Long-term forb dynamics in semi-arid grasslands of the Sevilleta LTER, northern Chihuahuan Desert, New Mexico, USA.

J.M. Mulhouse* and S.L. Collins *mulhouse@sevilleta.unm.edu Sevilleta LTER, Department of Biology, University of New Mexico, NM 87131, United States

ACKNOWLEDGEMENTSThanks to the US Fish and Wildlife Service, numerous field crew members, and Doug Moore for meteorological data. This research was supported by NSF grant DEB-0620482 to the University of New Mexico for Long-term Ecological Research.

INTRODUCTIONAs the climate of the Southwestern U.S. changes and semi-arid grasslands are particularly affected by higher temperatures and lower precipitation (IPCC, 2007), an understanding of forb communities will allow us to better predict how overall vegetation composition might change.

For this 22-year study within a remote wildlife refuge in the northern Chihuahuan Desert questions regarding the forb community included: 1.) How have vegetation characteristics varied?; 2.) Which taxa have been most common? and; 3.) How has moisture driven dynamics?

FIELD METHODSIn 1989, two 400-m transects were established 2 km apart along a north-south orientation. One transect was designated Five Points (FP) and the other Deep Well (DW). Transects were sampled each spring (April-June) and fall (September-November) through 2010. Groundcover was assessed at 1 cm intervals and, when a forb was detected, species identity was recorded, as was the distance along the transect which that individual occupied. Precipitation was recorded at event scale via tipping bucket at two climate stations between 600 m and 3 km from the transects.

DATA ANALYSISData was summarized by season to determine species richness, number of individuals, and transect distance occupied by forbs. The percent of years in which a species occurred, species represented by the largest number of individuals, and species occupying the greatest distance were also calculated. Differences were tested for between transects and across seasons using paired t-tests. When data was non-normal, a Wilcoxon two sample signed-rank test was utilized. P-values were Bonferroni-corrected to avoid type 1 errors (p≤0.002).

Relationships between richness, occurrence, or distance and precipitation or time were modeled using linear regression. Spring data was modeled using non-monsoon (October-May) precipitation while fall was modeled using the monsoon period (June-September). Residuals were plotted to confirm normalcy. P-values were Bonferroni-corrected to avoid spurious significant results (p≤0.004).

RESULTSSeventy-three forb species were identified. The highest seasonal richness was 22 (FP, spring 2004); a richness of 1 was recorded in fall 2009 at DW following wildfire (Fig. 1). The number of individuals was highly variable, with the greatest (952) recorded in fall 1996 at FP; two were detected at DW following the fire. With inter-annual variability ranging from 0.05-68.6 m, mean transect distance occupied by forbs was consistently low (Figure 1). There were no differences in vegetation variables between transects or seasons. Less than 10% of taxa were present in ≥20 years, about 20% were detected in ≥15 years, and 60%+ were observed in ≤10 years (Fig. 2). The most common taxa were tansyaster (Machaeranthera spp.) and globemallow (Sphaeralcea spp.), observed every year at high levels. Of the 17 taxa recorded in ≥50% of sampling in a season, only 8 ever exceeded 70%, despite 12 being perennials. Linear regression detected relationships between precipitation and richness, number of individuals, and distance occupied for all spring comparisons except one (Table 1). Fall produced no significant relationships, nor did time.

DISCUSSIONIn Chihuahuan Desert grasslands, forb abundance is low relative to perennial grasses. In this study, while the forb species pool was large, seasonal richness was low, illustrating forbs ephemerality. A study of flowering plants at the Sevilleta NWR found average year-to-year species turnover of 71.4% (Wetherill et al. in revision), affirming this lack of persistence. Further, relationships between vegetation and time weren’t significant, indicating no consistent accumulation or decline. This stable cycle of extirpation and establishment has been noted in grasslands where short-lived species successively occupy the same niche (van der Maarel and Sykes, 1993). The absence of a relationship between forbs and precipitation pulses has also been suggested (Peters & Yao 2012) and it’s likely that other vegetation outcompetes forbs. Analyses of normalized difference vegetation index (NDVI) variability has found correlations between monsoon precipitation and “greenness” at the Sevilleta NWR, presumably due to shrub and perennial grass growth (Weiss et al. 2004).

CONCLUSIONAs forbs seem to be negatively impacted by grass and shrub competition during the monsoon, it’s unclear how they might respond to a warmer, drier climate. Such a shift might be deleterious to grasses at their southern extent (e.g. blue grama), releasing forbs from competition. Or, black grama, as well as some shrubs, might expand north (see: Muldavin 2002), increasing competitive pressure. It’s also possible that forbs may simply decline with less rain and warmer temperatures. Such questions will only be addressed by continued long-term research.

Globemallow bloom, Sevilleta NWR, central New Mexico, USA.

LITERATURE CITEDIPCC, 2007. Contribution of Working Groups I, II & III to the 4th Assessment Report of the

Intergovernmental Panel on Climate Change. Pachauri, R.K., Reisinger, A. (Eds.), IPCC, Geneva, Switzerland, pp 104.

Muldavin, E.H. 2002. Some floristic characteristics of the northern Chihuahuan Desert: a search for its northern boundary. Taxon 51, 453-462.

Peters, D., Yao, J. 2012. Long-term experimental loss of foundation species: consequences for dynamics at ecotones across heterogeneous landscapes. Ecosphere. in press.

van der Maarel, E., Sykes, M.T. 1993. Small-scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept. J. Veg. Sci. 4, 179-188.

Weiss, J.L. et al. 2004. Seasonal and inter-annual relationships between vegetation and climate in central New Mexico, USA. J. Arid. Environ. 57, 507-534.

Wetherill, K.R. et al. 2012. Turnover and predictability of flower communities across two deserts and mountains: a pollinator’s perspective. in revision.

Chihuahuan Desert Grassland, Sevilleta NWR, central New Mexico, USA.

Surveying the forb community along a 400-m transect at the Sevilleta NWR.