Ecosystems (2005) 8: 841-86 1 ' DOI: 10.1007/~10021-005-0043-4 Abundance and Production of Riparian Trees in the Lowland Floodplain of the Queets River,. Washington Estelle V. ~alian,'.~and Robert J. ~aimanl* '~choolof Aquatic & Fishery Sciences, University of Washington, campus Box 355020, Seattle, Washington 981 95, USA '~reshwaterBiology, Royal Belgian Institute of Natuml Sciences, 29 rue Vautier, 1000 Brussels, Belgium** ABSTRACT Riparian zones associated with alluvial rivers are floodplain, young terrace and mature terrace. Stem spatially dynamic, forming distinct vegetative density was highest in the active floodplain mosaics that exhibit sharp contrasts in structure (-27,000 stems1 ha), decreasing in the young ter- and processes related to the underlying biophysical race (-2,700 stems /ha) and the mature terrace template. The productivity of riparian plants, (-500 stemslha). Basal area and total stem biomass especially trees, influences streamside community were lowest in the active floodplain (-16 m2/ha characteristics as,well as the forms and fluxes of and -18 Mg dry weightiha, respectively) and organic matter to adjacent streams - thereby higher on the young terrace (-32 m2/ha and -1 34 strongly impacting patte'ms of channel morphol- Mg dry weighttha) and on the mature terrace (-69 ogy, water flow, sedimentation, and habitat in m2/ha and -540 Mg dry weight /ha). Total plot- rivers. As part of a comprehensive investigation of scale BAI was not significantly different among the riparian dynamics in coastal rain forest rivers of the physical templates with mean values ranging from Pacific Northwest (USA), we examined riparian approximately 1.4 (low terrace) to approximately tree abundance f density, basal area, and biomass) 2.8 m2/haly (active floodplain). In contrast, P was and rates of production (basal area grovvth [BAI] significantly higher on the mature terrace (10.3 and bole wood biomass increase [PI) of seven Mglha) than the active floodplain (3.2 Mglha) but common species - red alder (Alnus nrbra), Sitka there was no significant difference between young spruce (Picea sitchensis), bigleaf maple (Acer terrace (6.5 Mglha) and mature terrace. For the rnacrophyllum), western hemlock (Tsuga heterophy- entire Queets River floodplain (57 km2 over 77 km Ila ), black cottonwood (Popu!us tric&t@q~&~-vine of river length), the mature terrace contributed maple (Acer circinatum) and willow (Salix spp.) - in 81Oh of the total annual production (28,764 Mg) the lowland floodplain of the Queets River whereas the active floodplain and young terrace (Olympic National Park), Washington. Measure- accounted only for 5 and 14%, respectively. ments were made annually for three years (1999 - Overall, we show that riparian trees grow quickly 2001) in 16 permanent plots on three biophysical in this coastal Pacific Northwest system and that templates that formed a toposequence - active the older riparizn forests on mature terraces are the main contributors to stem production at the plot and floodplain scales for at least 350 years after stand initiation. This suggests that, in combination Received 6 April 2004; accepted 7 September 2004; published online with the rapid lateral migrations of many alluvial 21 October 2005. *Corresponding author; e-mail: [email protected] rivers, the older riparian forests on those terraces **Current address for Estelle V. Balian. are important and sustained sources of organic 842 E.V. Balian and R.J. Naiman matter (especially large woody debris, LWD) that, Key words: Pacific Northwest; basal area growth; over decades to centuries, shape the character of tree growth; stem production; Riparian forest; coastal rivers in the Pacific Northwest. production dynamics; alluvial river. duction exists. Previous observations primarily fo- cus on hardwoods and shrubs (Campbell and Riparian forests associated with alluvial rivers may Franklin 1979) or commercially valuable species be key to understanding how and why the riverine like Douglas-fir (Pseudotsuga menziesii; Means and systems are so dynamic and productive. Interactions others 1996). Most riparian production studies in between geomorphology, hydrology and riparian- the Pacific Northwest are from constrained high- derived large woody debris (LWD) in alluvial gradient headwater channels having narrow floodplains modify channel morphology and shape riparian corridors dominated by conifers (Edmonds the nature of riparian forests throughout much of and others 1993; Pabst and Spies 1999; Acker and America's coastal Northwest region (Gregory and . others .2003). Increasingly, attention is being fo- others 1991; Montgomery 1999; Naiman and others cused on large floodplains with heterogeneous 2000, 2005a, b) - as well as in many other forested physical conditions and species composition, and a mountain regions (for example, Gregory and others greater abundance of hardwoods (Pabst and Spies 2003; Tockner and others 2003). Riparian forests are 1999; Nierenberg and Hibbs 2000; Harner and especially important because they determine the Stanford 2003; Naiman and others 2005a). Never- characteristics and the rates of nutrients and organic theless, information on riparian tree abundance matter fluxes to streams, including LWD. Riparian- and production remains limited. derived materials shape biotic communities and In contrast, species composition (Lee 198 3; underpin processes related to channel morphology, Hanley and Hoe1 1996) and successional dynamics flow conditions, sedimentation, and habitat corn- (Fonda 1974; Agee 1988; Van Pelt 1991; Fetherston plexity (Bilby and Bisson 1998). Forest productivity and others 1995; Poage and Spies 1996) of alluvial is especially important in shaping and modifying floodplains in the Pacific Northwest are better many of these physical processes. known. Younger stands tend to be dominated by Comparative data on tree production dynamics red alder (Alnus nrbra) and Scouler7swillow (Salix are important for understanding the interplay be- scouleriana), and older stands tend to be dominated tween channel movement and the ability of ripar- by Sitka spruce (Picea sitchensis) and occasionally ian forests to supply LWD of sufficient sizes and western hemlock (Tsuga heterophylla) or western quantities to maintain riverine characteristics for redcedar (Thuja plicata). Cottonwood (Populus the long term. Coastal alluvial rivers may annually trichocarpa ) and bigleaf maple (Aeer macrophyllum ) move 10's of meters laterally, reshaping wide are of intermediate dominance in older stands. The floodplains every 380-900 years (O'Connor and combination of frequent disturbance, patchy soils, others 2003): In the process, forests are undercut uneven subsurface water flows,. and variable and trees topple into the river. The geneous patterns of river migrations present a conundrum. Riparian vegetation colonization and growth by creating trees on floodplains 'appear' to quickly grow to distinct biophysical templates on floodplains. Our large sizes but, before being undercut by lateral use of the term 'biophysical template' refers to the channel movement, are growth rates sufficient for physical environment that governs biotic responses the trees to attain sizes that can significantly con-' (Webster and Meyer 1997; Urban and others 2000) tribute to the initiation of ecologically important and, to some extent, may be equated with the terms LMrD accumulations? Further, are riparian areas 'zone' and 'patch'. with large trees sufficiently abundant to provide a We identified three major physical templates sustained source of LWD? These and other related differentiated by their geomorphology: active questions fueled our initial explorations into the floodplain (for example, gravel bars colonized by production ecology of riparian trees in a semi- shrubby willow and red alder), young terrace (for pristine lowland floodplain of the Pacific Coastal example, red alder dominated stands in old main or Rainforest within the Olympic National Park, side channels), and mature terrace (for example, Washington. stands dominated by coniferous species). We Riparian forests in the Pacific Northwest are examined the stand characteristics of these tem- highly valued but little infomation on tree pro- plates, of a variety of ages, to quantify production Abundance and Production of Riparian Trees 843 Figure 1. Location of the Queets River in the Olympic National Park, Washington. dynamics. The biophysical templates and the asso- ural and fire is rare (return interval -450 y). The ciated vegetation represent, in general, a century- river originates from glaciers on Mount Olympus. scale toposequence in this system (Van Pelt and The area is characterized by a uniformly wet and others 2005). Our specific objectives in this article mild climate with dry summers, and high precipi- are to: tation (-300-600 cmly) during autumn and winter (Franklin and Dyrness 1973). 1. Quantify the density, basal area, and standing The study site, a 1-km long island and adjacent biomass of dominant riparian trees and evaluate riGerbank approximately 26 km from the Pacific variation in these attributes among and within Ocean, experiences repeated winter floods that physical templates. modify channel morphology (Figure 2). Discharge 2. Empirically describe tree production dynamics varies from -8 m31s in July-September to more (that is, growth capacity and total production) than 3,000 m3/s in winter, with a year-round mean among and within the biophysical templates in discharge of -120 m3/s (US Geological