Root production and belowground seagrass biomass

Abstract

Root production and belowground seagrass biomassCarlos M. ~u a r t e '~' , Martin ~e r i n o~, Nona S. R. ~g a w i n~, Janet uri3, Miguel D. ~o r t e s~, Margarita E. Gallegos4, Nuria Marba5, Marten A. Hemminga5 ' ABSTRACT: The root and rhizome biomass of the seagrass species present in 3 mixed and 2 nlonospecific meadows representative of different floras (Spanish Med~terranean, Mexican Caribbean, Kenyan coast, and the South China Sea off The Philippines) was examined to test for the existence of general patterns in the distribution of their biomass in the sediments, and to test a simple approach based on age determinations to estimate root production. The thickness of the roots was scaled to the thickness of the seagrass rhizomes (r = 0.92, p < 0.001). Root and rhizome biomass were high (> 100 and >200 g D\,V m-2, respectively) for the mixed meadows examined; these belowground structures had a projected surface area often exceeding 1 m2 m-' when roots and rhizomes were considered together, and they formed a dense web of root material comprising several hundred meters per square meter. Belowground biomass showed considerable vertical stratification within the sediments, with a tendency for the larger species to extend deeper into the sediments than smaller ones. This tendency for segregation should reduce the potential interspecific competition for sediment resources, which is l~kely to be greater in the uppermost layers, where the belowground biomass is more evenly distributed among species. The rate of adventitious root production on vertical shoots varied from species that produced a root on almost every node to species that produced 1 adventitious root for every 10 nodes. Root production-both on horizontal rhizomes and vertical shoots-was substantial, with the combined root production approaching, or exceeding, 1000 g DW m-2 yr-' The resulting root turnover was quite high, with most values ranging between 2 and 10 yr-', indicative of a characteristic turnover time of months for the root compartment. The estimates of root production derived here often exceed those of rhizome production and reach values comparable to leaf production, clearly demonstrating that root production is an important component (up to 50%) of total seayrass production.

Direct measurements of root growth and productivity in the seagrasses Posidonia australis and P. sinuosa

“…The nature of the experimental conditions in this study (removal of roots prior to planting, culture in standardized medium in pots) means that root-growth values are not typical of plants growing in a meadow situation and are likely to be close to the maximum potential growth rates. If growth rates for the transplants of P. australis and P. sinuosa are scaled up to a unit area of meadow, then increases in root length (2-5 m root m 22 d 21 ) would be similar to estimates for a meadow of the small, fast-growing Mediterranean seagrass Cymodocea nodosa during summer and considerably higher than root-growth values of Posidonia oceanica derived from annual rhizome growth (historical rhizome reconstruction) or from tagging rhizomes and estimating root growth between two points (Duarte et al 1998;Marbá and Duarte 2001; Table 5). Similarly, annual root productivities in P. australis and P. Fig.…”

“…Buoyancy of the air-filled roots and the fluidity of marine sediments when they are excavated lead to considerable losses during sampling of root systems in subtidal environments. Estimates of root growth have so far been based on methods that do not directly measure root production (Cebriá n et al 1997; Duarte et al 1998;Marbà and Duarte 2001). The method in this study directly measures growth of the entire root system and, combined with leaf and rhizome growth, provides an accurate estimate of whole-plant growth responses to macronutrients and season.…”

“…For species with extensive root systems that grow in subtidal environments, accurate sampling presents several difficulties, such as buoyant roots that are easily lost underwater and sediments that behave like a liquid when excavated. The few estimates of root growth have so far been based on methods that do not directly measure root production, such as invasive rhizome tagging (Marbà and Duarte 2001) or historical rhizome reconstruction techniques in established meadows (Cebriá n et al 1997;Duarte et al 1998).…”

“…For species with extensive root systems that grow in subtidal environments, accurate sampling presents several difficulties, such as buoyant roots that are easily lost underwater and sediments that behave like a liquid when excavated. The few estimates of root growth have so far been based on methods that do not directly measure root production, such as invasive rhizome tagging (Marbà and Duarte 2001) or historical rhizome reconstruction techniques in established meadows (Cebriá n et al 1997;Duarte et al 1998).Root development in plants may be highly responsive to nutrient availability. Nitrogen-rich patches are typically invaded by thinner, lateral roots that branch off the main root axes, and this leads to an overall increase in root growth and root length (Robinson 1994;Hodge 2004).…”

Shoot organization in the seagrass zostera noltii: implications for space occupation and plant architecture

“…A minimum development of an etiolated L-5 (upto 5 mm) and without any new leaves in the axillary shoot has been recorded (F. G. Brun, personal observation). This is mainly because meristems and axillary shoots are buried in the sediment at variable depths depending on the local environmental conditions (Brun et al 2005;Duarte et al 1998;. Furthermore, the sediment is frequently hypoxic or anoxic, and contains bacterial-derived phytotoxic compounds (sulphide, methane, ammonium and volatile organic acids) (Holmer and Nielsen 1997;Terrados et al 1999;Enrı´quez et al 2001) that may reduce axillary shoot and meristem survival preventing plant branching and affecting the overall growth (Carlson et al 1994;Armstrong et al 1996a, b;Armstrong and Armstrong 2001;Brun et al 2002;Greve et al 2003).…”