Oecologia (2003) 134:132–143 DOI 10.1007/s00442-002-1098-y ECOSYSTEMS ECOLOGY Robert H. Jones · Robert J. Mitchell · Glen N. Stevens · Stephen D. Pecot Controls of fine root dynamics across a gradient of gap sizes in a pine woodland Received: 18 January 2002 / Accepted: 25 September 2002 / Published online: 1 November 2002  Springer-Verlag 2002 Abstract Controls of fine dynamics were investigated in a Pinus palustris Mill. (longleaf pine) woodland subjected to two understory vegetation treatments (control versus removed) and four overstory treatments (no gap control, and canopy gaps of three sizes with constant total gap area per stand). Fine root (<2 mm diameter) dynamics were measured over 11 months using ingrowth cores (all treatments) and minirhizotrons (understory removed in no gap control and large gap treatments only). At the fine (microsite) spatial scale, pine and non-pine root mass production responded negatively to each other (P=0.033). Each life form was significantly (P£0.028) related to nearby overstory density, and pine root production compensated for reductions in non-pine roots if under- story vegetation was removed. Soil moisture and NO 3 mineralization rate were negatively related to pine root mass production (ingrowth cores; P<0.001 and P=0.052) and positively related to pine root length production, mortality and turnover (minirhizotrons; P from <0.001 to 0.078). Temperature variance was negatively related to pine root lifespan (P<0.001) and positively related to pine root turnover (P=0.003). At the ecosystem scale, pattern of overstory disturbance (gap size and number) had no significant effect on non-pine, pine, or total root produc- tion. However, the presence of gaps (versus the no-gap control) increased non-pine root mass production (AN- OVA, P=0.055) in natural understory conditions, and reduced pine root mass production (P=0.035) where the understory was removed. Ecosystem-wide pine root length production, mortality and turnover were positively related to weekly soil temperature (P£0.02). In natural systems, fine root dynamics are highly variable and strongly affected by biotic factors. Roots quickly close belowground gaps because one life form (pine or non- pine) compensates for the absence of the other. When understory vegetation is removed, however, pine roots respond to the local abiotic environment, particularly moisture and NO 3 . Keywords Compensatory growth · Ingrowth core · Minirhizotron · Mixed life form · Pinus palustris Introduction Although fine roots contribute 50% or more toward total forest net primary productivity (Vogt et al. 1986) and control many ecosystem and community processes, they are one of the most difficult components of forest ecosystems to measure and to predict (Clark et al. 2001). Studies of root growth are hampered by high spatial and temporal variability, which results in noisy data, and by the fact that numerous abiotic and biotic factors influence roots. An additional problem relates to the spatial scale of observation. For example, at submeter scales, production may appear positively correlated with soil nutrient availability (Jackson and Caldwell 1989; Campbell et al. 1991; Pregitzer et al. 1993; Robinson 1994; Mou et al. 1995), whereas at the scale of whole ecosystems, production may appear to decline (in abso- lute terms or as a proportion of total stand production) when mean nutrient availability increases (Nadelhoffer 2000). Measuring root responses to different types and spatial patterns of ecosystem disturbance may be one way to: (1) increase signal to noise in root data; (2) determine independent effects of abiotic and biotic factors that control fine root dynamics; and (3) deal with influences of scale. Disturbances kill roots, but some plants may be more affected than others, which results in a shift in the balance between competing root systems. If enough roots are lost, belowground gaps are formed with reduced R.H. Jones ( ) ) · G.N. Stevens Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA e-mail: rhjones@vt.edu Tel.: +1-540-2319514 Fax: +1-540-2319307 R.J. Mitchell · S.D. Pecot Joseph W. Jones Ecological Research Center at Ichauway, Newton, GA 31770, USA