IMPACTS OF ROOT COMPETITION IN FORESTS AND WOODLANDS: A THEORETICAL FRAMEWORK AND REVIEW OF EXPERIMENTS

Abstract

Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, 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 competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 times as much light to penetrate as do wet forests, but there is surprisingly little evidence that they have greater fine-root densities in the topsoil. Tropical rain forests on highly infertile soils have only slightly more open canopies than those on fertile soils, but much greater fine-root densities. Northern temperate forests on highly acidic peats and sandy soils are often dominated by early-successional, open-canopied conifers (generally pines), mostly as a result of recurrent 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 competition for belowground resources becomes important on infertile soils and in drier regions. Secondly, we consider how root competition alters species' shade tolerances. Shade-house experiments demonstrate that species differ markedly in the minimum irradiance at which they respond to nutrient addition, but there generally tends to be a sizable response at >5% daylight and little response in <2% daylight. There is some evidence that species that have high potential growth rates and that respond markedly to increased irradiance are also most responsive to nutrient addition in 2–3% daylight. T. Smith and M. Huston have hypothesized that species cannot tolerate both shade and drought; this appears to be the case for species that tolerate shade chiefly by maximizing leaf area. However, many shade-tolerant woody plants in tropical and mediterranean-climate forests have thick, tough, long-lived leaves and a relatively high allocation to roots, and these species are much more drought tolerant. A few studies indicate that root trenching allows species to persist in deeper shade than that in which they are normally found and allows species from mesic sites to invade more xeric sites. Usually, the impact of trenching 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 reduced nutrient availability, emphasizing the inadequacy of models that consider the impact of “belowground resource availability” in a generic sense. Competition in a dry climate leads to widely spaced dominants, a lack of interstitial plants, high rates of seedling mortality in the understory, and a restriction of regeneration to patches where established matrix-forming plants have died. In contrast, vegetation on moist, infertile sites is characterized by closely packed, slender dominants, miniaturized interstitial plants, and slow rates of seedling growth in the understory, combined with relatively low rates of seedling mortality. Consequently, there is a continuum of sizes among the individuals of the dominant species, and a lack of reliance on gaps for establishment.