Nutrient Content and Substrate Effect on Fine Root Density and Size Distribution in a Nicaraguan Rain Forest1

THAT A HIGH DEGREE OF NUTRIENT SPATIAL HETEROGENEITY exists in soil resources, in a broad range of ecosystems, has been confirmed by studies dating back to the 1970s (Hall 1970, Jackson & Caldwell 1992, Gonzalez & Zak 1994, Gross & Pregitzer 1995). Further, this heterogeneity occurs at a scale that would allow an individual plant to experience a variety of belowground nutrient environments. Thus, the ability of a plant to increase nutrient uptake in nutrient-rich patches may be critical to its competitive success. There are both morphological and physiological methods of foraging for edaphic resources (review in Robinson 1994). In this study, morphological mechanisms of foraging are emphasized, in which changes in the rate and form of root growth allow a plant to take advantage of localized increases in nutrient availability. There are three mechanisms by which morphological foraging can occur: (1) increasing rooting density, (2) increasing the proportion of fine roots, and (3) increasing fine root turnover rates. Root foraging is often limited to the frequently observed phenomenon of a plant’s root system growing prolifically into areas of high nutrient concentration and relatively little into areas that are nutrient-poor. The existence of this type of foraging has been demonstrated in many studies (Hutchings 1988, Pregitzer et al. 1993, Cain 1994, Caldwell 1994, Einsmann et al. 1999); however, it is not universal and some plants show relatively little morphological change in response to increased nutrient availability (Jackson & Caldwell 1992, Robinson 1994). While most roots act as a device for nutrient absorption, it is believed that young fine roots are more effective at this than their older, thicker counterparts and therefore responsible for the bulk of nutrient uptake (Nye & Tinker 1977, Caldwell & Richards 1983). This leads to the second form of foraging, in which there is an increase in the proportion of fine roots grown into an area in the presence of an increased resource supply. Because younger, finer roots are most able to absorb nutrients, it may not be energetically worthwhile for a plant to maintain older, less efficient roots. Thus, fine root turnover may increase in a high nutrient environment (Brække 1992, Pregitzer et al. 1995). This third mechanism of morphological root foraging involves not only an increase in fine root growth in nutrient rich areas but also increased root mortality. Root density is not necessarily changed. This study examined the density and size distribution of roots (mechanisms 1 and 2) in response to three substrates of varying nutrient quality: clay, leaf litter, and dried cow manure. The nutrient content of clay was the lowest and that of manure the highest. The first two substrates, clay and leaf litter, are normally available to plants for rooting. The third treatment, cow manure, was used for its characteristic high availability and slow release of nutrients, traits that make it a popular form of organic fertilizer. Given the mechanisms of root foraging, the length and diameter of roots growing into a substrate should be directly related to the nutrient quality of the substrate. This suggests two hypotheses about root ingrowth into the substrates tested: (1) root length should be longest in the manure treatment and shortest in the clay treatment; and (2) root diameter should be greatest in the clay treatment and least in the manure treatment. Since ca 80 percent of root biomass in this forest is found within the upper 20 cm of the mineral soil (B. Blair, pers. obs.), these hypotheses were tested using a root ingrowth technique to examine root proliferation into the three substrates within the top 20 cm of soil. The experiment was conducted on a 20 3 50 m plot within a wet tropical forest on the Caribbean coast of Nicaragua (118539N, 838589W; 20 m elev.), ca 15 km northwest of the city of Bluefields. Characterized by a patchwork of forest and farmland, this region on Nicaragua’s agricultural frontier was struck in 1988 by a category four storm (Hurricane Joan) that severely damaged the forest. The forest’s tree species (.60 spp. in a 1 ha plot) are dominated by species characteristic of wet tropical, primary