Short- and Long-Term Vegetation Change Related to Grazing Systems, Precipitation, and Mesquite Cover

Abstract

Rangeland scientists struggle with how long rangeland experiments must continue in order to detect treatment effects, particularly in semiarid ecosystems characterized by slow responses and high spatiotemporal variability. We compared changes in eight grass and three shrub categories to grazing systems (yearlong vs. seasonal rotation with equivalent long-term stocking rates), and covariates (precipitation and mesquite [Prosopis velutina] gradients) over 12 yr (1972-1984) and 34 yr (1972-2006) on the Santa Rita Experimental Range, Arizona. We used split-plot analysis of variance, with year as the split, to make these comparisons. Grazing systems did not influence plant dynamics as shown by the lack of grazing system by year effect on all response variables in either time period. The absence of a detectable grazing effect on vegetation changes may be due to overriding influences of grazing intensity, pasture size, precipitation variability, and few replicates. Also, more time may be needed to detect the small accumulating and potentially temporary effects from grazing systems. The grazing system main effects present at the beginning and throughout the study suggest that pastures assigned to each grazing system had different potentials to support vegetation. Nearly twice the number of response variables were related to the precipitation covariate than to mesquite cover, but only about half of all the relationships were consistent between time periods. The struggle to know how long to observe before detecting a grazing system effect was not resolved with the additional 22 yr of observation because we cannot definitively reject that either more time is needed to detect small but cumulative effects or that the two grazing systems are not different. Resumen Científicos en la ciencia de manejo de pastizales tienen dificultades al determinar la extensión de los proyectos sobre manejo de pastizales de forma que se puedan determinar los efectos debido a los tratamientos, especialmente en ecosistemas de zonas áridas que se caracterizan por una respuesta demasiado lenta y una alta variabilidad a nivel de espacio y tiempo. Se comparó los cambios de ocho gramíneas y tres arbustivas entre diferentes sistemas de pastoreo (continuo Vs. rotacional estacional, con cargas animales similares a largo plazo). Se utilizó la precipitación y el Mesquite (Prosopis velutina) como covariables sobre 12 añ os (1972-1984) y 34 añ os (1972-2006), en la estación experimental de Santa Rita, en Arizona. Se usó un análisis de varianza de parcelas-dividas utilizando los añ os como parcelas, para llevar a cabo estas comparaciones. Los sistemas de pastoreo no afectaron la dinámica de las plantas como pudo observarse por no haber efecto de añ o para todas las variables de respuesta en ninguno de los dos períodos. La falta detección de los efectos del pastoreo en los cambios de la vegetación se puede haber debido a los efectos de la intensidad de pastoreo, tamañ o de los potreros, variación en la precipitación y el resultado de pocas repeticiones. Además ...

Integrating precipitation, grazing, past effects and interactions in long-term vegetation change

“…This finding is consistent with studies from drier climates (Fuhlendorf and Smeins, 1997;Fuhlendorf et al, 2001;Yao et al, 2006;Mashiri et al, 2008;Butterfield et al, 2010;Moran et al, 2014). Plot, grazing and precipitation had the largest coefficients, but the consistency with which past effects appear highlights their importance.…”

“…In addition, the use of plots as the experimental unit instead of grazing management unit presents a case of pseudo-replication (Hurlbert, 1984). Access to permanent plots in a greater number of grazing units, as in other studies (Mashiri et al, 2008), would have been preferred but was unavailable. Given the large allotment sizes (2000e22,000 ha) and dispersion of our plots we feel enough heterogeneity in soil type, elevation (2308e2810 m) and precipitation (325e485 mm year À1 ) was captured to interpret our results despite probable statistical dependence in both time and space of the plots.…”

“…For instance, state-and-transition models focus on disturbance as drivers of change ignoring the contribution of climate and other internal ecosystem factors (Briske et al, 2003). However, inter-annual climatic variation is often considered a controlling factor in vegetation change, even more so than management (Fynn and O'Connor, 2000;West, 2003b;Mashiri et al, 2008). Rare or extreme climatic events can drive change in semi-arid systems Walker, 1993;Holmgren and Scheffer, 2001) or in some cases the convergence of multiple rare events are responsible (Wiegand and Milton, 1996).…”

Spatial patterns of grassland–shrubland state transitions: a 74‐year record on grazed and protected areas

“…Year-round grazing was practiced from 1916-1972, with stocking rates steadily decreasing from a maximum of 0.17 animal unit years per hectare in 1918 (Browning et al 2008). A rotational grazing system was implemented in 1972 and maintained through the time of our study (see Mashiri et al 2008).…”

Investigating vegetation biophysical and spectral parameters for detecting light to moderate grazing effects: a case study in mixed grass prairie

“…The absence of significant differences in green grass cover between UG and G1, and standing dead cover between UG and G2 indicates that despite the importance of grazing intensity in determine the magnitude of grazing impacts, the length of grazing treatment may also contribute to the outcome of interaction between herbivore and vegetation. Masbiri et al [60] indicated that two criteria must be met for grazing effects to be detected: the effects must be larger than the variability in the system and they must reach this size during the period of observation. Compared to G3, the length of grazing treatment in the G1 and G2 sites are relatively short.…”