Nutrient use efficiency in evergreen and deciduous species from heathlands

The nutrient (N, P) use efficiency (NUE: g g^-1 nutrient), measured for the entire plant, of field populations of the evergreen shrubs Erica tetralix (in a wet heathland) and Calluna vulgaris (in a dry heathland) and the deciduous grass Molinia caerulea (both in a wet and a dry heathland) was compared. Erica and Calluna are crowded out by Molinia when nutrient availability increases. NUE was measured as the product of the mean residence time of a unit of nutrient in the population (MRT: yr) and nutrient productivity (A: annual productivity per unit of nutrient in the population, g g^-1 nutrient yr^-1. It was hypothesized that 1) in low-nutrient habitats selection is on features leading to a high MRT, whereas in high-nutrient habitats selection is on features leading to a high A; and that 2) due to evolutionary trade-offs plants cannot combine genotypically determined features which maximize both components of NUE.Both total productivity and litter production of the Molinia populations exceeded that of both evergreens about three-fold. Nitrogen and phosphorus resorption from senescing shoots was much lower in the evergreens compared with Molinia. In a split-root experiment no nutrient resorption from senescing roots was observed. Nutrient concentrations in the litter were equal for all species, except for litter P-concentration of Molinia at the wet site. Both Erica and Calluna had a long mean residence time of both nitrogen and phosphorus and a low nitrogen and phosphorus productivity. The Molinia populations showed a shorter mean residence time of N and P and a higher N- and P-productivity. These patterns resulted in an equal nitrogen use efficiency and an almost equal phosphorus use efficiency for the species under study. However, when only aboveground NUE was considered the Molinia populations had a much higher NUE than the evergreens.The results are consistent with the hypotheses. Thus, the low potential growth rate of species from low-nutrient habitats is probably the consequence of their nutrient conserving strategy rather than a feature on which direct selection takes place in these habitats.

The effect of water and nitrogen amendments on photosynthesis, leaf demography, and resource-use efficiency in Larrea tridentata, a desert evergreen shrub

In the Chihuahuan Desert of southern New Mexico, both water and nitrogen limit the primary productivity of Larrea tridentata, a xerophytic evergreen shrub. Net photosynthesis was positively correlated to leaf N, but only in plants that received supplemental water. Nutrient-use efficiency, defined as photosynthetic carbon gain per unit N invested in leaf tissue, declined with increasing leaf N. However, water-use efficiency, defined as the ratio of photosynthesis to transpiration, increased with increasing leaf N, and thus these two measures of resource-use efficiency were inversely correlated. Resorption efficiency was not significantly altered over the nutrient gradient, nor was it affected by irrigation treatments. Leaf longevity decreased significantly with fertilization although the absolute magnitude of this decrease was fairly small, in part due to a large background of insect-induced mortality. Age-specific gas exchange measurements support the hypothesis that leaf aging represents a redistribution of resources, rather than actual deterioration or declining resource-use efficiency.

The effect of increased nutrient availability on leaf turnover and aboveground productivity of two evergreen ericaceous shrubs

Leaf turnover and aboveground productivity in relation to nutrient availability were studied in the evergreen shrubs Erica tetralix and Calluna vulgaris. In monospecific stands of these species four levels of nutrient (NPK) availability were created during three growing seasons. Percentage survival and life expectancy of Erica leaves decreased with increasing nutrient availability. For Calluna there was no effect. Winter mortality of Erica leaves was smaller than growing season mortality. These was no difference for Calluna. The timing of leaf mortality of both species was not affected by nutrient treatment. At the end of the experimental period current year leaf biomass, total biomass and current year second year and third year biomass of both species showed a significant increase with increasing nutrient availability. The relative increase was greater for Calluna, except for second and third year biomass. Stem production and stem mortality of both species increased with increasing nutrient availability. The increased stem mortality resulted also for Calluna in an increased leaf turnover (per unit ground area) with increasing nutrient availability. Nutrient cycling in ecosystems dominated by these species will increase with increasing nutrient availability, because of increased leaf and stem turnover and productivity. This phenotypic effect is similar to the effect of the shift in dominance between different species which occurs along natural gradients of nutrient availability.

The effect of nitrogen and phosphorus addition on the growth, reproduction, and nutrient dynamics of two ericaceous shrubs

Aboveground growth, reproduction, and foliar nitrogen and phosphorus contents of two ericaceous shrub species were compared over two seasons in (a) an undisturbed shrub bog (pocosin), and (b) a factorial fertilization design in which three levels each of nitrogen and phosphorus were added in all possible combinations. One species, Zenobia pulverulenta, is deciduous whereas the other species, Lyonia lucida, is evergreen. In the nutrient-poor undisturbed pocosin the two species exhibited similar foliar nitrogen and phosphorus concentrations and aboveground growth rates. Neither species flowered. In response to nutrient-addition Zenobia increased growth rates more than Lyonia. Foliar phosphorus concentrations of both species increased in response to enhanced phosphorus availability. in the first season neither species flowered in any treatment. In the second season Zenobia flowered only in the fertilized plots, with the most flowering in the high phosphorus treatments. I conclude that, by virtue of high growth rates and efficient use of nutrients and despite differences in leaf phenology and morphology, both Lyonia and Zenobia are successful in a competitive community under conditions of extremely low phosphorus availability. However, unlike Lyonia, Zenobia can take advantage of temporarily increased nutrient availability, which occurs following fire in the pocosin, to increase growth and reproduction.

Mineral nutrition and leaf longevity in Ledum palustre: the role of individual nutrients and the timing of leaf mortality

The effects of fertilization on leaf longevity and leaf mortality in the Alaskan evergreen shrub, Ledum palustre (Ait.) Hult., were investigated in a field experiment. The fertilization treatments included N alone, P alone, N plus P, and N plus P plus K. After 5 years all treatments had the same effect on leaf longevity, decreasing life expectancy from about 2 years in controls to 1-1.5 years in the fertilized plants. In the NPK-fertilized plants, most of the decrease in leaf longevity was due to increased winter leaf mortality; fertilization actually decreased leaf losses during the growing season. The results are consistent with previous research suggesting that one function of overwintering evergreen leaves is to serve as nutrient storage organs, a function that is superfluous when nutrient supplies for new growth can be obtained from current uptake.

Construction and maintenance costs of mediterranean-climate evergreen and deciduous leaves I. Growth and CO2 exchange analysis

Gas exchange and leaf growth analysis were used in conjunction to determine leaf-construction and maintenance costs in three co-occurring shrubs of the california chaparral, one evergreen, Heteromeles arbutifolia, and two drougth deciduous species, Lepechinia calycina, and Diplacus aurantiacus. The construction costs per unit of leaf weight were similar among the three species and very close to values reported for other evergreens but considerably higher than leaf construction costs for other deciduous or herbaceous plants. Maintenance costs per unit of leaf weight were significantly greater in one deciduous species, L. calycina, than in the evergreen. Maintenance costs for all species were in the range reported for herbaceous species and considerably above those reported for other evergreens.

Mineral nutrition and leaf longevity in an evergreen shrub, Ledum palustre ssp. decumbens

Effects of variable mineral nutrient status on evergreen leaf longevity were investigated in a field fertilization experiment, and by comparison of plants from several sites. The species studied was Ledum palustre spp. decumbens, with a normal leaf life expectancy of 2.06 years and a maximum leaf longevity of 4.5-5.0 years. Most leaf losses took place during the growing season, not during the winter. Fertilization increased leaf production but decreased leaf survivorship. Total number of leaves per stem was unchanged with fertilization. In a comparison among sites, there was a moderate negative correlation between plant N and P concentrations and leaf longevity. These intraspecific responses are similar to known interspecific changes along nutrient gradients, i.e. with high nutrient availability a vegetation should become more "deciduous" and less "evergreen".