Perturbation Experiments in Community Plant Species during Recovery from Agricultural Abandonment in a Semi-Arid Region of Central-Western Spain

Background: Can high diversity mixtures of later succession plant species be able to suppress early successional (arable weed) species than low diversity species mixtures? Can the removal of plant functional groups have important consequences on the sys-tem stability? Location: The study area was located at 850 m a.s.l., fifteen kilometers (15 km) to the west of Salamanca city, Spain, in a dehesa-like woodland. Methods: The field experiment was installed in Spring 1996 (May). The plots represented former agricultural lands cropped with (a rotation of) monocultures and abandoned right at the beginning of the experiment after the last crop was harvested in 1995. Before in-stalling plots, the fields were harrowed and equalized. The treatments (NC-natural colonization, LD-low diversity seed mixture and HD-high diversity seed mixture) were randomly allocated to the plots in each block. The experiment was organised accord-ing to a block design with five replicate blocks. Within each block, four plots measur-ing 10 × 10 m were marked out and each of the three treatments was randomly as-signed to one plot. All plots were separated by 2 m walkways. Results: The competitive ability of the sown species in different mixtures in LD, valued on the basis of their performance in HD, is a good predictor of their ability to suppress the weed crops spe-cies. This was also corroborated by the high relationship between the efficiency of the sown species in both treatments of species mixture (R2 = 0.51, F (1, 33) = 34.56, P < 0.001). Conclusions: The increase of colonizing species cover was significantly lower in LD, which again supported the increase of biomass in 1998, being also lower in HD. Therefore, this increase in the community is more prominent in LD treatments. The effect of sown species was not so important in this increase.

Can the Stepping Stone Enhance the Establishment, Competition and Distribution of Sown Grassland Species during Recovery on Ex-Arable Lands?

Most diversity restoration projects are not to improve diversity per se, but rather to enhance the presence and abundance of species that are characteristic of reference or target community. The use of Bromus inermis suppresses annual noxious grasses and increases the control of other-forb group these species are also noxious weeds;these may be substituted with another perennial species of the same functional group all through the whole experimental period, as it occurs with other perennial-forb Carduus tenuifolius. A field experiment was conducted on abandoned arable land with sown low and high diversity treatments and natural colonization following typical farming practice for the site. Experimental plots were installed on former agricultural land that had been cropped with (a rotation of) monocultures until the end of 1995. The experiment was organized according to a block design with five replicate blocks. An opposite trend was performed among the colonizer species, because the colonizer grasses were relegated by the high dominance of sown grasses. But at the same time, the sown grasses facilitated the dominance of other colonizer-forbs species;therefore its functional replacement in the community due to sown effect was again tested. However, in natural conditions the other-forbs group was the dominant group, without taking into account the stepping-stone treatment and there was also a functional change of dominance. Our study has demonstrated the restoration effectiveness of species richness at abandoned arable land and may be enhanced by sowing late successional species.