Facilitation Among Woody Plants Establishing in an Old Field

Abstract

Trees and shrubs establishing in grasslands frequently occur in clumps associated with increased soil moisture and N availability. We tested whether the presence of conspecifics or enhanced soil resources increased the growth and survivorship of a shrub (Symphoricarpos occidentalis) and a tree (Picea glauca) in the presence of the perennial grass (Bromus inermis). Rhizomes of Symphoricarpos and seedlings of Picea were transplanted into plots either singly or with four conspecific neighbors, and with grass neighbors present or removed. Half the plots received additional water, N, and shade to simulate a forest environment. Roots and shoots of transplants were harvested after two growing seasons. Soil moisture and available N were lowest at high transplant density, and highest in the simulated‐forest environment and in treatments where grass was removed. Transplant survivorship was generally enhanced in plots where grasses were removed and in the simulated‐forest environment. Picea survivorship was reduced by grasses in plots without resource manipulations but was unaffected by grasses in the simulated‐forest environment, suggesting that the simulated‐forest environment enhanced the ability of Picea to compete with grasses. Picea growth, however, was reduced by conspecific neighbors in all cases. For Symphoricarpos conspecific neighbors significantly decreased growth rates with grasses absent but significantly increased growth rates with grasses present. Thus the direct effect of high density in monoculture was to reduce Symphoricarpos growth, but the indirect effect of high density in the presence of grass was to facilitate Symphoricarpos growth. In summary, conspecific neighbors increased the growth rate of Symphoricarpos in competition with grass, and a simulated‐forest environment enhanced the ability of Picea to survive in the presence of grasses. The results support the idea that contagious distributions of woody species invading grasslands enhance woody plant establishment.

Changes in light and nitrogen availability under pioneer trees may indirectly facilitate tree invasions of grasslands

“…In such cases, the increasing rate of woody plant expansion is a demographic phenomenon, possibly with relatively constant success on a per seed or seedling basis throughout the conversion phase. However, the rapid increase in woody cover after the earliest stages of invasion may in some cases reflect a positive feedback involving changes in resource conditions that favour woody seedlings in competition with background herbaceous vegetation (Connell & Slatyer 1977;Li & Wilson 1998;Wilson 1998). In some cases, the average success of seeds and seedlings of woody plants may increase in combination with higher seed input.…”

“…Greater nitrogen availability tends to intensify above-ground competition for light and favours woody plants in competition with grasses (e.g. Tilman & Pacala 1993;Wilson 1993;Grace 1995;Li & Wilson 1998;Wilson 1998). Further, C3 plants such as Sapium have lower nitrogen use efficiencies and higher nitrogen requirements than C4 plants such as the perennial bunchgrasses that are abundant in tallgrass prairies (Sage & Pearcy 1987).…”

“…This positive feedback could promote rapid grassland-forest transitions. Li & Wilson (1998) showed that simultaneously shading, increasing water availability and fertilizing with nitrogen favoured woody seedlings in competition with grasses. In addition, because C4 plants expend additional energy in photosynthesis to avoid photorespiration in hot and /or dry conditions, shaded conditions may favour C3 plants such as Sapium that do not have this additional metabolic cost (Mooney 1972;Lambers et al .…”

A test of the indirect facilitation model in a temperate hardwood forest of the northern French Alps

“…Forest regeneration has been shown to be hindered by these fast-growing, competitive species (Humphrey & Swaine 1997;Kuuluvainen & Rouvinen 2000;Perez-Salicrup 2001). A greater density of tree seedlings below adult trees than in gaps (Van Auken & Bush 1991;Takahashi 1997;Li & Wilson 1998;Cuevas 2000;Kubota 2000;Slocum 2001) may be explained by indirect facilitation. We tested the model of indirect facilitation in a mature deciduous forest by comparing the response of seedlings of five tree species to tree canopy removal in the presence and absence of a dominant herbaceous neighbour, purple moor-grass ( Molinia caerulea (L.) Moench).…”

Cancer: an emergent property of disturbed resource‐rich environments? Ecology meets personalized medicine

“…Several theoretical models and empirical studies support this hypothesis. For instance, increasing water supplies in soil indirectly enhances the susceptibility of herbaceous communities by invasive woody and herbaceous vegetation (Grime and Curtis 1976;Harrington 1991;Li and Wilson 1998;Davis et al 1999). Conversely, imposed drought decreases the invasibility of the same communities (Davis et al 1998).…”