Its the total amount of biomass on the planet that contributes to the Carbon Cycle. Because of agriculture, this biomass is actually increasing with time (which contributes to the methane problem). Trees are temporary storage only; they fall down and rot and release Carbon eventually

Combustion + deforestation produce an imbalance and a net flux. Without them we would be experiencing a mild reduction of CO₂ as uptake on the land + ocean = 194 units while land and ocean sources are 190 units. This small imbalance is typical of the glacial/interglacial cycle. The natural system is never in exact equilibrium with respect to maintaining a constant level of CO₂. However, there is no sink presently available that absorbs the human induced source increase.
Residence time in the atmosphere varies between 5 and 200 years depending on many, many factors.

Note - the biggest uncertainty with the whole carbon cycle is the role of deep ocean transport:
The oceans' uptake of CO2 primarily depends on how fast CO2 can be transported downwards from the ocean surface; if "too much" CO2 accumulates in the surface layers at one time, absorption slows down.
Global data on the concentrations of CO2 in the oceans are still very sketchy, and many contributing factors, such as ocean biology, are still not well understood. Absorption is also known to depend on local weather conditions. For example, winter storms accelerate CO2 exchange by exposing sea-water to the atmosphere through bubbles and spray.

Not all of the carbon dioxide that has been emitted by human activities remains in the atmosphere. The oceans have absorbed some of it because as the carbon dioxide in the atmosphere increases it drives diffusion of carbon dioxide into the oceans. However, when we try to account for sources and sinks for carbon dioxide in the atmosphere we uncover some mysteries. For example, notice in Figure 1 (schematic of the carbon cycle) that fossil fuel burning releases roughly 5.5 gigatons of carbon (GtC [giga=1 billion]) per year into the atmosphere and that land-use changes such as deforestation contribute roughly 1.6 GtC per year. Measurements of atmospheric carbon dioxide levels (going on since 1957) suggest that of the approximate total amount of 7.1 GtC released per year by human activities, approximately 3.2 GtC remain in the atmosphere, resulting in an increase in atmospheric carbon dioxide. In addition, approximately 2 GtC diffuses into the world’s oceans, thus leaving 1.9 GtC unaccounted for.

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