Seed‐bank structure and plant‐recruitment conditions regulate the dynamics of a grassland‐shrubland Chihuahuan ecotone

Abstract

(2016) 'Seed-bank structure and plant-recruitment conditions regulate the dynamics of a grassland-shrubland Chihuahuan ecotone. ', Ecology., 97 (9). pp. 2303-2318. Further information on publisher's website:http://dx.doi.org/10.1002/ecy.1446Publisher's copyright statement:Copyright by the Ecological Society of America Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract. Large areas of desert grasslands in the southwestern United States have shifted to sparse shrublands dominated by drought-tolerant woody species over the last 150 yr, accompanied by accelerated soil erosion. An important step toward the understanding of patterns in species dominance and vegetation change at desert grassland-shrubland transitions is the study of environmental limitations imposed by the shrub-encroachment phenomenon on plant establishment. Here, we analyze the structure of soil seed banks, environmental limitations for seed germination (i.e., soil-water availability and temperature), and simulated seedling emergence and early establishment of dominant species (black grama, Bouteloua eriopoda, and creosotebush, Larrea tridentata) across a Chihuahuan grassland-shrubland ecotone (Sevilleta National Wildlife Refuge, New Mexico, USA). Average viable seed density in soils across the ecotone is generally low (200-400 seeds/ m 2 ), although is largely concentrated in densely vegetated areas (with peaks up to 800-1,200 seeds/m 2 in vegetated patches). Species composition in the seed bank is strongly affected by shrub encroachment, with seed densities of grass species sharply decreasing in shrubdominated sites. Environmental conditions for seed germination and seedling emergence are synchronized with the summer monsoon. Soil-moisture conditions for seedling establishment of B. eriopoda take place with a recurrence interval ranging between 5 and 8 yr for grassland and shrubland sites, respectively, and are favored by strong monsoonal precipitation. Limited L. tridentata seed dispersal and a narrow range of rainfall conditions for early seedling establishment (50-100 mm for five to six consecutive weeks) constrain shrub-recruitment pulses to localized and episodic decadal events (9-25 yr recurrence intervals) generally associated with late-summer rainfall. Re-establishment of B. eriopoda in areas now domi nated by L. tridentata is strongly limited by the lack of seeds and decreased plant-available soil moisture for seedling establishment.

Regional grassland productivity responses to precipitation during multiyear above‐ and below‐average rainfall periods

“…This transition results from long-term livestock overgrazing and multiyear drought in arid grasslands (Bestelmeyer et al, 2011), compared to high PPT and reduced incidence of fire in mesic grasslands (Briggs et al, 2005;Knapp et al, 2008). In semiarid warm grasslands, an increase in the number or length of below-average periods would likely increase their vulnerability to desertification, which requires sustained drought combined with additional disturbances such as fire and soil erosion (Moreno-de las Heras, Turnbull, & Wainwright, 2016;Peters, Herrick, Monger, & Huang, 2010;Van Auken, 2009). In semiarid warm grasslands, an increase in the number or length of below-average periods would likely increase their vulnerability to desertification, which requires sustained drought combined with additional disturbances such as fire and soil erosion (Moreno-de las Heras, Turnbull, & Wainwright, 2016;Peters, Herrick, Monger, & Huang, 2010;Van Auken, 2009).…”

Local‐regional similarity in drylands increases during multiyear wet and dry periods and in response to extreme events

“…, Moreno‐de las Heras et al. ). The recovery time for desert grasslands following degradation events is not yet fully clear (Drewa et al.…”

“…Additionally, large grassland areas in the Chihuahuan Desert region have been degraded over the past 100 yr due to multiple disturbances (U.S. Bureau of Land Management 2017), reducing vegetative cover and NDVI (Alvarez et al 2011, Browning andSteele 2013). Severe degradation events at JRN occurred following overgrazing in the late 1800s and early 1900s and during a severe drought in the 1950s (Gosz and Gosz 1996, Bestelmeyer et al 2011, Moreno-de las Heras et al 2016). The recovery time for desert grasslands following degradation events is not yet fully clear (Drewa et al 2006, Parmenter 2008, and requisite soil stability is actually still increasing at many JRN locations.…”

Minimal mortality and rapid recovery of the dominant shrub Larrea tridentata following an extreme cold event in the northern Chihuahuan Desert

“…Even though L. tridentata seed viability can be low (McGee & Marshall, 1993) and successful seedling establishment is rare (Ackerman, 1979), seed production following the stressful winter provided the potential for seedling establishment and shrubland expansion. (Ackerman, 1979;Moreno-de las Heras, Turnbull, & Wainwright, 2016;Reynolds et al, 1999). Therefore, although one event may have limited impacts on shrub expansion, clustered cold events, such as the several cold events between 1930s and 1950s ( Figure 3c), could both decrease mature shrub performance and kill seedlings.…”