Aim: Global studies show strong climatic control of leaf area index (LAI) and related ecosystem properties. However, alternative states stabilized by positive feedbacks have the potential to produce appreciable differences in LAI in a given environment. On certain sites in New Zealand, ectomycorrhizal (EM) southern beech forest (Nothofagaceae) appear to form a stable alternative to more species‐rich conifer‐broadleaved forest dominated by arbuscular mycorrhizal (AM) trees. We asked which climate variables best predict LAI of New Zealand's remaining old‐growth forests, and if stands dominated by Nothofagaceae have lower LAI and better‐lit understories than conifer‐broadleaved forests, when climate and other site factors are held constant. Location: New Zealand. Taxon: Seventy‐two tree species belonging to 37 angiosperm, conifer and pteridophyte families, the most frequent occurrences being Cunoniaceae, Cyatheaceae, Dicksoniaceae, Lauraceae, Nothofagaceae, Podocarpaceae and Violaceae. Methods: We quantified overstorey composition of 45 old‐growth stands, and used the LAI‐2200C Plant Canopy Analyzer to measure LAI and understorey light. We used generalized linear mixed models to determine how these variables were influenced by environment and by dominance of Nothofagaceae. Results: LAI ranged from 3.5 to 6.9, and increased linearly with mean annual temperature (MAT); percentage light transmission to the understorey spanned an order of magnitude and decreased exponentially with increasing MAT. However, at a given MAT, LAI of stands dominated by Nothofagaceae averaged 0.8–0.9 units lower than conifer‐broadleaf stands, resulting in twofold higher understorey light levels in the former. Soil water deficits also significantly depressed LAI, and increased understorey light; atmospheric water deficits had the opposite effect. Main conclusions: Temperature is the best climatic predictor of LAI and understorey light in New Zealand's remaining old‐growth forests. However, the lower LAI and canopy light interception in southern beech stands presumably reflects the light‐demanding nature of Nothofagaceae, and their apparent ability to inhibit invasion by more shade‐tolerant AM trees with heavier crowns. This inhibitory effect may reflect reduced N supply to AM plants because of suppression of N mineralization by EM fungi, as observed in some EM systems elsewhere.