Restoration Ecology

Assessment of leaf characteristics and fine root dynamics as drivers of demographic responses

Plant mechanistic responses to competition are critical to understanding factors that promote positive population growth. Leaf demography and leaf nutrient content are strong indicators of plant relative growth rate, a parameter that is closely associated with the ability of exotic species to invade non-native habitats (Hill et al. 1995) . Short leaf life-span, high specific leaf area (SLA), and high leaf nutrient content are associated with high individual growth rates of invasive exotic species compared to native species (Harrington et al. 1989, Pattison et al. 1998, Baruch and Goldstein 1999) and may allow exotics to make especially efficient and opportunistic use of resources that normally limit their populations but occur at some times and places in excess. The high leaf nutrient content of invasive species promotes increased individual growth in sites that have been invaded (Harrington et al. 1989a, Vitousek 1990, Pattison et al. 1998, Baruch and Goldstein 1999) and invaded sites tend to have higher soil nutrient levels than uninvaded sites (e.g. Vitousek 1990) . For example, elevated soil nutrients, including calcium and nitrogen, were positively correlated with the occurrence of invasive species in a number of Northeastern hardwood forest fragments in eastern New York; uninvaded sites exhibited comparably lower soil nutrients.Consistent with these observations, sites with low available resources, such as intact prairies, do not favor invasive species while nutrient rich ecosystems tend to be dominated by plants with high leaf nutrient concentrations (Cornelissen et al. 1997) . Greater leaf nutrient concentrations promote greater relative growth rates, which may allow invasives to allocate resources to reproduction earlier than do native plants. This can serve as a mechanism for recruitment limitation by allowing earlier seed production and dissemination and translating to higher population growth rates when nutrients are not limiting. These leaf traits are readily measured, may serve as indicators of population performance, and may provide reliable criteria for species selection in ecological restorations.

Nitrogen and calcium are key nutrients that influence a number of important leaf traits and plants vary in their use and allocation of these nutrients (Reich et al. 1995, McLaughlin and Wimmer 1999) . Soil nitrogen and calcium levels influence total leaf area and leaf demography (Millard and Proe 1991) and high leaf nitrogen across species is associated with high relative growth rate (Reich et al. 1995) . Foliar calcium influences photosynthetic rate (Reich et al. 1995, McLaughlin and Wimmer 1999) and calcium availability influences leaf longevity by delaying senescence, a factor that will increase in importance as resource partitioning becomes more critical (McLaughlin and Wimmer 1999) . For instance, Harrington et al. (1989) found that delayed leaf senescence led to increased carbon gain in invasive species found in the low light conditions of the forest understory compared to those in early successional habitats. Long leaf life-spans were not observed under high light conditions, suggesting that delayed leaf senescence is at least in part an adaptation for the use of limiting light resources. Finally, calcium stimulates the production of fine roots, which may increase nutrient uptake and improve leaf nitrogen content in nitrophilic plants.

Plant responses to soil nitrogen and calcium levels may affect competitive interactions among plants and ultimately determine restoration success. Stimulation of fine root production by soil calcium uptake may magnify nitrogen uptake of prairie-invasives, improving their growth advantage over prairie-natives. However, if invasives do not respond to soil calcium more strongly than native species, the competitive arena may equilibrate, giving greater importance to other plant characteristics such as reproductive output or seed germination rates in determining community composition. The role of fine root dynamics in mediating species interactions and influencing population growth trajectories is very poorly understood at present, but may represent a key factor linking soil resource availability with leaf demographic and reproductive processes (Körner and Renhardt 1987, Craine and Lee 2003) .

References

Baruch, Z., and G. Goldstein. 1999. Leaf construction cost, nutrient concentration and net CO 2 assimilation of native and invasive species in Hawaii. Oecologia 121 :183-192.

Cornelissen, J. H. C., M. J. A. Werger, P. Castro-Díez, J. W. A. v. Rheenen, and A. P. Rowland. 1997. Foliar nutrients in relation to growth, allocation and leaf traits in seedlings of a wide range of woody plant species and types. Oecologia 111 .

Craine, J. M., and W. G. Lee. 2003. Covariation in leaf and root traits for native and non-native grasses along an altitudinal gradient in New Zealand. Oecologia 134 :471-478.

Harrington, R. A., B. J. Brown, and P. B. Reich. 1989a. Ecophysiology of exotic and native shrubs in Southern Wisconsin I. Relationship of leaf characteristics, resource availability, and phenology to seasonal patterns of carbon gain. Oecologia 80 :356-367.

Hill, J. D., C. D. Canham, and D. M. Wood. 1995. Patterns and causes of resistance to tree invasion in rights-of-way. Ecological Applications 5 :459-470.

Körner, C., and U. Renhardt. 1987. Dry-matter partitioning and root length leaf-area ratios in herbaceous perennial plants with diverse altitudinal distribution. Oecologia 74 :411-418.

McLaughlin, S. B., and R. Wimmer. 1999. Tansley Review No. 104 Calcium physiology and terrestrial ecosystem processes. New Phytologist 142 :373-417.

Millard, P., and M. F. Proe. 1991. Leaf demography and the seasonal internal cycling of nitrogen in sycamore ( Acer pseudoplatauns L.) seedlings in relation to nitrogen supply. New Phytologist 117 :587-596.

Pattison, R. R., G. Goldstein, and A. Ares. 1998. Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species. Oecologia 117 :449-459.

Reich, P. B., D. S. Ellsworth, and C. Uhl. 1995. Leaf carbon and nutrient assimilation and conservation in species of differing successional status in an oligotrophic Amazonian forest. Functional Ecology 9 :65-76.

Vitousek, P. M. 1990. Biological invasions and ecosystem processes: towards an integration of population biology and ecosystem studies. Oikos 57 :7-13.