Process studies in aPinus radiata-pasture agroforestry system in a subhumid temperature environment. II. Analysis of dry matter yields in the third year

Abstract

In this paper we analyzed the dry matter yields (DM) produced in an agroforestry trial consisting of pine trees grown over (1) Control (bare ground), (2) ryegrass/clovers (Lolium perene/Trifolium spp.), (3) ryegrass only, and (4) lucerne (Medicago sativa) during the third growing season between 1992 and 1993. In addition these pastures were grown alone in separate plots in the open. The results show that:

• 1. Pasture yields during the one-year period in the agroforestry plots were little affected by the presence of trees although there were seasonal trends: yields were generally unaffected or increased in summer, but reduced in spring as a result of tree shade. Total pasture yield during the one-year period was in the following order: lucerne > ryegrass/clovers > ryegrass. This trend was the exact opposite of that shown by the above-ground tree weight which was reduced in the pasture ground covers by between 16 and 52%. The reduction in tree weights was associated with reduced soil moisture availability arising from competition with the pasture species. Thus the relationship between the yields of trees and pasture species in the agroforestry plots was reciprocal.

• 2. The combined DM for both the trees and pastures in the agroforestry plots during the one-year period was in the following order: lucerne (20 t ha⁻¹) > ryegrass/clovers (16 t) > ryegrass (11 t) > control (6 t) which was consistent with the total water used and photosynthetically active radiation (PAR) intercepted. The trees accounted for 55, 44, 34 and 24% of water used respectively in control, ryegrass, ryegras/cclovers and lucerne ground covers. The balance was largely accounted for by pasture transpiration, except in the control where soil evaporation was significant. The fraction of intercepted PAR accounted for by the trees followed a similar trend to that of the water used.

• 3. The crop coefficient for water use efficiency (k) (Pa) was 2.3 for radiata pine, 3.6 for ryegrass/clovers, 2.8 for ryegrass and 4.8 for lucerne. The DM produced per unit of water used (kg mm⁻¹ ha⁻¹) during the one-year period was 24.5 for radiata pine, 41.1 for lucerne, 30.0 for ryegrass/clovers and 23.1 for ryegrass. Radiation use efficiency (g DM MJ⁻¹ m⁻²) was 1.33 for pine, 1.83 for ryegrass/clovers, 1.49 for ryegrass and 2.34 for lucerne.

• 4. The land equivalent ratio (LER), i.e. the sum of ratios of DM produced by the plant species in the agroforestry plots to those produced in the sole plots, was 1.95 for ryegrass, 1.71 for ryegrass/clovers and 1.45 for lucerne ground covers.

• 5. These results indicate the susceptibility of pasture species to shading and interception of rainfall by the tree crowns (aboveground interactions), and the trees to competition for soil moisture (underground interaction).