A joint individual-based model coupling growth and mortality reveals that tree vigor is a key component of tropical forest dynamics

Tree vigor is often used as a covariate when tree mortality is predicted from tree growth in tropical forest dynamic models, but it is rarely explicitly accounted for in a coherent modeling framework. We quantify tree vigor at the individual tree level, based on the difference between expected and observed growth. The available methods to join nonlinear tree growth and mortality processes are not commonly used by forest ecologists so that we develop an inference methodology based on an MCMC approach, allowing us to sample the parameters of the growth and mortality model according to their posterior distribution using the joint model likelihood. We apply our framework to a set of data on the 20-year dynamics of a forest in Paracou, French Guiana, taking advantage of functional trait-based growth and mortality models already developed independently. Our results showed that growth and mortality are intimately linked and that the vigor estimator is an essential predictor of mortality, highlighting that trees growing more than expected have a far lower probability of dying. Our joint model methodology is sufficiently generic to be used to join two longitudinal and punctual linked processes and thus may be applied to a wide range of growth and mortality models. In the context of global changes, such joint models are urgently needed in tropical forests to analyze, and then predict, the effects of the ongoing changes on the tree dynamics in hyperdiverse tropical forests. (Résumé d'auteur

A silviculture-oriented spatio-temporal model for germination in Pinus pinea L. in the Spanish Northern Plateau based on a direct seeding experiment

Natural regeneration in Pinus pinea stands commonly fails throughout the Spanish Northern Plateau under current intensive regeneration treatments. As a result, extensive direct seeding is commonly conducted to guarantee regeneration occurrence. In a period of rationalization of the resources devoted to forest management, this kind of techniques may become unaffordable. Given that the climatic and stand factors driving germination remain unknown, tools are required to understand the process and temper the use of direct seeding. In this study, the spatio-temporal pattern of germination of P. pinea was modelled with those purposes. The resulting findings will allow us to (1) determine the main ecological variables involved in germination in the species and (2) infer adequate silvicultural alternatives. The modelling approach focuses on covariates which are readily available to forest managers. A two-step nonlinear mixed model was fitted to predict germination occurrence and abundance in P. pinea under varying climatic, environmental and stand conditions, based on a germination data set covering a 5-year period. The results obtained reveal that the process is primarily driven by climate variables. Favourable conditions for germination commonly occur in fall although the optimum window is often narrow and may not occur at all in some years. At spatial level, it would appear that germination is facilitated by high stand densities, suggesting that current felling intensity should be reduced. In accordance with other studies on P. pinea dispersal, it seems that denser stands during the regeneration period will reduce the present dependence on direct seeding

Trade-offs between vegetative growth and acorn production in Quercus lobata during a mast year: the relevance of crop size and hierarchical level within the canopy

The concept of trade-offs between reproduction and other fitness traits is a fundamental principle of life history theory. For many plant species, the cost of sexual reproduction affects vegetative growth in years of high seed production through the allocation of resources to reproduction at different hierarchical levels of canopy organization. We have examined these tradeoffs at the shoot and branch level in an endemic California oak, Quercus lobata, during a mast year. To determine whether acorn production caused a reduction in vegetative growth, we studied trees that were high and low acorn producers, respectively. We observed that in both low and high acorn producers, shoots without acorns located adjacent to reproductive shoots showed reduced vegetative growth but that reduced branch-level growth on acorn-bearing branches occurred only in low acorn producers. The availability of local resources, measured as previous year growth, was the main factor determining acorn biomass. These findings show that the costs of reproduction varied among hierarchical levels, suggesting some degree of physiological autonomy of shoots in terms of acorn production. Costs also differed among trees with different acorn crops, suggesting that trees with large acorn crops had more available resources to allocate for growth and acorn production and to compensate for immediate local costs of seed production. These findings provide new insight into the proximate mechanisms for mast-seeding as a reproductive strategy

Analysis and interpretation of forest fertilizer experiments

Sir Ronald Fisher's contributions to the design and analysis of field experiments have had a profound influence on the quality of forest fertilizer trials. Nevertheless, forestry experiments have unique features which are not always amenable to adoption of the routine analytical methods that are discussed and illustrated in many statistical texts. The aim of this thesis is to explore the nature and statistical standards appropriate for the analysis of forest fertilizer trials. The general linear model (GLM), embracing the techniques of analysis of variance, covariance, and regression analysis, represents a powerful tool with which to analyse forest nutrition experiments. An examination of pertinent literature reveals, however, that substantial numbers of forestry researchers have difficulty in applying GLM methodology to forest trial data. Indeed, there is clear evidence that some scientists are unaware of the utility of covariance as a means of removing the confounding effect of differences in initial plot growing stock; other researchers fail to extract all the data inherent in forest trial data, while a few advocate abandoning GLM methodology altogether, claiming it to be an insensitive tool for analysing forest fertilizer experiments. Re-analysis here shows how inappropriate some of these published results and claims are. A system of analysis is presented which is considered a reliable and sensitive procedure for examining later-age forest fertilizer trials. For non-factorial layouts, the method employs applications of regression analysis, allowing responses in each treatment to be represented by a unique intercept and regression slope. Tests of hypotheses are introduced to determine the need for disparate slopes and independent intercepts; alike parameters are pooled to achieve a minimum available residual error. Secondary covariates such as stand competition or plot fertility values are added to models whenever pertinent, to decrease experimental errors further, and to aid interpretation of trial results. Factorial experiments are analysed very similarly, but utilizing factorial linear models and (multiple) covariance. Appropriate manipulation of response variables and covariates is integral to the recommended system. The use of yield or growth as a response variable is demonstrated to give essentially equivalent results. Adoption of average yield per tree often achieves a more decisive analysis than with per hectare variables. Use of weighted least-squares can aid the interpretation of trial results when some plots have been partially damaged. The presented system is tested by examining data from eight later age (some long term) fertilizer trials established by N.Z.Forest Products Limited. Analysis and derived results completely vindicate the value of the proposed methodology; in particular, use of two covariates increases precision in some analyses by up to 76%. The analyses irrefutably confirm the potential of nitrogen fertilizer to boost yields in thinned Pinus radiata stands belonging to the Company. These responses are demonstrated to be associated frequently with significant, but small and transient changes in stand form-factor. Examination of the basal-area responses in each of the eight experiments highlights the fact that point estimation of fertilizer gain is inadequate for management planning and forecasting; yield tables of nutrient response are required, expressed as quantity of wood that can be realised in the future. The desired form can sometimes be achieved by modelling trial data to obtain a growth and yield model, then applying suitable realisation factors. Such a simulator has therefore been derived for the N.Z.Forest Products data, recognising variable inputs of fertilization, initial basal-area, initial stocking, and stand competition; experimental fertilizer response is estimated to be about 110 m³/ha by age 30 years. The suggested methodology has several implications for fertilizer trial experimental design as well as analysis; thus, it is now recommended that trials should be installed deliberately with a range of initial plot growing stock and to contain treatments which are distinctly different in their composition. Also, because multiple covariance is demonstrated to be a powerful technique to obtain good precision and additional information in later-age forest fertilizer trials, care must be taken to collect prior initial information about the trial material