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Dayton, PK, Tegner MJ, Parnell PE, Edwards PB.  1992.  Temporal and spatial patterns of disturbance and recovery in a kelp forest community. Ecological Monographs. 62:421-445.   10.2307/2937118   AbstractWebsite

This paper addresses questions of community and patch stability as defined by the population biology of dominant plants in the context of different areas within a large kelp forest. We ask (1) "Do large-scale episodic events override biological mechanisms as major community structuring processes?", (2) "Are different local areas characterized by different processes?", and (3) "How persistent are the patches or biological structure over decadal and local spatial scales?" We evaluate these questions with regard to the effects of various types of disturbance for as much as three decades on the populations of several species of kelp in the large kelp forest off Point Loma, San Diego, California. The most sensitive population factors we studied include recruitment, density, and survivorship. Patch stability was evaluated with regard to the persistence of patches already well established in 1971-1972. The study sites offer a cross-shore transect through the central part of a large kelp forest at depths of 8, 12, 15, 18, and 21 m; two additional sites at the north and south ends of the forest offer a longshore transect along the 18-m contour. There were marked differences among the decades with regard to the intensity of the disturbances. Compared with the 1980s, the two preceding decades were relatively benign. The 1980s had two extreme disturbance events: the 1982-1984 El Nino-Southern Oscillation (ENSO) was the most severe El Nino event in the last century, which included very warm, nutrient-depleted water, and a short but intense storm in January 1988 appeared to have been the most severe in perhaps 200 yr. The storm changed age-specific kelp mortality patterns and caused the first large-scale understory mortality in several decades. By sweeping away drift algae it caused intense local urchin grazing. The storm was followed by a strong La Nina event marked by cool, nutrient-rich water in 1988-1989. Differences in kelp recruitment and survivorship in different areas of the kelp forest are influenced by gradients in longshore currents, temperature, light, wave energy, floc, planktonic propagules, and physical disturbance. The areas are characterized by different plant Population patterns and the effects of several species of herbivores. The massive disturbances of the 1980s obliterated much of the structure in the kelp forest. Certainly the disturbances caused many lag effects including outbreaks of understory algae such as Desmarestia ligulata, intraspecific competition, changes in grazing patterns, etc., which in tum resulted in between-area variation in recovery rates. However, in all cases this variation was overshadowed by the overwhelming competitive dominance of Macrocystis pyrifera. Most of the understory patches on the transect lines, some of which had persisted for 7 yr, died out by the end of 1990. The population biology of Macrocystis was remarkably similar in most areas, as the cohort longevity and survivorship curves were very similar, and the plant and stipe densities tended to level off in only a few years. Thus large-scale episodic events such as El Ninos, La Ninas, and rare storms exert dramatic impacts, but small-scale responses such as density-vague recruitment (neither density dependent nor density independent) and survival allow prompt recovery, often to preexisting patterns. The one exception was the southern site, which was marked by sea urchin grazing and poor kelp recruitment through the latter half of the 1980s, but a recent wa urchin disease event has led to kelp recruitment in fall 1991. A seeming paradox to the observed Macrocystis dominance is that in almost all areas, some understory patches of old plants have persisted through the 1980s.

Dayton, PK, Tegner MJ, Edwards PB, Riser KL.  1999.  Temporal and spatial scales of kelp demography: The role of oceanographic climate. Ecological Monographs. 69:219-250.   10.1890/0012-9615(1999)069[0219:tassok];2   AbstractWebsite

This paper integrates long-term descriptive and experimental studies of the effects of ocean climate on inter- and intraspecific competition, as expressed by recruitment, density, survivorship, growth, and reproduction of the most conspicuous kelp species in the Point Loma kelp forest community off San Diego, California, USA. The species included Macrocystis pyrifera, with a floating canopy; Pterygophora californica and Eisenia arborea, which rely on stipes to support their canopy; Laminaria farlowii, with a prostrate canopy; and a speciose red algal turf. To evaluate the roles of large-scale oceanographic processes on biological processes across important depth gradients, the study was carried out over nine years during a cold-water, nutrient-rich La Nina event (1988-1989) and a warm-water, nutrient-stressed El Nino period (1992-1994), over a depth range of 8-23 m. This depth range encompassed strong physical gradients involving factors that are critical for kelp growth, including bottom temperatures (correlated with nutrients) and light levels. To examine interactions among these kelps, we established clearings across the depth gradient and then manipulated Macrocystis recruit densities. The demographic responses offer an understanding of the "fundamental" vs. "realized" niches of these species. Evaluating these patterns, as they are influenced by inter- and intraspecific competition, offers insights into the "realized niches" of the kelps. With the exception of some understory effects on Macrocystis recruitment and some evidence of intraspecific competition during the nutrient-rich La Nina conditions, we found little influence of competitive effects on Macrocystis. The response of Pterygophora to manipulations and disturbances suggests light-limited recruitment, and competition with Macrocystis was exhibited via reduced growth and reproduction, but not survivorship. No nutrient stress was observed in Pterygophora reproduction. Eisenia recruitment is rare, but once established, juveniles had very good survivorship, with growth and reproduction reduced by depth; the Macrocystis treatment was more important than depth, suggesting the importance of light to Eisenia recruitment and growth. In general, Macrocystis had massive effects on Laminaria growth and reproduction, the strength varying with depth. In particular, there were very strong effects of competition with Macrocystis during the nutrient-rich La Nina period when Macrocystis had a dense surface canopy. In addition to the Macrocystis effects, there were some significant Pterygophora effects on Laminaria growth during El Nino. The strongest biological definition of realized niches occurred during the nutrient-rich La Nina period, especially in shallow depths. One of the most important conclusions of this paper is the appreciation of the importance of scaling in time to include oceanographic climate. There are many seasonal patterns, but the interannual scales that encompass Er Ninos and La Ninas, and ultimately the interdecadal-scale oceanographic regime shifts that affect the intensity of canopy competition with Macrocystis, are critical for this system because surface-water nutrients have pervasive long-term effects an the other kelps. Small-scale patterns are driven by local processes (competition, disturbance, dispersal, etc.) that potentially are important at larger scales; however, our most lasting effects result from very large-scale, low-frequency episodic changes in nutrients, with cascading competitive consequences to the other algal populations in the community.