Plant responses to fertilization experiments in lowland, species-rich, tropical forests.

We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.

Lianas always outperform tree seedlings regardless of soil nutrients: results from a long-term fertilization experiment.

Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the "leaf economic spectrum," characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient x growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested.

Plant water status and hydraulic conductance during flowering in the southern California coastal sage shrub Salvia mellifera (Lamiaceae).

PREMISE OF THE STUDY: Plant water status during flowering is important for plant reproduction, but the physiology of floral water use is not well understood. We investigated plant water status in relation to leaf and floral physiology in naturally occurring individuals of a semiarid shrub, Salvia mellifera E. Greene. METHODS: We measured stomatal (g(s)) and corolla (g(c)) conductance to water vapor, transpiration from leaves (E(leaf)) and corollas (E(corolla)), leaf-specific hydraulic conductance (K(H)), bulk shoot water potential (Ψ(shoot)), and shoot water content on irrigated and control plants to analyze whether water was limiting to leaf and floral water use. KEY RESULTS: Experimental irrigation caused a 203% increase in soil moisture content, a 20% increase in predawn Ψ(shoot), a 29% increase in midday Ψ(shoot), and a 92% increase in K(H). Floral and leaf gas exchange did not respond significantly to water addition, indicating that rates were at seasonal maxima and not limited by water availability. Total daily water use by corollas was ∼20% of total shoot water use. There were no significant differences in total daily shoot water use with water addition. Mean shoot water content (5.07 g) was close to mean daily shoot water use (6.71 g), indicating that the equivalent of total shoot water content turned over every 0.76 d. CONCLUSIONS: These results demonstrate that although irrigation improved whole-plant hydraulic conductance, gas exchange was not limited by water availability. Additionally, the high water use of flowers in this species might limit future flowering and reproductive success during dry years.

Post-fire primary production and plant community dynamics in chaparral stands exposed to varying levels of nitrogen deposition.

High levels of atmospheric nitrogen (N) deposition to southern California chaparral shrublands may interact with fire to affect biomass production and plant species composition during secondary succession. To determine the potential interactions between post fire recovery and N deposition we compared rates of aboveground net primary production (ANPP), shrub growth, and the relative abundance of Adenostoma fasciculatum, other sub-dominant shrubs, and herbaceous species of three chaparral stands exposed to different levels of atmospheric N deposition over the first 3 years of post-fire succession. Our data suggest that rates of ANPP (gdw m(-2) month(-1)) and aboveground N storage (gN m(-2) month(-1)) for these chaparral stands were not related to N deposition even though sites exposed to high levels of N deposition had significantly higher rates of shrub growth (gdw plant(-1) month(-1)) and N uptake (gN plant(-1) month(-1)). However, high-N stands were composed of larger shrubs with a lower density, and this trade-off between shrub size and density may explain the low correlation between N deposition and post-fire ANPP. Differences in relative plant species abundance between sites were significantly correlated with N deposition exposure, where stands exposed to high N deposition had a lower relative abundance of A. fasciculatum and a higher relative abundance of other shrub and herbaceous species. While many factors can affect rates and patterns of post-fire recovery, these results suggest that chronic exposure to N deposition may significantly alter plant growth and species composition in successional chaparral stands.

Experimental dry-season N deposition alters species composition in southern Californian mediterranean-type shrublands.

Nitrogen (N) deposition in heavily polluted southern Californian shrublands is estimated to be 20-45 kg N x ha(-1) x yr(-1), but more exposed locales can receive as much as 145 kg N x ha(-1) x yr(-1). This large anthropogenic N input has the capacity to alter the composition of plant communities. We conducted N-fertilization experiments in chaparral and coastal sage scrub (CSS) stands over a five-year period to test the hypothesis that plant community composition would change in response to dry-season N addition because of an increase in the relative abundance of herbaceous plant species. Our results indicate that dry-season addition of N significantly altered the community composition of CSS but not chaparral. Contrary to our original hypothesis, changes in community composition were due to changes in the relative abundance of dominant shrubs and not herbaceous plant species. Given that community-level responses to changes in resource availability may take years to decades in order to fully materialize, our results suggest that continued dry-season input of N will cause even larger changes in community composition over time. These results have implications for plant species composition and diversity of mediterranean-type shrublands as N deposition increases with population growth and fossil-fuel use.

CARBON AND NITROGEN STORAGE IN SOIL AND LITTER OF SOUTHERN CALIFORNIAN SEMI-ARID SHRUBLANDS.

Semi-arid shrublands of southern California, including chaparral and coastal sage, are found in widely varying elevation and microclimatic regimes and are subjected to disturbance such as fire and atmospheric N deposition that have the capacity to alter soil and litter C and N storage. Here we present a case study where soil and litter C and N were measured over 19 months in post-fire chaparral and mature coastal sage stands to assess whether differences in soil and litter C and N between these diverse shrublands could be attributed to differences in elevation, stand age, rainfall, and/or estimated N deposition exposure. Our results indicate that atmospheric N deposition exposure, either alone or in conjunction with other environmental variables (elevation, rainfall, and/or stand age), was the most frequent predictor of the spatial pattern in the soil and litter N and C variables observed. These results are consistent with those reported for high-elevation coniferous forests arrayed along an N deposition gradient in southern California, suggesting that N deposition may affect the soil N and C storage of semiarid shrublands and woodlands in a qualitatively similar manner.