This is actually a highly controversial area of nutrient research. Observed resorption is highly variable among plants and years Resorption is high at times of active sink when growth or reproduction has active demand for CHO and nutrients. In this case, there is more effective withdrawal from senescing leaves. Many evergreens lose old leaves at the time of producing new leaves. This increases retention of nutr by resorption.
Resorption is low at time of drought stress.
This is a major mechanism by which plants reduce nutrient loss. This is both an adaptation and acclimation. Plants that produce leaves slowly also lose leaves slowly. This is most obvious with grasses and evergreens where leaves can be kept for variable lengths of time. Many pines keep their needles for 2-5 years, but black spruce , which grows in some of the most nutrient poor soils in the boreal forest, keep their needles up to 25 years.