We are talking about prairie fire ecology for several reasons.

Transcription

1 We are talking about prairie fire ecology for several reasons. First, the historical fire regime was one of very frequent fires. You ll learn about fire effects in two different ecosystems that once experienced very frequent fires, prairies and ponderosa pine forests, and you can then contrast fire effects with other ecosystems that experienced different historical fire regimes. Second, we can illustrate some key concepts in fire ecology by looking at how fire effects on a single species can change depending on whether drought occurs before and after fires, or how and why the response of a species will vary across its range. These concepts apply in all ecosystems, not just prairies. Thid Third, these prairie ii ecosystems and dthi their fire ecology are similar il to many different prairie ecosystems around the world. Prairies are often endangered ecosystems that are very important to a host of wildlife species and many other values. 1

2 The prairie ecosystems of Great Plains of North America were once very extensive. Like prairies worldwide, they are located in the middle of continents and in the rain shadow of mountains. Currently, many sites that once supported prairie vegetation have been converted to intensive agriculture and urban areas. In the US, most of the relatively healthy prairies are found in national grasslands, national parks, and in some nature preserves, such as those owned and managed by The Nature Conservancy. Some are only found in cemeteries and along old railroad lines. Grassland soils are typically very high in organic matter, which contributes to their value for conversion to agricultural crops. The very high below-ground production of organic matter in grasslands leaves a legacy in the soils that is important in holding moisture, releasing nutrients, and improving soil productivity. 2

3 Prairie ecosystems are important ecologically, socially and economically. 3

4 Prairie ecosystems are very important to wildlife. Many migratory birds spend part of their year in the prairies, and there are major flyways north to south through the prairies. Most of the effects of fire on wildlife, especially for birds and mammals, are indirect. In other words, they are often mobile enough to fly or move away from the path of the flames, or they can escape into burrows in the ground. Of course, this isn t possible for some critters, and doesn t apply to fledgling birds that can t yet fly. Thus, the habitat changes as a result of a fire are very important if you want to understand how fire affects wildlife. 4

5 Prairies experience continental climates. Droughts are common. Summer moisture comes in the form of thunderstorms, often associated with monsoonal moisture from the south. The wind is always blowing. Winters are harsh, with wind and freezing temperatures for many months. 5

6 There are three major zones. We ll contrast fire effects among these. 6

7 Many people have asked why grasslands aren t forests, and thinking through this question is insightful. 7

8 Each of these can alter the dominance by grass over shrubs and trees. 8

9 Probably, a combination of factors helped to maintain grasslands, and the relative contribution of these different factors varied from place to place. Along the ecotone between tall-grass prairie and eastern deciduous forest, many people think the forest has increased in extent since there has been less burning over the last century. In the heart of the prairies, the climatic conditions weren t suited to tree growth. Even trees planted and carefully tended in shelterbelts and windbreaks have a hard time surviving. In many areas, woody vegetation was only found where more water was available (e.g. in draws and along rivers) or in broken, dissected d topography where fires wouldn t have spread readily. 9

10 Bison (also called buffalo) and other herbivores were once common in the Great Plains. Bison have been reintroduced on large native prairies managed by The Nature Conservancy because they think they were central to the healthy function and natural dynamics of grasslands. Many think they were a keystone species removing them had many cascading effects for other parts of ecosystems. 10

11 Note that herbivory patterns were probably once quite different than they are now different species, different timing, different duration. Remember that in terms of the amount of biomass removed, rodents, insects and soil nematodes are much more important than the large mammals (e.g. cows and bison) that we tend to focus on in our grazing management and discussions about herbivory. Grazing not only recycles biomass, it can also redistribute nutrients. The hoof action of large mammals can expose bare mineral soil and thus create microsites for plant establishment. Many people think that the buffalo wallows increased the diversity it of forbs in grasslands by giving i the forbs a place to grow with less competition from grass. 11

12 Not all grazing animals eat the same things. 12

13 In general, we know that grasses recover to their pre-burn structure and abundance faster than do shrubs and trees. Also, the younger and smaller woody plants are also more susceptible to fire than old, large ones. Many shrubs resprout readily following fires, but the degree to which h that t is successful depends on how frequently fires occur, on the vigor of the plant before the fire, and what happens after the fire in terms of grazing, browsing and drought. 13

14 Interestingly enough, although we are quite sure that fire was prevalent and important in grasslands, we don t know much in detail about the historical role of fire in prairies. We know native Americans used fire in many different ways. Fire was central to the culture of native American tribes, especially for domestic and ceremonial purposes. As is the case for many different indigenous cultures around the world, fire was one of the most powerful tools people had for manipulating vegetation to meet their needs. Indians would sometimes burn areas to attract animals to the new forage that grew after a fire (and then they could snare or hunt them). They also used fire to drive animals into a confined area or over a cliff. Indians often burned around their camps to make it easier to see approaching enemies, and to make it easier to see snakes. Fire was a tool of warfare people used fire to burn out others or to burn up the forage and cover in areas that others would otherwise use. 14

15 Most of our information about historical fire regimes in prairie ecosystems is inferred. We know that there was plenty of lightning and that people were often igniting fires. Fires probably burned over extensive areas given the rolling terrain, almost constant wind, and abundant fuel once above-ground biomass dried out dry deep grass is an ideal fuelbed! We also know from the experience of cavalry, early settlers, and others that fires were feared. From the fire effects when we burn grasslands now, we know that fires were important t in consuming accumulated dead d biomass that t holds nutrients t and shades and insulates soils. Because fires are patchy and because they don t affect all plant species to the same degree, fires alter vegetation composition and structure, often increasing plant species diversity. 15

16 Let s look at fire effects in each of these ecosystems in more depth. We know the most about fire effects in the tall grass prairie. 16

17 Unfortunately, many of the tall grass prairie remnants that exist today have probably changed. They are often subject to invasion by non-native species. This has been called an endangered ecosystem because so little of it remains. 17

18 The Curtis Prairie at the University of Wisconsin has been the site of much early work on fire ecology of tall grass prairie. There, Curtis and other scientists burned areas repeatedly, some every year, some every two years, others every three to five years and compared these to unburned prairie. They concluded that tall grass prairie can tolerate burning every year, that it deteriorates without burning, and that because the plant production, biodiversity and health of prairies was highest when they were burned every three to five years, that that much have been the historical fire frequency. 18

19 Fires had these effects in tall grass prairies. All of these are key concepts in fire ecology that apply in many other places than the tall grass prairie. You can use these are rules of thumb elsewhere: 1) First, frequent fires tend to favor grasses over trees. 2) Second, plants are damaged more by fire when they are actively growing when fires occur. The corollary is that t plants are more tolerant t of fire when they are burned when dormant. This applies equally to grasses in prairies (as we ll discuss with cool and warm season grasses) as to scorch in trees (all else being equal a tree that is actively growing will experience a higher degree of crown damage from flames than one in which buds have formed and have not yet broken). 19

20 Remember C3 and C4 pathways of photosynthesis? Photosynthesis in C3 plants results in 3-carbon chains and optimum temperatures are lower than for photosynthesis resulting in 4-carbon chains. Indirectly, this affects how plants respond to season of burning. C3 plants are favored over C4 plants by late summer and fall fires because at that time C3 plants are dormant and C4 plants are actively growing. C4 plants are favored over C3 plants if they are burned in spring fires because C3 plants are actively growing then while C4 plants are dormant. Many exotics in prairie grasslands are C3 plants, so when you burn is important if you re designing i prescribed burns to favor native over introduced d grasses. 20

21 Here are some other key concepts that we ll illustrate with tall grass prairie species, but the general rules of thumb apply almost everywhere. Generally, if it is dry during the growing season and following years, a plant will be slower to recover post-fire. Likewise, if a plant is growing on a site that is relatively dry (compared to all the other sites on which it is found), fires will harm it more and it will take more time to recover to preburn conditions. Rhizomatous plants, those with deep underground stems from which plants resprout, are less harmed by fire than stoloniferous plants (the ones like strawberries where a horizontal stem connects new plantlets to a mother plant), and those with shallow rhizomes are harmed more than those with deeper rhizomes. 21

22 Forbs are the flowering plants. They are often key to gaining conservation support for restoration and other projects since people like wildflowers. They are usually not very abundant or frequent in prairies, but their importance to many butterflies, birds, and small mammals may be disproportionate to the abundance. Many forbs will resprout, and many seed into disturbed microsites. 22

23 Trees and shrubs were locally abundant in prairie landscapes, but they were often limited to the riparian areas along streams and to broken topography. With less fire, trees and shrubs have encroached into grasslands in many areas. 23

24 As I said before, this is true in many different ecosystems, not just prairies. 24

25 The mixed grass prairie is in between tall and short grass prairie in many ways. Intermediate in precipitation, intermediate in terms of height of the grass, and the vegetation is often composed of a mixture of species from short and tall grass prairie communities. 25

26 Mixed grass prairie is found on a wide variety of sites. 26

27 The vegetation, which is mostly grass but with some shrubs and many forbs, is well adapted to fire. 27

28 Relatively little is known about fire effects. While some mixed prairie has been converted to agriculture, more of it is used as pasture. Grazing often limits the amount of fine fuel that is available to burn, and thus fires often don t spread far. Fire spread is also limited by roads and by active fire suppression efforts. Probably it once burned more often than it once did. However, it is drier than the tall grass and many of the plants that are common to both mixed and tall grass prairie communities do not recover as quickly from fire when they are growing in mixed-grass prairie sites. Droughts are more common here than in the tall grass prairie. i 28

29 Shrubs and trees are more abundant than in the tall grass prairie, especially in the southern Great Plains. 29

30 The short grass prairie is still drier, and droughts are even more common. As a result, the biomass production is low in most years and there is relatively little fuel to burn. 30

31 Because it is so much drier, relatively little of the short grass prairie is cultivated. Much was and is grazed with domestic livestock. Most species are initially harmed by fire, especially when fires occur during droughts. Most plants do recover, but it can take multiple years before plants recover their pre-burn productivity. 31