific life form, there would not be a large patches of blight. Thank you, KRyan!
The 3.5 Blighter Prestigious Class has a power called Deforestation that does specifically that.
Deforestation (Sp): A blighter can kill all nonsentient plant life within a radius of 20 feet per blighter level as a full-round action once per day. If a potentially affected plant is under the control of another (such as a druid’s liveoak or a dryad’s home tree), the controller can make a Fortitude save (DC 10 + blighter level + blighter’s Wis modifier) to keep it alive. Affected plants immediately cease photosynthesis, root tapping, and all other methods of sustenance. Like picked flowers, they appear vibrant for several hours, but within a day, they turn brown and wither. Except for plants selected by a controller, nothing can grow in a deforested area until it has a hallow spell cast upon it and it is reseeded.
Deforestation enables a blighter to cast her daily allotment of spells. This ability works in any terrain, but deforesting a sandy desert, ice floe, or other environment with only sparse vegetation does not empower the character to cast spells.