The carbon costs for host trees of a phloem‐feeding herbivore

Abstract

Summary Through its effect on plant productivity and fitness, herbivory can regulate forest diversity and ecosystem productivity. We used the interaction between phloem‐feeding scale insects [Ultracoelostoma assimile (Homoptera: Coelostomidiidae)] and the beech trees [Nothofagus solandri var. solandri (Hook f.) Oerst.] they feed on to estimate directly the carbon costs of herbivory for a forest tree species over two consecutive years. The scale insects live on the bark of tree trunks and branches, and excrete readily collected droplets of carbon‐rich honeydew – this allows us to make an estimate of the carbon cost of herbivory that is not complicated by feeding damage to leaves and buds. Canopy photosynthesis (A) was estimated with an environmentally driven process‐based model, developed with parameters derived from measurements of photosynthesis on trees with (HD+) and without (HD–) honeydew scale insects during year 1. Honeydew dry matter production (PHD) was estimated from values for air temperature and humidity, and accounting for variation in honeydew production due to host‐tree diameter. The carbon cost of scale insect herbivory, CHD, was estimated as PHD/AHD+× 100%, where AHD+is canopy photosynthesis for trees with scale insects. Over the two years annual canopy photosynthesis at the site was 1720 g C m−2 ground area. In spring–summer, values for parameters describing photosynthesis (Vcmax, the maximum rate of rubisco‐catalysed RuBP carboxylation, and Jmax, the rate of electron transport when irradiance is saturating) were 44% and 66% higher, respectively, in infested, as opposed to uninfested, trees. Consequently, for equivalent basal area, annual canopy photosynthesis was 4% greater for infested trees. Assuming that honeydew dry matter was 42% carbon by mass, annual honeydew production was 59 g C m−2 ground area, which represents 4% of infested‐tree gross photosynthesis (c. 8% of net photosynthesis). Daily values of honeydew production were highest on cloudy days in summer, when warm humid conditions resulted in high values for PHD but cloud cover reduced AHD+; on these days CHD could be > 50% of AHD+. Values for PHD are remarkably similar to the differences between AHD+ and AHD–, suggesting that HD+ trees may be able to compensate fully for the loss of carbon to herbivory. Our modelling results thus suggest that while the honeydew insects harvest relatively large amounts of carbohydrate from their host trees, the consequences of this for tree growth and reproduction may be small.