EFFECTS OF CHRONIC NITROGEN ADDITIONS ON UNDERSTORY SPECIES IN A RED PINE PLANTATION.

Abstract. Two plots in a red pine stand at the Harvard Forest, Massachusetts, have been fertilized for 7 yr with 5 g [multiplied by] [m.sup.-2] [multiplied by] [yr.sup.-1] or 15 g [multiplied by] [m.sup.-2] [multiplied by] [yr.sup.-1] of N and compared to an unfertilized control to study the effects of chronic N additions on nutrient cycling, plants, and soil. Movement of added N into soils and plant biomass was tracked using additions of an 15N label to the 5 g [multiplied by] [m.sup.-2] [multiplied by] [yr.sup.-1] and control plots for two of these years. We present data on changes in the understory plant community of these plots. We measured aboveground biomass, density, N and other elemental concentrations, and [sup.15]N tracer recoveries to catalogue the effects of the N additions on this community. Nitrogen contents increased while biomass and nutrient cation concentrations decreased in some species. Percent recoveries of [sup.15]N tracers were small but detectable. The natural abundances of [sup.15]N also increased in a fertilized treatment without tracer additions. Though this forest has not yet reached N saturation by some definitions, it is possible that the understory is already saturated with N. Understory species may be useful indicators for N saturation through their increasing N content, decreasing nutrient concentrations, and increasing [Delta][sup.15]N in tissues.

Key words: ground flora; Harvard Forest; N cycle; N deposition; [sup.15]N labeling; [sup.15]N natural abundance; N saturation; Pinus resinosa; red pine; temperate forests.

INTRODUCTION

Forest ecosystems in the northeastern United States experience elevated nitrogen deposition rates compared to more remote areas (Galloway et al. 1984, Ollinger et al. 1993), and exhibit increased nitrogen cycling rates along corresponding gradients of increasing N deposition (McNulty et al. 1990). It has been hypothesized that prolonged N inputs could lead to "N Saturation" (sensu Aber et al. 1989), a condition where the availability of inorganic N in forest soil exceeds the biotic demands of microbes and plant roots, leading to increased N losses to drainage water and the atmosphere. Symptoms of N saturation include elevated N concentrations in plant tissues, [NO.sub.3] losses to ground and surface waters accompanied by base cation leaching, increased soil acidity, and eventual decreases in growth and live tree biomass (Aber et al. 1989, Stoddard 1994).

Few North American studies have reported on the effects of N deposition on forest understory vegetation, despite its role in biodiversity, its importance in nutrient cycling and retention (Zak et al. 1990), and its potential value as an indicator of pollution effects (Tappeiner and Alm 1975, Muller and Bormann 1976, Blank et al. 1980, Thimonier and Dupouey 1995). Therefore, we examined plant community and biogeochemical responses of understory vegetation in a 70-yr-old red pine (Pinus resinosa) plantation to chronic N additions. These plots were fertilized with [NH.sub.4][NO.sub.3] for 7 yr prior to our study. Responses of overall forest N cycling and trees after 3 and 6 yr of N additions have been reported by Aber et al. (1993, 1995) and Magill et al. (1997). [N.sub.2]O fluxes were reported by Bowden et al. (1991). Movements of added N into tree tissues and soils as determined using [sup.15]N tracer additions to plots are reported in Nadelhoffer et al. (1999). Here we report on the responses of understory plants to N additions. We hypothesized that understory species' responses could be used as early indicators of ecosystem N saturation, and might reflect processes that would later be observed in overstory trees. Specifically, we tested whether 7 yr of N fertilization affected understory species composition, stem densities, biomass, N content, and nutrient concentrations. We also assessed the role of understory plants in retaining N additions by following the movements of [sup.15]N-[NH.sub.4] and [sup.15]N-[NO.sub.3] tracers into understory plants in nonfertilized and fertilized plots. Finally, we used the natural abundance [sup.15]N of understory species in a heavily fertilized, but nonlabeled plot to determine the sensitivity of stable N isotope techniques to ecosystem scale N cycling.

METHODS

Fertilization and [sup.15]N tracer additions

The experimental plots are located at the Harvard Forest in Petersham, Massachusetts, a National Science Foundation Long Term Ecological Research (LTER) site (Aber et al. 1993). Treatments were started in 1988 in a 62-yr-old red pine (Pinus resinosa) plantation. Treatments included regular fertilization with 5 g N [multiplied by] [m.sup.-2] [multiplied by] [yr.sup.-1] ("Low N") or 15 g N [multiplied by] [m.sup.-2] [multiplied by] [yr.sup.-1] ("High N") on 30 x 30 m plots, divided into 36, 5 x 5 m quadrats for sampling purposes. A nonfertilized control plot (hereafter referred to as "Ambient") served as a reference for assessing effects of fertilizer additions on understory species abundance, biomass, and nutrient content. …

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