Climate and population origin shape pine tree height-diameter allometry

Abstract

Tree height-diameter allometry, the link between tree height and trunk diameter, reflects the evolutionary response of a particular species’ allocation patterns to above and belowground resources. As a result, it differs among and within species due to both local adaptation and phenotypic plasticity. These phenotypic variations in tree height-diameter allometry determine tree productivity, resistance and resilience to climate variation and, ultimately, the success of plant material used in restoration projects. We tested the effect of climate change and population origin on the phenotypic variation of tree allometry in four pine species at an early stage of development (ca. 11 years old) based upon data originated from multi-site provenance tests and planted along a wide climatic range in south-western Europe. For a representative sample of populations from each species, we used already-developed species-specific height-diameter allometric models to assess changes in allometry between present and future climatic conditions. We found that Pinus halepensis and Pinus pinaster were the most plastic species, while Pinus sylvestris and Pinus nigra showed negligible plastic responses. In addition, our models stressed that pine tree height-diameter allometry will change and phenotypic variation could increase, except in P. sylvestris, under future environmental conditions. For some of the species, this might allow the selection of phenotypes better suited to novel climatic conditions. These foreseeable changes in tree height-diameter allometry (among and within-species) could entail eco-evolutionary effects on the early forest plantation dynamics. Therefore, restoration and reforestation plans should consider these effects, as they may interfere with production and/or environmental goals.

Appendix: Tree height-diameter allometry model

The tree height-diameter allometry model uses measurements of tree height (total height in cm, measured with a pole) and dbh [diameter at breast height (130 cm) in mm, measured with a caliper] collected at 11 ± 1 years of age. Competition can be an important factor to better understand tree allometry in trees (e.g. Weiner et al. 2001; Lines et al. 2012). However, in our case of study we selected a young developmental stage to minimize interpopulation competition effects in the experimental design. Moreover, a previous study using the same experimental setup did not find inter- or intra-population competition effects in any of the two variables measured (height and dbh) in 32-year-old P. pinaster individuals (Alía et al. 2001).

Based upon literature and exploratory analyses, we selected the subset of climatic variables most relevant at the testing sites to plant allometry for the four pine species studied. The selected variables were MMT (minimum temperature of coldest month,  °C), and AP (annual precipitation, mm). Both MMT and AP affect physiological and growth processes of plant species in the Mediterranean region (Thompson 2005), and have been consistently used in previous studies (e.g. Wang et al. 2006; O’Neill and Nigh 2011; Leites et al. 2012). Geographical variables of the populations’ site of origin, such as latitude, longitude and altitude, are surrogates for environmental conditions, e.g. the amount of heat energy received relative to the sun angle, temperature, humidity, and solar radiation; as they can usually reflect adaptation patterns to local conditions (see Alberto et al. 2013).

We built the best tree height-diameter allometry model by considering several variations of the basic model (see below Eq. 4), where a and c scaling parameters were constant, and they were estimated with different combinations of the variables associated with the growing sites and the origin of populations. The best final model structure was selected based on both biological relevance and the DIC criterion. There were some model structures that could not be fitted due to problems of model convergence.

The final model estimated tree height allometry as a combination of climate at the testing site (s) and geographic characteristics at the origin site of the population (p).

Considering an individual i, from population p growing in testing site s, its height-diameter allometry was modeled as:

Likelihood: height_i ~ log Normal (H _i , σ ²) and the following process model:

$${\text{H}}_{\text{i}} = \ln \left( {a_{p(i),s(i)} } \right) + c_{p(i)} \times dbh_{i}$$

(4)

where the scaling coefficient $\ln (a_{p(i),s(i)} )$ was estimated as:

$$\ln (a_{p(i),s(i)} ) = \alpha_{1p} + \alpha_{2p} \times MMT_{s} + \alpha_{3p} \times AP_{s}$$

(5)

and the scaling exponent, $c_{p(i)}$, was estimated as:

$$c_{p(i)} = \beta_{1} + \beta_{2} \times LAT_{\text{p}} + \beta_{3} \times ALT_{\text{p}}$$

(6)

Tree height-diameter allometry, therefore, is the outcome result of population genetic effects on the basal height, parameterized in α _1p ; plus a genetic (population) clinal climatic pattern of the scaling exponent on latitude and altitude (β ₂ × LAT _p , β ₃ × ALT _p ), and of genetic differential plastic responses along temperature or and precipitation gradients of the testing site (α _2p  × MMT _s , α _3p  × AP _s ). Because all explanatory variables were standardized, parameter α_1p was the allometric curve’s intercept at average climate conditions of across all growing sites and β ₁ is the intercept of average diameter.

We used this model with its associated parameters’ means, variances and covariances to generate the necessary output data for our study, which aims to assess the effect of climate change on tree height-diameter allometry. Specifically, we generated total height at a fixed dbh (100 mm) of a specific population sample from each species within testing sites under present and future climatic conditions (see main text). We used OpenBUGs 1.4 to generate the output data (Thomas et al. 2006).