Notes

Outline

Plant Succession Definition Stages Tolerance

Plant Succession - Definition The orderly process of one plant community gradually or rapidly replacing another Can result from the developmental changes in the ecosystem itself Or from disturbances such as wind, fire, volcanic activity, insects and disease or harvesting In a particular environment, there is a characteristic sequence of biotic communities that successively occupy and replace each other over time following disturbance

Plant Succession Assumptions: no change in regional climate no catastrophic events long time frame (hundreds of years) Some stages of the characteristic sequence may be skipped Disturbance moves the community to a different, usually earlier, successional stage.

Stages of Succession Primary succession Initiated by disturbances that expose substrates and are left with essentially no plant growth at the beginning Examples:  glacial retreat; volcanic ash deposition

Plant Succession Seral stages Climax stage

Stages of Succession Seral communities:  The various “temporary” communities in the succession Seral species  - those that will ultimately be replaced examples:  annual plants, shrubs, intolerant trees

Stages of Succession Climax community:  The “final” stage of the succession - consists of those plants that can reproduce successfully beneath their own shade and therefore maintain the community indefinitely under the prevailing conditions Climax species:  Those species that will occupy the site at the climax stage of succession Examples:  tolerant tree species such as cedar and hemlock

Why do some species replace others in the successional progression?

Shade Tolerance Refers to the ability of a tree to grow in the shade of other trees Tolerant species (T) can grow comparatively  well when little light is available and do not show large increases in growth with increasing light levels.  Climax species Intolerant species (I) cannot grow well in low light levels but grow very well at light levels approaching full sunlight.  Pioneer species Some species are intermediate (N) in their ability to grow in shade

Relative Shade Tolerance of Some North American Species Very intolerant western larch subalpine larch whitebark pine

Relative Shade Tolerance of Some North American Species Intolerant lodgepole pine ponderosa pine juniper species

Relative Shade Tolerance of Some North American Species Intermediate Douglas-fir western white pine giant sequoia

Relative Shade Tolerance of Some North American Species Tolerant grand fir subalpine fir Engelmann spruce

Relative Shade Tolerance of Some North American Species Very tolerant western redcedar western hemlock Pacific yew

Classic Forest Succession Grass-forb Forbs, grasses and shrubs dominate the site Shrub-seedling Trees tend to share and then begin to dominate the site.  The shade intolerant species grow rapidly and dominate over shade tolerant species Sapling-pole Trees eventually  overtop and out-compete the forb and shrubs.  The intolerant trees continue rapid height growth while  the shade-tolerant trees occupy their respective niche

Classic Forest Succession Young Growth is still rapid.  Tree-to-tree competition may be severe resulting in competition-caused mortality.  Any intolerant individuals that drop behind may die and their growing space may be occupied by the tolerant trees. Mature Competition-caused mortality continues.  Both intolerant and tolerant trees may share the main canopy.  In mixed conifer stands, there may be distinct layering of intolerants and tolerants.

Classic Forest Succession Climax Shade-tolerant trees dominate the site and the climax species will reproduce. Shade-intolerant trees cannot replace themselves. Most climax stands (old growth) are set back to the early seral grass-forb or shrub-seedling stage by fire in the Northern Rockies.

Buell-Small Successional Study Hutcheson Memorial Forest – Somerset County, New Jersey

Forest Succession - Summary Plant communities continually change The community is a continuum in time and space New species invade the site Existing species continue to reproduce or they eventually disappear Succession results in “a rich dynamic tapestry of vegetation, providing an array of habitats for animals and microbes.”  (Perry 1994)

Distribution of forest species along environmental gradients in eastern Washington/northern Idaho

Forest Classification Cover type refers to species that are currently present on the site Habitat types indicate the potential climax vegetation on a site if left undisturbed Habitat types are named for the climax tree species and an understory indicator species - example:  Pinus ponderosa/Physocarpus malvaceus (ponderosa pine/ninebark)

Comparison of the distributions of habitat series in northern Idaho

Misconceptions about Habitat Types An abundance of climax vegetation is present in the current landscape We should manage the resource to promote climax vegetation To apply the habitat type classification system requires climax vegetation to be present