Environmental factors controlling soil respiration in three semiarid ecosystems

Previous research suggests that soil organic C pools may be a feature of semiarid regions that are particularly sensitive to climatic changes. We instituted an 18-mo experiment along an elevation gradient in northern Arizona to evaluate the influence of temperature, moisture, and soil C pool size on soil respiration. Soils, from underneath different free canopy types and interspaces of three semiarid ecosystems, were moved upslope and/or downslope to modify soil climate. Soils moved downslope experienced increased temperature and decreased precipitation, resulting in decreased soil moisture and soil respiration las much as 23 acid 20%, respectively). Soils moved upslope to more mesic, cooler sites had greater soil water content and increased rates of soil respiration las much as 40%), despite decreased temperature. Soil respiration rates normalized for total C were not significantly different within any of the three incubation sites, indicating that under identical climatic conditions, soil respiration is directly related to soil C pool size for the incubated soils. Normalized soil respiration rates between sites differed significantly for all soil types and were always greater for soils incubated under more mesic, but cooler, conditions. Total soil C did not change significantly during the experiment, but estimates suggest that significant portions of the rapidly cycling C pool were lost. While long-term decreases in aboveground and belowground detrital inputs may ultimately be greater than decreased soil respiration, the initial response to increased temperature and decreased precipitation in these systems is a decrease in annual soil C efflux

Canopy cover mediates interactions between a specialist caterpillar and seedlings of a neotropical tree

1 Light availability may be crucial for understanding dynamics of plant–herbivore interactions in temperate and tropical forest communities. This is because local light availability can influence both tree seedling tolerance and susceptibility to herbivory – yet how they mediate levels of insect herbivory that vary with the density of host population is virtually unknown. Here we tested predictions of three key, non-mutually exclusive hypotheses of plant–herbivore interactions: the Limiting Resource Model (LRM), the Plant Vigour Hypothesis (PVH), and the Janzen-Connell Mechanism (JCM). 2 In an Amazonian forest, we planted Swietenia macrophylla seedlings (c. 5 months old) into natural canopy gaps and the shaded understorey and simulated the damage patterns of the specialist herbivore moth, Steniscadia poliophaea, by clipping seedling leaves. Over the next 8 months, we monitored seedling performance in terms of growth and survivorship and also quantified herbivory to new young leaves on a seasonal basis. 3 In support of the LRM, severe leaf damage (≥ 50%) was lethal for Swietenia macrophylla seedlings in the understorey, but in gaps only reduced seedling growth. In support of the PVH, gap seedlings suffered greater post-simulated herbivory (up to 100% defoliation) by S. poliophaea caterpillars than their understorey counterparts. 4 Adding a novel dimension to the Janzen–Connell hypothesis, we found that early wet season herbivory of seedlings in gaps increased with conspecific adult density within a 125-m radius; whereas in the understorey only those seedlings within 50 m of a Swietenia tree were attacked by caterpillars. 5 Synthesis. These results suggest lepidopterans that need young leaves for food may forage more widely in forests to find seedlings in light-rich canopy gaps. Moths may achieve this successfully by being first attracted to gaps, and then searching within them for suitable hosts. A conceptual model, integrating conspecific adult tree density with light-driven changes in seedling tolerance/vigour and their susceptibility to herbivory and mortality, is presented. Spatial variation in the light available to tree seedlings often affects their tolerance and vigour, which may have important consequences for leaf-chewing insects and the scale of density-dependent herbivory in forests

Variation in Recruitment and Early Demography in Pinus Rigida Following Crown Fire in the Pine Barrens of Long Island, New York

Following severe, stand-replacing fires in 1995, we quantified emergence, growth and survival in Pinus rigida seedlings in eight stands in three areas of the pine barrens of Long Island, New York, USA, and examined factors contributing to spatial and temporal variation in recruitment. We followed 6431 marked seedlings in the first cohort following fire, as well as additional cohorts in subsequent years, for 7 years to assess the effects of fire intensity, soil characteristics, intraspecific density, interspecific competition and facilitation on variation in early demography at three different scales. We found substantial variation in many of the demographic parameters measured between plots within stands (10–50 m apart), among stands within areas (600– 1500 m apart), and among forest areas (5 –30 km apart), as well as among years. Almost all adult P. rigida in most of the stands studied were killed by the 1995 fires and population recovery therefore depended upon recruitment from seed. Initial recruitment appears to be largely determined by seed limitation, which was affected by fire intensity combined with serotiny and other factors. Subsequent seedling survival and growth were determined largely by inherent differences among areas and sites in factors including cover of the shrub Quercus ilicifolia and soil texture and composition. Initial seedling density, and subsequent survival and growth, varied among plots within sites, among sites within areas, and among areas. Early survival was negatively correlated with intraspecific seedling density, but was enhanced by neighbouring Q. ilicifolia, although these shrubs inhibited later seedling growth. Early demographic variation may determine many of the differences observed among mature populations in this landscape. Variation in demographic parameters across space and time, and over a hierarchy of scales, may have critical consequences at the population, community and landscape levels. A comprehensive evaluation of the nature and extent of such demographic variation across different systems would have major implications for understanding vegetation patterns at the population, community and landscape levels

Physiological responses of two tropical weeds to shade: II. Leaf gas exchange and nitrogen content Respostas fisiológicas de duas plantas invasoras tropicais ao sombreamento: II. Troca gasosa e conteúdo de nitrogênio foliar

Ipomoea asarifolia (Desr.) Roem. & Schultz (Convolvulaceae) and Stachytarpheta cayennensis (Rich) Vahl. (Verbenaceae), two weeds found in pastures and crop areas in the Brazilian Amazonia, Brazil, were grown in controlled environment cabinets under high (800-1000 µmol m-² s-¹) and low (200-350 µmol m-² s-¹) light regimes during a 40-day period. The objective was to determine the effect of shade on photosynthetic features and leaf nitrogen content of I. asarifolia and S. cayennensis. High-irradiance grown I. asarifolia leaves had significantly higher dark respiration and light saturated rates of photosynthesis than low-irradiance leaves. No significant differences for these traits, between treatments, were observed in S. cayennensis. Low-irradiance leaves of both species displayed higher CO2 assimilation rates under low irradiance. High-irradiance grown leaves of both species had less nitrogen per unit of weight. Low-irradiance S. cayennensis had more nitrogen per unit of leaf area than high-irradiance plants; however, I. asarifolia showed no consistent pattern for this variable through time. For S. cayennensis, leaf nitrogen content and CO2 assimilation were inversely correlated to the amount of biomass allocated to developing reproductive structures. These results are discussed in relation to their ecological and weed management implications.<br>Ipomoea asarifolia (Desr.) Roem. & Schultz (Convolvulaceae) e Stachytarpheta cayennensis (Rich) Vahl. (Verbenaceae), duas plantas invasoras encontradas em pastagens e áreas agrícolas da Amazônia brasileira, foram cultivadas durante 40 dias, em câmaras de crescimento sob alto (800-1000 µmol m-² s-¹) e baixo (200-350 µmol m-² s-¹) regime de luz. O objetivo foi estudar o efeito do sombreamento nas características fotossintéticas e no teor de nitrogênio de I. asarifolia e S. cayennensis. As folhas de I. asarifolia cultivadas sob regime de luz alta apresentaram valores de respiração no escuro e taxa máxima de fotossíntese significativamente maiores que das folhas cultivadas sob baixa luz. Em S. cayennensis, não houve diferença significativa, entre tratamentos, quanto a esses parâmetros. Em ambas espécies, folhas desenvolvidas sob baixa irradiância apresentaram maiores taxas de assimilação de CO2 sob baixos níveis de irradiância. Em ambas as espécies, plantas cultivadas em luz alta apresentaram menores teores de nitrogênio por unidade de peso foliar. Em S. cayennensis, folhas de baixa irradiância tiveram maior teor de N por unidade de área que em folhas de plantas de luz alta; no entanto, em I. asarifolia não foi observado um padrão definido quanto a essa variável durante o período experimental. O conteúdo de N foliar e a assimilação de CO2 em S. cayennensis foram inversamente correlacionados com o percentual de biomassa alocado para estruturas reprodutivas. Esses resultados são discutidos com relação a suas importâncias ecológica e de manejo