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Competition for soil water and nitrogen between Holcus lanatus L. and young Eucalyptus globulus Labill.


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Adams, PR 2004 , 'Competition for soil water and nitrogen between Holcus lanatus L. and young Eucalyptus globulus Labill.', PhD thesis, University of Tasmania.

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Competition for below-ground resources was investigated between young Eucalyptus
globulus Labill. (Tasmanian blue gum) and Holcus lanatus L. (Yorkshire fog grass)
in a plantationin SE Tasmania, Australia. The objectives of the study were (1) to
measure the competitive effect of weeds on the growth of E. globulus under
contrasting levels of soil water and nitrogen availability and (2) to determine the
'critical period' of competition for soil water and nitrogen in a developing stand of E.
globulus. This information lead to improved understanding of the mechanisms of
competition between H. lanatus and young E. globulus.
A two hectare field experiment was established on a low rainfall (500 mm yr-1 ), ex-pasture
site with a sandy soil of low to moderate fertility. Two levels of both
irrigation and nitrogen (N) fertiliser (urea) were applied to provide contrasting soil
water and nitrogen levels. A range of periods of weed presence and absence
(following the Critical Period method) were used in specific combinations of water
and nitrogen availability. Weedy plots were rapidly dominated by H. lanatus, a very competitive grass, that had
up to 100% cover and grass biomass production of 9 - 14 t ha-1 one year after planting
E. globulus. There were strong responses to nitrogen fertiliser in the weedy
treatments, but there were only small responses to irrigation, regardless of weed
status. Lateral root growth of E. globulus was severely restricted by the vigorous
development of grass roots that completely dominated the surface soil horizons. After two years, E. globulus height and diameter growth was up to 3 and 4.8 times
greater, respectively, in weed-free plots compared to trees growing with grass. Where
grass was present, there was no tree growth response to level of irrigation, but
diameter was 1.8 times greater due to N-fertilisation, which indicated more severe
competition for N in the unfertilised treatment, even though grass growth was also
less vigorous in this treatment. Growth recovery after the application of weed control
was also slower in the unfertilised treatment, and was influenced by season, with
faster tree recovery following spring compared to autumn weed removal. Although
there was no significant growth response to irrigation, the response to nitrogen in
weedy treatments was stronger with the higher level of irrigation indicating a water
by nitrogen interaction in the surface soil. The critical period for grass control at this site was from planting to age 20 months to
avoid growth reduction caused by grass competition. There was no early period after
planting when weeds could be tolerated by the young eucalypts. Most of the growth
suppression occurred during the first 12 months, while maximum diameter growth
(80%) was approached after approximately one year of weed control. These effects
on growth were still evident 4.4 years after planting.
Grass growth reduced surface soil water content during the first year and mild to
moderate tree water stress (ψmax< - 0.55 MPa) occurred on a number of occasions
during the first 18 months of growth. There was high availability of water for plant
use in the top 1 m of soil for most of the period, reflecting the overriding influence of
the shallow water table at the site. However, in one block on the drier part of the site,
available water was reduced by up to 80% due to grass presence during the first year,
and this area exhibited the greatest degree of water stress.
Soil nitrogen was strongly affected by grass presence, with NH4 + concentrations
higher under grass, while NO3- was lower. Rates of nitrification and leaching were
high in the surface soil in weed-free conditions. The application of nitrogen fertiliser
( NHigh) caused rapid increases in the concentrations of NH4+ and NO3 - in soil solution
especially in weed-free plots, with greater increases after spring compared to autumn
application. Both NH4+ and NO3- concentrations decreased to low values (<0.10 mM)
during the first winter after planting (age six to nine months) except in Nhigh weed free
plots. Thereafter, mineral N concentrations were variable with wide fluctuations
that were probably related mostly to season, temperature, moisture and grass
presence. Concentrations of N in foliage reflected levels of soil available nitrogen. Trees grown
with grass had half the foliar nitrogen concentration of trees growing under weed-free
conditions early in the first year. Concentrations increased following the addition of
nitrogen fertiliser, and were equivalent to those growing under weed-free conditions
by the end of the first year. The increased nitrogen status did not compensate for the
lost growth. Vector analysis demonstrated that the competitive effects were mediated
by water and nitrogen.
Evidence from this research suggests that competition between H. lanatus and young
E. globulus at this site was mainly for nitrogen, and to a lesser extent soil water
during the first 12 — 18 months of tree growth. Where water was non-limiting in
surface soil, competition for nitrogen was high, while in the drier part of the site,
both water and nitrogen were limiting in the presence of grass. The use of the critical
period method has provided a definition of the period when grass should be
controlled to avoid E. globulus growth losses and has indicated the potential
importance of an interaction between nutrient and water availability to the expression
of the response.
This research indicates weed control requirements for E. globulus on ex-pasture sites
can be determined on the basis of site fertility and moisture regime. In this way
operations can be tailored on a soil and site basis. The trade-off will be increased
management complexity, however, improved knowledge and understanding of these
factors enhances more effective and efficient plantation weed management. The
extension of this approach to other species (weeds and trees) is also possible.

Item Type: Thesis - PhD
Authors/Creators:Adams, PR
Keywords: Eucalyptus globulus, Holcus, Weeds
Copyright Holders: The Author
Copyright Information:

Copyright 2004 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2004. Includes bibliographical references

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