There's no reason given as to why we should spare animals but not plants

The paper clearly gives reasons why animals should be spared harm, eg: "These animals are conscious, sentient, social beings. They experience pleasures and pains and a variety of complex negative and positive emotions".

Clearly, if you do not accept that we should be more concerned for the welfare of beings that have a greater capacity for suffering than other beings, then you don't accept a basic premise of the argument being made by the paper. The arguments being made are predicated upon a desire to avoid harm when possible. If you (genuinely) don't believe that this premise, then you clearly won't accept any arguments that follow from the premise.

Again, I don't believe that you really mean what you are saying. If you do, your arguments could be extended to humans, and to non-living things.

If dogs and carrots are equal from a moral standpoint, then why are dogs and rocks not also equal? Or humans and automobiles? Are you basing the value you ascribe to something upon its usefulness to humans? If so, would you therefore privlege the existence of a particularly useful truck over the life a a child who does nothing to contribute to society? We can view them both as a mere collection of atoms; it's only a matter of how their atomic structure differs, right?

All life is valuable

Is it? If so, why? On what basis do you value life?