Malcolm C. Press

Comparative responses of phenology and reproductive development to simulated environmental change in sub-Arctic and high Arctic plants

The effects of temperature, precipitation and nutrient perturbations, and their interactions, are being assessed on two contrasting arctic ecosystems to simulate impacts of climate change. One, a high arctic polar semi-desert community, is characterised by a sparse, low and aggregated vegetation cover where plant proliferation is by seedlings, whereas the other, a sub-arctic dwarf shrub heath, is characterised by a complete vegetation cover of erect, clonal dwarf shrubs which spread vegetatively. The developmental processes of seed production were shown to be highly sensitive, even within one growing season, to specific environmental perturbations which differed between sites

Growth responses of four sub-Arctic dwarf shrubs to simulated environmental change

Vegetative responses of Empetrum hermaphroditum, Vaccinium vitis-idaea, V. uliginosum and V. myrtillus to environmental change (temperature (T), water (W) and fertilizer (F)) were investigated in a factorial field perturbation study in sub-Arctic Sweden over two growing seasons (1991 and 1992). Total above-ground biomass was largely unresponsive to the perturbations due to dilution of current seasons growth by material produced in previous years. The mass of shoot material produced in 1991, increased in response to F within 11 weeks of the start of the experiment in the two evergreen species (V. vitis-idaea and E. hermaphroditum), but not in the only deciduous species (V. uliginosum) measured that year (...)

PLANT COMMUNITY RESPONSES TO SIMULATED ENVIRONMENTAL CHANGE AT A HIGH ARCTIC POLAR SEMI-DESERT

Impacts of climate change were simulated over five summer seasons in a high arctic polar semi-desert at Ny Alesund, Svalbard, by using polythene tents to increase temperature, and by increasing precipitation and soil nutrient (NPK) availability. The effects of these treatments on vegetation cover were assessed at the start of the 1991, 1993, and 1995 field seasons, and at peak biomass in the same years. Over the first season of the experiment (1991), changes in percentage total living vegetation cover were significantly greater, and changes in dead vegetation cover significantly lower, in the tented treatments. In subsequent seasons, changes in total living cover were also greater under treatments simulating climate change, although the significant factors and interactions were year- specific. Between years, at both the early and mid-season sampling periods, the fertilizer application had the strongest effect on changes in plant cover, significantly decreasing cover of living Dryas octopetala, Saxifraga oppositifolia, and bare ground between 1991 and 1995, while increasing cover of bryophytes, Salix Polaris, Polygonum viviparum, and total dead vegetation. Although cover of D. octopetala was greater during the first three years of fertilizer addition, marked winter injury occurred in this species on fertilized plots during winter 1993-1994. This resulted in reductions in total live cover and D. octopetala cover and an increase in total dead cover (by up to 22%) in watered and fertilized plots between 1991 and 1995. Seedlings of nitrophilous immigrant species were established naturally on bare ground in fertilized plots in the third year of the study and subsequently increased in number, so that after five seasons the community tended more toward bird-cliff vegetation rather than polar semi-desert vegetation. The tent treatment and the simulated increase in summer precipitation had little effect between seasons on the plant community, in com- parison with the fertilizer treatment.

Environmental constraints on the growth, photosynthesis and reproductive development of Dryas octopetala at a high Arctic polar semi-desert, Svalbard

Opportunities exist in high Arctic polar semidesert communities for colonisation of unvegetated ground by long-lived clonal plants such as Dryas octopetala. This can be achieved by lateral spread of vegetative ramets, or by sexual reproduction and seedling recruitment. The objectives of this study were (1) to determine whether these two means of proliferation show differential sensitivity to contrasting components of the abiotic environment (temperature, soil nutrient and water availability) and (2) to evaluate the potential for D. octopetala to respond to climate change by an increase in cover and biomass in polar semi-desert communities. Factorial environmental manipulations of growing season temperature, soil nutrient and water status were conducted over 3 years at a polar semi-desert community in Svalbard, Norway (78°56.12′N, 11°50.4′E) and both clonal and sexual reproductive performance, together with instantaneous net photosynthesis (Pn), were recorded during the third season (1993). D. octopetala capitalised rapidly on an amelioration in the availability of inorganic nutrients (N, P and K) by an expansion in leaf area and biomass supported by increased Pn per unit leaf weight, and by apparent luxury uptake of nutrients (particularly P). Several facets of sexual reproductive development and seed viability were markedly improved by elevated temperatures or soil nutrient availability. Thus although D. octopetala is a long-lived clonal plant, with many traits characteristic of stress resistance syndrome, it showed considerable phenotypic plasticity in response to environmental manipulations. The results support the hypothesis that clonal growth confers survival potential during unfavourable years, together with the ability to capitalise on nutrient flushes and recycle nutrients internally. Continued investment in sexual reproduction ensures that seed setting is successful during favourable years, even if these occur infrequently. Cimate warming in the high Arctic could thus result in changes in the cover, biomass and the relative significance of clonal versus sexual proliferation of D. octopetala (and thus the genetic diversity of the population) but the long-term responses will probably be constrained by lack of available nutrients.

DECOMPOSITION OF SUB-ARCTIC PLANTS WITH DIFFERING NITROGEN ECONOMIES: A FUNCTIONAL ROLE FOR HEMIPARASITES

Quested, H.M., Cornelissen, J.H.C., Press, M.C., Callaghan, T.V., Aerts, R., Trosien, F., Riemann, P., Gwynn-Jones, D., Kondratchuk, A., Jonasson, S.E. (2003). Decomposition of sub-arctic plants with differing nitrogen economies: A functional role for hemiparasites. Ecology, 84, (12), 3209-3221 Sponsorship: This work was funded by the UK Biology and Biotechnology Research Council (BBSRC), Nordic Council of Ministers Nordic Arctic Research Program (NARP), and the Swedish Royal Academy of Sciences (KVA)

Responses of plant litter decomposition and nitrogen mineralisation to simulated environmental change in a high arctic polar semi-desert and a subarctic dwarf shrub heath

Impacts of climate change were simulated in two contrasting European arctic ecosystems, a high arctic polar semi-desert and a subarctic dwarf shrub heath, by increasing temperature (using polythene tents), precipitation and soil nutrient (NPK) availability. The effects of these treatments and their interactions on plant litter decomposition and soil nutrient fluxes were assessed. Polythene tents increased air, litter and soil temperatures but reduced litter and soil moisture contents. At both sites, litter decomposition was significantly retarded in the tent treatments due probably to reduced litter moisture contents. The tent treatment had no effect on extractable soil N pools or net total N mineralisation at either site, although the treatment significantly reduced net seasonal nitrification values at the subarctic site. The additional precipitation treatment significantly increased litter decomposition at the dwarf shrub heath site and the net amount of N mineralised at the polar semi-desert site. Litter decomposition was increased, as was net N mineralisation, by the application of nutrients. The results suggest that soil temperature increases of up to 1°C, which may occur by the end of the next century as an effect of a predicted 4°C rise in air temperature, have only small effects on total N mineralisation in the short term in arctic soils.

Differential growth, allocation and photosynthetic responses of Polygonum viviparum to simulated environmental change at a high Arctic polar semi-desert

Growing season temperatures and precipitation, and soil nutrient status, were increased in situ at a polar semi-desert site in northwest Spitsbergen to simulate the possible impacts of climate change. During the second year of the experiment the responses of a perennial geophyte, Polygonum viviparum, were assessed both by biometric analyses of vegetative and reproductive structures and by measurements of instantaneous net photosynthesis (P n ). The objectives were to determine whether P. viviparum demonstrates conservative or opportunistic responses to increased temperature, water supply and nutrient availability, to assess whether vegetative and reproductive development show differential sensitivity, and thus whether allocation patterns are altered, and to evaluate whether changes in rate of photosynthesis underlied any changes in growth and allocation (...)

Striga infestation of cereal crops : an unsolved problem in resource limited agriculture

The parasitic weed Striga causes devastating losses in cereal yields in sub-Saharan Africa. The parasite lifecycle is intimately linked with its host via a complex interchange of signals. Understanding the molecular basis of these interactions and of host resistance to Striga is essential for the identification of genes for improving crop yield via biotechnological or marker assisted breeding strategies. Cloning and sequencing of ESTs from the model parasite Triphysaria versicolor is facilitating the identification of parasitism genes. The identification of resistance to Striga in sorghum and rice germplasm is allowing molecular dissection of these traits using genomic platforms and quantitative trait loci (QTL) analysis. QTL underlying different resistance phenotypes have been identified and the use of advanced backcross populations is allowing the exploitation of sources of resistance in wild relatives of cereals.

Photosynthetic characteristics of dipterocarp seedlings in three tropical rain forest light environments: a basis for niche partitioning?

Abstract In three tropical rain forest light environments in Sabah, Malaysia, we compared photosynthesis in seedlings of ten climax tree species with putatively differing shade tolerances. The objectives of the study were (a) to characterise the range of photosynthetic responses in ten species of the Dipterocarpaceae and (b) to elucidate those photosynthetic characteristics that might provide a basis for niche partitioning. Seedlings were acclimated (c. 7 months) in three light environments; understorey, partial shade and a gap (140u2009m2). The light environments represented a gradation in median diurnal (0630–1830u2009hours) photon flux density (PFD) ranging from understorey (4.7u2009μmol m−2 s−1), through partial shade (21.2u2009μmol m−2 s−1) to gap (113.7u2009μmol m−2 s−1). Integrated diurnal PFD were in the sequence gapu2009> partial shadeu2009>u2009understorey (15.2, 4.7, 1.3u2009mol m−2 day−1, respectively). In gap-acclimated plants, species differed in the photosynthetic light-response variables apparent quantum yield, dark respiration rate, light compensation point, net saturated leaf assimilation rate (Asat), and in stomatal conductance (gsu2009sat) when assimilation rate (A) was saturated. A light-demanding pioneer species (Macaranga hypoleuca) and a shade-demanding understorey species (Begonia sp.) had, respectively, higher and lower Asat and gsu2009sat than the dipterocarp species. In high-light conditions Asat and gsu2009sat were strongly positively correlated in dipterocarp species. Differing photosynthetic characteristics of gap-acclimated plants suggest that, in these dipterocarp species, different rates of carbon fixation may be an important factor contributing towards niche partitioning. Mean integrated diurnal A (Adiurnal) in the gap, partial shade and understory were, respectively, 122.9, 52.7, 20.5u2009mmol m−2 day−1. Differences occurred in Adiurnal of dipterocarp species between light environments. When Macaranga was included, differences in Adiurnal were evident in the gap and partial shade, and in both cases were attributed to the pioneer. For the variable Adiurnal, there was of a shift in the rank position of Macaranga among light environments, but a shift did not occur among the dipterocarp species. Results from this study are consistent with the idea that rates of carbon fixation per unit leaf area may contribute towards niche differentiation between the climax and single pioneer species, but not within the group of climax species. Other physiological and/or carbon allocation factors may be involved in any niche partitioning; dipterocarp species often have inherently different growth rates and susceptibility to herbivory. As an alternative to niche partitioning, dipterocarp species may co-exist in natural light environments as a result of habitat disequilibrium or purely stochastic processes.

The hemiparasitic angiosperm Bartsia alpina has the potential to accelerate decomposition in sub-arctic communities

We investigated the hypothesis that hemiparasites accelerate nutrient cycling in nutrient-poor communities. Hemiparasites concentrate nutrients in their leaves, thus potentially producing high quality litter that releases nutrients that would otherwise remain in host tissues or in slowly decomposing plant litter. This hypothesis was tested using species from a European sub-arctic community where root hemiparasites are abundant. The N content of green leaves, and the N, P and C content of leaf litter were measured in seven species of root hemiparasitic Scrophulariaceae, and nine species of commonly co-occurring dwarf shrubs, graminoids and herbs. Fresh leaves of the hemiparasites had greater N concentrations than leaves of dwarf shrubs, graminoids or herbs. This difference was even more marked in litter, with hemiparasite litter containing 1.8–4.1% N, between 1.8 and 8.5xa0times as much N as in the litter of commonly co-occurring species. Litter of the hemiparasitic plant Bartsia alpina and of three commonly co-occurring dominant species of dwarf shrub was decomposed alone and in two species mixtures, in a laboratory microcosm experiment. Bartsia litter decomposed faster and lost between 5.4 and 10.8xa0times more N than that of the dwarf shrubs over the 240xa0days of the experiment. Mixtures of dwarf shrub and hemiparasite litter showed significantly more mass loss and CO2 release than expected, while nutrient release was the same as or less than expected. It is concluded that hemiparasites have the potential to enhance decomposition and nutrient cycling in nutrient-poor environments.