Short and longterm responses in the ecosystem

The chapters about hurricanes and droughts in this section demonstrate that short-term climatic events may have short- and long-term responses in the ecosystem. Both the short- and long-term responses are important. The short-term responses have noteworthy economic influences in the agricultural ecosystem. One could argue that the ecosystems containing species with short life spans such as grasslands are able to respond and recover from a short-term climatic disturbance more quickly than those ecosystems with longer lived species such as trees. Corn responds quickly to variability in precipitation during important parts of the growing season. Gage believes the long-term effects of a short-term drought on annual rotational agronomic systems are generally minimal. Other LTER studies have documented strong relationships between annual precipitation and grassland aboveground net primary productivity (Knapp et al. 1998). Conversely, the Coweeta study brings to our attention the insidious, long-term effects of drought that quietly kills trees and leaves their dead necromass on the landscape for decades into the future. However, each ecosystem is responding at its own characteristic timescale. Boose notes that the mixed hardwood forests of central New England and the Tabonuco forests of Puerto Rico both exhibit remarkable resiliency to wind damage. In both cases, despite major structural reorganization after a hurricane, there was rapid regeneration of canopy cover through releafing, sprouting, or recruitment, which helped to reduce impacts on soil moisture, temperature, and nutrient cycling processes. Nevertheless, some signs of the hurricane impact are present for decades, although less so in Puerto Rico where decomposition and regeneration rates are much faster than in New England. Gage quotes work from the Cedar Creek LTER in Minnesota that also exhibits a difference in the time response to the 1988 drought between grasslands and the semiarboreal ecosystem. He reminds us that it was not until 1993, the fifth year following the 1988 drought, that there were no longer discernible effects of drought on species richness in successional grasslands. However, the effects of the 1988 drought were still clearly evident in the oak savanna complex in 1993. About 30% of mature pin oaks died during the drought, compared to only 10% of bur oaks. Most of these dead trees are still standing. Tilman and Downing (1994) concluded that this major shift in oak species composition and reduction in oak canopy cover will likely impact these savanna ecosystems for decades to come. Comparable to the Cedar Creek case is the Kloeppel et al. report that drought effects in the Appalachian forests are species specific and the necromass remains on the landscape.

One of the implications of these dynamics relates to the stability of the ecosystem. On the one hand, following a climatic event giving rise to a short-term response the ecosystem might return to approximately the state that it was in before. On the other hand, some of the long-term responses, especially those related to human activities such as increasing irrigation systems or logging, may change the ecosystem in important, semipermanent and permanent ways. The Cedar Creek LTER work pointed to another pivotal finding on ecosystem operation with respect to the 1988 drought. At this site plant communities with the greatest biodiversity were the least susceptible to year-to-year fluctuations in total plant growth and also were the least susceptible to nutrient loss from the soil by leaching (Tilman and Downing 1994). Cedar Creek researchers point out that this conclusion was the first experiment showing a clear connection between biodiversity and the functioning of ecosystems since Darwin first suggested it over a century ago.

Investigators of short-term climate events and related ecosystem responses are also concerned with the return period of the event. Boose comments that a critical factor controlling ecosystem impact is the relative length of hurricane return intervals and vegetation life spans. However, it is important that there was no clear evidence of centennial-scale trends for either region. In both regions Harvard Forest investigators found the same multidecadal variation that is well documented for North Atlantic hurricanes in general.

0 0

Post a comment