James O. Klemmedson

Effect of mesquite on physical and chemical properties of the soil.

Highlight: Soil under the crown of mesquite trees was compared to soil from adjacent openings at three depths for several physical and chemical properties near Tucson, Ariz. Bulk density was lower in soil under mesquite but increased with depth in that location. Organic matter, total nitrogen, total sulfur, and total soluble salts were up to three times greater in the surface 0 to 4.5 cm of mesquite soil than in open soil but declined with increasing depth to levels approximately the same as in open soil. Total potassium was higher under mesquite but increased with depth. Total phosphorus and hydrogen ion concentrations were the same in soil under mesquite as in soil from open areas. Results suggest that mesquite trees function to improve soil conditions under their canopies by redistribution of nutrient ions from areas beyond the canopy to areas beneath the canopy. This process helps to explain the greater abundance and improved growth of perennial grasses observed under mesquite. It also helps to explain grazing patterns and responses on desert grassland. Mesquite (Prosopis juliflora (Swartz.) DC.) is a native, deciduous, leguminous tree that has invaded vast areas of desert grassland range in the southwestern United States since the early 1900’s. Because of a lateral root system that spreads to distances of 10 m or more (Kearney and Peebles, 1960), mesquite competes with perennial grasses for moisture in natural openings. This results in reduced forage production and increased soil erosion. Effectiveness of mesquite in extraction of soil moisture and in competition with perennial grasses was demonstrated by Parker and Martin (1952). They found significant increases in moisture content of the upper 45 cm of soil at distances of 3, 6 and 10 m from killed mesquite trees compared with live trees. Also, soil moisture was available for a longer period of time where mesquite trees were killed than where they were alive. In comparison of surface soil properties between native desert grassland and invading mesquite stands, Paulsen (1950) found that native grassland soil had a more favorable physical regime and a higher nutrient capital than soil from the mesquite stands. However, an aspect of competition between mesquite trees and perennial grasses that has not been examined is the role of mesquite in altering physical and chemical properties of the

Solution of forest health problems with prescribed fire: are forest productivity and wildlife at risk?

Advanced forest succession and associated accumulations of forest biomass in the Blue Mountains of Oregon and Washington and Intermountain area have led to increased vulnerability of these forests to insects, diseases, and wildfire. One proposed solution is large-scale conversion of these forests to seral conditions that emulate those assumed to exist before European settlement: open-spaced stands (ca. 50 trees per ha), consisting primarily of ponderosa pine (Pinus ponderosa Laws.) and western larch (Larix occidentalis Nutt.). We question how well presettlement forest conditions are understood and the feasibility and desirability of conversion to a seral state that represents those conditions. Current and future expectations of forest outputs and values are far different from those at presettlement times. Emphasis on prescribed fire for achieving and maintaining this conversion raises questions about how well we understand fire effects on forest resources and values. We consider here potential effects of prescribed fire on two key aspects of forest management—productivity and wildlife. Use of large-scale prescribed fire presents complex problems with potential long-term effects on forest resources. Before implementing prescribed fire widely, we need to understand the range of its effects on all resources and values. Rather than attempting to convert forests to poorly described and understood presettlement seral conditions, it would seem prudent to examine present forest conditions and assess their potential to provide desired resource outputs and values. Once this is achieved, the full complement of forest management tools and strategies, including prescribed fire, should be used to accomplish the desired objectives. We suggest a more conservative approach until prescribed fire effects are better understood. # 2000 Elsevier Science B.V. All rights reserved.

Effect of mesquite trees on vegetation and soils in the desert grassland.

Highlight: Studies were conducted in the mesquite-desert grassland to assess effects of shade, roots, and litter of mesquite trees on understory vegetation and microenvironmental factors. Elimination of mesquite shade and root action increased foliar cover of understory vegetation in the canopy zone from 19% with intact mesquite to 24%. Replacement of mesquite shade with artificial shade screens further increased understory vegetative cover to 32%. Only forbs responded to elimination of mesquite roots in open areas. Vegetation responses indicated improved soil moisture in the canopy zone with both treatments, but there were no detectable soil moisture differences among treatments during the major part of the growing season. Greater vegetal cover with noshade and artificial shade treatments was apparently associated with differential utilization of moisture compared with the mesquite shade treatment. Increased soil moisture made available by mesquite removal and in excess of that lost by evaporation was reflected in greater vegetative cover. With artificial shade, potential evaporation was similar to that for natural shade-thus increased moisture was utilized for growth of understory vegetation.

Decomposition and nutrient release from mixtures of Gambel oak and ponderosa pine leaf litter

Abstract Studies were carried out to test the hypothesis that decay and release of nutrients of ponderosa pine (Pinus ponderosa Dougl. ex Laws) is accelerated in litter mixtures with Gambel oak (Quercus gambilii Nutt.). Litterbags containing different proportions of pine needles and oak leaves were placed in pine stands and collected over a 2-year period. At the end of the experiment mass losses from pure oak were 60% higher than those from pure pine with no evidence that the intrinsic rate of dry matter loss was affected in mixtures of litter. Litter type significantly influenced the concentration of all nutrients studied and amount of nutrients remaining for all nutrients except N and Ca. Increasing amounts of oak caused N, S, Ca and Mg to be retained more so than in 100% pine litter. Relative mobility of K and P were unaffected by litter composition. Nutrient release patterns were similar for N, S and Ca with little or no loss over the study period. Nutrient release patterns for P, Mg and K were similar with marked release (56 to 82% loss) over the two-year period. Possible reasons for lack of a synergistic effect between pine and oak are discussed.

Responses of desert grassland vegetation to mesquite removal and regrowth

Abstract The purpose of this study was to determine the long-term response of understory vegetation in the desert grassland of southeastern Arizona, USA, to removal and regrowth of mesquite Prosopis juliflora (Swartz) DC. var. velutina (Wooton) Sarg. trees. The study involved 3 treatments applied to mesquite in 1966; mesquite left intact (MI), mesquite removed (MR), and mesquite removed, sprouted, and regrown (MRS). Vegetation responses to 2 litter treatments, litter intact (LI) and litter removed (LR), also were examined. Cover of understory vegetation and juvenile mesquite (< 1.5 m height) were measured in canopy and open (intercanopy) locations. Major changes between 1967 and 1991 were increased cover of juvenile mesquite, shrubs, halfshrubs, bush muhly Muhlenbergia porteri Scribn., and Lehmann lovegrass Eragrostis lehmanniana Nees., and a decline in cover of Arizona cottontop Digitaria californica (Benth.) Chase, and plains bristlegrass Setaria macrostachya H.B.K. Arizona cottontop, plains bristlegras...

Effect of prescribed fire on nitrogen and phosphorus in Arizona chaparral soil‐plant systems

Abstract To improve understanding of how prescribed fire affects N and P in chaparral soil‐plant systems, a study was conducted to compare the distribution and availability of N and P in soil‐plant systems before and after a burn. Distribution of N and P in biomass components, litter, and soil was measured for five Cercocarpus betuloides and eight Quercus turbinella soil‐plant systems and compared to the distribution of biomass, N, and P in the same 13 soil‐plant systems following a prescribed burn. Individuals of C. betuloides accumulated more dry matter, N, and P in their shoots, litter, and upper 2 cm of mineral soil than those of Q. turbinella. Burning resulted in a loss of dry matter and N and a redistribution of N and P in soil‐plant systems of both species. Nutrient availability of soil collected under the 13 shrubs before and after the burn was assessed using a pot culture technique with barley as a test plant. The availability of P was greater in postburn soils than in preburn soils, while availa...

Soil organic matter in arid and semiarid ecosystems: Sources, accumulation, and distribution

Abstract Accumulation of organic carbon and formation of humus in arid and semi‐arid soils are reviewed with special attention to sources and distribution of organic matter, factors affecting humus formation and accumulation, transfer of organic debris to soil organic matter, and organic matter management. The character of vegetation is a major factor determining rate of organic decomposition and the quality, quantity, and distribution of soil organic matter, but the influence of other soil‐forming factors is sufficient to complicate the pattern and process of humus formation. Over a broad scale moisture and temperature are especially important. Patterns of soil organic matter distribution and litter decomposition are characteristic for arid and semiarid environments; they vary with the state of the system but involve complex interrelations. Trends in soil humus research and approaches needed to answer crucial ecological questions are reviewed.

Dendrochronological analysis of single-tree interactions in mixed pine-oak stands of central Arizona, USA

Abstract Radial growth of ponderosa pine after stand establishment was dendrochronologically analyzed in relation to varying amounts of Gambel oak presence on the Beaver Creek Experimental Watershed, Arizona. Climate, topography, soil parent material, vegetation structure and soil type were uniform across selected sites. Intra- and inter-specific competition affecting individual pines was quantified using an age-independent index, also modified for asymmetric competition. Pine mean ring width and mean ring area were regressed against competition indices, pine age, and height index after accounting for sites and plots in sites. Dendrochronological techniques provided accurate high resolution data, which were used to reconstruct the growth history of individual pines and stands. Site interactions were never significant, and fitted models explained from 50 to 66% of the total variance. Regression statistics were best when competition indices assumed symmetric interactions. Mean radial growth of pine over 5 and 10 years was negatively related to intra-specific competition, but was not significantly related to inter-specific competition between pine and oak. This could be attributed to the positive influence of oak on soil nutrient status and/or to increased pine spacing caused by the presence of oak.

Aspect and species influences on nitrogen and phosphorus accumulation in Arizona chaparral soil‐plant systems

Abstract To improve understanding of nutrient relationships and factors affecting nutrient patterns in chaparral ecosystems a study was conducted to quantify the effect of aspect and shrub species on accumulation of N and P in a mature chaparral ecosystem. Components of shrub biomass, litter, and soil were sampled from 32 randomly selected soil‐plant systems, eight each of Cercocarpus betuloides and Quercus turbinella on north and south aspects, and analyzed for N and P. Shrub species influenced dry matter accumulation, with Cercocarpus accumulating more biomass in leaves, stems, and litter than Quercus. Similarly, N concentration and accumulation were greater in Cercocarpus systems than in Quercus systems. This was attributed to the ability of Cercocarpus to form symbiotic relationships with actinomycetes capable of N fixation. Phosphorus accumulation was much greater in Cercocarpus biomass than in Quercus. Aspect significantly influenced P concentration of soil, with higher concentrations occurring on n...

Nodulation and nitrogen accretion response of Cercocarpus betuloides seedlings to phosphorus supplementation and water availability

Greenhouse experiments were conducted to assess the effect of phosphorus (P) supplementation and water availability on incidence of nodulation and nitrogen (N) fixation by Cercocarpus betuloides seedlings. Phosphorus supplementation resulted in a two-fold increase in shoot and total dry matter of seedlings grown in a P-deficient soil. Seedlings grown in P-supplemented soil supported a larger number (4.8 vs. 0.3) and fresh mass (0.179 vs. 0.009 g) of nodules than did controls. Nitrogenase activity (acetylene reduction), N concentration and N accretion were greater for seedlings grown in P-amended soil than for controls.Seedlings were grown at four soil water potentials −0.1, −0.2, −0.4, and −0.7 MPa for 214 days. Biomass production, nodulation, acetylene reduction activity and N accretion were greatest at −0.2 MPa and declined at higher and lower soil water potentials. Independently-measured dependent variables were highly correlated, suggesting that water availability was influencing a primary plant process, most likely photosynthesis, on which other plant processes are dependent.