Vol. 524: 95–106, 2015 MARINE ECOLOGY PROGRESS SERIES Published March 30 doi: 10.3354/meps11206 Mar Ecol Prog Ser Impact of landscape structure on propagule dispersal in mangrove forests T. Van der Stocken1,2,*, D. J. R. De Ryck1,2, B. Vanschoenwinkel1, E. Deboelpaep1, T. J. Bouma3, F. Dahdouh-Guebas1,2, N. Koedam1 1Laboratory of Plant Biology and Nature Management, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium 2Laboratory of Systems Ecology and Resource Management, Université Libre de Bruxelles (ULB), Av. F.D. Roosevelt 50, CPI 264/1, 1050 Brussels, Belgium 3Department of Spatial Ecology, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 140, 4400 AC Yerseke, The Netherlands ABSTRACT: Although many riparian and semi-aquatic plant species disperse via water currents, little is known about how this process interacts with the landscape matrix. In mangroves, the dense aerial root network could act as a strong dispersal barrier for the morphologically diverse propag- ules found in these trees. In this study, we combined field and laboratory experiments to test the effect of root density, propagule morphology and hydrodynamic variables on retention rates and trajectories of the propagules of 4 common species. Overall, flume experiments showed that larger propagules were more frequently retained than smaller ones. For the larger propa gules, retention rates increased with increasing obstacle density in the landscape matrix. In elongated propagules, intraspecific variation was linked to floating orientation. Experimental wave action and increased water flow velocity reduced retention. Dispersal in the field was constrained by major tidal cur- rents and experiments confirmed less retention of smaller propagules, which moved farther than larger ones. Overall, our results reveal that the pronounced morphological variation in mangrove propagules interacts with the landscape matrix, contributing to strong differences in dispersal capacity among species and morphotypes. These results may help to explain observed mangrove distribution patterns, including zonation at local, regional and global scales. Additionally, given that many mangrove biotopes are currently strongly threatened by human pressure and fragmen- tation, this information is important as an input variable for dispersal models that aim to predict dispersal patterns at multiple scales and species responses to environmental change. KEY WORDS: Retention · Hydrochory · Flume tank · Tidal system · Community structure Resale or republication not permitted without written consent of the publisher INTRODUCTION matrix. Geomorphology, currents and other land - scape elements constitute barriers that may constrain, In general, propagules (i.e. dispersal units) of pas- delay or prevent dispersal altogether. For instance, sively dispersing organisms have a low probability of Baums et al. (2006) showed that for the reef building reaching a suitable destination, and this probability coral Acropora palmata, topo graphically induced strongly decreases with increasing spatial scale gyres in the Mona Passage between the Dominican (Clobert et al. 2012). Besides the dilution effect asso- Republic and Puerto Rico act as a seasonal filter for ciated with dispersal over wider areas, the main larval dispersal, determining population connectivity causes explaining the failure of propagules to com- and structure. Davies & Sheley (2007) demonstrated plete their mission are interactions with the landscape that high vegetation can strongly limit the dispersal *Corresponding author: [email protected] © Inter-Research 2015 · www.int-res.com 96 Mar Ecol Prog Ser 524: 95–106, 2015 distance of wind -dispersed seeds. Mangrove systems in a natural macrotidal mangrove system (in situ). In provide ex cellent examples of fragmented popula- the experimental treatments, different root densities tions that are dependent on dispersal for exploiting were mimicked and multiple water flow velocities the availability of suitable local and remote locations. applied. Waves were induced as an additional hydro- These ecosystems, however, are also notorious for dynamic variable. We hypothesized that (1) retention their impenetrable nature. Specialized mangrove of propagules would increase with increasing root growth forms with aerial roots generate dense net- density; (2) larger propagules would be more easily works of branches and roots that allow for persistence retained than smaller ones; (3) propagule retention in a harsh environment with a high disturbance would decrease with increasing water flow velocity; regime. At the same time, the root network is also and (4) wave action would reduce propagule reten- likely to interfere with hydrochorous transport of tion. Finally, we assessed the impact of mangrove mangrove propagules, affecting both emigration and roots and tidal forces on the dispersal behaviour of immigration. Although the complexity of the land- the propagules of 2 mangrove species by measuring scape matrix and the interplay with morphological the dispersal distance and direction in a release− propagule traits and hydrodynamic variables have recapture experiment in a natural mangrove system. been shown to influence hydrochorous dispersal in Here, we hypothesized that (5) the dispersal trajecto- wetland plants (Schneider & Sharitz 1988, Chang et ries of different propagule types in a natural system al. 2008, Nilsson et al. 2010), it is unknown whether would be constrained by the interplay between this process is important for mangrove propagules. propagule morphology, root density (dispersal dis- Past studies have revealed that the interaction of dis- tance) and hydrodynamics (dispersal distance and persing mangrove propagules with salt-marsh vege- direction). tation can facilitate recruitment (McKee et al. 2007, Peterson & Bell 2012). The efficiency of such physical structures in trapping hydrochorous propagules in- MATERIALS AND METHODS creases with propagule size (Sousa et al. 2007) and depends on structural differences in vegetation Flume experiment (Peterson & Bell 2012). In addition, higher water levels can strongly reduce or completely overcome Retention rates and dispersal characteristics of the trapping capacity of vegetation structures, with propagules in a mangrove forest environment were potential effects on deposition patterns (Peterson studied in a flume facility for a variety of barrier den- & Bell 2012). Besides the interaction with physical sities and hydrodynamic conditions. The racetrack structures, water flow direction, water depth and flume at the Royal Netherlands Institute for Sea propagule traits have been linked to long-term com- Research (see Bouma et al. 2005 for a detailed munity dynamics (Rabinowitz 1978b). Rabinowitz description) was adjusted with a wooden frame (6 m (1978b) showed that smaller propagules are trans- long, 0.6 m wide and 0.3 m high), of which the bottom ported farther inland than larger ones, but the final and top were covered with poultry netting through deposition pattern depends on site-specific conditions which bamboo sticks were inserted to mimic tree (rainfall, overland runoff, tidal regime) (Sousa et al. stems and vertical stilt roots (Fig. 1). The 2 layers of 2007). Although these studies indicate that propag- poultry netting ensured that bamboo sticks were ules interact with the landscape matrix, the effect of kept in place against the force of the water current. propagule traits and hydrodynamic variables on the Bamboo sticks (0.04 and 0.06 m diameter) were more process of retention and dispersal has, to our knowl- or less regularly interspersed over the whole length edge, never been studied for mangroves using a com- of the construction, mimicking 3 different root densi- bined experimental (i.e. under controlled conditions) ties (10, 20 and 30 roots m−2). Dispersal behaviour and field based approach. was studied for propa gules of 4 species: Rhizophora In this study, we empirically tested the impact of mucronata, Ceriops tagal, Heritiera littoralis and root density, propagule morphology, water flow vel - Xylocarpus granatum (Fig. S1 in the Supplement ocity and waves on the retention and the dispersal at www.int-res.com/articles/ suppl/m524 p095_supp. distance and direction of mangrove propagules. pdf). In general, propa gules of the first 2 species float Therefore, we mimicked mangrove root complexes horizontally after release from the parent tree and in a controlled experimental flume tank while manip- progressively change to a vertical position as the ulating velocity and wave action (ex situ). Addition- density distribution of their tissue changes (Davis ally, we conducted release−recapture experiments 1940, Rabinowitz 1978a). Hence, for these species, Van der Stocken et al.: Landscape structure impact on dispersal 97 pumps at the bottom of the flume (one at each 1 m interval of the test sec- tion). Dispersal velocities through the mimicked mangrove forest were cal- culated by measuring the time it took propagules to travel through the arti - ficial root system. To interpret the data, we imposed the following rules: (1) a propagule was considered as ‘retained’ when it was stuck for >3 min, and not touching the walls of the flume; (2) measurements for a propagule that was hindered by the walls of the flume were not considered in the results and were repeated; and (3) when a propag- ule was retained during 5 subsequent Fig. 1. Mimicked mangrove roots used in our flume experiments. Bamboo sticks (0.04 and 0.06 m diameter)