Net Primary Productivity
Net primary production (NPP) is a fundamental ecological variable describing rates of carbon consumption and fixation. Estimates of NPP are important in understanding the spatial and temporal responses of communities to a wide range of ecological processes including decomposition, fertility, and producer-consumer dynamics. Net primary production is influenced by many factors, including temperature, soil nutrient content, soil texture, pollination, herbivory, and granivory. In semi-arid environments, NPP is most highly correlated with precipitation and soil moisture and may become decoupled from other processes in the wake of highly variable climatic conditions. Thus, a variety of projects have been initiated within the Sevilleta National Wildlife Refuge (SNWR) investigating the impacts of plant community type, soil nutrient content, disturbance, nighttime warming, and monsoon precipitation patterns on NPP.
Core: In 1999, a long-term study was established at the Sevilleta LTER to monitor NPP across four distinct ecosystems: L. tridentata (creosote) dominant shrubland (C), Bouteloua eriopoda (black grama) dominant grassland (G), B. gracilis (blue grama) dominant grassland (B), Pinus edulis/Juniperus monosperma (Pinyon/Juniper) woodland (P).
Fertilizer: In December 1995, a second NPP study was initiated. This study examines fertilization effects on NPP in a semi-arid mixed-grassland. The study includes twenty experimental plots, of which half are fertilized and half are not. Fertilizer is applied in the spring and fall as granular NH4NO3.
Burn: In 2003, the U.S. Fish and Wildlife Service conducted a prescribed burn over a large portion of the northeast corner of the SNWR. A study was designed to look at the effects of this fire on NPP in three different vegetation types: A mixed grass (MG) area dominated by B. eriopoda and B. gracilis with occurrences of Sporobolus spp. (dropseed) and Pleuraphis jamesii (galleta grass); a mixed shrub (MS) area dominated by L. tridenta but with a relatively dense understory of B. eriopoda; and a grassland dominated by B. eriopoda (G) with occurrences of Sporobolus spp. Forty quadrats were set up in both burned and unburned sections of each habitat type.
Warming: In 2006, a project was installed at the SNWR to better understand the potential effects of environmental change on grassland community composition and the germination and growth of shrub seeds and seedlings. Thus, the focus is on the response of B. eriopoda, B. gracilis, and L. tridenta, all of which are near their range margins, to environmental change, including fluctuations in precipitation, increased soil nutrient content, and higher nighttime tempartures. This project was designed as a multi-factorial experiment with three fully crossed factors: nighttime warming, water addition, and nitrogen addition. For addional information , please see the entry for this project under "Current Research Projects."
Monsoon: Finally, also in 2006, the Monsoon Rainfall Manipulation Experiment (MRME) study was initiated. The study is designed to enable the quantification of changes in the structure and function of a semi-arid grassland ecosystem caused by increased rainfall variability. It is predicted that changes in the size and variability of precipitation events will have differential effects on grassland dynamics, including the invasion of L. tridentata. While an increased frequency of small rainfall events might increase germination, such events may not result in the soil moisture retention needed to aid establishment. Conversely, following a small number of large events, growth of vegetation may be enhanced through deep soil moisture even while germination is not. The "MONSOON" study delivers moisture to plots as a series of small events occurring relatively frequently and a fewer number of large events, in addition to ambient controls. For additional information , please see the entry for this project under "Current Research Projects."
Cerro Montoso Pinyon-Juniper Dendrometer, Height, and Crown Area Measurements: Allometry is a standard method of determining biomass and Net Primary Production of many trees. One of the standard variables used in such allometric regressions is bole diameter. On straight trunk trees measurements at breast height (DBH) taken using a DBH tape is adequate for quantifying changes in diameter over time. However, in scrub forests such as the Pinon Juniper PJ woodlands, common on the Sevilleta, both the pinon and particularly the junipers are relatively short and multi-trunked so that measurements must be taken near the ground and the consistency between measurements often lead to erroneous growth analysis. To reduce such discrepancy in readings, dendrometer bands were installed on 20 pinons and 20 junipers in the Cerro Montoso area where understory ANPP has been measured for some time. These dendrometers quantify the expansion (and contraction) of the tree bole through time. The beginning diameter at the time of installation was also measured and recorded. Heights of the 40 trees were also measured. The diameter of the foliage was measured across these trees at the widest point and then again on an axis perpendicular to this first reading. Readings of the dendrometers are repeated on about a monthly basis through the growing season. Heights of the trees are done on an annual basis. Measurements are collected to quantify the increase in bole diameter for a set of Pinons and Junipers in this area over time.
Point-Quarter Distance and Dimension Measurements to Calculate Shrub Density and Estimate Shrub ANPP: In an effort to better quantify NPP of Creosotebush in the Five-Points region, it was decided to test the Point-Quarter method against the standard 1-m2 quadrat method that has been in use since 1998. Transects were laid out across the 5 mammal trapping webs as well as across burned and unburned plots of the Mixed Shrub site (MS). Repeated measures of the same bushes are performed seasonally. Whole shrubs of various size classes are collected, sorted, and weighed to develop regressions for biomass.