The feeding height preferences of two goat breeds fed Grewia occidentalis L. (Tiliaceae) in the Eastern Cape,South Africa

Nguni goats and Boer goats, which are farmed together in the savannas of the Eastern Cape Province, depend on the same feed resources. The feeding height intake rates and preferences of one of their most preferred browse species, Grewia occidentalis L. (Tiliaceae), were studied in order to indirectly determine resource partitioning between these goat breeds on the basis of feeding height. The two hypotheses tested were that taller Boer goats feed at higher levels than do Nguni goats and that both breeds have feeding height preferences that are related to differences in intake rates. Four individuals of each breed were fed branches attached to a height foraging board. The two breeds showed no particular height preference in either season. Bite rates varied with feeding height, while bite sizes and intake rates increased with feeding height. Boer goats had lower bite rates compared to Nguni goats, although both breeds achieved similar bite sizes and intake rates. Bites sizes and intake rates for both breeds were higher in summer than in winter, possibly because more browse biomass was available per branch in summer than in winter since G. occidentalis is deciduous. There was no evidence to support that feeding height preference was related to differences in intake rates. It is concluded that Boer goats may compete for forage resources during times of resource limitation. Thus, Nguni goats might be a more favourable breed in semi-arid savannas because of their smaller body-size and, therefore, absolute nutrient requirements as well as their better reproductive performance compared to Boer goats.     

Leaf chemistry of woody plants in relation to season,canopy retention and goat browsing in a semiarid subtropical savanna

Abstract It is assumed that the phytochemistry of browse species protects their biomass and nutrients against herbivory. In this study we were primarily interested in (i) seasonal and phenology‐related variations in leaf chemistry, and (ii) chemistry‐related variations in the feeding behaviour of domestic goats. Such knowledge would guide management‐orientated modelling of browse–browser interactions in seasonal, subtropical zones where goats are abundant. The browse species studied were typical of semiarid savannas in southern Africa: Grewia occidentalis L. (Tiliaceae), Scutia myrtina (Burm. f) Kurz (Rhamnaceae), Diospyros lycioides Desf. ssp. lycioides (Ebenaceae), Rhus longispina Eckl. and Zeyh. (Anacardiaceae), Ehretia rigida (Thunb.) Druce (Boraginaceae) and Acacia karroo Hayne (Mimosoideae). Nitrogen (N), phosphorus (P), condensed tannins (CT), protein‐precipitating tannins (PPT), total phenols (TP), cellulose and lignin concentrations were estimated for each species during the late dormant and early growing seasons. N, P, CT and TP were elevated during the growth season, while cellulose, lignin and PPT decreased. Unlike cytoplasm contents, which varied seasonally, cell wall and vacuole contents varied both seasonally and among species. Except that seasonal variation in N of deciduous species was greater than that of evergreen species, leaf phenology was not related to variations in forage quality. Short‐term intake rates were not related to primary metabolite concentrations, but were positively related to secondary metabolites. Elevated intake rates of putative defences were concluded to be side‐effects of attempts by goats to increase nutrient intake rate, indicating tolerance of chemical defences. Without support for the hypothesis that chemical defences are correlated to canopy retention, we propose an alternative hypothesis, that defences are distributed among woody plants in semiarid, subtropical savannas according to shoot morphology because it affects the vulnerability of plant parts to browsers.     

The diet of a small group of extralimital giraffe

Giraffe are extralimital in the Eastern Cape Province, South Africa where recent local introductions have persisted despite limited research into their impact on the indigenous flora. The diet of 15 giraffe at the Shamwari Game Reserve was recorded by direct observation during summer (March/April) and winter (July/August) 2001, quantifying diet by frequency of occurrence (individual records scored and expressed as a percentage of the total). Preference indices were also calculated. Habitat use was measured by the number of hours giraffe fed in different habitats. The diet comprised of 14 plant species, the most important species being Rhus longispina (47.9%), Acacia karroo (25.7%) and Euclea undulata (17.6%). Importance of R. longispina, A. karroo and Tarchonanthus camphoratus fluctuated seasonally. Rhus longispina was more important in winter with a corresponding decrease in feeding on A. karroo. Tarchonanthus camphoratus was only consumed during summer. Acacia karroo thickets (previously disturbed areas) were utilized most (summer 12 h; winter 9 h), with alternative habitats utilized more often in winter than in summer. We suggest that the seasonal fluctuation in the importance of R. longispina & A. karroo reflects the deciduous nature of A. karroo.     

Chilling-Induced Photoinhibition in Two Oak Species: Are Evergreen Leaves Inherently Better Protected Than Deciduous Leaves?

We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure to 1 h high irradiance (1000 µmol m^-2 s^-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective q_E and less irreversible quenching (q_I) than the deciduous Q. michauxii. The large q_E which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter.     

Extended leaf senescence promotes carbon gain and nutrient resorption: importance of maintaining winter photosynthesis in subtropical forests

The relative advantages of being deciduous or evergreen in subtropical forests and the relationship between leaf phenology and nutrient resorption efficiency are not well understood. The most successful deciduous species (Lyonia ovalifolia) in an evergreen-dominated subtropical montane cloud forest in southwest (SW) China maintains red senescing leaves throughout much of the winter. The aim of this study was to investigate whether red senescing leaves of this species were able to assimilate carbon in winter, to infer the importance of maintaining a positive winter carbon balance in subtropical forests, and to test whether an extended leaf life span is associated with enhanced nutrient resorption and yearly carbon gain. The red senescing leaves of L. ovalifolia assimilated considerable carbon during part of the winter, resulting in a higher yearly carbon gain than co-occurring deciduous species. Its leaf N and P resorption efficiency was higher than for co-occurring non-anthocyanic deciduous species that dropped leaves in autumn, supporting the hypothesis that anthocyanin accumulation and/or extended leaf senescence help in nutrient resorption. Substantial winter carbon gain and efficient nutrient resorption may partially explain the success of L. ovalifolia versus that of the other deciduous species in this subtropical forest. The importance of maintaining a positive carbon balance for ecological success in this forest also provides indirect evidence for the dominance of evergreen species in the subtropical forests of SW China.     

Seasonal differences in green leaf breakdown and nutrient content of deciduous and evergreen tree species and grass in a granitic headwater stream

Litter processing was examined in autumn–winter and spring–summer in a second order stream in Galicia (NW Spain). We compared decay rate and nutrient dynamics of green leaves of several deciduous (riparian: Alnus glutinosa, Betula alba and Populus×canadensis; terrestrial: Castanea sativa, Quercus robur), and evergreen tree species (terrestrial: Pinus radiata and Eucalyptus nitens), in addition to ray-grass (Lolium perenne). In the autumn–winter period, the decay rates (–k) ranged between 0.0086 degree-days^–1 for poplar, and 0.0019 degree-days^–1 for birch leaves. Alder showed the most rapid breakdown in spring–summer (0.0124 degree-days^–1), and pine the slowest (0.0016 degree-days^–1). Deciduous species exhibited general higher processing rates than evergreen species and ray-grass. The initial nitrogen and phosphorus contents were higher in riparian species leaves and ray-grass, being higher in spring (2.28±0.14% and 0.24±0.04% of nitrogen and phosphorus, respectively) than in autumn (1.88±0.36% of nitrogen and 0.18±0.03% of phosphorus). A significant correlation coefficient was found only between mean nitrogen leaf packs contents during incubation and decay rates (r=0.61; p=0.012).In deciduous species, processing was faster during the spring–summer than in the autumn–winter period, which may be attributed to the greater nutritional value and less consistency of the leaves during this season. Within evergreen species, pine had a significantly faster processing rate in autumn, attributed in this study to greater physical fragmentation of the needles. Ray-grass and eucalyptus did not exhibit any seasonal differences in processing rate.During the spring–summer period, litterfall inputs are quantitatively less important than during the autumn–winter, but due to high retention and fast breakdown during the spring–summer, green inputs should contribute substantially to nutrient incorporation and cycling in benthic communities.     

A Cost-Benefit Analysis of Leaves of Eight Australian Savanna Tree Species of Differing Leaf Life-Span

Cost-benefit analysis of foliar construction and maintenance costs and of carbon assimilation of leaves of differing life-span were conducted using two evergreen, three semi-deciduous, and three deciduous tree species of savannas of north Australia. Rates of radiant-energy-saturated CO₂ assimilation (P _max) and dark respiration were measured and leaves were analysed for total nitrogen, fat, and ash concentrations, and for heat of combustion. Specific leaf area, and leaf N and ash contents were significantly lower in longer-lived leaves (evergreen) than shorter-lived leaves (deciduous) species. Leaves of evergreen species also had significantly higher heat of combustion and lower crude fat content than leaves of deciduous species. On a leaf area basis, P _max was highest in leaves of evergreen species, but on a leaf dry mass basis it was highest in leaves of deciduous species. P _max and total Kieldahl N content were linearly correlated across all eight species, and foliar N content was higher in leaves of deciduous than evergreen species. Leaf construction cost was significantly higher and maintenance costs were lower for leaves of evergreen than deciduous species. Maintenance and construction costs were linearly related to each other across all species. Leaves of evergreen species had a higher cost-benefit ratio compared to leaves of deciduous species but with longer lived leaves, the payback interval was longer in evergreen than deciduous species. These results support the hypotheses that longer lived leaves are more expensive to construct than short-lived leaves, and that a higher investment of N into short-lived leaves occurs which supports a higher P _max over a shorter payback interval.     

Leaf and stem physiological responses to summer and winter extremes of woody species across temperate ecosystems

Winter cold limits temperate plant performance, as does summer water stress in drought‐prone ecosystems. The relative impact of seasonal extremes on plant performance has received considerable attention for individual systems. An integrated study compiling the existing literature was needed to identify overall trends. First, we conducted a meta‐analysis of the impacts of summer and winter on ecophysiology for three woody plant functional types (winter deciduous angiosperms, evergreen angiosperms and conifers), including data for 210 records from 75 studies of ecosystems with and without summer drought across the temperate zone. Second, we tested predictions by conducting a case study in a drought‐prone Mediterranean ecosystem subject to winter freezing. As indicators of physiological response of leaves and xylem to seasonal stress, we focused on stomatal conductance (g_s), percent loss of stem xylem hydraulic conductivity (PLC) and photochemical efficiency of photosystem II (F_v/F_m). Our meta‐analysis showed that in ecosystems without summer drought, g_s was higher during summer than winter. By contrast, in drought‐prone ecosystems many species maintained open stomata during winter, with potential strong consequences for plant carbon gain over the year. Further, PLC tended to increase and F_v/F_m to decrease from summer to winter for most functional types and ecosystems due to low temperatures. Overall, deciduous angiosperms were most sensitive to climatic stress. Leaf gas exchange and stem xylem hydraulics showed a coordinated seasonal response at ecosystems without summer drought. In our Mediterranean site subjected to winter freezing the species showed similar responses to those typically found for ecosystems without summer drought. We conclude that winter stress is most extreme for systems without summer drought and systems with summer drought and winter freezing, and less extreme for drought‐prone systems without freezing. In all cases the evergreen species show less pronounced seasonal responses in both leaves and stems than deciduous species.     

Some observations of phenology and ecophysiology ofDaphne kamtschatica Maxim.var.jezoensis (Maxim.) Ohwi, a shade deciduous shrub, in the forest of northern Japan

The phenology and leaf traits ofDaphne kamtschatica Maxim. var.jezoensis (Maxim.) Ohwi, the only summer deciduous shrub (20–40 cm) in the temperate forest of northern Japan, are examined. This plant carries through the winter mature leaves and well formed flower buds. It flowers in early spring during snowmelt and begins photosynthesis under relatively high irradiance under an open forest canopy. Our results show that there is significant carbon gain during the period when new leaves and fruit maturation also take place. Beginning in June, as the forest canopy closes, leaves onDaphne shoots senesce acropetally and the plants become completely bare in mid-July. After a period of 20-day dormancy, the shoots begin to resprout. Leaves become mature in early October and remain on the stem over winter. Leaf traits and photosynthesis measurements suggest as follows. 1) By becoming summer deciduous,D. kamtschatica avoids the cost of maintaining leaves inefficient under deep shade. 2) The onset and breaking of the summer dormancy is triggered by photoperiod since plants at the forest edge also become dormant even when light remained relatively high. However, the decreased duration of dormancy with higher light levels suggests that there is a tendency towards shorter dormancy where summer shade is absent and this could eventually lead to an evergreen habit such as that found in the alpine speciesDaphne miyabeana.     

The failure of nitrogen and lignin control of decomposition in a North American desert

We measured mass losses of both buried and surface litter of six litter types: leaves of the perennial evergreen shrub, Larrea tridentata, leaves of the winter deciduous perennials Fluorensia cernua, Prosopis glandulosa and Chilopsis linearis (a desert riparian species), an evergreen monocot, Yucca elata, and a mixture of annual plants. These species differed in lignin content and carbon-nitrogen ratios. There was no correlation between rates of mass loss and percent lignin, carbon-nitrogen ratio, or lignin-nitrogen ratio. The leaves of F. cernua and the mixed annuals exhibited the highest rates of mass loss. Surface litter of Y. elata, the mixed annuals and C. linearis exhibited higher mass loss than buried litter of the same species. The patterns of mass loss of buried and surface litter differed with buried litter mass loss occurring as a negative exponential and surface litter exhibiting low rates in winter and spring and high rates in summer. There was no correlation between mass loss in surface bags that were field exposed for 1 month and actual evapotranspiration (AET) but there was a correlation between AET and mass losses in buried litter. A model relating mass loss to AET and initial lignin content underestimated mass losses in all species examined.     

Proteins as a potential nitrogen storage compound in bark and leaves of several softwoods

Summary The occurrence of vegetative storage proteins in the leaf and bark tissues of several softwood species during overwintering was investigated by sodium dodecyl sulphate polyacrylamide electrophoresis. Monthly protein profiles from leaves and bark of six evergreen softwood species (Pinus strobus, P. sylvestris, Picea abies, P. glauca, Abies balsamea, and Thuja occidentalis) and the bark of one deciduous softwood species (Larix decidua) suggest that storage proteins are present in bark tissues of L. decidua, Pinus sylvestris, and P. strobus. The remaining species did not show similar specific proteins. However, the total soluble protein content which was determined during active growth and during overwintering in the same tissues indicated that protein levels were higher in the winter compared to the summer in the bark of all species and in the leaves of Pinus spp. and T. occidentalis. While vegetative storage proteins do not appear prevalent in all softwood species, proteins may constitute a major form of overwintering nitrogen storage for many species.     

Periodicity of leaf growth and leaf dry mass changes in the evergreen and deciduous species of Southern Assam,India

The study described patterns of leaf dry mass change, leaf mass per area (LMA), relative growth rate and leaf life span (LL) for 14 evergreen and 7 deciduous species of a tropical forest of Southern Assam, India. Leaf expansion in both the groups was, in general, completed before June (i.e. well before the onset of monsoon rains). Although leaf dry mass during leaf initiation phase was significantly higher (P < 0.01) in evergreen species than in deciduous species, at the time of full leaf expansion, average leaf dry mass relative to the peak leaf dry mass, realised by the evergreen species was lower (66 %) than for deciduous species (76 %). Leaf dry mass increase in both groups continued after leaf full expansion. Evergreen species had a longer leaf dry mass steady phase than deciduous species (2–6 vs 2–3 months). Average LMA of mature leaves for evergreen species (77.43 g m^?2) was significantly greater than that of deciduous species (48.43 g m^?2). LL ranged from 165 days in Gmelina arborea (deciduous) to 509 days in Dipterocarpus turbinatus (evergreen). LMA was correlated positively with LL, indicating that evergreen species with higher leaf construction cost retain leaves for longer period to pay back. The average leaf dry mass loss before leaf shedding was greater (P < 0.01) for deciduous species (30.29 %) than for evergreen species (18.31 %). Although the cost of leaf construction in deciduous species was lower than for evergreen species, they replace leaves at a faster rate. Deciduous species perhaps compensate the cost involved in faster leaf replacement through higher reabsorption of dry mass during senescence, which they remobilise to initiate growth in the following spring when soil resources remain limiting.     

Seasonal changes in composition and intake of chaparral by Spanish goats

Seasonal changes in utilization of one-year regrowth of chaparral vegetation by Spanish goats were investigated. The dominant shrub species were: oak (Quercus dumosa and Q. wislizenii), chamise (Adenostoma fasciculatum), manzanita (Arctostaphylos glandulosa) and ceanothus (Ceanothus cuneatus, and C. foliosus). Oesophageal and faecal samples were obtained during the spring, summer and autumn seasons of 1979. Mean daily temperatures for the three seasons were 19, 18 and 12° C, respectively. Winter sampling was restricted by cold weather at the 985-m elevation site. Cover was measured from ten permanent transects. Preference of Spanish goats was highly directed towards oak (> 50%) and chamise (> 30%) in all seasons.Correlation of selectivity (percentage botanical composition in the diet) and availability data indicated that goats may be generalist in their feeding strategies, but display some sort of specialized behaviour in selecting between the most dominant species. The observation that selection of herbaceous vegetation was restricted by availability conforms with the recent classification of goats as adaptive mixed feeders rather than browsers.Daily feed intake (g DM/kg BW^0.75) was calculated by dividing faecal residue by (100—digestibility in vitro). Intake values were equivalent to maintenance requirements during spring (60 g) and summer (57 g). However, autumn chaparral provided sub-maintenance intake levels (36 g). Condensed tannins (ADF—NAD), estimated as the difference between acid detergent fibre (ADF) and sequential acid detergent residue of neutral detergent residue (NAD), were variable in the clipped shrub samples but low in all dietary samples (2.1 ± 1.8%). The low levels of tannins did not appear to be affecting utilization of this browse.Predictive equations for intake (DMI) and digestibility in vitro (IVOMD) were derived from step-wise regression analysis using the following independent variables: cell wall content (CWC), NAD, cellulose (NADC), NAD sulphuric acid insoluble lignin (NADL_s), hemicellulose (NADHC), lignin: ADF—NAD tannins, ash and nitrogen (N) The equation for intake was: DMI (g DM/kg BW^0.75) = ?25.67 + 1.80 OMD (n = 56, Syx = 17.72, r = 0.59, P < 0.005), while digestibility (%) was predicted as OMD = 56.98 ? 0.46 CWC ? 0.45 NADL_s + 0.65 N + 0.09 DMI (n = 56, Syx = 2.77, r = 0.88, P < 0.005).It is concluded that recently-burnt chaparral vegetation would provide a maintenance browse for Spanish goats during the spring and summer seasons only.     

Optimisation or satiation,testing diet selection rules in goats

Several hypotheses have been formulated to explain diet selection by herbivores, focusing on the maximization of nutrient intake, the minimization of plant secondary compounds, or the satiety hypothesis. This research aimed at studying diet selection revealing which chemical characteristics of plants form the bases for dietary preferences of goats. This was done by setting up a feeding experiment with three different combinations of tree species Acacia karroo, A. nilotica and A. sieberana. The chemical characteristics of these three Acacia species were used to predict diet selection. To test the validity of the satiety hypothesis, goats were placed on a conditioning diet of one of the three species. We found a clear preference for A. karroo and an avoidance of A. nilotica when these two were offered to the goats. In trials where A. nilotica was present, tannin minimization was the best explaining diet selection rule. In trials where A. nilotica was not present, however, tannin minimization was not the best explanation. Our findings suggest that tannins are not avoided but kept below a certain threshold. Below this threshold, goats based their dietary choices on other chemical characteristics of the Acacia species. Acid detergent fibre (ADF) minimization could then best explain preferences in trials with Acacia karroo and A. sieberana that have generally low tannin content. Goats did not maximize nutrient intake or digestibility, and we found no support for the satiety hypothesis.     

Protein-storing vacuoles in inner bark and leaves of softwoods

Summary The seasonal occurrence of protein-storage vacuoles in parenchyma cells of the inner bark and leaf tissues of seven softwood species was examined. Previously published results showed that these organelles often fill the phloem parenchyma cells of the inner bark tissues in overwintering hardwoods, whereas they are absent from this tissue during the summer. We hypothesize that the organelles are involved in the storage of reduced nitrogen during wintering, in a manner analogous to protein bodies of seeds. A survey of the phloem and cambial parenchyma tissues in six evergreen softwood species (Pinus strobus, P. sylvestris, Picea abies, P. glauca, Abies balsamea, and Thuja occidentalis) and in one deciduous softwood species (Larix decidua) was conducted. There was a large variation in the degree and timing of protein-storage vacuole formation between the individual genera and species. The organelles were not seen in summer samples of inner bark tissues of any of the genera or species examined. Protein-storage vacuoles were common in the bark tissues of Pinus, Abies and Thuja, occasionally seen in Picea, and rarely found in Larix during the winter. One-year-old leaves were also examined, since in all but Larix they are overwintering structures and can act as potential sites of nitrogen storage. Protein-storage vacuoles were present in Pinus and Thuja leaf tissue in both summer and winter, in Abies during winter only, and were absent from Picea leaf tissue at all times. These results indicate that the formation of protein-storage vacuoles prior to overwintering is not a ubiquitous phenomenon in softwoods.     

Chilling-Induced Photoinhibition in Two Oak Species: Are Evergreen Leaves Inherently Better Protected Than Deciduous Leaves?

We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure to 1 h high irradiance (1000 μmol m^-2 s^-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective q_E and less irreversible quenching (q_I) than the deciduous Q. michauxii. The large q_E which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonal reversibility of acclimation to irradiance in leaves of common box (Buxus sempervirens L.) in a deciduous forest

Understorey shade plants are seasonally exposed to dramatic changes in light conditions in deciduous forests related with the dynamics of the overstorey leaf phenology. These transitions are commonly followed by changes in herb plant communities, but shade-tolerant evergreen species must be able to adapt to changing light conditions. In this work we checked the photoprotective responses of evergreen species to acclimate to the shady summer environment and reversibly de-acclimate to a more illuminated environment after leaf fall on deciduous overstoreys. For that purpose we have followed the process of light acclimation in leaves of common box (Buxus sempervirens) during the winter to spring transition, which decrease irradiance in the understorey, and conversely during the transition from summer to autumn. Four parameters indicative of the structure and degree of acclimation of the photosynthetic apparatus have been studied: chlorophyll a/b ratio which is supposed to be inversely proportional to the antenna size, α/β-carotene which increases in shade acclimated leaves and the pools of α-tocopherol and xanthophyll cycle pigments (VAZ) which are two of the main photoprotection mechanisms in plants. Among these parameters, chlorophyll a/b ratio and VAZ pool responded finely to changes in irradiance indicating that modifications in the light harvesting size and photoprotective capacity contribute to the continuous acclimation and de-acclimation of long-lived evergreen leaves.     

Initial Stages of Tundra Shrub Litter Decomposition May Be Accelerated by Deeper Winter Snow But Slowed Down by Spring Warming

The Arctic climate is projected to change during the coming century, with expected higher air temperatures and increased winter snowfall. These climatic changes might alter litter decomposition rates, which in turn could affect carbon (C) and nitrogen (N) cycling rates in tundra ecosystems. However, little is known of seasonal climate change effects on plant litter decomposition rates and N dynamics, hampering predictions of future arctic vegetation composition and the tundra C balance. We tested the effects of snow addition (snow fences), warming (open top chambers), and shrub removal (clipping), using a full-factorial experiment, on mass loss and N dynamics of two shrub tissue types with contrasting quality: deciduous shrub leaf litter (Salix glauca) and evergreen shrub shoots (Cassiope tetragona). We performed a 10.5-month decomposition experiment in a low-arctic shrub tundra heath in West-Greenland. Field incubations started in late fall, with harvests made after 249, 273, and 319 days of field incubation during early spring, summer and fall of the next year, respectively. We observed a positive effect of deeper snow on winter mass loss which is considered a result of observed higher soil winter temperatures and corresponding increased winter microbial litter decomposition in deep-snow plots. In contrast, warming reduced litter mass loss during spring, possibly because the dry spring conditions might have dried out the litter layer and thereby limited microbial litter decomposition. Shrub removal had a small positive effect on litter mass loss for C. tetragona during summer, but not for S. glauca. Nitrogen dynamics in decomposing leaves and shoots were not affected by the treatments but did show differences in temporal patterns between tissue types: there was a net immobilization of N by C. tetragona shoots after the winter incubation, while S. glauca leaf N-pools were unaltered over time. Our results support the widely hypothesized positive linkage between winter snow depth and litter decomposition rates in tundra ecosystems, but our results do not reveal changes in N dynamics during initial decomposition stages. Our study also shows contrasting impacts of spring warming and snow addition on shrub decomposition rates that might have important consequences for plant community composition and vegetation-climate feedbacks in rapidly changing tundra ecosystems.     

Seasonal Changes in Nitrogen Fractions of Coptis japonica, a Temperate Forest Evergreen Chamaephyte, and their Ecological Implications

Seasonal changes in several forms of nitrogen were investigatedin Coptis japonica, an evergreen rosette hemicryptophyte intemperate deciduous forest. The concentration of total nitrogenin rhizomes and roots decreased during the period of new shootgrowth from winter to spring. In the rhizomes, total solubleprotein stored by early summer decreased gradually until winter,coupled with an increase in free amino acids. Nitrogen was largelystored in free amino acids in the roots, especially during summer.The total soluble protein in current-year leaves decreased fromspring to summer and then increased during winter. The seasonalchanges in nitrogen components were coincident with the changein light-saturated photosynthetic rates recorded in a previousstudy. The ratio of total soluble protein to total nitrogendecreased from spring to summer and then increased from latesummer to winter in the current-year leaves. In contrast, chlorophyllcontent and the ratio of chlorophyll to total nitrogen werehigher in summer than in other seasons. The results indicatethat nitrogen was used in a manner that better utilizes thevery weak light in summer and the higher light intensities inother seasons. The major component of the free amino acid poolwas asparagine, in every organ throughout the season, exceptfor the senescent leaves. Since asparagine has a high N:C ratio,we suppose that the asparagine-dominated amino acid pool isadvantageous in the carbon-limited environment of the forestfloor.Copyright 1994, 1999 Academic Press Free amino acid composition, total nitrogen, total soluble protein, photosynthesis, evergreen hemicryptophyte     

A Cost-Benefit Analysis of Leaves of Eight Australian Savanna Tree Species of Differing Leaf Life-Span

Cost-benefit analysis of foliar construction and maintenance costs and of carbon assimilation of leaves of differing life-span were conducted using two evergreen, three semi-deciduous, and three deciduous tree species of savannas of north Australia. Rates of radiant-energy-saturated CO₂ assimilation (P _max) and dark respiration were measured and leaves were analysed for total nitrogen, fat, and ash concentrations, and for heat of combustion. Specific leaf area, and leaf N and ash contents were significantly lower in longer-lived leaves (evergreen) than shorter-lived leaves (deciduous) species. Leaves of evergreen species also had significantly higher heat of combustion and lower crude fat content than leaves of deciduous species. On a leaf area basis, P _max was highest in leaves of evergreen species, but on a leaf dry mass basis it was highest in leaves of deciduous species. P _max and total Kieldahl N content were linearly correlated across all eight species, and foliar N content was higher in leaves of deciduous than evergreen species. Leaf construction cost was significantly higher and maintenance costs were lower for leaves of evergreen than deciduous species. Maintenance and construction costs were linearly related to each other across all species. Leaves of evergreen species had a higher cost-benefit ratio compared to leaves of deciduous species but with longer lived leaves, the payback interval was longer in evergreen than deciduous species. These results support the hypotheses that longer lived leaves are more expensive to construct than short-lived leaves, and that a higher investment of N into short-lived leaves occurs which supports a higher P _max over a shorter payback interval. This revised version was published online in June 2006 with corrections to the Cover Date.