Nutrient conservation strategies in native Andean‐Patagonian forests

Abstract. Nutrient conservation in vegetation affects rates of litter decomposition and soil nutrient availability. Although resorption has been traditionally considered one of the most important plant strategies to conserve nutrients in temperate forests, long leaf life‐span and low nutrient requirements have been postulated as better indicators. We aimed at identifying nutrient conservation strategies within characteristic functional groups of NW Patagonian forests on Andisols. We analysed C‐, N‐, P‐, K‐ and lignin‐concentrations in mature and senescent leaves of ten native woody species within the functional groups: broad‐leaved deciduous species, broad‐leaved evergreens and conifers. We also examined mycorrhizal associations in all species. Nutrient concentration in mature leaves and N‐ resorption were higher in broad‐leaved deciduous species than in the other two functional groups. Conifers had low mature leaf nutrient concentrations, low N‐resorption and high lignin/N ratios in senescent leaves. P‐ and K‐resorptions did not differ among functional groups. Broad‐leaved evergreens exhibited a species‐dependent response. Nitrogen in mature leaves was positively correlated with both N resorption and soil N‐fertility. Despite the high P‐retention capacity of Andisols, N appeared to be the more limiting nutrient, with most species being proficient in resorbing N but not P. The presence of endomycorrhizae in all conifers and the broad‐leaved evergreen Maytenus boaria, ectomycorrhizae in all Nothofagus species (four deciduous, one evergreen), and cluster roots in the broad‐leaved evergreen Lomatia hirsuta, would be possibly explaining why P is less limiting than N in these forests.     

Optional tool use: The case of wild bearded capuchins (Sapajus libidinosus) cracking cashew nuts by biting or by using percussors

Tool use in humans can be optional, that is, the same person can use different tools or no tool to achieve a given goal. Strategies to reach the same goal may differ across individuals and cultures and at the intra‐individual level. This is the first experimental study at the intra‐individual level on the optional use of a tool in wild nonhuman primates. We investigated optional tool use by wild bearded capuchins (Sapajus libidinosus) of Fazenda Boa Vista (FBV; Piauí, Brazil). These monkeys habitually succeed in cracking open the mesocarp of dry cashew nuts (Anacardium spp.) by pounding them with stones and/or by biting. We assessed whether availability of a stone and resistance of the nut affected capuchins' choice to pound or to bite the nuts and their rates of success. Sixteen capuchins (1–16 years) received small and large dry cashew nuts by an anvil together with a stone (Stone condition) or without a stone (No‐Stone condition). In the Stone conditions, subjects used it to crack the nut in 89.1% (large nuts) and 90.1% (small nut) of the trials. Nut size significantly affected the number of strikes used to open it. Availability of the stone significantly increased the average percent of success. In the No‐Stone conditions, monkeys searched for and used other percussors to crack the nuts in 54% of trials. In all conditions, age affects percentage of success and number of strikes to reach success. We argue that exclusive use of stones in other sites may be due to the higher abundance of stones at these sites compared with FBV. Since capuchins opened cashews with a tool 1–2 years earlier than they succeed at cracking more resistant palm nuts, we suggest that success at opening cashew nuts with percussors may support the monkeys' persistent efforts to crack palm nuts.     

Carbon trading for phosphorus gain: the balance between rhizosphere carboxylates and arbuscular mycorrhizal symbiosis in plant phosphorus acquisition

Two key plant adaptations for phosphorus (P) acquisition are carboxylate exudation into the rhizosphere and mycorrhizal symbioses. These target different soil P resources, presumably with different plant carbon costs. We examined the effect of inoculation with arbuscular mycorrhizal fungi (AMF) on amount of rhizosphere carboxylates and plant P uptake for 10 species of low‐P adapted Kennedia grown for 23 weeks in low‐P sand. Inoculation decreased carboxylates in some species (up to 50%), decreased plant dry weight (21%) and increased plant P content (23%). There was a positive logarithmic relationship between plant P content and the amount of rhizosphere citric acid for inoculated and uninoculated plants. Causality was indicated by experiments using sand where little citric acid was lost from the soil solution over 2 h and citric acid at low concentrations desorbed P into the soil solution. Senesced leaf P concentration was often low and P‐resorption efficiencies reached >90%. In conclusion, we propose that mycorrhizally mediated resource partitioning occurred because inoculation reduced rhizosphere carboxylates, but increased plant P uptake. Hence, presumably, the proportion of plant P acquired from strongly sorbed sources decreased with inoculation, while the proportion from labile inorganic P increased. Implications for plant fitness under field conditions now require investigation.     

Nutrient resorption patterns of plant functional groups in a tropical savanna: variation and functional significance

Green and senesced leaf nitrogen (N) and phosphorus (P) concentrations of different plant functional groups in savanna communities of Kruger National Park, South Africa were analyzed to determine if nutrient resorption was regulated by plant nutritional status and foliar N:P ratios. The N and P concentrations in green leaves and the N concentrations in senesced leaves differed significantly between the dominant plant functional groups in these savannas: fine-leaved trees, broad-leaved trees and grasses. However, all three functional groups reduced P to comparable and very low levels in senesced leaves, suggesting that P was tightly conserved in this tropical semi-arid savanna ecosystem. Across all functional groups, there was evidence for nutritional control of resorption in this system, with both N and P resorption efficiencies decreasing as green leaf nutrient concentrations increased. However, specific patterns of resorption and the functional relationships between nutrient concentrations in green and senesced leaves varied by nutrient and plant functional group. Functional relationships between N concentrations in green and senesced leaves were indistinguishable between the dominant groups, suggesting that variation in N resorption efficiency was largely the result of inter-life form differences in green leaf N concentrations. In contrast, observed differences in P resorption efficiencies between life forms appear to be the result of both differences in green leaf P concentrations as well as inherent differences between life forms in the fraction of green leaf P resorbed from senescing leaves. Our results indicate that foliar N:P ratios are poor predictors of resorption efficiency in this ecosystem, in contrast to N and P resorption proficiencies, which are more responsive to foliar N:P ratios.     

Soil type affects nitrogen conservation in foliage of small Pinus sylvestris L. trees

Nutrient conservation in plants and soil fertility may be intricately linked. We studied nitrogen conservation in small Scots pine (Pinus sylvestrisL.) trees growing in stands on organogenic Dystric Histosols and on mineral Podzols. Nitrogen-resorption efficiency (NRE) and proficiency (NRP) of senescent needles, and mean residence time of nitrogen (MRT) were studied in relation to needle surface area, needle longevity, and leaf mass per area (LMA). Trees on Podzols had higher nitrogen concentration in green needles than the trees on Dystric Histosols, but the nitrogen concentration of yellowing needles was similar for trees on both soil types. NRE averaged 65±3.5% (mean±SD) and 56±7.2% for the trees on Podzols and Dystric Histosols, respectively. Neither NRP (0.44±0.05% and 0.35±0.07%, respectively) nor MRT (8.4±2.3 and 6.1±1.2 years) differed significantly between the stands on the two soil types. Mean needle surface area was significantly smaller in trees on Dystric Histosols (76±29 mm²) than on Podzols (131±38 mm²), whereas needle longevity varied between 2 and 4 years independently of the soil type. Trees invested, on average, the same amount of dry matter per unit of needle area on both soil types. Growth of trees, measured as increment of shoot length, was more restricted on Dystric Histosols (55±18 mm yr^–1) than on Podzols (184±44 mm yr^–1). The results of the correlation analysis applied to pooled data were inconsistent with the relations between traits of stress resistance syndrome observed in inter-specific comparisons. The study indicated that Scots pine trees relocated nitrogen from senescent foliage more efficiently on mineral Podzols than on organogenic Dystric Histosols, but the minimum nitrogen concentration of needles appeared to be similar on both soil types.     

Language and zoo biology training outcomes: A cautionary note

The Jersey Wildlife Preservation Trust (JWPT)/University of Kent Diploma in Endangered Species Management (DESMAN) was initiated in 1985 to provide an academically recognized qualification to trainees attending the JWPT's International Training Centre (ITC). Since its inception, 59 of 118 trainees enrolled were awarded the diploma. Examination and practical work performance of 141 trainees from 1994 to 1998 showed a significant variance in grades. DESMAN participants were by far the highest achievers with high mean examination grades and high practical work grades. More than one third (38%) of trainees, all of whom were DESMAN participants, obtained above median grades in practical work and the examination. Another third of all trainees attained below median grades in both examination and practical work. Given that most course participants (93%) had a strong academic background in biology or the veterinary sciences, low course performance could have been related to the inability of some trainees to fully understand the language used. Before the start of a course, trainees are requested to produce evidence of English language proficiency, at least level 7 of the International English Language Testing System (IELTS). When individual trainee language skills were tested in Jersey, through a self‐assessment exercise (named English Language Proficiency or ELP), a significant variation in language skills was found. These ELP scores positively correlated with the trainees' mean and median examination performance. Thus, entrance into the DESMAN seems to be affected by trainee's fluency in English. Since most trainees are graduates or have some form of tertiary education, the perceived influence of language proficiency on the attainment of a university‐accredited qualification such as the DESMAN needs to be taken most seriously. Zoo Biol 18:71–76, 1999. © 1999 Wiley‐Liss, Inc.     

Changes in nitrogen resorption traits of six temperate grassland species along a multi-level N addition gradient

Nitrogen (N) resorption from senescing leaves is an important mechanism of N conservation for terrestrial plant species, but changes in N-resorption traits over wide-range and multi-level N addition gradients have not been well characterized. Here, a 3-year N addition experiment was conducted to determine the effects of N addition on N resorption of six temperate grassland species belonging to three different life-forms: Stipa krylovii Roshev. (grass), Cleistogenes squarrosa (T.) Keng (grass), Artemisia frigida Willd. (semishrub), Melissitus ruthenica C.W.Wang (semishrub and N-fixer), Potentilla acaulis L. (forb) and Allium bidentatum Fisch.ex Prokh. (forb). Generally, N concentrations in green leaves increased asymptotically for all species. N concentrations in senescent leaves for most species (5/6) also increased asymptotically, except that the N concentration in senescent leaves of A. bidentatum was independent of N addition. N-resorption efficiency decreased with increasing N addition level only for S. krylovii and A. frigida, while no clear responses were found for other species. These results suggest that long-term N fertilization increased N uptake and decreased N-resorption proficiency, but the effects on N-resorption efficiency were species-specific for different temperate grassland species in northern China. These inter-specific differences in N resorption may influence the positive feedback between species dominance and N availability and thus soil N cycling in the grassland ecosystem in this region.     

Leaf nutrient concentration as an indicator of Populus and Tamarix response to flooding

Plants growing in infertile environments are able to produce more biomass per unit of nutrient taken up than plants of fertile habitats, and also to minimize nutrients loss by resorbing them from senescing leaves. The leaf nutrient concentration variability of two co-existing riparian tree genera (Populus and Tamarix) along a flood inundation gradient was examined to infer nutrient limitation and to compare nutrient use strategies in the two genera. To that end, seasonal and spatial variability in leaf nitrogen (N) and phosphorus (P) concentration (i.e., % dry mass of N and P) were analyzed in 720 samples of leaves (2 tree genera × 3 seasons × 12 sites × 10 tree replicates). Both Populus and Tamarix showed strong seasonal variability in leaf N and P concentrations, with values decreasing throughout the growing season. However, while N:P atomic ratio remained seasonally constant in Populus (N:P = 33), Tamarix shifted from N:P = 29 in spring to N:P = 36 and 37 in summer and fall. %N, %P and N:P atomic ratios were also spatially variable, but leaf litter N and P concentration (i.e., nutrient resorption proficiency) and leaf litter N:P generally followed the local flood inundation gradient as shown by linear mixed effects models. In particular, nutrient resorption was usually less proficient (higher terminal nutrient concentrations) at higher flood durations (in gravel bars and natural levees), whereas N:P increased in the drier sites (floodplain terrace). At floodplain level, a P-limitation that is higher than N-limitation seems to characterize the plant nutrient circulation in the riparian ecosystem studied. Tamarix was slightly more proficient in P resorption than Populus. The study shows that leaf nutrient concentration (e.g., N and P) derived from nutrient availability is partly controlled by the flood inundation regime and can be used as an indicator of nutrient limitation in forested floodplains. Subtle differences between tree genera provide an additional, novel explanation for the recent expansion of Tamarix in many arid and semi-arid rivers with altered hydrogeomorphic regimes.