The analysis suggests that roots should have long lifespans if they have low maintenance respiration or are located in favorable patches of nutrient-rich soil. Shedding roots in dry soil may not be necessary to root efficiency if reductions in maintenance respiration can match reductions in uptake. Accurate predictions of optimal root lifespan are presently limited by insufficient information on the changing C costs and nutrient uptake capacity of roots with age. The costs and benefits of root hairs, root exudates and mycorrhizal fungi also need to be included. The current model of root efficiency omits some important factors that may exert controls on root lifespan.
Fine roots have other functions besides absorption, including transport of water and nutrients.
Seasonality of climate and the need for carbon and nutrient storage could constrain the root lifespan that optimizes plant fitness to differ from that which maximizes root efficiency. Death of roots may not entail the loss of all the C and nutrients they contain, if material is resorbed from senescing tissues or if nutrient cycling is tight and the lost material eventually benefits the plant. Finally, root herbivory and parasitism are probably much greater than generally appreciated. Young roots generally lack structural defenses and are at high risk of attack by soil organisms. The lifespan of stressed roots with low carbohydrate reserves is probably affected by weak pathogens and primary saprophytes that reside in the rhizosphere.