There are many reasons we should use more crops and less timber for construction. Both are good at pulling CO2 from the air as they photosynthesis, and then locking it up or 'sequestering' the carbon. In construction, these materials then store the carbon for at least the lifetime of the building.
I explored this in detail in my MSc thesis for sustainable architecture. Here is a recent article From Future Earth arguing the need to stop cutting down forests as they are such an important carbon sink: http://www.futureearth.org/blog/2018-jan-31/how-much-carbon-does-planets-vegetation-hold
Photo credit James Wheeler - this is taken from where I grew up in British Columbia, Canada, where clearcutting forests is common practice - devastating not only the biodiversity, but removing huge areas of carbon sink suddenly.
1. A long time to grow trees. Like oil, we need to leave trees in the ground. It takes decades for a young tree to obtain the same carbon capture and storage capacity as a mature tree - this time lag is a problem when we are now experiencing a climate crisis.
Fast growing crops can be grown annually. Hemp for example grows at 5 x the rate of timber and is much more efficient at carbon capture.
2. Not so much carbon left in final timber products. There is also a fallacy about how much carbon timber can sequester. One study about this embodied energy of timber evidenced that when you include the energy / carbon needed to harvest, transport, mill and transport to gate timber there is actually only about 15% of the carbon left in the total finished timber products relative to the original live tree (Ingerson, 2007. U.S. Forest Carbon and Climate Change).
3. Timber supplants food, crops provide food and fibre. Unlike timber which can supplant food crops, crops can also provide food as well as fibre for textiles and construction.
4. Forests are in trouble. Forest cover is declining at a high rate globally. Part of this is from logging, but a huge amount is due to climate change - leading to more forest fires and the increase in pests destroying forests because they aren't killed off in cold winters. In British Columbia for example, the pine beetle wiped out 53% of commercially valuable pine from 1999 to 2012 alone, and fires destroyed more forest than was harvested (Natural Resources Canada, 2013. Area of forest disturbed by fire, insects, disease and harvesting.)
So, although timber is a renewable resource, we need to minimise its use - and start using crops more.