Natural Regrowth Of Vegetation Research Articles

Sesbania Tree Fallows on Phosphorus‐Deficient Sites: Maize Yield and Financial Benefit

AbstractRotation of Sesbania sesban (L.) Merr., a fast‐growing N2‐fixing tree, with maize (Zea mays L.) has potential for increasing fertility of tropical soils, where fertilizer use by resource‐poor farmers is limited. At two sites in Kenya (Ochinga, with a Kandiudalfic Eutrudox soil, and Muange, with a Kandic Paleustalf), we compared maize yields and financial returns for (i) sesbania grown for three or four seasons followed by three maize crops (sesbania fallow), (ii) one maize crop followed by natural regrowth of vegetation for three seasons and then three maize crops (natural fallow), and (iii) maize monoculture for seven seasons. After the fallows, plots were split with and without added P. Maize responded to P at both sites. Cumulative grain yields for seven seasons of maize monoculture were 8.4 Mg ha−1 at Ochinga and 5.6 Mg ha−1 at Muange. They were comparable to cumulative maize yields for sesbania fallow (Ochinga, 10.6 Mg ha−1 Muange, 4.5 Mg ha−1) and natural fallow (Ochinga, 7.7 Mg ha−1; Muange, 4.2 Mg ha−1), even though maize was grown for only three or four seasons in the fallow treatments. Sesbania fallow was financially attractive at Ochinga (≥500 mm rain in each season) but not at Muange, where low rainfall (<300 mm in each postfallow season) limited maize yield. Phosphorus fertilization of maize at Ochinga increased (P < 0.2) net benefit for sesbania fallow. Improved fallows have potential to supply nutrients to crops, but they are unlikely to eliminate the need for P fertilizers on P‐deficient soils.

Soil nitrogen availability as affected by fallow-maize systems on two soils in Kenya

Simple methods for the measurement of nitrogen (N) availability are needed to assess the effect of low-input, organically based land management systems on the N supply of tropical soils. Our objectives were to determine the effect of contrasting land-use systems (LUS) on soil N availability and to identify measures of N availability that correlated with maize (Zea mays L.) grain yield. The LUS at the two sites in Kenya involved growth of a maize crop following 17 months of either: (1) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (2) natural regrowth of vegetation without cultivation (natural fallow), (3) three crops of unfertilized maize (maize monoculture), or (4) bare uncultivated soil (bare fallow). Soil was collected before the post-fallow maize crop was sown. The LUS had no effect on total soil N or amount of N in the heavy fraction soil organic matter (SOM) (>150 μm, >1.37 Mg m–3). Sesbania and natural fallows, as compared to maize monoculture, increased the N in light fraction SOM (>150 μm, 150 μm, 1.13 to 1.37 Mg m–3), ammonium-N and aerobic N mineralization at a depth of 0–15 cm. Maize yields were highest following the sesbania fallow. Nitrate-N, inorganic-N (ammonium plus nitrate) and anaerobic N mineralization correlated with maize grain yield at both sites. The relationship between maize yield and pre-season nitrate-N improved when the depth of soil sampling was increased to 1 m at one site (an Alfisol), but did not improve at the site with anion adsorption in the subsoil (an Oxisol). The sesbania fallow was more effective than the natural fallow in increasing available soil N. Maize yield was better related to pre-season nitrate than N in size-density fractions of SOM.

Succession from Heather Moorland to Birch Woodland. III. Seed Availability, Germination and Early Growth

SUMMARY (1) This paper examines seed input, germination and early growth of species found during the succession from heather moorland to mature birch woodland. (2) Numbers of seeds of Calluna vulgaris in the seed rain decreased rapidly with increasing age of birch stands. Large numbers of Calluna seeds were found, however, in soil beneath all ages of birch as well as the open moor, illustrating the longevity of seeds of this species. (3) Large numbers of Deschampsia flexuosa seeds arrived in the seed rain but very few germinated from the soil, indicating dependence on immediate seed regeneration, unlike Agrostis capillaris which had large, persistent soil seed stores. (4) The presence of seeds of Anthoxanthum odoratum, Agrostis capillaris and Rumex acetosella in seed rain and soil seed stores showed that the absence of these species from open moorland and young birch was not due to unavailability of propagules. (5) Germination of the species studied was apparently not affected by stand age, with the exception of Agrostis capillaris and Cerastium holosteoides which showed reduced germination on the open moor. Early growth of most species was faster, however, beneath the birch than on the open moor and improved beneath increasing ages of birch. (6) The natural regrowth of vegetation in the cleared plots was faster beneath the older birch than the young birch or the open moor. (7) The implications of differences in seed availability, germination and early growth are discussed in relation to observations and experiments on species changes beneath developing birch reported in the first two papers of this series.

Sediment Losses from Forest Management: Mechanical vs. Chemical Site Preparation After Clearcutting

AbstractThe comparative effects of mechanical and chemical site preparation on water yields and sediment losses following forest clearcutting were evaluated over a 4‐yr period in the Athens Plateau area of southwestern Arkansas. After 1 yr of pretreatment measurements, three forested watersheds were clearcut and the residual vegetation and debris were sheared and windrowed but not burned. Three watersheds were clearcut in a similar manner, but received chemical site preparation. Residual trees on two watersheds were injected with 2–4,D amine; the third watershed was aerially sprayed with a mixture of Tordon® (active ingredient; picloram [4‐amino‐3,5,6‐trichloropicoline acid]) and Garlon® (active ingredient; triclopyr [3,5,6‐trichloro‐2‐pyridinyloxyacetic acid]). Three additional watersheds were left undisturbed for controls. Mean annual sediment losses on the mechanically, site prepared watersheds during the first posttreatment year were significantly higher than those from either the chemically site prepared watersheds or controls. Chemical site preparation did not significantly increase sediment losses. Although 2nd yr losses for the mechanical site preparation and control treatments doubled over 1st‐yr levels, no significant treatment effect was detected for either site preparation treatment. Third‐year losses decreased below 1st‐yr losses for all treatments but not to pretreatment year levels. The relatively sharp declines in sediment losses during the third posttreatment year were attributed to rapid regrowth of natural vegetation on the sites.