Its always frustrating to have an unusable area on the allotment. The problem with conifers is that they tend to have rapid growth which over time can exhaust the soil. The dense canopy also prevents rainfall reaching the ground leaving the soil parched, dry and shaded. There are few plants that will grow in these conditions; even weeds do not thrive in the shadow of a conifer hedge. Make use this area for storage, thereby freeing up more productive land elsewhere on the allotment. Build a pot store for your pots, garden canes and other equipment, and stack your bags of compost there to keep the worst of the weather off of them. Unless this area gets some sun then this is not the ideal place for the compost heap, but by starting some wormeries it is pissible to make your own compost. If you want to encourage wildlife try building a pile of decomposing logs to shelter toads and insects. With a bit of creative thought this area could be turned into a useful part of your allotment.