Size and Structure of Fine Root Systems in Old‐growth and Secondary Tropical Montane Forests (Costa Rica)

Abstract

The fine root systems of three tropical montane forests differing in age and history were investigated in the Cordillera Talamanca, Costa Rica. We analyzed abundance, vertical distribution, and morphology of fine roots in an early successional forest (10–15 years old, ESF), a mid‐successional forest (40 years old, MSP), and a nearby undisturbed old‐growth forest (OGF), and related the root data to soil morphological and chemical parameters. The OGF stand contained a 19 cm deep organic layer on the forest floor (i.e., 530 mol C/m2), which was two and five times thicker than that of the MSF (10 cm) and ESF stands (4 cm), respectively. There was a corresponding decrease in fine root biomass in this horizon from 1128 g dry matter/m2 in the old‐growth forest to 337 (MSF) and 31 g/m2 (ESF) in the secondary forests, although the stands had similar leaf areas. The organic layer was a preferred substrate for fine root growth in the old‐growth forest as indicated by more than four times higher fine root densities (root mass per soil volume) than in the mineral topsoil (0–10 cm); in the two secondary forests, root densities in the organic layer were equal to or lower than in the mineral soil. Specific fine root surface areas and specific root tip abundance (tips per unit root dry mass) were significantly greater in the roots of the ESF than the MSF and OGF stands. Most roots of the ESF trees (8 abundant species) were infected by VA mycorrhizal fungi; ectomycorrhizal species (Quercus copeyemis and Q. costaricensis) were dominant in the MSF and OGF stands. Replacement of tropical montane oak forest by secondary forest in Costa Rica has resulted in (1) a large reduction of tree fine root biomass; (2) a substantial decrease in depth of the organic layer (and thus in preferred rooting space); and (3) a great loss of soil carbon and nutrients. Whether old–growth Quercus forests maintain a very high fine root biomass because their ectomycorrhizal rootlets are less effective in nutrient absorption than those of VA mycorrhizal secondary forests, or if their nutrient demand is much higher than that of secondary forests (despite a similar leaf area and leaf mass production), remains unclear.

Fine root biomass and root length density in a lowland and a montane tropical rain forest, SP, Brazil

“…11,no. 3 Our FRB data in the top 30 cm (219.11-875.22 g.m -2 ) are within the range reported for other tropical forests from 150 to ~1100 g.m -2 (Cavelier et al 1996, Herbert & Fownes 1999, Yavitt & Wright 2001, Hertel et al 2003, Leuschner et al 2007). In fact, Jackson et al (1997) reported an average value of 330 g.m -2 for tropical forests.…”

Increased Litterfall Changes Fine Root Distribution in a Moist Tropical Forest

“…Root mats are generally formed on tropical forest soils with a clearly defined Oa horizon (Vogt et al, 1983); the thick organic layers (Oe and Oa horizons) that have accumulated in some late successional forests are also known to contain higher fine root biomass per area than the mineral soil both in the tropics (Hertel et al, 2003) and in the temperate zone (Berendse et al, 1989;Coomes and Grubb, 2000;Fahey and Hughes, 1994). In addition to the concentration of fine root growth into old established Oe and Oa layers, we have shown that roots also respond rapidly to relatively recent increases in fresh leaf litter.…”

“…Tropical forest soils are generally nutrient-poor (Jordan, 1985;Richards, 1996), nutrient cycling is tight (Vitousek, 1984) and the direct cycling of nutrients from organic matter therefore plays a major role in their nutrient budget Stark and Jordan, 1978). In some, but not all, tropical forests growing on infertile soils extensive root mats form in the humus (Oa) and fermentation (Oe) horizons (Chuyong et al, 2002;Coomes and Grubb, 1996;LaClau et al, 2004;Stark and Jordan, 1978;Stark and Spratt, 1977) and even where root mats are not formed, root proliferation into the organic layer is often substantial (Hertel et al, 2003). There exists evidence that roots colonise not only older decomposing organic matter but also freshly fallen leaves (Jordan and Escalante, 1980;Herrera et al, 1978), and in a site in Southern Venezuela it has been shown that root contact with fresh litter accelerates the decomposition process and the release of Ca and Mg from the litter (Cuevas and Medina, 1988).…”

High litterfall in old-growth and secondary upper montane forest of Costa Rica

“…Oaks of this size predominately occur on the west facing slopes and in areas over 2,400 m high, and are not found in other regions of the country. In addition to a presumed high aboveground biomass, these forests also have a high fine root biomass (Hertel et al 2003). Although data are not available, one may assume that productivity is also relatively high in this high-elevation forest type.…”