2S8 



Ripuriiui Ecosystems — Our Liviiii; Rfsouires 



No. of cottonwoods aged on a surface 

 H^ Trees Seedlings 



Bl Trees at eslablished surface 



Establishment Patterns of 

 Riparian Tree Populations 



The structural diversity of riparian cotton- 

 wood and willow stands is a function of spatial 

 and temporal patterns of occurrence. These pat- 

 terns are largely determined by events during 

 the establishment phase (Stromberg et al. 1991; 

 Scott et al. 1993). Where stream regulation lim- 

 its flooding and channel movement (e.g.. the 

 lower Colorado River), opportunities for seed 

 gennination are limited. In such systems, com- 

 munity structure may become less dynamic 

 unless novel forms of disturbance such as fire 

 increase in importance relative to the natural 

 disturbance regime. 



The magnitudes of flows associated with 

 Cottonwood establishment are intluenced by 

 local channel processes. Along the upper 

 Missouri River, sections of meandering channel 

 alternate with sections where lateral migration 

 does not occur In meandering sections, suc- 

 cessful establishment occurs at relatively low 

 elevations above the channel (Fig. 3a). produc- 

 ing several bands of even-aged trees ( Bradley 

 and Smith 1986). 



If. however, lateral movement of the channel 

 is constrained by a narrow valley, successful 

 establishment occurs only at high elevations, 

 often producing a single, narrow band of trees 

 (Fig. 3b): seedlings initially established at lower 

 positions are removed by water or ice scour. 

 Where the channel is free to move, plant estab- 

 lishment occurs relatively frequently in associa- 



surface 



50 



100 



150 



200 250 



Distance (m) 



300 



350 



400 



450 



Surveyed, not aged 



Water 



S 



surface 



50 



100 



150 



200 250 



Distance (m) 



300 



350 



400 



450 



Fig. 3. Cross section in the la) meandering ctiannel reacti, Missouri River, Montana, and (b) the 

 constrained channel reach. All seedlings established within 10 cm (3.9 in) of the present surface 

 (from Scott et al. 1994). For (a), at 160 m (525 ft) six trees were aged, but depth to establishment 

 surface was measured for only one. 



tion with both moderate and high river flows, 

 but where the channel is constrained, plant 

 establishment is associated with infrequent high 

 Hows in excess of 1,400 m% (50.000 ft-Vs). 

 Elimination of such high flows would largely 

 eliminate cottonwood and willow stands from 

 the constrained reaches of the upper Missouri 

 River and decrease the frequency of stand estab- 

 lishment in the meandering reaches. From a 

 water-management perspective, then, it is 

 important to recognize how flow variability, 

 including infrequent large flows, shapes the dis- 

 tribution and abundance of riparian tree popula- 

 tions. 



Meandering reach of the Missouri River, Montana. 



Disturbance Regimes and the 

 Invasion of Non-native Species 



Riparian ecosystems are dependent upon 

 disturbance caused by occasional high flows. 

 Along rivers where these flows have been 

 reduced in frequency and magnitude, natural 

 riparian ecosystems are being lost along with 

 associated invertebrate and vertebrate species. 

 Resource managers concerned with maintaining 

 floodplain ecosystems need to consider ways of 

 preserving flows that produce establishment, 

 growth, and survival of native riparian species. 

 If not, species such as tamarisk can exploit 

 resources more efficiently than native riparian 

 species, thereby altering whole ecosystem prop- 

 erties (Vitousek 1990). Thus, as Hobbs and 

 Huenneke (1992) suggest, modification of the 

 historical disturbance regimes will result in a 

 decline in native species diversity. Although 

 successful plant invasions are often associated 

 with increased disturbance (Hobbs 1989; 

 Rejmanek 1989; Hobbs and Huenneke 1992; 

 Parker et al. 1993), in situations where the fre- 

 quency or intensity of a natural disturbance is 

 decreased, the invasion of competitively superi- 

 or non-natives may be promoted (Hobbs and 

 Huenneke 1992). 



