Biomass and mineral element responses of a Serengeti short-grass species to nitrogen supply and defoliation: compensation requires a critical [N]

Large mammalian herbivores in grassland ecosystems influence plant growth dynamics in many ways, including the removal of plant biomass and the return of nutrients to the soil. A 10-week growth chamber experiment examined the responses of Sporobolus kentrophyllus from the heavily grazed short-grass plains of Serengeti National Park, Tanzania, to simulated grazing and varying nitrogen nutrition. Plants were subjected to two clipping treatments (clipped and unclipped) and five nitrogen levels (weekly applications at levels equivalent to 0, 1, 5, 10, and 40 g N m⁻²), the highest being equivalent to a urine hit. Tiller and stolon production were measured weekly. Total biomass at harvest was partitioned by plant organ and analyzed for nitrogen and mineral element composition. Tiller and stolon production reached a peak at 3–5 weeks in unclipped plants, then declined drastically, but tiller number increased continually in clipped plants; this differential effect was enhanced at higher N levels. Total plant production increased substantially with N supply, was dominated by aboveground production, and was similar in clipped and unclipped plants, except at high nitrogen levels where clipped plants produced more. Much of the standing biomass of unclipped plants was standing dead and stem; most of the standing biomass of clipped plants was live leaf with clipped plants having significantly more leaf than unclipped plants. However, leaf nitrogen was stimulated by clipping only in plants receiving levels of N application above 1 g N m⁻² which corresponded to a tissue concentration of 2.5% N. Leaf N concentration was lower in unclipped plants and increased with level of N. Aboveground N and mineral concentrations were consistently greater than belowground levels and while clipping commonly promoted aboveground concentrations, it generally diminished those belowground. In general, clipped plants exhibited increased leaf elemental concentrations of K, P, and Mg. Concentrations of B, Ca, K, Mg, and Zn increased with the level of N. No evidence was found that the much greater growth associated with higher N levels diminished the concentration of any other nutrient and that clipping coupled with N fertilization increased the total mineral content available in leaf tissue. The results suggest that plants can (1) compensate for leaf removal, but only when N is above a critical point (tissue [N] 2.8%) and (2) grazing coupled with N fertilization can increase the quality and quantity of tissue available for herbivore removal. Received: 25 August 1997 / Accepted: 14 April 1998     

Interactive regulation of grass yield and chemical properties by defoliation,a salivary chemical,and inorganic nutrition

Summary Clones of 2 C₄ grass species, Sprobolus ioclados and S. pyramidalis, were obtained from more and less heavily grazed grasslands, respectively, in Tanzania's Serengeti National Park. Plants were grown in a factorial experiment to determine the effects of severe defoliation, nutrient limitation, and a salivary chemical (thiamine) on plant growth, nitrogen content, and non-structural carbohydrate content. The experimental design included: (1) species; (2) clipping, with plants either unclipped or clipped weekly to a height of 5 cm; (3) 0.2 ml of distilled water of 0.2 ml of 10 ppb aqueous thiamine sprayed on plants from an atomizer after clipping and identical treatments at the same time to unclipped plants; (4) phosphorus (P) at 0.2 or 1 mM; (5) nitrogen (N) at 3 or 15 mM. Clipping was the major variable affecting plant growth. Total and litter yields were reduced to half and residual plant yield was reduced to 30% of the values for unclipped plants. Clipping interacted strongly with other variables since they commonly had minor effects on clipped plants and major effects on unclipped plants. Exceptions to this generalization were generally due to better performance by S. ioclados under clipping. Compared to lower treatment levels, higher treatment levels promoted total yield of unclipped plants by 52% for N, 43% for thiamine, and 33% for P. In general, thiamine had greater effects than P but lesser effects than N. Thiamine promoted yield and modified the chemical balance of plants by promoting carbohydrate (CHO) concentrations and reducing N concentrations. N and P deficiencies promoted CHO accumulation. Clipping promoted the N of leaves and crowns and reduced the N levels in roots. Leaf blade water and N contents were positively correlated with very little scatter. The slope of the line was different for S. ioclados and S. pyramidalis. Leaf blade water and CHO contents were negatively related but there was more scatter and the species could not be distinguished. The species from more heavily grazed grasslands was conspicuously more sensitive to thiamine application. The results indicated that leaf treatment with thiamine, the only likely source of which in natural grasslands is saliva deposited by feeding herbivores, can have major effects on plant yield and metabolic balances at very low application levels. But under defoliation levels as severe as those imposed in this experiment, which reduced above ground plant biomass to a fourth of the level produced by unclipped plants, growth was so strongly limited by defoliation that neither thiamine nor inorganic nutrients affected plant yield residual from clipping. Therefore, whether chemicals such as thiamine that may be introduced onto grass foliage by grazing ungulates and other herbivores will influence the growth of grazed plants will depend upon the grazing intensity associated with the transfer.     

Effects of ramet clipping and nutrient availability on growth and biomass allocation of yellow nutsedge

A glasshouse experiment was conducted to examine how the interactions of nutrient availability and partial ramet clipping affect growth, reproduction and biomass allocation of Cyperus esculentus, an invasive sedge. The plants sprouting from tubers were grown at low and high nutrient levels, and were subject either to no clipping, one, two or three clippings, with each clipping cutting half of the existing ramets at soil level. Our results show that nutrient availability and clipping frequency tended to independently affect most of growth, reproduction and biomass allocation parameters of Cyperus esculentus examined in the present study. Increased supply of nutrients led to an increase in plant productivity and its associated traits. All of the traits, except for the number of ramets, displayed a decreasing pattern with increasing clipping frequency, indicating that Cyperus esculentus had undercompensatory responses to ramet clipping. It is likely that the patterns of plants response to clipping are species specific, and depend on morphological characters of species. Its susceptibility to ramet clipping can offer opportunities for controlling this invasive species through mechanical methods such as mowing. Clipping had little effects on biomass allocation; however, root weight fraction increased with increasing clipping frequency. While nutrient availability and clipping frequency had no influence on leaf carbon concentration at harvest, both of them increased leaf nitrogen concentration, and hence reduced leaf C/N ratio.     

Ecological conditions that determine when grazing stimulates grass production

Summary We report the results of a pot experiment that examined the effects of three ecologically important factors controlling plant growth rates in savanna grasslands: defoliation, soil nitrogen and soil water availability. The experiment was conducted in the Amboseli region in east Africa, and was designed to simulate natural conditions as far as possible, using local soils and a grass species that is heavily grazed by abundant large herbivores. Productivity by different plant components was reduced, stimulated or unchanged by defoliation, depending on specific watering and fertilization treatments. Total above-ground production was stimulated by defoliation and was maximized at moderate clipping intensities, but this was statistically significant only when plants were watered infrequently (every 8 days), and most important, periods between clipping events were extended (at least 24 days). Under these conditions, plant growth rates were limited by water availability at the time of clipping, and soil water conserved in clipped, compared to unclipped plants. Within a given fertilization treatment, whole-plant production was never stimulated by defoliation because root growth was unaffected or inhibited by clipping. However, when fertilization was coupled to defoliation, as they are in the field, whole-plant production by fertilized and moderately clipped plants exceeded production by infertilized, unclipped plants. Under this interpretation, maximum whole-plant production coincided with optimum conditions for herbivores (maximum nitrogen concentration in grass leaves) when watering was frequent, and plants were moderately defoliated. However, these conditions were not the same as those that maximized relative above-ground stimulation of growth (infrequent watering and clipping).The results indicate that above-ground grass production can be stimulated by grazing, and when that is likely to occur. However, the results emphasize that plant production responses to defoliation can vary widely, contigent upon a complex interaction of ecological factors.     

Net photosynthesis,root respiration,and regrowth of Bouteloua gracilis following simulated grazing

Summary Net photosynthesis (P_N), root respiration (R_R), and regrowth of Bouteloua gracilis (H.B.K.) Lag. were examined in the laboratory over a 10-day period following clipping to a 4-cm height to simulate grazing by large herbivores. Net photosynthesis rates of tissue remaining immediately following defoliation were only about 40% as great as preclipping rates. Three days after clipping, P_N rates of defoliated plants had increased to values about 21% greater (per unit leaf area) than those of unclipped controls and remained at that level through Day 10. No statistically significant changes in R_R occurred following defoliation. Biomass of unclipped plants nearly doubled during the 10-day study period, while that of defoliated plants increased 67%. Over half the new growth of defoliated plants was allocated to new leaf blades and only 18% to new roots, while only 33% of the new growth of control plants was allocated to new leaf blades but 29% went to new roots. As a consequence of increased P_N rates and increased carbon allocation to synthesis of additional photosynthetic tissue following defoliation, net CO₂ uptake per plant increased from 9% to 80% of that of the controls from Day 0 through Day 10.     

Compensatory growth responses to clipping defoliation in Leymus chinensis (Poaceae) under nutrient addition and water deficiency conditions

Compensatory growth responses of Leymus chinensis, a dominant species in Inner Mongolia steppe, to clipping defoliation were evaluated in a pot-cultivated experiment under different nutrient (N and P) and water availability conditions. Leymus chinensis exhibited over-compensatory growth at the light and moderate clipping intensities (20% and 40% aerial mass removed) with a greater accumulated aboveground biomass, higher relative growth rate (RGR), more rhizomatic tillers and a stimulation of compensatory photosynthesis to the remnant leaves as compared with those of the unclipped plants. Intense clipping (80% aerial mass removed), which removed most of the aboveground tissues, greatly reduced the growth of aboveground biomass in comparison with that of the unclipped plants. Nitrogen addition only slightly improved the biomass production and RGR in light and moderately clipped plants, and it did not allow plants in the intense clipping condition to over-compensate. Phosphorus addition had no obvious influences on the growth and physiological responses to clipping defoliation. These results indicated that nutrient addition could not compensate for the negative effects of severe clipping on the defoliated grass. On the other hand, there were no distinct positive responses under water deficiency condition for L. chinensis at all clipping intensities with a significant reduction of aboveground and belowground biomass, lower RGR, fewer rhizomatic tillers, and a lower net photosynthetic rate than other wet treatments. Additionally, the chlorophyll contents of remnant leaves gradually increased with the increase of clipping intensities in each treatment. In conclusion, although L. chinensis could compensate for tissues removal by some morphological and physiological responses, intense clipping and drought can result in a significant decrease of biomass and growth rate, even under enriched nutrition conditions.     

Urea as a promotive coupler of plant-herbivore interactions

Summary Growth responses of Kyllinga nervosa Steud., a sedge from the Serengeti short-grass plains, were examined in a factorial experiment which included clipped and unclipped plants, and nitrogen supplied as either urea or ammonium nitrate. Results were expressed in relation to three transfer processes: flow to grazers, flow to producers and flow to reproduction. Clipping increased biomass and nitrogen flow to grazers by significantly increasing nitrogen uptake, aboveground nitrogen flow, and the weights of and proportional allocation to green leaf production. This was at the expense of flow to vegetative and sexual reproduction, since the weights and proportional investments in roots, crowns and reproductive structures were reduced. Urea nutrition increased flow to grazers and plant reproduction through increases in green leaf weight, flower weight, allocation to green leaves, flowers and stems, and aboveground: belowground biomass ratios. Stimulation of aboveground productivity by urea was a consequence of increased tillering rates.Interactive responses of clipping and nitrogen source regulated plant growth, thus controlling flow to each transfer process. Combined effects of clipping and urea resulted in compensatory production of both green leaves and flowers, and maximized biomass and nitrogen flow to grazers. Both urea and clipping tightened herbivore-producer recycling by significantly reducing litter nitrogen and carbon masses. In contrast, when plants were unclipped and grown on NH₄NO₃, biomass allocation and weights of roots and crowns were increased at the expense of aboveground tissues, thus increasing flow to primary producers. Plant growth responses to experimental treatment combinations simulating nutritional status of grazed and ungrazed field plants indicate that urea represents a potential importance beyond it nitrogen contribution by introducing a positive feedback to herbivores.     

The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus,a short-grass species from the serengeti plains

Summary Sporobolus kentrophyllus, a grazing-tolerant C₄ grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The ¹⁵N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average ¹⁵N uptake rate of clipped plants was twice that of unclipped plants. The relative ¹⁵N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed ¹⁵N-ammonium or ¹⁵N-nitrate during the ¹⁵N pulse experiment had greater ¹⁵N tissue concentrations compared with urea-fed plants, and ¹⁵N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.     

Interactive effects of soil-dwelling ants, ant mounds and simulated grazing on local plant community composition

Interactions between aboveground vertebrate herbivores and subterranean yellow meadow ants (Lasius flavus) can drive plant community patterns in grassland ecosystems. Here, we study the relative importance of the presence of ants (L. flavus) and ant mounds under different simulated grazing regimes for biomass production and species composition in plant communities. We set up a greenhouse experiment using intact soil cores with their associated vegetation.We found that plant biomass production in the short term was affected by an interaction between simulated grazing (clipping) and ant mound presence. Clipping homogenized production on and off mounds, while in unclipped situations production was higher off than on mounds. During the experiment, these differences in unclipped situations disappeared, because production on unclipped mounds increased. Plant species richness was on average higher in clipped treatments and patterns did not change significantly over the experimental period. Plant community composition was mainly affected by clipping, which increased the cover of grazing-tolerant plant species. The actual presence of yellow meadow ants did not affect plant community composition and production.We conclude that the interaction between ant mounds and clipping determined plant community composition and biomass production, while the actual presence of ants themselves was not important. Moreover, clipping can overrule effects of ant mounds on biomass production. Only shortly after the cessation of clipping biomass production was affected by ant mound presence, suggesting that only under low intensity clipping ant mounds may become important determining plant production. Therefore, under low intensity grazing ant mounds may drive the formation of small-scale plant patches.     

Effects of light and nutrient availability on the growth,allocation, carbon/nitrogen balance,phenolic chemistry,and resistance to herbivory of two freshwater macrophytes

Phenotypic responses of Potamogeton amplifolius and Nuphar advena to different light (7% and 35% of surface irradiance) and nutrient environments were assessed with field manipulation experiments. Higher light and nutrient availability enhanced the growth of P. amplifolius by 154% and 255%, respectively. Additionally, biomass was allocated differently depending on the resource: high light availability resulted in a higher root/shoot ratio, whereas high nutrient availability resulted in a lower root/shoot ratio. Low light availability and high nutrient availability increased the nitrogen content of leaf tissue by 53% and 40% respectively, resulting in a 37% and 31% decrease in the C/N ratio. Root nitrogen content was also increased by low light and high nutrient availability, by 50% (P=0.0807) and 77% respectively, resulting in a 20% and 40% decrease in root C/N ratio. Leaf phenolics were significantly increased 72% by high light and 31% by high nutrient availability, but root phenolic concentrations were not altered significantly. None of these changes in tissue constituents resulted in altered palatability to crayfish. N. advena was killed by the same high nutrient treatment that stimulated growth in P. amplifolius, preventing assessment of phenotypic responses to nutrient availability. However, high light availability increased overall growth by 24%, but this was mainly due to increased growth of the rhizome (increased 100%), resulting in a higher root/shoot ratio. High light tended to increase the production of floating leaves (P=0.09) and significantly decreased the production of submersed leaves. High light availability decreased the nitrogen content by 15% and 25% and increased the phenolic concentration by 88% and 255% in floating and submersed leaves, respectively. These differences in leaf traits did not result in detectable differences in damage by herbivores.     

Intraspecific variation in the response of Themeda triandra to defoliation: the effect of time of recovery and growth rates on compensatory growth

Summary The response to a single defoliation was studied on three clones of Themeda triandra collected in the short, mid, and tall grassland regions of the Serengeti National Park (Tanzania). These sites represent a gradient of decreasing grazing intensity. Growth, allocation pattern, and several morphometric traits were monitored during an 80-day period. Clipped plants of the short and medium clones fully compensated for the reduction of biomass, while plants of the tall clone showed overcompensation. During the first two weeks after clipping, clipped plants showed lower relative growth rates than unclipped ones, whereas the opposite was observed later on. Clipped plants compensated for the removal of leaf area by producing new leaves with lower specific weights and higher nitrogen content. They also produced more, smaller tillers. Although clipped plants mobilized nonstructural carbohydrates from roots and crowns, this did not account for a significant amount of growth. Relative growth rates of unclipped plants of the short clone were higher. The relative growth rate of the short clone diminished less after clipping, but also exhibited the lowest increase later. The tall clone was the most negatively affected early, but showed the highest compensation later. Compared to the other clones, the short ecotype showed many of the characteristics that defoliation induced in each individual of any clone: higher allocation to leaf area production, higher relative growth rate, higher number but smaller size of tillers, and lower leaf specific weights.     

Interactive effects of clipping and nutrient availability on the compensatory growth of a grass species

Resource availability is an important factor affecting the capacity of compensatory growth after grazing. We performed a greenhouse experiment with Poa bulbosa, a small perennial grass of the Mediterranean and Central Asian grasslands, to test the importance of nutrient availability for compensatory growth after clipping. We also compared the results with predictions of the limited resource model (LRM). Plants were grown at low and high fertilization levels and subjected to a clipping treatment. Contrary to the LMR, we found that in Poa plants compensatory growth occurred under the high fertilization level, while it did not occur under the low level. The LMR predicts a higher tolerance for grazing in the stressful environment. Our plants showed a significant decrease in their relative growth rates (RGR) after clipping. Although the plants allocated a 32–188% greater fraction of the mass to lamina growth after clipping, this greater allocation to the leaves did not fully compensate for the initial reduction in leaf area ratio (LAR). A sensitivity analysis showed for the clipped plants under the high fertilization treatment, that changes in leaf allocation (f _lam) enabled the plants to compensate for a part of the potential loss caused by defoliation. Probably, the increased biomass allocation comes largely from the bulbs. We conclude that the inconsistency of the LRM with our results originates in the lack of compensatory mechanisms in the model. To better understand how environmental conditions affect tolerance to herbivory, the effects of compensatory growth should be taken into account.     

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation

Summary The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.     

The effects of light and nitrogen on photosynthesis,leaf characteristics,and dry matter allocation in the chaparral shrub,Diplacus aurantiacus

Summary Plants of Diplacus aurantiacus, a successional shrub common in California chaparral, were grown under controlled conditions in which either quantum flux density or nitrogen availability was varied. Photosynthesis and leaf nitrogen content were determined on a leaf area and a leaf weight basis, and whole plant growth was monitored.There was a direct relationship between photosynthesis and leaf nitrogen content on both area and weight bases. Reduced light intensity of the growth environment resulted in reductions in light-saturated photosynthesis and nitrogen content on an area basis, but not on a weight basis. With reduced nitrogen availability, photosynthesis and leaf nitrogen content per unit leaf weight decreased.Resource use efficiency increased as the resource became more limiting. The results are consistent with a model of plant growth in which net carbon gain of the leaf is maximized. Abbreviations. For brevity, the following set of abbreviations is used in presenting and discussing the results. P/area and N/area are, respectively, photosynthesis and leaf nitrogen content per unit leaf area. P/wt and N/wt are the same quantities per unit leaf dry weight. SLW (specific leaf weight) is dry weight per unit leaf area. RGR (relative growth rate) is the relative rate of increase in shoot dry matter per day     

Biomass allocation and growth responses of Scots pine saplings to simulated herbivory depend on plant age and light availability

This study experimentally analyses the response to simulated herbivory of juvenile Scots pine of two different ages in contrasting abiotic scenarios, focusing on the potential dual role of browsing ungulates: negative, by removing aerial biomass, and positive, by stimulating compensation capacity and providing nutrients by depositing their excrement. Compensation against herbivory was investigated by experimentally clipping a set of Scots pine (Pinus sylvestris L. nevadensis) juveniles, grown under different levels of light and nutrient availability. The responses analysed were survival, trunk-diameter growth, leader-shoot growth, increment in number of meristems, RGR, biomass of needles, shoots, root and whole plant, and root-to-shoot ratio. Clipping consistently resulted in a worse survival and performance of pines with respect to unclipped ones. From the factors analysed, light availability was responsible mainly for the variations in plant performance, while the addition of nutrients was much less important. Age was also important, with older pines showing in general better performance after clipping. Overall, clipping invariably had a negative effect on Scots pine, since none of the combinations of abiotic factors used resulted in overcompensation. However, the intensity on this negative effect proved quite variable, from almost an exact compensation in clipped older pines under full sunlight availability to very poor performance and high death probability in younger pines in shade. Scots pine cannot overcompensate after clipping, but, depending of the environmental conditions, the negative result of clipping varies from severe undercompensation to almost exact compensation. Also, small differences in sapling age can promote significant differences in sapling response to clipping and light environment.     

Branching responses in Silphium integrifolium (Asteraceae) following mechanical or gall damage to apical meristems and neighbor removal

Branching in plants increases plant access to light and provides pathways for regrowth following damage or loss of the apical meristem. We conducted two experiments in an eastern Kansas tallgrass prairie to determine how apical meristem loss (by clipping), apical meristem damage (by insect galling), and increased light availability affected growth, reproduction, and branching in Silphium integrifolium (Asteraceae). The first experiment compared clipping with galling. Clipping increased axillary shoot numbers, while galling increased axillary shoot lengths, reflecting different allocation responses among damage types and inhibition of branching by galls. However, total capitulum production was less in all gall/clip treatments than in intact shoots. The second experiment compared clipping with mowing the surrounding vegetation to increase light availability. Mowing increased total leaf, total capitulum, and axillary shoot length and axillary capitulum production in clipped and unclipped plants and in large vs. small shoots. The presence of the neighboring canopy, not of an intact apical meristem, was therefore the stronger limitation on leaf and capitulum production. These experiments suggest that damage and light competition affected both branching frequency and the partitioning of resources among shoots, branches, and leaves. Because Silphium's growth form is widespread, similar responses may occur in other grassland forbs.     

Attack frequency and the tolerance to herbivory of Neotropical savanna trees

Tolerance is the ability of a plant to regrow or reproduce following damage. While experimental studies typically measure tolerance in response to the intensity of herbivory (i.e., the amount of leaf tissue removed in one attack), the impact of how many times plants are attacked during a growing season (i.e., the frequency of damage) is virtually unexplored. Using experimental defoliations that mimicked patterns of attack by leaf-cutter ants (Atta spp.), we examined how the frequency of herbivory influenced plant tolerance traits in six tree species in Brazil’s Cerrado. For 2 years we quantified how monthly and quarterly damage influenced individual survivorship, relative growth rate, plant architecture, flowering, and foliar chemistry. We found that the content of leaf nitrogen (N) increased among clipped individuals of most species, suggesting that Atta influences the allocation of resources in damaged plants. Furthermore, our clipping treatments affected tree architecture in ways thought to promote tolerance. However, none of our focal species exhibited a compensatory increase in growth (increment in trunk diameter) in response to herbivory as relative growth rates were significantly lower in clipped than in unclipped individuals. In addition, the probability of survival was much lower for clipped plants, and lower for plants clipped monthly than those clipped quarterly. For plants that did survive, simulated herbivory dramatically reduced the probability of flowering. Our results were similar across a phylogenetically distinct suite of species, suggesting a potential extendability of these findings to other plant species in this system.     

Growth and chemical responses of trembling aspen to simulated browsing and ungulate saliva

Aims Woody plant-browser systems represent an understudied facet of herbivory. We subjected four genotypes of trembling aspen to artificial browsing, similar to that of a large mammalian herbivore, and applied deer saliva to clipped and unclipped trees to assess: (i) the effects of artificial browsing on aspen growth and phytochemistry of leaves and stems, (ii) genotypic variation in responses and (iii) potential alterations of responses by mammalian saliva.Methods Potted aspen trees were grown outdoors on the University of Wisconsin-Madison campus. The experiment consisted of a fully-crossed, 2 × 2 × 4 randomized complete block design, with two levels of artificial browsing (unclipped and clipped), two levels of saliva application (no saliva and saliva) and four aspen genotypes. To simulate ungulate browsing damage, we removed the upper 50% of the stem of half of the trees by pinching the stem with needle-nosed pliers and then separating it by tearing. For half of the damaged trees, we immediately swabbed the wound with deer saliva. Trees in the unclipped plus saliva treatment were swabbed with saliva at the 50% height mark. To assess the effects of clipping and saliva application, we harvested all trees after 2 months and measured various growth and chemical properties. Growth measurements included height, vertical growth, mass of leaves, stems and roots, leaf number and area and bud set. Chemical parameters included defensive, nutritional and structural components of both foliage and stems.Important findings Clipping affected most of the growth parameters measured, decreasing tree height, leaf, stem, root and total tree mass and leaf area. Clipped trees had greater vertical growth, more leaves and higher specific leaf area (SLA) than unclipped trees. Deer saliva had little to no effect on plant growth response to the clipping treatment. Terminal budset was delayed by clipping and varied among genotypes but not in response to saliva application. Clipping also affected most of the phytochemical variables measured, reducing defensive compounds (phenolic glycosides and condensed tannins (CTs)) and nutrients (N), but increasing structural components (cellulose and lignin) in both leaves and stems. Saliva had very little effect on tree chemistry, causing only a slight decrease in the amount of CTs in leaves. In general, leaves contained more defensive compounds and nutrients, but much less cellulose, compared with stems. Genotypes differed for all physical and chemical indices, and in tolerance to damage as measured by vertical growth. In addition, for most of the physical and chemical variables measured, genotype interacted with the clipping treatment, suggesting that in natural stands some genotypes will resist or tolerate browsing better than others, affecting forest genetic composition and ultimately forest dynamics.     

WATER RELATIONS AND GROWTH CHARACTERISTICS OF PROSOPIS GLANDULOSA VAR. TORREYANA IN A SIMULATED PHREATOPHYTIC ENVIRONMENT

Recent studies of Prosopis glandulosa have demonstrated a unique system of a deeply rooted species with significant water stress tolerance. Several growth and developmental characteristics have been correlated with water stress and nitrogen availability during field studies. Here we present a lab experiment in which a phreatophytic regime is simulated and the availability of nitrogen and water are varied. Increased ground water salinity caused lower plant water potentials and greater osmotic adjustment without significant increases in leaf Na⁺ concentrations. Leaf conductance was higher in the higher salinity treatments. Low water potential was also associated with reduced leaf size, reduced leaf area per plant and increased root to shoot ratio. Specific leaf weight and the transpiration ratio were unaffected by the low water potentials induced by increased salinity. Increasing nitrogen availability caused increased growth rates but did not influence water use efficiency. Net assimilation rates increased with increasing nitrogen availability but relative growth rates were more dependent on overall plant size than treatment conditions. The responses of P. glandulosa to the simulated phreatophytic environment were similar to those predicted by field measurements.     

不同光照条件下喜阴植物谢君魔芋对稳态和动态光源的响应特征

谢君魔芋(Amorphophallus xiei)是起源于云南西南地区热带雨林的典型喜阴植物,近年来得到了广泛种植和推广,在种植过程中,谢君魔芋需要采用遮荫栽培模式。为了揭示谢君魔芋对光照强度的适应策略,该研究探讨了生长在不同光照强度下(透光率为50%、29%、17%、7%)谢君魔芋叶片的光合作用特征、光合诱导特征、光合色素含量以及叶片氮素(N)含量和N分配。结果表明:随着生长环境光照强度的降低,单位叶面积和单位叶质量最大净光合速率、光合色素含量、最大羧化速率、最大电子传递速率及比叶面积均增大,而暗呼吸和光补偿点均减小。在光合诱导过程中,生长在透光率为17%光环境中的谢君魔芋完成50%光合诱导所需的时间最短,约为81.4 s;在光诱导进行10 min时,诱导状态最高,为87.3%。完成50%和90%光合诱导所需的时间与低光下初始气孔导度呈负相关关系。随着生长光照强度降低,叶片中的N分配到羧化组分和生物能转化组分中的比例先增大后减小,在透光率为17%的光环境下具有最大值;而叶片中的N分配到捕光色素组分中的比例随着生长环境光照强度降低而增加。该研究结果表明,喜阴植物谢君魔芋通过加强对低光和动态光源的利用能力及有效的N资源分配策略来适应低光照环境。