TLDR; The same way any tree does!
The key to any plant based life form is numbers. You'll need to decide what you want your species' main "predator" to be.
Is it herbivores that like to feed on your offspring (seeds)? Depending on the herbivore, they have their own predators to worry about, and won't be able to eat ALL of your offspring before retreating to their own home. Are they a predatory animal that uses your race for habitat but preys on animals that feed on your offspring? Chances are your offspring are safe from these, as the population of this animal can grow as yours does. The mutual benefit of not only protecting your species, but making sure that your species grows is a boon to both parties, as yours is the reason their food gravitates to them.
Regardless of what you choose, being a tree, young saplings are less likely to be feasted on by the other kingdom.
The next step for your species is to create a symbiotic relationship with fungus, as the decomposition of your species' corpses will renew the nutrients and provide more fertile soil.
On the other end of the spectrum, plants and fungi engage in mutually beneficial relationships, the most important of which is called mycorriza (plural = mycorrhizae) where fungi live on and in the plant’s roots. In this case both the plant and the fungus depend on this relationship to develop and survive.
So you've now solved predators, and soil/nutrients. Seasons are the last problem for you to solve, but it is admittedly the easiest. Hibernation is something that not only plants, but animals do as well. Those that don't are either predators, feasting on small animals, or don't leave their burrows for anything but leftover food (essentially the offspring of yours that didn't make it). When winter comes, the majority of animals either save their energy for other pursuits (growing fur for hunting, and saving lipids), hibernate (or frequently take naps), or migrate to warmer climates.
Your species during these winter months will do what all trees do! Stop producing offspring, shed leaves, and go into dormancy. Sunlight in this state is not really necessary, and your species will have the "instincts" to do this as a group, similar to animals. For more information about how the tree knows when to lose it's leaves,
Dormancy is like hibernation in that everything within the plant slows down. Metabolism, energy consumption, growth and so on. The first part of dormancy is when trees lose their leaves. They don't make food in the winter, so they have no use for masses of leaves that would require energy to maintain. When it's time for trees to lose their leaves, a chemical called ABA (Abscisic acid) is produced in terminal buds (the part at the tip of the stem that connects to the leaf). The terminal bud is where the leaf breaks off when it falls, so when ABA gathers there, it signals the leaf to break off. (This occurs only in deciduous trees — not in coniferous trees.)
In the end, unless you're a coniferous tree, you have nothing to worry about but living a long successful life (until humanoids.....).
If you're species is coniferous, the greater number of "leaves", and the reduction in the water they need is key. As a conifer, you also have a special substance called cutin, which when combined with the lesser amount of water, will prevent the leaves from freezing.
These are very small and thin needles that can be thought of as a leaf tightly coiled together. They cover themselves in a waxy substance called cutin. These needles also require less water to stay alive and perform photosynthesis than leaf. The small amount of water and protective Cutin coating stop any water from freezing and killing any pine needles.
In the end of it all, no matter what tree you are, you'll have a larger problem uprooting yourself than survival, and will move infrequently as a result of photosynthetic energy being less than that of other methods.
If you compete with other trees, it'll just be a numbers game. If you produce more offspring or have a more aggressive root system, then your tree will take over new territory than your possibly non sentient neighbors.
Using computer software to predict the growth patterns in forests over time, Dr. Dybzinski and his colleagues are now reporting that the overabundant roots act as weapons to prevent other trees from growing.