Answer: Citrus trees are frost-tender, and since we had such a severe winter with freezing temperatures on consecutive days, they suffered without frost protection. Citrus varieties are grafted on a different variety of rootstock. Rootstocks are chosen for their hardiness, ability to thrive in specific soil conditions, and so on. The fruit varieties are chosen for flavor, fewer seeds, and so on. If the grafted variety succumbs to cold or disease, the rootstock often sends up shoots. The graft union is a slightly bulging area at the base of the plant, usually about a foot or so above ground. If the shoots are growing from below the graft unions, the rootstocks have taken over. However, you mention the trunk is sprouting with leaves, so the variety may still be alive. Provide your lemon with appropriate water and fertilizer and wait to see if the variety comes back.
Curling leaves may be a sign of citrus thrips, a tiny insect hard to see with the naked eye. They feed on foliage and the rinds of tiny fruit, usually at late winter/early spring. Curled and deformed leaves is a result. Hold a sheet of white paper and tap or shake it. Thrips are about 1/16 inch long and look like wood splinters. Do not spray insecticides, which will kill the pollinating insects. Often the damage from thrips is seen after they are long gone, so attempts at control are useless. Good luck!
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