Pasi Rautio

Associational effects of plant defences in relation to within- and between-patch food choice by a mammalian herbivore: neighbour contrast susceptibility and defence.

A basic idea of plant defences is that a plant should gain protection from its own defence. In addition, there is evidence that defence traits of the neighbouring plants can influence the degree of protection of an individual plant. These associational effects depend in part on the spatial scale of herbivore selectivity. A strong between-patch selectivity together with a weak within-patch selectivity leads to a situation where a palatable plant could avoid being grazed by growing in a patch with unpalatable plants, which is referred to as associational defence. Quite different associational effects will come about if the herbivore instead is unselective between patches and selective within a patch. We studied these effects in a manipulative experiment where we followed the food choice of fallow deer when they encountered two patches of overall different quality. One of the two patches consisted of pellets with low-tannin concentration in seven out of eight buckets and with high concentration in the remaining bucket. The other patch instead had seven high- and one low-tannin bucket. We performed the experiment both with individuals one at a time and with a group of 16–17 deer. We found that the deer were unselective between patches, but selective within a patch, and that the single low-tannin bucket among seven high-tannin buckets was used more than a low-tannin bucket among other low-tannin buckets. This corresponds to a situation where a palatable plant that grows among unpalatable plants is attacked more than if it was growing among its own kind, and for this effect we suggest the term neighbour contrast susceptibility, which is the opposite of associational defence. We also found that the high-tannin bucket in the less defended patch was less used than the high-tannin buckets in the other patch, which corresponds to neighbour contrast defence. The neighbour contrast susceptibility was present both for individual and group foraging, but the strength of the effect was somewhat weaker for groups due to weaker within-patch selectivity.

Tolerance of Gentianella campestris in relation to damage intensity: an interplay between apical dominance and herbivory

Meristem allocation models suggest that the patterns of compensatory regrowth responses following grazing vary, depending on (i) the number of latent meristems that escape from being damaged, and (ii) the activation sensitivity of the meristems in relation to the degree of damage. We examined the shape of compensatory responses in two late-flowering populations (59°20′N and 65°45′N) of the field gentian. Plants of equal initial sizes were randomly assigned to four treatment groups with 0, 10, 50 and 75% removal of the main stalk. The plants were clipped before flowering, and their performance was studied at the end of the growing season. The northern population showed a linear decrease in shoot biomass and fecundity with increasing biomass removal, while the response in the southern population was quadratic with maximum performance at the damage level of 50% clipping. This nonlinear shape depended upon the activation sensitivity of dormant meristems in relation to their position along the main stem. The highest plant performance was achieved by inflicting intermediate damage which induced regrowth from basally located meristems. In contrast, the topmost branches took over the dominance role of the main stem after minor apical damage (10% clipping). Consequently, the breakage of apical dominance is a necessary precondition of vigorous regrowth in this species. However, compensation in the field gentian is unlikely to be a mere incidental by-product of apical dominance. The ability to regrow from basally located meristems that escape from being damaged by grazing may well be a sign of adaptation to moderate levels of shoot damage.

Tree mineral nutrition is deteriorating in Europe

The response of forest ecosystems to increased atmospheric CO2 is constrained by nutrient availability. It is thus crucial to account for nutrient limitation when studying the forest response to climate change. The objectives of this study were to describe the nutritional status of the main European tree species, to identify growth-limiting nutrients and to assess changes in tree nutrition during the past two decades. We analysed the foliar nutrition data collected during 1992-2009 on the intensive forest monitoring plots of the ICP Forests programme. Of the 22 significant temporal trends that were observed in foliar nutrient concentrations, 20 were decreasing and two were increasing. Some of these trends were alarming, among which the foliar P concentration in F. sylvatica, Q. Petraea and P. sylvestris that significantly deteriorated during 1992-2009. In Q. Petraea and P. sylvestris, the decrease in foliar P concentration was more pronounced on plots with low foliar P status, meaning that trees with latent P deficiency could become deficient in the near future. Increased tree productivity, possibly resulting from high N deposition and from the global increase in atmospheric CO2, has led to higher nutrient demand by trees. As the soil nutrient supply was not always sufficient to meet the demands of faster growing trees, this could partly explain the deterioration of tree mineral nutrition. The results suggest that when evaluating forest carbon storage capacity and when planning to reduce CO2 emissions by increasing use of wood biomass for bioenergy, it is crucial that nutrient limitations for forest growth are considered.

Stress indications in copper- and nickel-exposed Scots pine seedlings.

Scots pine nursery seedlings were planted in pots, five seedlings per treatment, and placed in an experimental field at the University of Oulu in northern Finland at the beginning of June 1997. Copper and nickel sulphates were mixed with forest mineral soil before seedling planting. The metal levels ranged from 0 to 25 mg Ni kg(-1) dry soil and 0 to 50 mg Cu kg(-1) in dry soil and in combinations of both metals. Current years needles for element analyses, EDS microanalyses, microscopy and glutathione and peroxidase activity analyses were collected from 1-5 seedlings per treatment in September. Seedling biomass in controls, Cu25 and Cu50 differed significantly from the Ni25Cu50 treatment. The root/shoot ratio was highest in the Ni5 treatment, indicating good root growth, though the roots were visibly healthier in the Cu25 treatment than in the Ni5 treatment. At higher Ni levels, the condition of roots deteriorated. The proportion of plasmolysed mesophyll cells was highest in the Ni25 treatment. Copper-treated seedlings did not suffer from Cu stress, because no severe injuries were seen in either the roots or the needles in Cu-exposed seedlings. The needle concentrations of Cu increased only slightly due to treatments. Ni accumulation in needles increased with increasing concentrations in soil. Needles of Cu-treated seedlings had less oxidized glutathione than those of Ni-treated seedlings, but the roots had higher, not significantly, peroxidase activity levels. Light-colored, swollen thylakoids were occasionally observed in the Ni25Cu50 treatment, indicating some interaction between Ni and Cu. Ni seemed to cause more oxidative stress to the seedlings than copper, which was manifested as a decreased GSH level and an increased proportion of GSSG in the Ni treatments. Copper together with nickel strongly decreased root growth, the root/shoot ratio being lowest in the Ni25Cu50 treatment.

SEVERE DEFOLIATION OF SCOTS PINE REDUCES REPRODUCTIVE INVESTMENT BY ECTOMYCORRHIZAL SYMBIONTS

Reduction in the photosynthetic capacity of plants is presumed to negatively affect their fungal symbionts. To test this hypothesis under natural conditions, we artificially removed 100% of previous year needles in two successive years on Scots pine trees (Pinus sylvestris L.) to simulate pine sawfly attack. Despite a decline in the shoot growth of defoliated trees, root biomass did not differ from control trees. The ergosterol (fungal biomass) and starch concentration of fine roots, however, slightly declined in defoliated trees. Percent ectomycorrhizal colonization of fine root tips remained high in both defoliated and control trees. The dominant tubercle morphotypes were slightly more abundant in the control than in defoliated trees. In contrast to the relatively weak effects on vegetative ectomycorrhizae, reproduction declined near the defoliated pines. Average sporocarp numbers and, consequently, the relative fungal investment to reproduction of the estimated total fungal biomass were more than three times higher near controls than defoliated trees in the first treatment year. Defoliation also reduced the diversity of ectomycorrhizal species producing sporocarps. Mutualistic fungal symbionts may thus alter their reproductive investment in response to restrictions on host resources. Because fungal biomass in the roots as well as colonization percentage remained unchanged, Scots pine evidently continues to invest in the maintenance of the symbiosis despite the reduced photosynthetic capacity due to defoliation.

Grazing tolerance of Gentianella amarella and other monocarpic herbs: why is tolerance highest at low damage levels?

Plants have adapted to compensate for the loss of vegetative biomass and reproductive potential caused by grazing. Shoot damage breaks down the correlative inhibition maintained by apical dominance. The consequent increased branching may lead to increased production of flowers and fruits in damaged plants, provided that enough resources, both in terms of meristems and nutrients, are available. In Gentianella amarella, the removal of the apex of the main stem (10% clipping) had no pronounced effect on branching and plant performance. In one of the two study populations, however, apically damaged plants produced more fruits than undamaged control plants. The plants also fully compensated for 50% removal of the main stem in terms of above-ground biomass, but their fruit production was reduced compared to control and apically damaged plants. After 75% clipping, fruit production was not significantly reduced compared to 50% clipping. Consequently, G. amarella showed highest tolerance in the presence of minor shoot damage. The pattern is qualitatively similar in some other monocarpic species (Gentianella campestris, Erysimum strictum and Rhinanthus minor). Multiple constraints as well as selective forces may shape these compensatory responses: (1) A lack of basal meristems may constrain tolerance of high damage levels. (2) Species with basal meristems may have a potential to tolerate major damage, but a shortage of resources or otherwise unfavourable growth conditions may constrain their compensatory ability. (3) It may be adaptive to have maximum tolerance of low and moderate damage levels if chemical defences reduce the risk of extensive shoot damage as well as the risk of repeated grazing. (4) The compensatory ability of monocarpic species may be affected by selective forces that favour fast vertical growth early in the season and unbranched architecture in undamaged conditions. Therefore, it is not the mere grazing history, but also other factors associated with growth conditions that are required to explain the variation in grazing tolerance.

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of Betula pubescens

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.

A test of the compensatory continuum: fertilization increases and below-ground competition decreases the grazing tolerance of tall wormseed mustard (Erysimum strictum).

Contrary to the general expectation, the compensatory continuum hypothesis proposes that grazing may not always affect plant performance adversely. Instead, the effects may vary from negative (undercompensation) to positive (overcompensation), depending on the local availability of resources and the intensity of competition experienced by individual plants. We tested this hypothesis in a common garden experiment by growing tall wormseed mustard, Erysimum strictum, under a factorial design involving simulated grazing (0, 10, or 50% of the main stem clipped), supplemental fertilization and below-ground competition. The results supported the hypothesis. On an average, fertilization increased and competition decreased plant performance. Overcompensation was only observed among the fertilized plants growing free of competition. Simulated grazing increased seed yield 1.6 (10% clipping) and 1.4 times (50% clipping) as compared to unclipped plants when the plants were grown with fertilization and without competition. In contrast, clipping did not significantly increase seed yield in the plants grown without fertilization and/or with competition. The breakage of apical dominance provides a proximate mechanism of these regrowth responses. This is consistent with the fact that most plants (85%) had an unbranched shoot architecture in our study population. However, it is not clear why E. strictum has a relatively unbranched architecture in natural populations. We briefly discuss the alternative ecological factors – competition for light, adaptation to herbivory and optimal timing of flowering as a bet-hedging strategy in monocarpic plants – which might maintain unbranched architecture in this species.

Restorative mowing on an abandoned semi‐natural meadow: short‐term and predicted long‐term effects

When management, in the form of cattle grazing and mowing, ceases the abundance of competitively superior plant species tends to increase in abandoned semi-natural meadows. Litter accumulation elevates the soil nutrient levels and hinders seedling recruitment. We surveyed changes in plant cover and species composition of a formerly grazed meadow in permanent plots for six years. Some plots were unmown, while others were mown and raked annually in August. The cover of grasses decreased and herb cover remained unchanged regardless of the treatment. Mowing and raking significantly reduced litter accumulation and increased the number of ground layer species. The expected long-term effects of abandonment and restorative mowing were studied by calculating the transition probabilities for unmown and mown plots and simulating the course of suc- cession as projected by the transition matrices. During a simulation period of 30 yr, abandonment led to (1) a de- crease in the cover of small herbs, (2) a slight increase in the cover of tall herbs and (3) a slight decrease in the cover of grasses. In contrast, the cover of small herbs on the mown plots remained unchanged or slightly increased during the course of simulation. These results suggest that mowing late in the season is primarily a management tool for the mainte- nance of the existing species diversity and composition. However, it may not be an effective restorative tool to induce overall changes in the resident vegetation of abandoned grass-dominated meadows. Grazing or mowing early in the season may be more effective in this respect. Consequently, mowing early or, alternatively, late in the season may pro- vide management strategies for the maintenance and restora- tion of species diversity, respectively.

Total vs. internal element concentrations in Scots pine needles along a sulphur and metal pollution gradient

Analysis of foliar elements is a commonly used method for studying tree nutrition and for monitoring the impacts of air pollutants on forest ecosystems. Interpretations based on the results of foliar element analysis may, however, be different in nutrition vs. monitoring studies. We studied the impacts of severe sulphur and metal (mainly Cu and Ni) pollution on the element concentrations (Al, Ca, Cu, Fe, K, Mg, Mn, Ni, P, Pb, S and Zn) in Scots pine (Pinus sylvestris L.) foliage along an airborne sulphur and metal pollution gradient. Emphasis was put on determining the contribution of air-borne particles that have accumulated on needle surfaces to the total foliage concentrations. A comparison of two soil extraction methods was carried out in order to obtain a reliable estimate of plant-available element concentrations in the soil. Element concentrations in the soil showed only a weak relationship with internal foliar concentrations. There were no clear differences between the total and internal needle S concentrations along the gradient, whereas at the plot closest to the metal smelter complex the total Cu concentrations in the youngest needles were 1.3-fold and Ni concentrations over 1.6-fold higher than the internal needle concentrations. Chloroform-extracted surface wax was found to have Ni and Cu concentrations of as high as 3000 and 600 microg/g of wax, respectively. Our results suggest that bioindicator studies (e.g. monitoring studies) may require different foliar analysis techniques from those used in studies on the nutritional status of trees.