Canals R.M., Durán M., San Emeterio L. and Múgica L.
Dpto. Producción Agraria, UPNA, Campus Arrosadia s/n, 31006 Pamplona, Spain;
rmcanals@unavarra.es
Abstract
Grassland systems have evolved under the effect of fire and herbivory. Disruption of a sustainable disturbance regime poses a serious threat to the maintenance of diversity and may lead to degradative processes. In Western Pyrenees, the native perennial tall-grass Brachypodium pinnatum is experiencing an increased expansion that is accompanied by a severe loss of sympatric species. As a consequence, low-diverse and stable covers develop which are very productive despite the prevailing stressful conditions (high-altitude, extremely acidic and poor soils, metals solubilised). In order to gain insight into the mechanisms explaining the temporal stability of degraded covers, we selected eight populations of the species developing within the same regional area, four in grazed-diverse and four in ungrazed-degraded covers, and analysed the aerial and subterranean dynamics of mineral and metal allocation during a growing season. The belowground reservoir played a key role in the translocation and reallocation of nutrients and in the accumulation of toxic metals (mainly aluminium), which made the species very competitive. The results are discussed focusing on the role of herbivory and on the build-up of the vigorous, complex belowground system.
Keywords: highlands, Brachypodium pinnatum, oligotrophic grasslands, nutrients, metals
Introduction
Open highland landscapes in Europe are experiencing deep changes linked to the global change and, particularly, to the rural exodus and land-use abandonment. In Pyrenees, shrub encroachment and afforestation are frequent processes occurring all over the range, due to the relaxation of grazing practices (low stocking rates and lack of guided grazing). As a consequence, the use of traditional tools to control grazers’ rejections, such as controlled burnings, is nowadays increasing in both intensity and frequency.
The expansion of the native tall-grass Brachypodium pinnatum in Western Pyrenees is likely related to the increased fire regime and decreased herbivory (Canals et al., 2015). The species grows usually in diverse chalk grasslands but conditions of change make it very competitive and aggressive to sympatric species (Bobbink and Willems, 1987; Hurst and John, 1999). The success of B. pinnatum gives rise to extensive areas with an herbaceous cover fully dominated by the species, which are stable and lack of successional processes towards shrub or tree encroachment, despite the absence of herbivores. This research was planned to understand the mechanisms explaining the persistence of these covers, which create a situation very difficult to revert. We analysed the performance of eight on-site populations of B. pinnatum growing in two contrasting covers, as a sympatric component of rich, grazed grasslands or as a dominant component of low-diverse, ungrazed covers. Both community types were similar in terms of abiotic factors – climate, substrate and soil – but differed in their recent historical management.
We studied the performance of the populations in terms of biomass production and analysed nutrients – nitrogen (N), phosphorus (P) and potassium (K) – and metal – aluminium (Al) and lead (Pb) – dynamics and compartmentalisation along a growing season.
Materials and methods
The research was done in Aezkoa valley commons (42°57’N 1°10´W and 43°3´N 1°13´W; 800-1,450 m a.s.l.). The area, included in the SIC Roncesvalles-Selva de Irati (ES0000126), encompasses a mosaic of beech forests, heathlands and grasslands and is dominated by a cold, rainy and misty climate (Tm=
9.3 °C; P=1,856 mm per year). Rangelands support nowadays a free mixed grazing 6 months per year with low stocking rates (<1.2 AU ha-1). Relaxed grazing has promoted in the last decades an uncontrolled use of fire in the most remote areas.
In spring 2013, we selected eight sites that encompassed the variety of B. pinnatum covers in the area:
four low-diverse, B. pinnatum dominant covers (80-100%) and four high-diverse communities (30-50% cover of B. pinnatum). Despite the contrasting grassland communities, soils were similar between covers for the main physical and chemical traits. Soils were very organic (9% OM) and acidic (pH in water=4.74) and developed on sandstones and calcareous clays. At each site, a sampling itinerary of 600-800 m was scheduled, in which 5 clumps dominated by B. pinnatum vegetation were selected. At each point, a 15×15×15 cm block of soil and vegetation was collected and kept cool. In the laboratory, aerial (necromass and green) and subterranean tissues (including roots and rhizomes) were separated from the soil. Tissues were oven dried, finely sieved and sent to the CEBAS-CSIC for main nutrients analyses.
N was determined by elemental combustion and the rest of elements by ICP-OES atomic absorption spectrophotometry (ICAP 6500 model, Thermo Electron IRIS Intrepid II XDL Duo). Field samplings and laboratorial procedures were repeated three times during the season, after snowmelt (late April), in midsummer (early July) and in autumn (late September). As a whole, 120 blocks were processed, 40 blocks per sampling date.
Statistical analyses used R software (RC Team, 2011). The models included cover type and sampling date as fixed factors and sampling point nested within site as random factor. A correlation structure to account for the repeated measures on the same sampling plot and/or a variance structure if the residual spread differed between sampling dates were included if necessary. Differences among LSM of sampling dates and comparisons between vegetation cover types within each sampling date were done by Tukey tests (lsmean package).
Results and discussion
Results show B. pinnatum as a high-productive grass, with a considerable belowground system formed by a dense network of rhizomes and roots which is most of its total biomass. Herbivory appears to play a key role in the above and belowground biomass dynamics. Early in season, stands in low-diverse, ungrazed covers averaged 400 g m-2 of necromass compared to 100 g m-2 in high-diverse, grazed covers.
The belowground biomass was much higher in stands of low-diverse covers (c. 850 g m-2) than in the rest of sites (c. 500 g m-2). Because of the absence of defoliation by grazing, low-diverse covers accumulated high amounts of aboveground tissues, which appeared to influence belowground patterns in the same way.
Nutrient concentrations in aboveground tissues varied temporally (Pdate<0.001 for all nutrients) and showed similar seasonal patterns, reaching a peak in spring and decreasing along the season (Table 1).
Nutrients in belowground tissues also varied temporally (Pdate<0.001 for N and K, Pdate<0.008 for P) showing the highest concentrations in spring. The results suggest that herbivory influenced the timing of internal nutrient regulation, and particularly the remobilization of nutrients to the rhizome, since different patterns of temporal translocation were found between covers. Aboveground concentrations of N were higher in high-diverse compared to low-diverse covers most of the time (Pcover=0.004, Pinteraction=0.051), whereas the concentrations of P and K slightly differed between covers, except at the
end of the season (Pinteraction=0.006 for P; Pinteraction<0.001 for K). As a consequence, at the end of the growing season, individuals in high-diverse, grazed covers have more nutrient-enriched aboveground tissues than individuals in low-diverse, ungrazed covers. Patterns of metal allocation in B. pinnatum were consistent for the two elements studied (Al and Pb) and differed radically from patterns of nutrient allocation. Belowground structures accumulated the highest quantity of metals (Table 1), that were translocated to aerial parts in small amounts along the season, (Pdate>0.001 for Al, Pdate>0.001 for Pb).
Conclusions
Altogether, the results suggest that belowground organs perform complex functions, not only as foraging structures and nutrient reservoirs but also as efficient sinks for the harmful metals that may solubilize in acid soils.
Acknowledgements
Financial support was provided by the Spanish Ministry of Science and Innovation (CGL 2010-21963 and CGL2011-29746).
References
Bobbink, R. and Willems, J.H. (1987) Increasing dominance of Brachypodium pinnatum (L) Beauv in chalk grasslands: a threat to a species-rich ecosystem. Biological Conservation 40, 301-314.
Canals, R.M., Pedro, J., Ruperez, E. and San-Emeterio, L. (2014) Nutrient pulses after prescribed winter fires and preferential patterns of N uptake may contribute to the expansion of Brachypodium pinnatum (L.) P. Beauv. in highland grasslands. Applied Vegetation Science 17, 419-428.
Hurst, A. and John, E. (1999) The biotic and abiotic changes associated with Brachypodium pinnatum dominance in chalk grassland in south-east England. Biological Conservation 88, 75-84.
RC Team (2011) R:A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.
Table 1. Nutrients (N, P, K) and metals (Al, Pb) concentrations in aboveground and belowground tissues of plants of B. pinnatum growing in low-diverse and high-diverse covers. In spring samplings, necromass (previous season material) was separated from green tissues (early season material). Values are the mean ± standard errors for 20 replicates.
Cover previous Aboveground Belowground
early mid late early mid late
N (%) low 0.9±0.04 2.68±0.05 2.02±0.06 1.39±0.06 1.01±0.04 0.82±0.03 0.94±0.04
high 1.10±0.04 2.82±0.09 2.28±0.05 1.75±0.06 1.11±0.04 0.80±0.03 0.86±0.03
P (%) low 0.03±0.002 0.16±0.01 0.12±0.01 0.07±0.003 0.05±0.003 0.05±0.002 0.06±0.003
high 0.04±0.001 0.16±0.01 0.14±0.01 0.10±0.01 0.07±0.01 0.05±0.01 0.06±0.01
K (%) low 0.17±0.01 1.86±0.05 1.60±0.11 0.84±0.04 0.17±0.01 1.86±0.05 1.60±0.11
high 0.25±0.02 1.67±0.06 1.53±0.08 1.07±0.06 0.67±0.04 0.52±0.05 0.41±0.03
Al (mg kg-1)
low 1,696.61±223.37 329.85±42.58 979.66±157.27 1,198.93±306.32 2,958.75±233.54 3,558.48±726.76 2,704.55±208.19 high 1,876.22±372.69 394.01±45.62 1,524.70±233.49 1,867.55±411.63 4,283.71±579.53 2,350.17±186.10 2,850.45±231.22 Pb
(mg kg-1)
low 2.99±0.22 0.61±0.07 0.91±0.13 2.08±0.24 4.52±0.38 4.14±0.65 5.07±0.74
high 2.94±0.30 0.56±0.05 1.22±0.19 2.62±0.34 4.94±0.84 2.83±0.30 3.93±0.32