Lec19_Succession_08 - HORT 460 Cropping Systems Ecology MODULE IV Cropping Systems Ecology Over Time Plant succession A Introduction The

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Unformatted text preview: HORT 460 Cropping Systems Ecology MODULE IV. Cropping Systems Ecology Over Time Plant succession A. Introduction The term "succession" was first introduced by the famous philosopher and naturalist, Henry David Thoreau, in an 1860 essay entitled, "The succession of forest trees" (in the 1863 essay collection, Excursions). Every practicing ecologist has their own definition. Here is mine: Succession is the directional change in the biotic and abiotic composition, structure, and function of an ecosystem over time. Many ecologists might accept this simple definition, but the subject has been, and continues to be, very controversial. B. The Clements model of succession A book on the subject published in 1916 by the ecologist, Frederick Clements, dominated the thinking about succession for much of 20th century. Clements had a "superorganism" view of plant communities, and saw succession as an obligatory, highly predictable process, that converged toward a specific "climax" species composition, illustrated below. Disturbance—->pioneer species A--> B --> C --> climax species D The letters A, B, etc. should be taken to represent a group of species adapted to a specific successional stage. Species that occur later in the succession are dependent on species earlier who have "paved the way" for them, in a series of steps: Step 1: Disturbance--> invasion by "pioneer" species A that can out-compete others on the primitive, low organic matter soils with little ground cover. Step 2: Productivity of pioneer species A gradually increases soil organic matter, and causes other changes, to create an environment where species B can invade and outcompete and dominate A Step 3: Species B has effects that gradually change the environment in such a way that species C has an opportunity to invade and dominate. Step 4: etc., until a final dominate climax community of species D takes over, and the environment becomes stable such that D continues to replace itself. This very simplistic view (that you may have learned in high school) is rejected by most ecologists today. Many years of careful observation have revealed tha,‘ succession shows a bewildering variety of patterns, many of which do not fit the simple Clements model. The ideal of convergent and undisturbed reestablishment of a stable dominant community is not general in nature, if it exists at all. C. Modern views on succession 1. In addition to plant life histories, morphology, and biology, succession is highly sensitive to: a. the frequency of disturbance, b. initial plant composition and environmental conditions, c. and many more or less random changes and historical events. 2. Most ecosystems are very "patchy" with regard to successional stage. To the extent that convergence does occur, it is a statistical phenomenon (i.e., based on probabilities of various events), and there may be alternative "climax" states (Fig. 1). 3. Despite the complexity and controversy, particularly with regard to the mechanisms that drive succession, some fairly simple models of succession work in some cases. 4. An example is the succession pattern of some hardwood forest communities in the NE US, which, if undisturbed, can be approximately modeled from relatively few inputs: a. initial tree species composition b. longevity of each species (probability of a particular tree still standing after some period of time, e.g., 50 years) c. probability of a species replacing itself after some period of time. See Tables 1 and 2 D. Some generalities about succession 1. The role of plant-plant interactions A popular current theory called the “dynamic equilibrium model” of succession stresses the importance of competition between individual plants, and physiological traits such as tolerance to environmental stress. It also recognizes that plants change the environment (6. g. resource availability) for future generations, and that no species will be dominant under all environments, leading to a change in species composition over time. Some plant effects have a major impact on subsequent generations and the pattern of succession. These include: 0 light capture (shade) by canopy leaves 0 input of carbon and energy in the form of organic matter (affecting soil microbial processes, nutrient cycling, water and cation exchange capacity, soil structure, etc.) o additions of nitrogen by symbiotic nitrogen fixation o uptake of water and nutrients Succession dominated by these biotic effects is called "autogenic", as opposed to "allogenic" succession, dominated by environmental factors (e. g. weather) not directly under the control of the plants themselves. H U 2. Plant growth characteristics ( r vs. "K" strategists) Plant species that tend to dominate early in succession are often "r" strategists. These are weedy annual plants with high relative growth rates and high rates of seed production dispersed by wind. Plants dominating late in succession tend to be tall woody perennials, with relatively slow growth rates, slow tissue turnover rates, and relatively few large seeds often dispersed by animals rather than wind. A common pattern along these lines in abandoned farmland succession is: mixed weeds--> grassland--> shrubland --> forest 3. Disturbance is so frequent in most ecosytems that a stable climax is never reached, and cyclic patterns are established (Fig. 2). Even within a mature forest, light gaps created by fallen trees result in patches of early successional stages. 4. There are a number of vegetation and ecosystem traits that often change in a progressive way during succession (Table 3). Some of these traits, such as biodiversity and nutrient storage or recycling, may peak at intermediate and not final stages of succession. 5. Stability, defined as lack of change and resistance to minor environmental changes, tends to increase at later successional stages. However, stability defined another way-- as being able to return to a preexisting state after major disturbance-- is less at late successional stages. E. Managing succession in agroecosystems. 1. Conventional agroecosystems are in a perpeptual state of early successional stage, dominated by fast—growing crops and weeds, because disturbance is frequent and intense. This maximizes net primary productivity (which is high at early successional stages), but minimizes many other ecological characteristics, such as organic matter and biomass storage (Fig. 3) beneficial for long—term sustainability. 2. Many desirable characteristics, such as mycorrhizal networks and other mutualisms, efficient nutrient cycling, and high biodiversity and organic matter, become manifest late in succession (Table 4). 3. Some researchers (e. g., Gliessman 1998) are optimistic that perennial or semi-perennial agricultural production systems can be developed that mimic succession, and use controlled above- ground disturbance to get the best of both worlds. The goal is to create a 1(an of successional mosaic, that keeps the below-ground system at later stages of succession, while growing highly productive species in patches or at certain times in the progression, and harvesting them for high net productivity. The basic steps in this process might be: Bare soil—-> mono- or polyculture of annual crops --> polyculture of mixed annuals and short-lived perennials --> annual/perennial polyculture with tree seedlings -—> agroforestry (See Fig. 4) Some traditional agroforestry systems approach this goal. One example is the Javanese "Talun-kebun" system, described in the previous lecture on agroforestry. Another is the "taungya" system that has been used in one form or anogher in India and surrounding countries for more than a hundred years. As practiced today, it is often a temporary arrangement in which farmers are allowed to produce annual agricultural crops on recently clear-cut forests. The annual crop plants are intercropped with young tree seedlings. The growers take care of the tree seedlings as well as their cash crops in exchange for use of the land. Eventually the trees dominate the system and cash crop production is halted until the forest is again clear-cut for timber or other purposes. But, can these types of systems feed an ever increasing world population? REFERENCES Barbour, MG et al. 1999. Succession. IN: Barbour et al. Terrestrial Plant Ecology. Third Edition. Addison—Wesley Longman. Menlo Park, CA. pp. 288-302. Gliessman, SR. 1998. Disturbance, succession, and agrcecosytem management. IN: Gliessman, SR. Agroecology. Ann Arbor Press. Chelsea, MI pp. 249-258. | ‘ Secondary (old-field) succession in the Piedmont region. Only the common names of the dominant species are listed; many other taxa are associated with each seral stage. From Oosting 1942 (reprinted by permission of American Midland Naturalist), Bard 1952 (by permission of the Ecological Society of America), and Richardson 1977 (Dimen- sions of Ecology, by permission of Williams and Wilkins, Baltimore). Years after abandonment North Carolina Piedmont 0 Cropland t 1 Crabgrass, horseweed t 2 White aster, ragweed t 3 Broomsedge t 5 Broomsedge, pine seedlings t 10 Young pines, broomsedge Shortleaf pine Loblolly pine (drier sites) (moister sites) 20 30 40 60 Shortleaf pine, hardwood Loblolly pine, hardwood understory understory 100 j l 150 White oak, post oak, White oak, many hickory, dogwood, etc. hickories, dogwood, sourwood, etc. (VS‘vaV’UL/j OLGA“? “4 chlcl .. TAVWMlL—n-L ECoiOi‘y’ P‘ A i \ \ Fifty year tree-by-tree transition matrix for Gray Birch, Blackgum, Red Maple and Beech. 50 years hence Now Birch gum Gray Birch 5 + o 36 Blackgum I 37 +'2o Red Maple o 1 4 Beech o 1 Diagonal is percentage of trees still standing plus percentage that have been re— placed in 50 years by another of their own kind. Off-diagonal terms are percentage of trees replaced by another species in 50 years. The percentage of trees still standing was estimated by assuming that trees die at a constant rate such that 5 per cent are left standing after 50 years for Gray Birch, 150 years for Blackgum and Red Maple, and 300 years for Beech. For those trees that die in 50 years, replacement of one species by another is assumed to be proportional to the number of saplings of the latter under a large sample of canopy trees of the former. The forests in Which these data were gathered have many more species (Horn 1975b); so this matrix is only an illustrative caricature. ’— K “‘4, KQ 2. , Predicted composition of a succession. Age of Forest Very Old (years) 0 50 100 1 50 200 ... 00 Forest Gray Birch oo 5 1 0 o I o A Blackgum o 36 29 23 18 5 3 Red Maple o 550 D 39 3o 24 9 4 Beech o , ' 9 31 47 58 86 93 The succession starts with a field full of Gray Birch and then obeys the transitions specified by the matrix of Table 11.1. The stationary composition after an infinite amount of time is gotten by solving the set of equations in which the composition equals the composition multiplied by the matrix. The composition of the ‘very old forest” is the proportion of trees of the four species in the canopy of an actual forest that may not have suffered an extensive disturbance for 3 50 years or more. Several species have been left out of the analysis for simplicity (See Horn 1975b for more details). EXCKDM (Q ~ “’4' Twle \~ ‘Fvcm New land l A periodic . disturbance Micro- (anary I such as fire cyclic \ succeSSion) Cydlf can maintain succession l successron subdimax fl Pioneer Seral Subclimax , ——~—>. ——-> Climax seral stages seral stage stage v Small, noncumulative changes Disturbance at any stage can set back succession and initiate secondary succession l l Diagrammatic pathway of different types of succession: pr’ ary, secondary, and cyclic. The climax stage is in a state of dynamic equilibrium. (SauJ‘CL: Sou/xx, M Ft‘jl) Viz-70) TEX hi 2 Some vegetation and ecosystem traits that often change during progressive succession. The status of each trait is shown for early and late stages of succession (not for pioneer and climax stages necessarily, because some trends peak at intermediate seral stages). Each trend is briefly discussed in the text. Trait Early stages Late stages Biomass Small Large Physiognomy Simple Complex Leaf orientation Multilayered Monolayered Major site of nutrient storage Soil Biomass Role of detritus Minor Important Mineral cycles Open (leaky), Closed (tight), rapid transfer slow transfer Net primary production High L0w Site quality Extreme Mesic Importance of the macroenvironment Great Moderated and dampened; less Stability (absence or slowness of change) Low High Plant species diversity Low High Species life history character r K Propagule dispersal vector Wind Animals Propagule longevity Long Short Efficient nutrient cycling - NPP (tons/ha/ .. 100 year) Biomass 1 (tons/ha) 0 0 Tlme 2~ . “ g 3 Ch e over time in the relationship between annual net primary productivity (NPP) and accumulated living and dead biomass in a representative successionally developing ecosystem. A time interval (e.g., one season) in the early stages of succession (such as t2-t1) will witness a rapid increase in NPP, whereas NPP will decline slightly during a time interval of similar length (such as t4-t3) during the latter stages of succession. Modified from Whittaker (1975) and Odum (1993). (S.°\U"\‘Q~‘ QlC—Lsxwvndi‘lfi Ayfl'K'kclogy/ ppr‘2§7\ ’Tth, 42 L/ Desirable ecological characteristics of agroecosystems in relation to successional development Successional stage of greatest development Characteristic Early Middle Late Benefit to agroecosystem High species diversity Reduced risk of catastrophic crop loss High total biomass Larger source of soil organic matter High net primary Greater potential for production of productivity harvestable biomass Complexity of Greater potential for biological control species interactions Diminished need for external nutrient :LjLn ' if, 7 r - - r 1 - Greater stability; diminished need for exter- nal inputs Mutualistic interference - mm W 1. Bare soll 2. Annual monoculture 0: ‘v v; 1- 'v 3. Annual polycullure 4. Polyculture of mixed annuals and short-lived perennials . r: r- W 5. Annual/perennlal polyculture with 6. Agroforestry tree seedlings WW 7. Tree crop agroecosyslem Steps in the suecessiona] development of an agroecosystem. At any stage in the process, disturbance can be introduced to bring all or part of the system back to an earlier stage of development. “We: SW 09 F7: a, f. 2%) ...
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This note was uploaded on 05/13/2008 for the course HORT 4600 taught by Professor Wolfe,d.w. during the Spring '08 term at Cornell University (Engineering School).

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Lec19_Succession_08 - HORT 460 Cropping Systems Ecology MODULE IV Cropping Systems Ecology Over Time Plant succession A Introduction The

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