DOI: 10.5061/dryad.c866t1g5x
title: What drives temporal stability of biomass production? Testing the roles of species diversity, dominance, asynchrony and spatial scale in annual plant communities

description: Aims: Primary biomass production is a fundamental process for ecosystem
 functioning. Yet, little is known on the mechanisms driving temporal
 stability of biomass production in annual plant communities, particularly
 in communities subjected to highly variable environments and undergoing
 temporal changes in species composition. We aimed to disentangle the
 relative importance of biomass production, species diversity, dominance
 and asynchrony of species fluctuations as drivers of biomass stability in
 Mediterranean and semiarid annual plant communities. Location:
 Mediterranean (N31o42’; E35o03’) and Semiarid (N31o23’; E34o54’) sites,
 Israel. Methods: Aboveground biomass and species abundance were monitored
 in 15 plots of 250m2 per site during eight consecutive years.
 Relationships between stability drivers and community stability were
 studied at the regional (between-sites) and local (within-sites) spatial
 scales. Results: Community biomass stability (temporal mean/SD) increased
 from the Semiarid to the Mediterranean site concomitantly with higher
 biomass production, richness, and evenness, but was not associated with
 changes in species synchrony. Differences in stability between sites were
 due to opposite effects of site conditions on the mean and SD of community
 biomass, leading to higher stability in the Mediterranean site. Within
 sites, species asynchrony was the key driver of stability at the local
 spatial-scale. Richness and biomass production affected stability
 indirectly through asynchrony, but in different ways at each site. At the
 Mediterranean site, these factors had indirect negative effects on
 stability by reducing asynchrony, but did not rescind a positive effect of
 asynchrony on community stability. At the Semiarid site, biomass
 production had indirect positive effects on stability through asynchrony,
 while richness had no effect on asynchrony and stability. Stability was
 not driven by species evenness in either site.  Conclusions: Our
 study provides new insights into the complex control of biomass stability
 in the dynamics of Mediterranean and semiarid annual plant communities,
 with different mechanisms driving stability across the regional vs. local
 spatial scales. 
description: <b> Study
 sites </b> Study sites were located in
 contrasting ecosystems: Mediterranean (Matta LTER, N
 31<sup>o</sup>42’; E 35<sup>o</sup>03’) and
 Semiarid (N 31<sup>o</sup>23’; E
 34<sup>o</sup>54’). The climate in both sites is
 typical Mediterranean, with hot dry summers and mild wet winters,
 characterized by high rainfall variability which increases with
 diminishing rainfall. Average rainfall for the hydrological year (October
 to September) during the experimental period (seasons 2001-2002 to
 2009-2010) was 510 mm (CV 25%, range 413-799 mm) and 222 mm (CV 33%, range
 132-377 mm) in the Mediterranean and Semiarid sites, respectively. Soil at
 the Mediterranean site is Terra Rossa, while light brown Rendzina prevails
 at the Semiarid site. The length of the growing season
 is determined by the distribution of rainfall, commencing in
 October-November and ending in April-May. The landscape in both sites is a
 sparse, stony shrubland, with the spiny dwarf-shrub <i>Sarcopoterium
 spinosum</i> (L.) Spach as the dominant shrub. The herbaceous
 vegetation emerging at the onset of the rainy season in open patches
 between the shrubs is dominated by annual species, and is a main forage
 source for livestock. Shrubs contribute much less to biomass production
 due to their lower cover and slower growth. Both sites were subjected to
 livestock grazing, mainly sheep and goats, before the onset of the
 experiment, with higher grazing intensity in the Semiarid site.
 <b>2.2- Sampling and data
 collection</b>
 We sampled the herbaceous
 vegetation at peak biomass – late March in the semiarid site
 and mid-April in the Mediterranean site, during growth seasons 2001-2002
 to 2009-2010, except for season 2004-2005. The 2001–2002 season was not
 included in the stability analyses to avoid immediate disturbance effects
 caused by the initial setting the rain manipulation treatments. Ten random
 samples (20 x 20 cm quadrats) of the herbaceous vegetation were taken in
 the open patches in each one of the 15 plots. This quadrat size is
 commonly used to study Mediterranean herbaceous communities
 (Osem, Kigel and Perevolotski, 2002; Peco, Sanchez and
 Azcarate, 2006) and is appropriate for the small range at which
 interactions occur between herbaceous plants in both sites. To avoid edge
 effects, samples were taken at least 1 m away from plot boundaries. Plant
 roots were cut just below the soil surface to avoid plant fragmentation
 and allow counting of individual plants to assess plant density. Plant
 species were identified and their aboveground biomass per quadrat was
 weighed after 48 h oven drying (75<sup>o</sup>C). Since the
 growth season is short in both sites, biomass loss by litter production
 prior to sampling is relatively small. Thus, the average aboveground
 biomass per unit area at peak season can be taken as a proxy of the actual
 annual primary production (ANPP) in each site. Temporal variation over
 years in density and biomass of individual species and in community
 biomass (i.e. sum of species biomasses) was assessed for each plot, by
 averaging the 10 samples taken in each growing season. Mean biomass of
 individual plants at the community level was obtained by dividing quadrat
 biomass by total number of plants in the quadrat. Species
 nomenclature follows Feinbrun-Dothan and Danin
 (1998).