|
|
- Info
Natural processes
| Author: |
Pickett, Steward T.A. |
| Date: |
1998 |
| Periodical: |
In: Mac, M.J.; Olper, P.A.; Haecker, C.E.P. [and others],eds. Status and trends of the Nation\'s biological resources. Washington, DC: U.S. Department of the Interior, Geological Survey |
| Abstract: |
Biological Diversity and Heterogeneity- We had decided to look at an unusual place an-- enormous blowdown in the old-growth forest in the Tionesta Scenic Natural Area in western Pennsylvania. Chris Peterson (at that time a graduate student at Rutgers University) and I were visiting Tionesta Scenic Natural Area to see if he would like to conduct the research for his doctoral dissertation there. I was enthusiastic because it was a rare opportunity to study an intense, natural disturbance in a virgin forest (Peterson and Pickett 1991).
But it might be a tough place to work. It was nearly a day\'s drive from New Brunswick, and the tornado, estimated to have packed winds in excess of 386 kilometers per hour, had made a jumble of the forest. Many of the largest trees were between 1 and 1.5 meters in diameter and had stood more than 30 meters tall-- these were now scattered like pick-up sticks. We had to make our way over the interlocking mass of downed logs; this intermittent elevated highway of logs was the easiest way to move through the blowdown because the massive crowns of the downed forest giants made a nearly impassible tangle on the surface. The ground surface itself was now punctuated by traps and barriers. Some of the trees were uprooted, and next to the roots were the deep pits from which those roots had been wrenched. If we were lucky, we could see the pits, rather than stepping through a seemingly solid mat of leaves and branches into the soggy hole they covered. Other trees were twisted and broken, and their splintered trunks pointed at the clear blue sky. These snags became our landmarks as we navigated across the 900-meters-wide blowdown.
In spite of the difficulty of working in the blowdown at Tionesta Scenic Natural Area, Chris agreed that it was a fascinating place, and over the next 7 years we came to understand this stunning place (Peterson and Pickett 1995). The piles of woody debris and leaf litter we poked through during that first visit would prove to be barriers that protected some tree seedlings from hungry deer. The pits would become ringed with ferns and mosses, and many tree seedlings would die on the dry, clayey mounds. Small clusters of American beech sprouts would turn into dense, shady patches where competition would be intense. All of these insights taught us the lesson of Tionesta Scenic Natural Area: the severe tornado of 31 May 1985 set up a patchy template of physical and biological structure, called enviromental heterogeneity. The heterogeneity interacted with the organisms to determine how the forest would regrow, and the template itself changed with time. That lesson may sound specific to the effects of a tornado in an old-growth forest in western Pennsylvania, but the lesson is general and applies to the entire biological heritage of the United States. The diversity of organisms and of the communities, ecosystems, and landscapes in which they participate is a response to the processes that generate heterogeneity (Wiens 1977; Chesson 1985, Kareiva 1990; Caswell and Cohen 1991; Naeem and Colwell 1991; Tilman 1994). Heterogeneity appears in many guises and is generated by a handful of important ecological processes thatoperate everywhere in one form or another. |
|
