Abstract: We examined the relationships between biomass or growth rates and leaf characteristics of 2-year-old trees of two clones of Populus. Leaf characteristics were: total plant leaf area or leaf weight, mean size (or weight) of fully expanded terminal leaves, and foliar concentrations and contents of N, P, K, Ca, Mg, total chlorophyll and total available carbohydrates. Sample trees (n=156) were chosen from two irrigation regimes and several fertilization treatments to provide a wide range in environmental conditions and growth rates for each clone. Total plant leaf area or weight were strongly correlated with total above-ground biomass (r=0.98 - 0.99); however, mean size (area or weight) of the fully expanded terminal leaves was also quite strongly correlated with biomass (0.64 - 0.72), height growth (0.54 - 0.72) and diameter growth (0.53 - 0.73). With one exception (correlation between foliar K concentration and height growth of one clone, r=0.67), leaf size characteristics were more strongly correlated with biomass or growth than were concentrations or contents of foliar chemicals. Since size of the terminal leaves is easy to measure, it may be useful as a simple indicator of potential productivity.

Introduction

It has long been recognized that plants growing under substantial soil moisture or nutrient stress have smaller leaves and lower growth rates than plants of the same genotype growing under more favorable conditions. In addition, rapid production of leaf area appears to be an important attribute of fast-growing plants. Previous work on Populus has shown that: (1) mean leaf size per clone and clonal performance are correlated (Ridge et al. 1986, Isebrands et al. 1988, Ceulemans 1990), (2) mean leaf size per clone increases as ortet location becomes more mesic and the expression of this relationship is greater at a more mesic test site (Dunlap et al. 1995) and (3) mean leaf size and leaf growth rates were greater in irrigated than non-irrigated trees (Roden et al. 1990). This past work generally involved relatively few samples (2 to 5 trees) per clone and did not examine within-clone variation in leaf size and productivity.

In this study we examined the relationships between growth rates or attained size and leaf characteristics of 2-year-old trees of two Populus clones. We concentrated on evaluating the size, weight and selected chemical characteristics of the fully expanded leaves produced on the current terminal shoot. In young, fast-growing, short-rotation, intensively cultured plantings, leaves on the current terminal are considered to be the most important suppliers of photosynthate for height and diameter growth (Isebrands and Nelson 1983). In addition, we examined the relationship of total plant leaf area or weight to total above-ground biomass, diameter growth, and height growth.

Materials and Methods

Unrooted cuttings of two Populus clones were planted at 2- x 2-m spacing in alternate rows in a 0.7 ha block near Olympia, Washington, USA. The clones were 11-11, a Populus deltoides Bartr. ex Marsh x Populus trichocarpa Torr. & Gray hybrid and 7-75, a Populus trichocarpa selection from a natural stand near Orting, WA (approximately 40 km from the study area). Both clones were developed (or selected) by the University of Washington/Washington State University Poplar Research Program (Quinsey et al. 1991). The study area is relatively flat, elevation is 50 m and the soil is Nisqually loamy fine sand (sandy, mixed, mesic Pachic Xerumbrepts), a very deep, somewhat excessively drained soil which formed in sandy glacial outwash (USDA Soil Conservation Service 1990). Four plots in each of three contiguous blocks were randomly assigned to an irrigation regime (low or high) and 4-tree subplots were assigned to a fertilization treatments which applied varying amounts of N (0 to 500 kg N ha-1 as ammonium nitrate), and P (0 to 1000 kg P ha-1 as triple superphosphate), K (0 to 1000 kg K ha-1 as muriate of potash), and 0 to 10 t lime ha -1. The fertilization subplot treatments were laid out in a continuous function design (Shoulders and Tiarks 1983). Irrigation was applied with a drip system; during the 2nd growing season (May 1 through August 30), the low regime received 14 cm and the high regime 51 cm of rainfall plus irrigation.

Samples for this study were collected from selected fertilizer subplot treatments to represent the range of nutrient environments established in the plantation. Equal numbers of trees per clone and irrigation regime were sampled in each treatment to provide a balanced data set (n=156; 39 for each clone and irrigation regime). The data set also encompassed the range of tree sizes and current growth rates occurring in the plantation. Fully expanded terminal leaves (leaf plastochron index ³ 6 per Larson and Isebrands 1971) were collected from 2-year-old trees during the last week of August. Most of the annual growth and nutrient uptake had occurred and the foliage had not yet deteriorated. Sampling was done in early morning and consisted of 3 to 4 leaves per sample tree.

Immediately after harvest, fresh weight, leaf area, and number of leaves per sample tree were determined. Chlorophyll a and b were extracted from the blade portions of a subsample of fresh leaves by maceration in 80% acetone, optical densities measured spectrophotometrically, and contents computed according to Arnon (1949). Remaining leaf samples (blades and petioles) were dried to constant weight at 65O C, ground to 40 mesh, then analyzed for total N (including nitrate) by the micro-Kjeldahl procedure (Bremner and Mulvaney 1982); P by the molybdenum- blue technique (Chapman and Pratt 1961); and K, calcium (Ca), and magnesium (Mg) by atomic absorption (Perkin-Elmer Corporation 1976). Determination of total available carbohydrates was done by extraction and hydrolysis in sulfuric acid (Smith et al. 1964). Concentrations of total available carbohydrate were calculated as percent glucose in ovendried leaf tissue.

Tree height and basal diameter (0.15 m) were measured at the end of the first growing season and in the second growing season when the leaves were sampled. The plantation was thinned the same week the terminal leaves were collected and total above-ground biomass was determined for each tree by weighing the entire above-ground portion of the plant. In addition, half of these thinned trees were partitioned into stem, branches, and leaves, and component weights and leaf areas were determined. Subsamples were taken to determine moisture content and fresh weight/dry weight relationships. For the trees that were not partitioned, total plant leaf area and leaf weight were predicted from total plant weight using regression equations developed for each clone and irrigation regime.

Data Analysis

Analyses were done to explore relationships among leaf characteristics and tree productivity rather than to test specific models or determine significant differences among discrete fertilizer treatments. Lateral root development was rapid and roots of trees planted in one subplot treatment commonly extended into adjacent fertilizer and irrigation treatments. Thus, individual tree characteristics did not reflect responses to distinct treatments; however, the treatments provided an extensive variety of growing conditions that resulted in a wide range of tree sizes, growth rates, and leaf characteristics.

Plottings of tree biomass or growth (height and diameter growth during the second growing season) versus leaf characteristics were examined. Non-linear relationships between variables were not apparent and data transformations were assumed unnecessary for subsequent analyses. Simple correlation coefficients were determined between the biomass or growth variables and the leaf characteristics for each clone and irrigation regime. If differences in the relationships observed between irrigation regimes or clones were non-significant or minimal, the data were pooled and the correlations determined for the combined observations. Only correlations for variable combinations for which at least one of the clones had an r value > 0.60 are presented. Simple and multiple regression equations using leaf size and chemical characteristics were examined for their ability to predict 2nd year height growth. The biomass and growth variables were also examined using analysis of variance with clone, irrigation, and clone by irrigation as the model sources of variation.

Results

Mean growth of both clones in the study plantation was good, but due to the imposed range in nutrient conditions, plant biomass and growth rates varied substantially within each clone and irrigation regime (Table 1). Mean total above-ground biomass, height growth and diameter growth were significantly greater for clone 11-11 than for clone 7-75 and greater for the high versus the low irrigation regime.

Clone 11-11 had greater mean area and weight of terminal leaves, and greater total plant leaf area (or weight) than clone 7-75 (Table 2). Leaf length was similar but shape of the leaves differed between clones with 11-11 having a broader leaf base and a more deltoid shape. Clone 7-75 had higher mean concentrations of N, P, K, and chlorophyll and lower concentrations of Ca than clone 11-11, but concentrations of Mg and total available carbohydrate were similar in both clones. On average, high irrigation increased mean area and weight of terminal leaves, total plant leaf area, and P concentrations, but decreased concentrations of most other nutrients and chlorophyll. Total available carbohydrates were essentially unaffected by irrigation regime. Potassium levels in clone 11-11 were also unaffected by irrigation; in clone 7-75, however, K levels were substantially increased in the high irrigation regime.

Mean area and weight of terminal leaves were positively correlated with total above-ground biomass, height growth and diameter growth of both clones (Table 3). The correlations between mean weight of terminal leaves and total biomass or growth variables were similar to those for mean terminal leaf area because area and weight of terminal leaves were very strongly correlated (r=0.97 for clone 11-11; r=0.89 for clone 7-75). Correlations between total plant leaf area or leaf weight and total biomass were stronger than those of terminal leaf traits with total biomass or diameter; however, correlations of total plant leaf traits with height growth were weaker than those of terminal leaf traits with height growth.

Foliar K concentrations were correlated with biomass and growth of clone 7-75, but not with biomass or growth of clone 11-11 (Table 3). Foliar concentration or content (i.e., concentration multiplied by mean leaf weight) of most nutrients, chlorophyll, and available carbohydrates were significantly correlated (P < 0.05) with total biomass and growth of both clones. In all cases except K concentration and height growth of clone 7-75, however, correlations between biomass or growth and mean area and weight of terminal leaves or total leaves were substantially higher than correlations with concentrations or contents of nutrients and chlorophyll. For clone 7-75, the multiple regression equation predicting height growth which included K concentration and mean terminal leaf area (R2=0.53) accounted for substantially more variation than the equation using only mean terminal leaf area (R2=0.39). For clone 11-11, none of the multiple regression equations which included additional foliar variables increased R2 values more than 0.02 over the equation with mean terminal leaf area as the independent variable.

Relationships between mean terminal leaf area and height growth were similar for high irrigation and low irrigation regimes of clone 11-11, despite the fact that trees in the high irrigation regime generally had much larger leaves (Fig. 1). Moreover, the relationship between leaf area and height growth for trees of clone 7-75 in the low irrigation regime (Fig. 1) was essentially identical to the relationship for clone 11-11. The relationship for trees of clone 7-75 in the high irrigation regime, however, was not as strong (i.e., the slope of the line was not as steep). Maximum area per terminal leaf was clearly lower for clone 7-75 than for clone 11-11; none of the clone 7-75 trees had mean leaf areas > 500 cm2 whereas 14 percent of clone 11-11 had leaf areas exceeding that size.

Figure 1. The relationship between mean area of fully expanded terminal leaves and 2nd-year height growth for clone 11-11 and clone 7-75. For 11-11, one regression relationship was sufficient for both low and high irrigation regimes (y=0.0043 x + 0.8). For 7-75, the low irrigation regime could be described by the same equation as for 11-11, but the high irrigation regime was best described by y=0.0014 x + 2.1.

Discussion

A strong correlation between total leaf area or total leaf weight and tree productivity (size or growth) has been reported previously (Larson and Isebrands 1971, Larson et al. 1976) as have high positive correlations between mean size of mature leaves and relative growth of various Populus clones (Ridge et al. 1986, Isebrands et al. 1988, Ceulemans 1990). The present study expands on these general relationships to demonstrate a very simple, yet strong correlation between mean size of the fully expanded terminal leaves and productivity of two Populus clones. Our findings apply to within clonal differences in productivity (size and growth) "created" by manipulating several growth factors -- N, P, K, and lime amendments and water availability.

Despite the range of nutritional status affecting tree size and growth and significant correlations between production variables and chemical concentrations or contents, mean size of terminal leaves (area or weight) was more strongly correlated with productivity than concentration or content of any single chemical with the one exception of K concentration and height growth in clone 7-75. Thus, this simple, easy-to-measure characteristic may be a very useful indicator of potential productivity or future growth. It merits further testing as a possible tool to aid in site selection, matching clones to sites, or monitoring tree response to cultural treatments as well as providing an early indicator of the relative performance of various clones.