Responses of tree fine roots to temperature

Soil temperature can influence the functioning of roots in many ways. If so il moisture and nutrient availability are adequate, rates of root length ex tension and root mortality increase with increasing soil temperature, at le ast up to an optimal temperature for root growth, which seems to vary among taxa. Root growth and root mortality are highly seasonal in perennial plan ts, with a flush of growth in spring and significant mortality in the fall. At present we do not understand whether root growth phenology responds to the same temperature cues that are known to control shoot growth. We also d o not understand whether the flush of root growth in the spring depends on the utilization of stored nonstructural carbohydrates, or if it is fueled b y current photosynthate. Root respiration increases exponentially with temp erature, but Q(10) values range widely from c. 1.5 to >3.0. Significant que stions yet to be resolved are: whether rates of root respiration acclimate to soil temperature, and what mechanisms control acclimation if it occurs. Limited data suggest that fine roots depend heavily on the import of new ca rbon (C) from the canopy during the growing season. We hypothesize that roo t growth and root respiration are tightly linked to whole-canopy assimilati on through complex source-sink relationships within the plant. Our understa nding of how the whole plant responds to dynamic changes in soil temperatur e, moisture and nutrient availability is poor, even though it is well known that multiple growth-limiting resources change simultaneously through time during a typical growing season. We review the interactions between soil t emperature and other growth-limiting factors to illustrate how simple gener alizations about temperature and root functioning can be misleading.