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Effects of Scale and Disturbance View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Texas A&M University Ecological Monographs, 73(2), 2003, pp. 223±239 q 2003 by the Ecological Society of America ECOHYDROLOGY OF A RESOURCE-CONSERVING SEMIARID WOODLAND: EFFECTS OF SCALE AND DISTURBANCE BRADFORD P. W ILCOX,1 DAVID D. BRESHEARS,2 AND CRAIG D. ALLEN3 1Rangeland Ecology and Management Department, Texas A&M University, College Station, Texas 77845-2126 USA 2Environmental Dynamics and Spatial Analysis Group, Mail Stop J495, Earth and Environmental Science Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA 3U.S. Geological Survey, Midcontinent Ecological Science Center, Jemez Mountains Field Station, Los Alamos, New Mexico 87544 USA Abstract. In semiarid landscapes, the linkage between runoff and vegetation is a par- ticularly close one. In this paper we report on the results of a long-term and multiple-scale study of interactions between runoff, erosion, and vegetation in a pinÄon±juniper woodland in New Mexico. We use our results to address three knowledge gaps: (1) the temporal scaling relationships between precipitation and runoff; (2) the effects of spatial scale on runoff and erosion, as in¯uenced by vegetation; and (3) the in¯uence of disturbance on these relationships. On the basis of our results, we tested three assumptions that represent current thinking in these areas (as evidenced, for example, by explicit or implicit assump- tions embedded in commonly used models). The ®rst assumption, that aggregated precip- itation can be used as a surrogate for total runoff in semiarid environments, was not veri®ed by our ®ndings. We found that when runoff is generated mainly by overland ¯ow in these systems, aggregated precipitation amounts alone (by year, season, or individual event) are a poor predictor of runoff amounts. The second assumption, that at the hillslope and smaller scales runoff and erosion are independent of spatial scale, was likewise not veri®ed. We found that the redistribution of water and sediment within the hillslope was substantial and that there was a strong and nonlinear reduction in unit-area runoff and erosion with in- creasing scale (our scales were slope lengths ranging from1mto105m).Thethird assumption, that disturbance-related increases in runoff and erosion remain constant with time, was partially veri®ed. We found that for low-slope-gradient sites, disturbance led to accelerated runoff and erosion, and these conditions may persist for a decade or longer. On the basis of our ®ndings, we further suggest that (a) disturbance alters the effects of scale on runoff and erosion in a predictable wayÐscale relationships in degraded areas will be fundamentally different from those in nondegraded areas because more runoff will escape off site and erosion rates will be much higher; and (b) there exists a slope threshold, below which semiarid landscapes will eventually recover following disturbance and above which there will be no recovery without mitigation or remediation. Key words: banded vegetation; dryland hydrology; ecohydrology; erosion; landscape ecology; pinyon; pinÄon±juniper; runoff; semiarid hydrology; vegetation patches; water yield. INTRODUCTION and dry subhumid zones (Middleton and Thomas Ecological and hydrological processes are tightly in- 1997). In these environments, a positive-feedback or terrelated, and the complex ways in which they interact self-reinforcing mechanism links water and vegetation represents an important research frontierÐone that re- (Cammeraat and Imeson 1999, Ludwig et al. 2000). In quires close collaboration between ecologists and hy- other words, how water is redistributed and where it drologists. Understanding these dynamics is crucial if becomes concentrated are important determinants of we are to effectively address landscape change result- vegetation patterns; and conversely, vegetation patterns ing from climate change and land use. Recent critiques also directly modify the nature of runoff (Ludwig et of current hydrologic research have emphasized the al. 1997, Shachak et al. 1998, Bergkamp et al. 1999, need for more research at the interface of ecology and Puigdefabregas et al. 1999, Valentin et al. 1999). These interactions are the focus of the emerging ®eld of eco- hydrology (Entekhabi et al. 1999, National Research hydrology. Council 1999). Understanding the linkages between vegetation and Ecological and hydrological processes are particu- runoff processes takes on added importance in light of larly tightly coupled in water-limited environments, or the current pace of change in both climate and land drylands, which encompass hyperarid, arid, semiarid, use. Both types of change are likely to engender non- Manuscript received 26 September 2001; revised 29 April linear responses, with threshold conditions being im- 2002; accepted 30 April 2002; ®nal version received 22 May portant (Davenport et al. 1998, Imeson and Lavee 2002. Corresponding Editor: W. K. Lauenroth. 1998). The linkages between runoff and vegetation in 223 224 BRADFORD P. WILCOX ET AL. Ecological Monographs Vol. 73, No. 2 drylands, however, are complex (Hutjes et al. 1998). sociated erosion), including the frequency, magnitude, A conceptual framework (Ludwig et al. 1997) clari®es and timing of runoff and erosion; (2) the effects of these linkages; it proposes that the redistribution of spatial scale on runoff and erosion, including the re- resources from source areas (bare patches) to sink areas lationships between redistribution of resources and re- (vegetation patches) is a fundamental process within source losses; and (3) the effects of disturbance on drylands that may be disrupted if the vegetation patch these processes and relationships, in terms of both mag- structure is disturbed. ``Resource-conserving'' dry- nitude and persistence. The objective of our study was lands are organized such that runoff is quickly captured to evaluate, for each knowledge gap, common as- by, and concentrated in, vegetation patches, minimiz- sumptions (explicit and implicit) related to that area. ing the loss of resources. This concentration of re- Using observations of runoff and erosion in a semiarid sources increases the ef®ciency of their use and allows pinÄon±juniper woodland, we addressed knowledge for higher net primary productivity. For example, when gaps 1 and 3 with a long-term data set (8 yr) and knowl- water from bare patches becomes concentrated in veg- edge gap 2 with a shorter term (26 mo), multiple-scale etated patches, it is stored at greater depths and is less data set. subject to evaporation (Newman et al. 1997), which means more water is available to plants (Seghieri and KNOWLEDGE GAPS AND ASSUMPTIONS Galle 1999, Galle et al. 2001). Temporal relationships between precipitation The framework further proposes that if a disturbance, and runoff such as overgrazing, reduces the density and/or size of vegetation patches, the system will become ``leaky'' or Runoff prediction is often based on a simpli®ed con- ``nonconserving''Ðless ef®cient at trapping runoff, cept: that a given magnitude of runoff will be produced leading to a loss of valuable water and nutrient re- by a given amount of precipitation occurring over some sources (Ludwig and Tongway 2000). A positive-feed- period of time. One common methodology in hydro- back loop may then reinforce the degradation process: logic modeling, the curve number (Pilgrim and Cordery the higher runoff rates will mean less water available 1993, Arnold et al. 1998), predicts runoff volume in to plants and higher erosion rates (see also Davenport this way. It is employed in ecological models as well et al. 1998). The degradation cycle may proceed where- (e.g., Mauchamp 1994). Although reasonable for many by overland-¯ow runoff increases in both amount and humid environments, where runoff occurs as subsur- energy, plant vigor declines, and the microclimate be- face ¯ow and is essentially that portion of precipitation comes more harsh. Although few studies have explic- greater than soil storage capacity, these precipitation± itly documented changes in overland runoff caused by runoff relationships may not be appropriate for semi- disturbance, there is indirect evidence of such changes arid landscapes where runoff generally occurs as in- (Whisenant 1999). ®ltration-excess overland ¯ow. For these regions, the Some of the fundamental assumptions of this frame- optimal period over which precipitation amounts work, however, have not been tested or quanti®ed, par- should be aggregated is unknown; and even more im- ticularly in regard to the temporal and spatial nature portant, runoff is strongly in¯uenced by factors other of runoff. One of these assumptions is that some in- than precipitation amountÐsuch as surface in®ltration herent relationships exist between spatial scale and run- characteristics, soil moisture, and precipitation inten- off and erosion. For example, in a conserving system, sity. In other words, data on precipitation alone may unit-area runoff and erosion should dramatically de- be insuf®cient for reliable prediction of runoff. Given crease as scale increases, because runoff is quickly cap- the ecological importance of runoff in semiarid land- tured. For a degraded or nonconserving system, the scapes, gaining an understanding of the true relation- decrease in unit-area runoff with increasing scale ship between precipitation and runoff is essential. should be less precipitous, because capture is less ef- To address this
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