UNIVERSITY OF NEVADA COOPERATIVE EXTENSION

 

Nevada Rangeland Monitoring Handbook | Third Edition


Appendix C - Weather Variability

Climate and weather must be considered for the interpretation of monitoring. In arid regions especially, timing and effectiveness of precipitation, which can vary by season and size of each precipitation event, is an important climatic factor that must be considered as changes are evaluated. The bottom line for plants is the soil moisture (and soil temperature) during their thermal growing season. Drought, along with fires and unusually wet conditions of flooding or prolonged rapid plant growth, are common reasons why flexibility in management is so important.

Drought

Drought is defined in a number of ways (NOAA 2012), but is often described as a series of years when low rainfall and moderate to high temperatures exceed some average. Drought may be considered as a period of abnormally dry weather sufficiently prolonged for the lack of water to cause a significant reduction in plant growing conditions and productivity or a serious hydrologic imbalance in the affected area. These two effects can happen simultaneously or, either can happen in the absence of the effects of the other. Each has different management implications. Extreme drought may or may not modify the structure of rangelands by changing vegetation composition. However, in a summer-dry climate such as most of Nevada, moisture limitations end the growing season for most rangeland plants every year. Plants express growth and phenology to reflect the limited amount and duration of soil moisture. No two droughts are the same, so the management response to drought should vary to reflect the unique conditions of the current drought.

The management of plants before, during and after drought influences the impact of drought and rate of plant recovery following relief from drought. Drought may or may not modify ecological processes by influencing species composition, biomass production, nutrient cycling and soil properties. Understanding how individual plant species respond to drought, and how ecological processes are affected by drought, informs flexibility in management and interpretation of monitoring data.

Monitoring helps managers detect, record and understand drought effects and separate the respective influences of drought and management. Plants that may have had time to recover after grazing may not have soil moisture to do so. Observations on growing conditions may lead to altered management within the season to minimize impact to range plants.

Furthermore, the level of use often increases during drought unless management changes are implemented. This may be especially evident near riparian areas where use is concentrated because intermittent streams or stream reaches have dried up early. Conversely, upstream or downstream areas without water may receive less or shorter use. Also, the physiological effect of grazing on dormant plants after soil dehydration is much reduced from grazing effects while plants are still growing.

Careful management in a post drought growing season may be especially important for recovery after the stress of drought. Hence there is a need to track where drought induced management stress is or will be located so that managers can avoid or mitigate it. Maintaining short-term monitoring records through droughts helps interpret long-term monitoring data.

Very Wet Years

Very wet years may represent an even more significant challenge than drought. In the past four decades, Nevada has had six cycles of many large fires in the year(s) following very wet conditions that allowed abundant fine fuels production. Residual fuels, as well as the abundance of litter that facilitates cheatgrass production, perpetuate the risk of fire. Then expansive dry lightning caused more fire starts than could be successfully controlled. Abundant highly flammable fine fuels (often combined with accumulated woody fuels) across a landscape allow the uncontrolled fires to get very large (mega-fires) before eventual containment. Where the fine fuels cause connectivity among woody fuels, the resulting hot fire may cause excess perennial plant mortality. Sagebrush is not fire tolerant, and these large fires after wet years is perhaps the biggest issue for sagebrush ecosystems and associated wildlife, as well as multiple other land uses.

To address the abundance of fine fuels after wet years, it is helpful to recognize the abundant plant growth early to enable flexibility in management such as temporary nonrenewable (TNR) grazing and targeted grazing to create linear fuel breaks. Plans should be developed before the wet years to monitor the abundance of plant growth in the wet springs for the purpose of triggering criteria-based follow-up management. Monitoring of fuel breaks is also important. See Monitoring Fuel Breaks side bar.