Dendrometers are being increasingly used to measure stem-radius changes in trees and to unravel the mechanisms underlying stem daily rhythms of radial expansion and contraction. Nevertheless, automated dendrometers have not been often used to measure root-radius dynamics, their relationship with environmental variables, and the influence of endogenous processes, especially in drought-prone Mediterranean areas. Here, we measured root-radius dynamics of two coexisting oak species (the evergreen Quercus ilex and the deciduous Quercus faginea). Our goals were to describe annual, seasonal and diurnal scale root-radius patterns, and, to disentangle the role of different environmental parameters as drivers. Long-term high-resolution measurements (every 15 minutes over 7 years) were collected with automated point dendrometers on the main tree roots of five individuals per species. Root-radius annual change patterns were bimodal and similar for both oak species. Q. faginea showed three times larger root increment in the spring than Q. ilex, but the bimodal pattern was stronger in Q. ilex which showed a larger root increment in autumn. Q. faginea showed an earlier root phenological activation in the spring and in late summer compared with Q. ilex. The effects of environmental drivers across species were similar at daily scales: root-radius increased with air temperature and soil moisture and it decreased with rising vapor pressure deficit. Furthermore, daily root-radius variations for both oak species were maintained after extracting statistically the environmental effects, which points towards a significant role of endogenous drivers. These differences in root-radius change patterns at seasonal to daily scales likely result from the differences in leaf phenology and growth strategy. Q. faginea is deciduous and faster growing rate in spring than the evergreen Q. ilex, which can grow more in summer.