ESRM 401, Spring 2010
Spring Comes to the Cascades
Instructors: Tom Hinckley & Julie Combs
| Home & Critical Page
Student Issues Teaching Team Background material on:
Field Trips Historical
|
Gradients (03-19-08)
What do we mean by gradients? As one drives from Port Angeles to Hurricane Ridge, from Eatonville to Paradise, from Enumclaw to Sunrise, from North Bend to Snoqualmie Pass (or Cle Elum to Snoqualmie Pass or Cle Elum to the parking area for the North Fork of the Teanaway River) and from Gold Bar to Stevens Pass, vegetation changes. It changes in both composition and size. Douglas-fir grows at low elevation and Pacific silver fir at higher elevations. Douglas-fir at low elevation is different from that at high elevation. For example, Douglas-fir tends to be taller at lower as compared to higher elevations. These changes are the result of environmental gradients. For example, from Seattle to North Bend and from North Bend to Snoqualmie Pass, precipitation almost doubles with each step. Temperature changes, the form of precipitation changes. Below is a discussion of gradients and plants. Justification: If one examines almost any surface of the earth, one is presented with an array of different environmental gradients (see definitions below). For example, as one goes from the Pacific Ocean and coast over the Olympics to the Sound and then over the Cascades and to the Columbia, there are four major gradients over a relatively short distance. A change in elevation with a change in distance is, by definition, a gradient. However, associated with changes in elevation are small to large changes in other environmental variables and it is these changes that in turn affect the types of plant and animal species present. Even apparently abrupt changes, for example, at the coast line of the Pacific Ocean or the Puget Sound, there are gradients (for example, humidity). Indeed, at the scale that biological organisms respond, these apparently abrupt changes are really gradual. In the College's new set of core courses, we present concepts and then use these concepts to help us understand the patterns we see. In wildland environments, perhaps we feel more comfortable with gradients whereas in urban and production land environments, we may beless comfortable with the idea of gradients (abruptness seems pervasive in both managed and urban landscapes). We also see that scale, another important concept, influences both our and the organism of interest's "perceptions" of environmental change (whether gradual or abrupt). Finally, much of our management activities tend to be applied in discrete (i.e., abrupt) rather than gradual ways (e.g., hazard trees are removed, stands are harvested, jurisdictional boundaries are ubiquitous, etc.). Gradient Definition: From the 3rd Edition (2000) of the American Heritage College Dictionary: 1. A rate of inclination; a slope. 2. An ascending or descending part; an incline. 3. Phys. The rate at which a physical quantity, such as pressure, changes with respect to a given variable, especially distance. 4. Math. A vector having coordinate components that are the partial derivatives of a function with respect to its variables. 5. Biol. A series of progressively changing differences in the growth rate, metabolism, or physiological activity of a cell, an organ, or an organism. From an elementary Webster's Dictionary: Gradient = a continuous graded change in a measured activity or substance (e.g., vertical temperature gradient in a lake). Background: Although considerable ecological research uses gradient analysis (e.g., Curtis 1959, Von Numers and van der Maarel 1998, Wheeler and Proctor 2000), most scientists recognize that distinct communities develop along these gradients. In some cases, the boundaries between these communities are extraordinarily clear whereas in other cases, one community type grades into another and there is not a distinct boundary (see Figures 1 and 2). The gradients may be straight-forward such as latitude, elevation, temperature, light, soil depth, soil nutrition, soil moisture, pH, water level, precipitation, or more complicated such as the acid base-poor vs. neutral, base- and bicarbonate-rich gradient, soil redox potential, oxygen concentration, etc.
So then how does one go from a gradient to where there appears to be such distinctive communities (defined as an association of interacting plant and animal populations, usually defined by the nature of their interactions, the place which they live or some combination of the dominant overstory and understory plants [for example, a Douglas-fir, western hemlock forest with an understory of sword fern and Oregon grape])? One can understand that each organism has a particular environment in which it can survive and grow. It would then seem that along a gradient one should see gradual increases and decreases in different species. To a certain extent, one can see examples of gradual changes; however, more often one sees rather abrupt or dramatic changes. How does this occur? In order to understand how relatively distinct communities form, one must first understand how the individual species responds and then how the response is modified in the presence of other species due to competition. The first step is both straight-forward and relatively easily understood. The second is not. Step 1 (response of the individual species): One can describe for a population of a given plant species, the ability of the individuals making up that population to grow (height at age 50 or height increment, or biomass per individual per year are just a few examples of how growth might be determined). If one graphs the average height at age 10 of the individuals growing on various sites along a soil moisture gradient the heights might look like this:
Management Implications: In spite of the myriad of gradients that exist, most plant and animal communities appear to be organized into distinctive communities. Typically management occurs at one of several levels of biological organization: an individual organism (e.g., a hazard or diseased tree), a population (e.g., a list population of a particular salmon species, an elk herd), a stand, an ecosystem, a watershed or a landscape level. Because of either disturbances (e.g., fire, clearcut) or jurisdictional boundaries, many plants are managed on a stand level basis. As we increasingly appreciate the interactions between various organisms and we better understand the services provided by assemblages of organisms, management becomes more sophisticated and approaches a situation where several different levels of biological organization are considered. This would be even true for hazard tree evaluation as such a tree could provide a range of services beyond itself if left standing.
In a sense, birds utilize the environmental gradients that exist in a forest stand (gradients of moisture, wind, heat, light, etc., see Figure above). Stand structure creates these gradients. Glossary of Terms
References:
|
