Impacts of root competition in forests and woodlands: A theoretical framework and review of experiments

Light is widely considered to be the most important factor limiting the per formance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to p redict the types of forest in which root competition affects seedling perfo rmance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competi tion, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-roo t mass along gradients of soil fertility and seasonal drought, keeping in m ind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major n utrients. The changes are far more complex than those considered by allocat ion models. Dry woodlands typically allow 20 times as much light to penetra te as do wet forests, but there is surprisingly little evidence that they h ave greater fine-root densities in the topsoil. Tropical rain forests on hi ghly infertile soils have only slightly more open canopies than those on fe rtile soils, but much greater fine-root densities. Northern temperate fores ts on highly acidic peats and sandy soils are often dominated by early-succ essional, open-canopied conifers (generally pines), mostly as a result of r ecurrent fires, and transmit about five times as much light as surrounding deciduous forests. A review of trenching experiments shows that light alone limits seedling growth in forests on moist, nutrient-rich soils, but compe tition for belowground resources becomes important on infertile soils and i n drier regions. Secondly, we consider how root competition alters species' shade tolerances . Shade-house experiments demonstrate that species differ markedly in the m inimum irradiance at which they respond to nutrient addition, but there gen erally tends to be a sizable response at >5% daylight and little response i n <2% daylight. There is some evidence that species that have high potentia l growth rates and that respond markedly to increased irradiance are also m ost responsive to nutrient addition in 2-3% daylight. T Smith and M. Huston have hypothesized that species cannot tolerate both shade and drought; thi s appears to be the case for species that tolerate shade chiefly by maximiz ing leaf area. However, many shade-tolerant woody plants in tropical and me diterranean-climate forests have thick, tough, long-lived leaves and a rela tively high allocation to roots, and these species are much more drought to lerant. A few studies indicate that root trenching allows species to persis t in deeper shade than that in which they are normally found and allows spe cies from mesic sites to invade more xeric sites. Usually, the impact of tr enching on growth rate is much greater in gaps than in the understory. Finally, we discuss the ways in which life-form composition and population structure of plant communities are shaped by reduced water: supply and redu ced nutrient availability, emphasizing the inadequacy of models that consid er the impact of "belowground resource availability" in a generic sense. Co mpetition in a dry climate leads to widely spaced dominants, a lack of inte rstitial plants, high rates of seedling mortality in the understory, and a restriction of regeneration to patches where established matrix-forming pla nts have died. In contrast, vegetation on moist, infertile sites is charact erized by closely packed, slender dominants, miniaturized interstitial plan ts, and slow rates of seedling growth in the understory, combined with rela tively low rates of seedling mortality. Consequently, there is a continuum of sizes among the individuals of the dominant species, and a lack of relia nce on gaps for establishment.