In general, many biogeographers and ecophysiologists believe that species are limited at their upper elevational or poleward latitudinal limits by climatic effects on reproduction and establishment (Woodward, 1992). The limiting effect of high temperature on the distribution and dynamics of forests is less clear, however. High temperatures do not seem to be directly physiologically limiting in most ecosystems (Larcher, 1980, Woodward, 1987). Trees have been long noted to grow south of their latitudinal limits in the northern hemisphere (Loehle & LeBlanc, 1996), and physiological (Bonan & Sirois, 1992), dendrochonological (Cook & Cole, 1990), and tree growth (Larsen, 1965) data all show that maximum relative growth rates often occur at a species southern range limit. Lower elevational or lower latitudinal limits appear to have other causes than climate.
Similar pattern can be observed along the site water gradient. Generally, it is limited water supply that reduces the growth of individual trees. If water supply is improved this will result in better growth. Only in cases of above optimal soil water content with an elevated water table a shortage in oxygen will result in a decline of growth.
From theory, the better adapted will outcompete the less adapted species (). On the other hand, it can be argued that adaptation towards "minimum in supply" (of resources like water, light, nutrients) and towards "extremes" (of direct gradients like temperature) results in a significant reduction of competitive ability(). Thus, if two plant species with individual adaptations and constraints compete for resources at a given site the result will be determined by the balance between stress-tolerance (adaptation towards minima) and competitive ability (). Along a resource (or direct) gradient at the point of competitive balance between these two different species it will be "too stressful" for the more competitive species, and "too competitive" for the more stress-adapted species. Under the exclusion of competition at this point of competitive balance however, the stress-tolerant species is generally not physiologically limited.
To define parameters for mechanistic forest succession models we, thus, should focus on physiological contraints (fundamental niche), rather than on fixing limits towards minima and maxima of resources and direct gradients simultaneously (realized niche). We argue that the observed limits towards shortage in supply is close to the physiological limit of that species while the limit towards optimal supply is not. We propose to derive data on upper physiological limits from experimental analysis instead (see e.g. Lange et al., 1981).
This concept allows to test the feasability to define fundametal niches for trees from survey data. We attempt to compare simple predictive spatial models for trees of the Shoshone National Forest based on the environmental envelope technique(). Environmental envelopes are defined either from principles of the realized niche (upper and lower observed limits per species) and of the fundamental niche (lower observed limits complemented with competitive indices per species).
.......
Over the last five years in collaboration with Forest Service land managers and scientists at Utah State University an extensive vegetation and soils inventory for the Shoshone National Forest, Wyoming was performed. A large amount of data was collected in the field, as well as from remotely-sensed imagery and GIS-based analyses of a digital elevation models (DEM). Specifically, data on approximately 1200 geo-referenced sites, distributed in an optimal-stratified design were sampled.
Specific data recorded include soil parent material, and depth, texture, and coarse fragment content by horizon. Specifically of interest to this study, these sites provide information on individual tree species cover and height, elevation, aspect, slope, geology, and soil water holding capacity. These data provide us with the information to perform a preliminary analysis of species distributions along environmental gradients, including the derived climatic gradients described below, and to extend the calculations as proposed below.
Using the available digital elevation model (DEM) and available climate data, we have developed a detailed model of climate and landscape pattern for the Shoshone National Forest.
The database of the Shoshone Ecosystem Project containing information on the abundance and distribution of both individual species and habitat types was dropped through bioclimatic maps. In different projects a series of hypotheses were tested concerning the driving forces that govern the distribution and dynamics of plant species (Wendel, in prep.; Roberts et al., in prep.). These analyses revealed the importance of heat sum, cold temperatures, evapotranspiration and site water balance to predict the distribution of individual species.
Consequently, we used the climate variables to define the tree species realized and fundamental niches using the environmental envelope approach (Holdridge, 1967; Box, 1982; Busby, 1986>; Walker & Cocks, 1991; Carpenter et al., 1993; Shao & Halpin, 1995). Two-dimensional rectilinear environmental envelopes for tree species of the Shoshone Nat. Forest were derived from heat sum and site water balance.
Following are the simulated distribution pattern of tree species for the Shoshone National Forest. The predicted distribution is based on rectilinear bivariate (DDEG &: SWB) environmental envelopes. Highest probabilities are modelled for the core 80% of observed diestribution along these gradients.
| Biophysical parameters | Modelled realized niche | Modelled fundamental niche |
|---|---|---|
| Not yet available |
Box, E.O. 1981. Macroclimate and Plant Forms: An Introduction to Predictive Modelling in Phytogeography. Junk, The Hague, NL.
Busby, J.R. 1986. A biogeographical analysis of Nothofagus cunninghamii (Hook.) Oerst. In southeastern Australia. Australian Journal of Ecology 11: 1-7.
Carpenter, G., Gillison, A.N. & Winter, J. 1993. DOMAIN: a flexible modelling procedure for mapping potential distributions of plants and animals. Biodiversity and Conservation 2: 667-680..
Holdridge, L.R. 1967. Life Zone Ecology , Tropical Science Center, San José, Costa Rica.
Lange, O.L., Noble, P.S., Osmond, C.B. & Ziegler, H. (eds.) 1981.Plant Physiological Ecology. I. Responses to the physical Environment. Springer, Berlin.
Shao, G. & Halpin, P.N. 1995. Climatic controls of eastern North American coastal tree and shrub distributions. Journal of Biogeography 22: 1083-1089.
Walker, P.A. & Cocks, K.D. 1991.
HABITAT: a procedure for modeling a disjoint environmental envelope for
a plant or animal species. Global Ecology & Biogeography Letter
1: 108-118.