Geomorphology 123 2010 61 73 Contents lists available at ScienceDirect Geomorphology j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e g e o m o r p h Wood retention and transport in tropical headwater streams La Selva Biological Station Costa Rica Daniel Cadol Ellen Wohl Department of Geosciences Colorado State University Fort Collins CO 80523 1482 USA a r t i c l e i n f o Article history Received 18 January 2010 Received in revised form 23 June 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords In stream wood Large woody debris Wood dynamics Forest streams Tropical streams Costa Rica a b s t r a c t Wood in tropical streams has the potential to be more mobile than wood in otherwise similar temperate streams because of the warm and humid conditions that promote decay and the more frequent and ashier oods of the tropics To test this hypothesis we monitored all large wood pieces for 2 3 years in 10 50 mlong reaches of old growth headwater streams in La Selva Biological Station Costa Rica Annual wood retention rates for pieces ranged from 0 55 to 0 91 among the sites and retention rates by volume ranged from 0 67 to 0 99 Assuming steady state wood load which is reasonable for La Selva these rates are equivalent to mean residence times of 2 2 10 6 years for pieces and 3 0 83 2 years for a volume of wood Calculating mean residence time from the weighted average of retention rates gives an average residence time of 4 9 years for a piece of wood and 6 9 years for a volume of wood These values are less than those reported for old growth temperate forests supporting our hypothesis Mobility of individual pieces was best predicted by piece length relative to stream width lr higher lr led to lower mobility channel gradient s higher s led to higher mobility and piece integration into the channel unattached pieces were 2 6 times more mobile than attached ramp or bridge pieces Temporal variation in retention rates was well explained by variation in peak ow All four of these factors have also been observed to in uence mobility in the temperate zone The higher mobility of wood in our study site relative to the temperate rainforest of the Paci c Northwest may be explained by the ashy and frequent oods the high decay rate or the branching morphology of the native trees but differentiating the role of these factors particularly ow and decay will be complicated by their covariation across climates 2010 Elsevier B V All rights reserved 1 Introduction A growing body of research has established that in stream wood can be important to both the geomorphology and ecological function of a broad range of temperate zone streams Most geomorphic effects of wood occur around stationary pieces or jams within the channel that de ect or impound ow Such pieces may do any or all of the following i increase resistance to ow Curran and Wohl 2003 Wilcox and Wohl 2006 ii de ect ow toward channel margins Daniels and Rhoads 2003 iii shield channel margins Brooks et al 2003 iv form steps and pools Richmond and Fausch 1995 Beechie and Sibley 1997 Gurnell and Sweet 1998 v induce pool scour Fausch and Northcote 1992 Baillie and Davies 2002 vi trap sediment or nutrients Smith et al 1993 Hart 2002 Faustini and Jones 2003 vii force avulsions Maser and Sedell 1994 and viii increase overbank ow Jeffries et al 2003 Ecologically the presence Corresponding author Tel 1 970 491 2524 E mail address cadol cnr colostate edu D Cadol 0169 555X see front matter 2010 Elsevier B V All rights reserved doi 10 1016 j geomorph 2010 06 015 of wood usually leads to increased stream habitat complexity Bisson et al 1987 Fausch and Northcote 1992 Kail 2003 Wood also promotes retention of both ne particulate organic matter FPOM Daniels 2006 and coarse particulate organic matter CPOM Bilby and Likens 1980 Webster et al 1994 which are major sources of energy and nutrients in many streams The wood hosts macroinvertebrates Anderson et al 1978 and provides substrate for algae fungi and microbes that contribute to the basal layer of aquatic food webs Maser and Sedell 1994 Tank and Webster 1998 In order to perform most of these functions the wood must remain stable and the duration of piece stability re ects the degree of in uence the piece will have on uvial processes and channel morphology In this sense we expect to see a correlation between wood residence time and geomorphic effectiveness For example a frequently moving piece of wood is less likely to cause persistent storage of CPOM or induce deep scour Daniels 2006 And although jams may persist even if the individual pieces turn over quickly thereby maintaining ow de ection characteristics a jam that frequently loses wood will likely pass even coarse sediment as well Haschenburger and Rice 2004 Moreover wood depletion rate is a necessary component of a comprehensive wood budget Benda and Sias 2003 Finally wood mobility is an important factor to consider 62 D Cadol E Wohl Geomorphology 123 2010 61 73 when balancing the needs of natural stream function and infrastructure protection and maintenance Comiti et al 2006 Mazzorana et al 2009 For these reasons quanti cation of wood mobility in terms of retention rates or residence times is desirable Numerous researchers have considered wood residence time in temperate zone streams using techniques such as dendro chronology Keller and Tally 1979 Murphy and Koski 1989 Dahlstr m et al 2005 Powell et al 2009 radiocarbon dating Hyatt and Naiman 2001 Guyette et al 2002 wood input monitoring Lienkaemper and Swanson 1987 and wood transport monitoring Berg et al 1998 Wohl and Goode 2008 Few studies have considered in stream wood in tropical settings Wood loads in La Selva Biological Station Costa Rica a tropical rainforest are lower than in the temperate rainforests of the Paci c Northwest Cadol et al 2009 In the previous paper we inferred that the lower wood loads re ect increased mobility of in stream wood because both regions have equally large trees and because primary productivity is expected to be as high or higher in the tropical rainforest as in temperate forests Raich et al 1991 Melillo et al 1993 although errors are potentially large in these estimates of productivity Clark et al 2001 Here we test the inference of greater mobility of wood in old growth tropical forest streams relative to analogous temperate forest streams Rates of wood transport in tropical streams may be different for several reasons including decay and ow