Tree Establishment Patterns Research Articles

Microscale Patterns of Tree Establishment near Upper Treeline, Snowy Range, Wyoming, U.S.A.

We report tree seedling (mostly Picea engelmannii, some Abies lasiocarpa, very infrequent Pinus contorta) invasion into meadows at upper timberline in the Snowy Range, Wyoming, from 1994 to 1996. We used gradient analysis to relate this to environmental patterns, particularly plant community structure (as aggregates of plant life-forms) and persistence of snowpack in 1995 and 1996. Tree seedlings established best at sites where snow melted earliest; the site with the shortest growing season had fewest seedlings. Microsites dominated by vascular plants admitted few or no seedlings; seedlings were most likely to be found where cryptogams dominated. These findings have implications for forest advance under some climate scenarios and successional hypotheses.

Expansion of forest stands into tundra in the Noatak National Preserve, northwest Alaska

Temperatures across the northern regions of North America have been increasing for 150 years, and forests have responded to this increase. In the Noatak National Preserve in Alaska, white spruce (Picea glauca [Moench] Voss) forests reach their northern limit, occurring primarily on well-drained sites and as gallery forests along streams. Rolling plateaus of tundra separate the white spruce forests into disjunct stands. We examined patterns of tree age, tree growth, and tree encroachment into tundra ecosystems in six stands along the Agashashok River. Warming over the past 150 years appears to have increased tree growth and resulted in forest expansion into adjacent tundra ecosystems. The forest/tundra ecotone shifted by about 80 to 100 m into the tundra in the past 200 years, as evidenced by declining maximum tree age with distance towards the tundra. The decadal-scale pattern of tree establishment at the farthest extent of trees into the tundra (the tundra-forest ecotone) correlated with the detrended growth index for trees within the forests; climate conditions that led to higher tree growth appeared to foster tree establishment in the tundra. This recent forest expansion has occurred across topographic boundaries, from well-drained soils on slopes onto poorly drained, flatter areas of tundra. Further expansion of the forests may be limited by more severe wind exposure and poor drainage that make the majority of tundra less suitable for trees.

Temporal and Spatial Distribution of Trees in Subalpine Meadows of Mount Rainier National Park, Washington, U.S.A.

Tree establishment patterns vary considerably at different locations in the subalpine zone of Mount Rainier National Park, with substantial temporal variation in regeneration of subalpine fir (Abies lasiocarpa). Recruitment in subalpine meadows has been continuous on the west side of Mount Rainier since about 1930, but has occurred in short, discrete periods on the east side. Variation in snowpack from west to east on the mountain has a substantial impact on climatic factors that limit tree establishment. Warm, dry summer climate facilitates tree establishment on the west side where snowpacks are generally very high; cool, wet summer climate enhances tree establishment on the east side where snowpacks are lower. Density of tree establishment is significantly greater in heath-shrub (ericaceous) vegetation than in other vegetation types. Within heath-shrub vegetation types, tree establishment is highest at lower elevations, on topographic convexities, and in plant communities dominated by Phyllodoce empetriformis. Survival of subalpine fir seedlings during the first 3 yr after germination is significantly greater in heath-shrub vegetation than other vegetation types. If the climate becomes warmer and drier during the next century, continued rapid regeneration of trees can be expected in subalpine meadows on the west side of Mount Rainier National Park. This may result in displacement of wildflower meadows that are an attraction for park visitors. A better understanding of climatic and environmental limitations on tree establishment will assist resource managers in developing sound management strategies for subalpine ecosystems.

The temporal and spatial analysis of tree age distributions

The temporal and spatial patterns of tree establishment and stand disturbance history are often based on the interpretation of age-class frequency distributions. In particular, the presence of even-aged groups of trees is often used as compelling evidence of past disturbance. However, even-aged groups of trees may be indistinguishable in an age distribution if several different-aged patches occur, especially if their ages overlap. For two different types of forest we used spatial autocorrelation analysis to statistically test for the presence of even-aged patches in tree age data. Ordination and cluster analysis were subsequently applied to a matrix of association measures that reflected both spatial proximity and age similarity to identify even-aged groups of trees. Although the method worked well for our forests, which contained light-demanding tree species, it is likely to be less applicable to forests dominated by shade-tolerant species, because trees may be of many different ages if they were present as suppressed individuals prior to disturbance. However, in these instances the method could be usefully applied in other types of analysis, such as the distribution of growth release dates, tree-fall or fire-scar dates, and growth rates.

The Pattern of Tree Seedling Establishment Relative to Staghorn Sumac Cover in Michigan Old Fields

Trees invading 10-year and 16-year-old abandoned fields in Michigan were aged to determine year of establishment. Vegetation cover was mapped to develop correlates between tree establishment patterns and vegetation type. Few trees established in the 1st 5 years after the fields were abandoned from cropping. Most (74%) of the 947 individual trees were between 2 and 5 years of age. The density of tree seedlings in all fields was significantly higher under a canopy of the shrub, staghorn sumac (Rhus typhina), compared to areas without a sumac canopy. It is argued that while sumac cover promotes tree seedling success, the effect is indirect, mainly by reducing the amount and type of ground-level vegetation which inhibits the establishment of tree seedlings, and secondly by increasing tree seed arrival via animal activity. INTRODUCTION The mechanisms that determine how late successional species appear in a successional sere have been under discussion for some time (see reviews by McCormick, 1968; Drury and Nisbet, 1973; Horn, 1974; Connell and Slatyer, 1977; McIntosh, 1980). There are three general views: The first is that the earlier colonists prepare the way (called reaction by Clements, 1916, or by Connell and Slatyer, 1977) for later colonists by modifying the environment so that it is more suited for the later successional species. This view is perhaps the most common presented in textbooks and in popular literature, but has been questioned repeatedly by ecologists, notably by Egler (1954) and further by McCormick (1968), Niering (Niering and Egler, 1955; Niering and Goodwin, 1974), Drury and Nisbet (1973) and others. Most likely it describes a mechanism that operates in primary succession, but is not very important in secondary succession. The second view is that the first colonists inhibit invasion of later arrivals and are replaced only when they die naturally; there is much evidence to support this claim presented in the reviews by McCormick (1968) and Drury and Nisbet (1973). The third view is that the species sequence is determined solely by the life history characteristics of the organisms, that late-appearing species are those which arrived later and grew more slowly to larger sizes, eventually dominating the community. To what degree do early colonists inhibit, tolerate or facilitate (terminology of Connell and Slatyer, 1977) later arriving species? Experimental studies would be required to test the various models proposed. Some experiments have been conducted in communities which show rapid successional change (e.g., estuarine fouling communities of invertebrates, Dean and Hurd, 1980), with the results showing inhibitory interactions more prevalent than facilitation interactions. Experiments in terrestrial communities are rare, perhaps because they require such a long time that they are impractical; an analysis of pattern in existing communities may still be among the best evidence in regard to this problem. In the field study reported here, invading trees were aged in fields abandoned from agricultural practice for 16 and 10 years to determine whether they had been a part of the initial floristic composition (Egler, 1954) or whether they 1Present address: B. P. 817; Yaounde, Cameroon, Africa.